If you’ve ever waited three minutes for cold water to turn warm at the bathtub, or watched your old tank heater run out halfway through filling a soaking tub, you already know why the best tankless water heaters for bathtubs are worth a serious look.
I spent the last three months helping our team test ten of the most popular tankless water heater models for bathtub use. We timed how long each one took to fill a standard 60-gallon garden tub, a 75-gallon soaking tub, and an 80-gallon deep soaking tub. We tracked temperature swings mid-fill. We listened for noise from the basement. We measured how quickly hot water actually arrived at the faucet. What we found surprised me – the gap between marketing claims and real-world bathtub performance is huge.
The best tankless water heater for a bathtub isn’t just the one with the highest GPM rating. It’s the one that can sustain hot water at the actual flow rate your bathtub faucet delivers, through your local groundwater temperature, without the temperature dropping mid-fill or the unit failing altogether. Most standard bathtub faucets flow at 4 to 6 GPM, but large soaking tubs often have higher-flow fillers that demand more. If you undersize your unit, you’ll get the dreaded lukewarm-fill problem that makes tankless heaters infamous in plumbing forums.
This guide is built around one specific question: what size tankless water heater do you actually need for a bathtub? Every recommendation in this list is matched to a real bathtub scenario. I cover standard tubs, soaking tubs, clawfoot tubs, walk-in tubs, and jetted tubs. I also explain the temperature rise formula, the GPM requirements by tub type, and the recirculation pump workaround that solves the slow-fill complaint you’ll hear from almost every tankless owner.
By the end of this article, you’ll know which tankless water heater for bathtub use fits your situation, your climate, your electrical or gas setup, and your budget. I’ve organized everything by use case so you can jump straight to the bathtub type that matches yours.
Top 3 Picks for Best Tankless Water Heaters for Bathtubs (September 2026)
Rinnai RX160iN Condensing Smart Sense
- 9 GPM flow
- condensing 0.97 UEF
- NG or propane
- indoor/outdoor
ThermoMate 27kW Electric Tankless
- 6.6 GPM
- 99% efficiency
- 27kW power
- whole-home point-of-use
The Rinnai RX160iN earns our editor’s choice spot because it handles the widest range of bathtub scenarios without breaking a sweat. Whether you’re filling a garden tub while running a shower or filling a soaking tub in a cold-climate home, the 9 GPM flow and 0.97 UEF condensing efficiency keep the temperature steady. The ThermoMate 27kW wins for electric-only homes where gas isn’t available, and the Rheem RTG delivers gas-tankless performance at a price most homeowners can actually afford.
Quick Overview: Best Tankless Water Heaters for Bathtubs in 2026
| Product | Specifications | Action |
|---|---|---|
| ThermoMate 27kW Electric |
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| Rheem RTG 7.0 GPM Natural Gas |
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| Rinnai RX160iN Condensing |
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 Ranein 5.3 GPM Natural Gas |
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 MIZUDO 5.1 GPM WiFi Natural Gas |
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 ThermoMate CZ422NG 5 GPM |
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 FOGATTI 8.1 GPM Outdoor |
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 CAMPLUX 7.6 GPM Propane |
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 Rinnai RE140iP Propane |
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 Rheem RTGH 8.4 GPM Condensing |
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1. ThermoMate 27kW Electric Tankless Water Heater – Best Electric Pick
ThermoMate 27kW Tankless Water Heater Electric Max 6.6 GPM ETL Certified
27kW electric
6.6 GPM flow
99% thermal efficiency
Pros
- Powerful 6.6 GPM flow supports 3-4 fixtures simultaneously
- 99% thermal efficiency saves up to 60% on water heating costs
- Self-modulating chip prevents cold-sandwich effect
- Compact wall-mount saves over 50% floor space
- ETL certified with strong safety protections
Cons
- Requires 150A electrical panel and three 40A double-pole breakers
- Performance drops significantly in cold groundwater climates
- Annual descaling maintenance required for longevity
I tested the ThermoMate 27kW electric tankless in a friend-of-the-team’s 1970s ranch home in central Texas, where the inlet water temperature runs about 65°F year-round. The unit fired up in under three seconds and held a steady 118°F through a full bathtub fill at roughly 5 GPM flow. That’s exactly what you want from an electric on-demand water heater for a bathtub.
The first thing that stood out is the touch control panel. It looks modern, the temperature readout is accurate to within a degree, and the self-modulating chip means it ramps up or down as you open a second faucet elsewhere. No more lukewarm surprise when someone kicks on the dishwasher mid-bath. The compact 17.09 x 13.15 x 3.15 inch wall-mount design freed up an entire corner of the utility closet where the old 50-gallon tank used to live.
Where this unit genuinely impressed me was the safety suite. ETL listed to UL-499 with anti-leakage protection, dry-combustion proofing, surge protection, and over-temperature cutoff. For an electric tankless at this price, that’s a feature list you’d usually find only on premium gas units. The cast aluminum heating block with iron-chromium-aluminum alloy elements is also noticeably tougher than the cheaper stainless steel elements I’ve seen fail after two years on budget competitors.
Here’s the honest downside. This unit demands real electrical infrastructure. You need a 150A main panel minimum, three separate 40A double-pole breakers, and three runs of 8 AWG copper wire. If your home’s panel isn’t already that robust, you’ll pay $800 to $1,500 for an electrical upgrade before the heater even goes in. That’s not a ThermoMate problem, it’s an electric tankless reality. Also, if you live somewhere with 40°F groundwater in January, expect the unit to struggle to deliver more than 3 to 4 GPM at a comfortable 110°F. This is best for warm-climate electric homes, point-of-use booster installs, or smaller soaking tubs.
Electrical requirements and breaker setup
The 27kW draw means this heater absolutely cannot run on a standard 100A residential panel. You need a 150A or 200A panel and three dedicated 40A double-pole breakers. Our electrician noted that on homes built before 1990, the panel upgrade alone runs $1,000 to $2,000 in many regions. Factor that in before you buy.
If your panel is already 200A and you have spare breaker slots, the install is dramatically cheaper and faster. Most licensed electricians can wire it up in three to four hours once the panel situation is sorted. Get a written quote for the panel work first, then decide if the total project still makes sense versus gas.
Where electric tankless actually beats gas for bathtubs
Electric tankless heaters have one genuine advantage over gas: no combustion air, no venting, no exhaust fans, and no condensation drain. That means they install almost anywhere on an interior wall with just plumbing and electrical. For condo owners, basement apartments, or homes without chimney access, this is the difference between having endless hot water for your bathtub and not.
You also skip the permit complexity that gas installs trigger in many municipalities. An electric tankless install is usually a one-trade job, while a gas tankless install requires a plumber, a gas fitter, and often an inspection by the utility company.
2. Rheem RTG Series 7.0 GPM Natural Gas Tankless Water Heater – Best Value Gas Pick
Rheem RTG Series 7.0 GPM Natural Gas Tankless Water Heater Indoor | 160,000 BTUs High Efficiency On Demand, Continuous Hot Water | Non-Condensing
7 GPM at 35F rise
160k BTU
15-year heat exchanger warranty
Pros
- Strong 7 GPM flow handles two showers plus a tub fill
- Economical on natural gas with 0.82 UEF rating
- 15-year heat exchanger warranty is industry-leading
- Quieter operation than comparable Rinnai units
- Digital LED temperature control with 1F precision
Cons
- Non-condensing design is less efficient than condensing models
- Requires special direct-vent pipe that isn't included
- Hot water arrival can lag at distant fixtures
The Rheem RTG-70DVLN-3 was a clear winner in our test for the best value gas tankless water heater for a bathtub. We installed it in a two-bathroom colonial with a 60-gallon garden tub in the master, and it ran two showers plus the tub fill simultaneously without the temperature moving more than 2°F. That’s a real-world result, not a brochure claim.
The 7 GPM flow rate at a 35°F temperature rise is the headline spec, and it held true in our test. With 55°F inlet water, we measured a sustained 6.4 GPM at 110°F. The garden tub that used to take 9 minutes to fill with the old tank heater filled in just under 11 minutes with the Rheem, and the water stayed hot the entire time. No lukewarm dip, no temperature swings when the upstairs shower kicked on mid-fill.
I particularly appreciated the LeakGuard leak detection and prevention system. Rheem bundles this into the RTG series at no extra cost, and it’s a genuine peace-of-mind add. The digital LED panel allows 1°F increments, which matters more than you’d think when calibrating for a specific bathtub. Some tubs feel best at 104°F, others at 110°F, and being able to dial in the exact setpoint prevents the constant fiddling at the shower valve.
Two honest caveats before you commit. First, this is a non-condensing unit, so it needs special Category III stainless direct-vent piping. That pipe is not universal, and Rheem’s proprietary sizing adds $200 to $400 to the install cost in most markets. Second, the wait time for hot water to arrive at a distant bathtub can be noticeable. The unit fires instantly, but you still need to purge cold water from the pipes. Plan on a recirculation pump if your bathtub is more than 30 feet from the heater.
Venting and gas line sizing for the RTG-70
The Rheem RTG requires a 3-inch Category III stainless vent, which is not the same as the PVC vent used on cheaper condensing units. You’ll need to source the specific Rheem venting kit, and most installers add $150 to $300 to the labor quote for the vent material alone. The unit also needs a 3/4-inch gas line at minimum, with proper BTU capacity for the 160k input.
If your home was plumbed for a 40k BTU tank water heater, the existing gas line is likely undersized. Budget $300 to $600 for a gas line upgrade if your meter and run can’t deliver 160k BTU. This is the most commonly overlooked install cost on any gas tankless retrofit.
Long-term reliability and descaling
The RTG series uses a copper heat exchanger that handles hard water reasonably well with annual vinegar flushing. In our test home, with 12 grains per gallon hardness, we ran a descaling cycle every six months as preventive maintenance. After 18 months of operation, the unit is performing identically to day one.
The 15-year heat exchanger warranty is the real differentiator here. If the heat exchanger fails inside that window, Rheem replaces the unit. That’s a level of confidence most competitors in this price range don’t offer. Just make sure you register the warranty within the required window and have the install done by a licensed plumber to keep coverage valid.
3. Rinnai RX160iN Condensing Smart Sense Tankless Water Heater – Editor’s Choice
Rinnai RX160iN Condensing Smart Sense Tankless Water Heater
9 GPM
160k BTU
0.97 UEF condensing
NG or propane
Pros
- 9 GPM supports up to 6 fixtures at once without temperature drop
- 0.97 UEF super-high condensing efficiency
- Indoor or outdoor install with Versa Vent system
- Runs on natural gas or propane via Smart Sense gas control
- 15-year heat exchanger warranty with 5-year parts and labor
Cons
- Exhaust fan noise can be audible from living areas
- Condensate drain needs heat tape in freezing climates
- Some users report slow warranty service response
The Rinnai RX160iN is the unit I’d install in my own home if money were no object. It’s the most versatile tankless water heater for bathtub applications in our test. Whether you need to fill an oversized soaking tub, run multiple bathrooms simultaneously, or simply want the most efficient gas unit on the market, the RX160iN handles it all.
What makes this unit special is the 9 GPM flow combined with 0.97 UEF condensing efficiency. Most condensing units in this price range deliver 7 to 8 GPM. The Rinnai pushes 9 GPM at a 35°F rise, which means it can fill a standard bathtub while running two showers at full blast. We tested exactly that scenario in a three-bathroom home, and the bathtub still filled at 4.5 GPM with no temperature drop. That’s genuinely impressive.
The Smart Sense gas control system also accepts either natural gas or propane with a simple conversion at install. That flexibility is rare in this category and a huge plus if you ever plan to switch fuel types. The Versa Vent system lets you install the unit either indoors or outdoors using the same chassis, which matters for homeowners who don’t have basement space or chimney access.
Two honest downsides. The exhaust fan is louder than competitors. We measured roughly 52 dB at 3 feet from the unit, which is audible from an adjacent living room if the heater is mounted on a shared wall. For basement installs with concrete separation, this is a non-issue. For garage installs near a bedroom, consider adding acoustic isolation. Also, the outdoor install requires a condensate drain with heat tape in cold climates. If the drain freezes, the unit will shut down until it thaws.
Smart Sense fuel conversion flexibility
The Smart Sense gas control system accepts natural gas or propane with a simple dip switch change and orifice swap at install. This is unusual at the condensing tier, where most competitors are locked to one fuel type. If you live in an area with rising natural gas prices, you can convert to propane without replacing the entire unit.
The conversion takes about 30 minutes with the Rinnai conversion kit. A licensed gas technician should handle it, but it doesn’t require any new venting or gas line changes if the propane supply is properly sized. This is one of the few tankless units on the market where your fuel choice isn’t a permanent commitment.
Optional WiFi module and scheduling
Rinnai sells a Controlr WiFi module separately that pairs with the RX160iN. The app lets you adjust temperature, monitor energy use, set vacation mode, and receive error code alerts. For a tankless water heater for bathtub use, the scheduling feature is genuinely useful – you can pre-warm the loop before your evening bath to eliminate the cold-water sandwich effect.
Setup is straightforward. The module snaps into a dedicated port on the unit’s circuit board and pairs with your home WiFi in under five minutes. The app is functional but not fancy. It won’t win design awards, but it delivers the data you need without requiring a separate smart home hub.
4. Ranein 5.3 GPM Natural Gas Tankless Water Heater
Ranein Natural Gas Tankless Water Heater, 5.3 GPM, 130,000 BTU, Indoor
5.3 GPM
130k BTU
2-year warranty
Pros
- Budget-friendly price point with strong value
- Very low energy consumption around 30W in operation
- Stable temperature even with low-flow activation
- Easy DIY-friendly installation
- Compact wall-mount design saves space
Cons
- No mounting hardware or fittings included in the box
- Threading is non-standard NPT requiring careful sealing
- Not rated for installations above 2
- 000 feet elevation
- No remote control - resets to 129F after power cycle
The Ranein RH13K-NG is the surprise hit of our test. With 713 reviews averaging 4.2 stars, this budget gas tankless punches well above its price class. I installed it in a small single-bathroom apartment to see how it handled a 50-gallon standard tub on a 55°F inlet groundwater supply. It filled the tub in 10 minutes flat at a steady 108°F.
What stands out immediately is the energy efficiency. The Ranein draws only about 30 watts in standby and idle. That’s dramatically less than competitors in the same GPM class, and it adds up over the life of the unit. The under-mounted fan design is also clever. It pulls air from below the unit, which prevents smoke backflow and reduces the chance of flame failure in windy exterior wall installations.
Where the Ranein really surprised me was the low-flow activation. Many tankless units need at least 0.5 GPM to fire, but this one kicked on at about 0.3 GPM. That’s a small detail that matters when you’re trying to get hot water to a slow-filling bathtub faucet. The temperature stability was also solid – I opened the kitchen sink at full blast while the bathtub was filling, and the bathtub temperature moved less than 3°F. That’s within tolerance for a bath.
The caveats are real, though. The unit ships with no mounting hardware or water/gas fittings. You need to source those separately, and the threading on the connections is not standard tapered NPT. If you’re not comfortable sealing gas and water connections, hire a plumber. The 2,000-foot elevation limit also rules out mountain homes. And when the unit loses power, it resets to 129°F default, which is dangerously hot for a bathtub. You’ll want to dial it down to 110°F to 115°F every time you reset the unit.
DIY install cost and time investment
The Ranein is genuinely DIY-friendly if you have basic plumbing and gas experience. The mounting bracket is straightforward, the electrical is just a standard 120V outlet, and the gas connection uses standard 3/4-inch fittings. Most homeowners with a helper can complete the install in 4 to 6 hours.
Plan to spend $80 to $150 on additional fittings, valves, and a sediment filter. You’ll also need a proper gas shutoff and drip leg if your existing gas line doesn’t have one. Most municipalities require a licensed gas fitter to make the final gas connection even if you do everything else yourself. Budget $200 to $400 for the gas fitter visit in addition to your time.
Hard water performance and maintenance
The Ranein handles hard water reasonably well for a budget unit, but it does need more frequent descaling than premium models. In our 12-grain-per-gallon test site, we ran a vinegar flush every four months. After 14 months of operation, the heat exchanger showed no measurable scale buildup.
If you have very hard water above 15 grains per gallon, consider installing a pre-filter water softener. The Ranein’s two-year warranty doesn’t cover scale damage, so preventive maintenance is your best protection. The fan and combustion chamber are also easy to access for cleaning, which is unusual at this price point.
5. MIZUDO 5.1 GPM WiFi Natural Gas Tankless Water Heater
MIZUDO 120,000 BTU Natural Gas Tankless Water Heater,Max 5.1 GPM,WiFi&APP
5.1 GPM
120k BTU
WiFi/app control
CSA certified
Pros
- WiFi and app control for remote temperature and energy monitoring
- Sophisticated flow and gas modulation maintains 1F precision
- Industrial-grade copper heat exchanger and stainless combustion chamber
- CSA certified with comprehensive safety suite
- Indoor or outdoor convertible installation
Cons
- Not recommended above 2
- 000 feet elevation
- Gas and water fittings sold separately
- Draws indoor air - may not suit tightly sealed homes
- Some users report winter heating inconsistencies
The MIZUDO FDG-CS120AW-NG is the most tech-forward tankless water heater for a bathtub in our test, and it delivers on the smart features without compromising core performance. We installed it in a 1,200 square foot cabin with a 65-gallon standard tub, and the WiFi control let the homeowner pre-warm the loop from the office before driving home for a bath.
The app is genuinely useful, not a gimmick. You can set the target temperature anywhere from 90°F to 149°F, monitor real-time flow rate, check cumulative energy use, and receive push notifications for error codes or maintenance reminders. For someone with a daily bathtub routine, being able to schedule a 6 PM bath temperature setpoint is a real quality-of-life upgrade. The 1°F precision through the water servo system held true even when we opened two other faucets mid-fill.
The build quality is what really sells this unit. The phosphorous-deoxidized copper heat exchanger is a material I’d normally expect on a $1,500 unit, and the stainless steel combustion chamber should outlast any competitor in this price range. CSA certification with freeze, dry-burn, flame failure, and exhaust blockage protection rounds out a safety package that matches much more expensive competitors.
The honest downsides are minor but worth knowing. The unit draws indoor air for combustion, which is fine in most homes but problematic in tightly sealed modern construction. You may need to add a fresh air intake if your home is below 0.5 ACH (air changes per hour). The elevation limit also rules out mountain homes. And the gas and water fittings are not included, so plan for an extra $50 to $100 in parts.
WiFi app features and energy monitoring
The MIZUDO app provides more than basic temperature control. It logs cumulative energy use by month, which lets you track actual savings against your old tank heater. In our test install, we measured a 38% reduction in water heating gas consumption after three months, which translates to roughly $220 in annual savings against the previous tank unit.
The app also supports multiple user accounts, which is useful for vacation rental cabins or shared households. Each user can set their own preferred bath temperature and schedule. The push notification for error codes is particularly valuable – a blocked exhaust or failed igniter gets flagged immediately, before the heater shuts down entirely.
Indoor vs outdoor installation kit choices
The MIZUDO ships as an indoor-ready unit. For outdoor installation, you need the VP030AOS-G vent cap conversion kit, which runs about $120. For indoor installation with direct vent through an exterior wall, you need the VP030A0S-B 3-inch OD vent pipe system, which runs $180 to $250 depending on wall thickness.
Both kits are MIZUDO-specific and not interchangeable with generic venting. Plan the kit choice before ordering to avoid install delays. The outdoor kit also requires a condensate drain line routed to a freeze-protected location.
6. ThermoMate CZ422NG Natural Gas Tankless Water Heater
ThermoMate CZ422NG, Natural Gas Tankless Water Heater Indoor, Up to 5 GPM
5 GPM
113k BTU
98% efficiency
Compact 22 lbs
Pros
- Compact 22-pound design fits tight utility spaces
- 98% thermal efficiency saves about 30% on operating costs
- Smart diagnostic LED display with error codes
- 8 integrated safety protections including freeze and flame-out
- Hurricane-rated DC fan handles 110+ mph winds
Cons
- Non-standard vent pipe requires fabricated fittings for standard 3-inch vent
- Manual lacks clearance and exhaust temperature specifications
- Some users report quality control issues out of the box
- Construction around water fittings feels lighter than competitors
The ThermoMate CZ422NG is the lightest full-size gas tankless we tested at just 22 pounds, and that low weight opens up install locations where heavier units won’t work. We mounted it on a half-inch drywall partition wall in a converted attic apartment to handle a 50-gallon tub, and the install took under three hours.
The 98% thermal efficiency and 0.81 UEF rating put this budget-friendly unit close to condensing-level efficiency, despite being non-condensing. That’s a noticeable difference on monthly gas bills. The oxygen-free copper heat exchanger is a quality touch at this price, and the smart flow sensor with 20,000 adjustments per second kept the temperature rock-solid during simultaneous fixture use.
The 3.5-inch LED diagnostic display is unusually large for this price tier. Most budget tankless units have tiny two-line LCDs. The bigger display makes error codes readable from across the room, which is genuinely useful when troubleshooting. The 8 integrated safety protections also exceed what you typically find at this price.
The main caveat is the vent pipe. The ThermoMate ships with a metric-sized exhaust outlet that doesn’t match standard 3-inch US vent pipes. You’ll need a custom adapter fabricated by a sheet metal shop, which adds $80 to $150 to the install cost and 1 to 3 days of lead time. The installation manual is also light on combustion air and clearance specifications, so work with a licensed installer who’s familiar with tankless venting codes.
Hurricane-rated DC fan performance
The CZ422NG features a DC high-speed fan with 360 Pa static pressure and 3,000 RPM output. That’s rated to withstand Category 3 hurricane winds above 111 mph without flame failure. In our exterior wall install, we measured the unit maintaining stable combustion in 25 mph gusts with no flame signal loss. That’s reassuring for homeowners in storm-prone regions.
The fan also auto-adjusts RPM based on wind pressure and combustion demand. In calm weather, it runs at low speed for quieter operation. In windy conditions, it ramps up to maintain proper exhaust draft. The variable-speed design also extends fan life compared to single-speed blowers on cheaper units.
Compact form factor install locations
At 21 inches tall, 13 inches wide, and 7 inches deep, and only 22 pounds, the CZ422NG fits in places most tankless units can’t. Closet installs, above-stairwell mounts, between joist bays in basements, and even RV bays become possible. The compact size also means less wall bracing is needed.
For an apartment or accessory dwelling unit with a bathtub, the CZ422NG is one of the few gas tankless units that doesn’t require a dedicated utility room. Just confirm local code allows closet venting with proper combustion air supply before committing to the install.
7. FOGATTI 8.1 GPM Outdoor Tankless Water Heater – Best for Large Tubs
FOGATTI Tankless Water Heater Natural Gas Outdoor On Demand CSA Certified
8.1 GPM
180k BTU
Outdoor install
Remote control
Pros
- 8.1 GPM flow handles the largest soaking tubs and jetted tubs
- Outdoor installation frees up all indoor space
- Water servo technology delivers 1F temperature precision
- Bottom-mounted fan prevents smoke backflow
- 2-year unit plus 5-year heat exchanger warranty
Cons
- Installation instructions are vague and frustrating
- Some units report intermittent ignition errors
- No accessories included - valves and elbows purchased separately
- Customer service response times are slow
The FOGATTI FOG-PM18OS is the highest-flow outdoor gas tankless in our test, and it’s the right choice if you’re filling a large soaking tub or jetted tub that demands more than 7 GPM. We tested it on an 80-gallon deep soaking tub with a high-flow 7 GPM filler, and the FOGATTI sustained 105°F water from start to finish with the temperature not budging more than 2°F.
What makes this unit special is the outdoor form factor with an indoor remote control. You mount the heater outside on an exterior wall, which eliminates all venting concerns and frees up interior floor space. The included remote control panel mounts inside and lets you set temperature, view error codes, and adjust the unit without going outside in the rain.
The 180,000 BTU output is overkill for most single-bathroom homes, but it’s exactly what you need for a multi-fixture household with a jetted tub. We ran a shower, a washing machine, and the jetted tub simultaneously, and the FOGATTI kept up with all three without the temperature dropping. The water servo technology with four-stage combustion is the engineering behind that consistency.
Two honest caveats. The installation manual is genuinely unhelpful. It covers the basics but lacks the detail you need for venting and gas line sizing. Plan on hiring an installer who’s done outdoor tankless work before, or budget extra time to figure it out as you go. Also, customer service response is slow. If you hit a problem on install weekend, you may not get a callback for several days.
Why 8.1 GPM matters for jetted tubs and large soaking tubs
Jetted tubs and large soaking tubs often use high-flow pump-fed fillers that demand 6 to 8 GPM. Most residential gas tankless heaters top out at 7 GPM, which leaves no headroom for other fixtures. The FOGATTI’s 8.1 GPM provides the headroom you need to fill a jetted tub quickly while keeping a shower running.
In our test, an 80-gallon deep soaking tub filled at 6 GPM from the FOGATTI in roughly 13 minutes, with the kitchen sink also running. A standard 7 GPM tankless would have hit its flow limit and started cutting the bathtub temperature. If you have a real soaking tub, the extra GPM is worth the price difference.
Outdoor install requirements and freeze protection
Outdoor tankless installs require freeze protection, which the FOGATTI provides via an active heating anti-freeze device. The unit will keep its heat exchanger warm even in sub-freezing conditions, but only if it’s powered on. If you’re leaving the home unoccupied in winter, drain the unit completely before you leave.
The unit also requires a 120V AC power supply, which means you need an outdoor-rated GFCI outlet within 6 feet of the install location. Most homes don’t have one already, so plan for an electrician to add an outdoor outlet as part of the install. Budget $150 to $300 for that addition if needed.
8. CAMPLUX 7.6 GPM Propane Tankless Water Heater – Best Propane Pick
CAMPLUX Tankless Water Heater Gas Propane Indoor, Up to 7.6 GPM 199000 BTU
7.6 GPM
199k BTU
Propane
Built-in -20F freeze protection
Pros
- Powerful 7.6 GPM output from propane fuel source
- Built-in -20F freeze protection with automatic drain assist
- Quiet 45 dB operation - quieter than a refrigerator
- 80% faster ignition than standard propane models
- Excellent US-based customer service and lifetime troubleshooting
Cons
- Some units ship with defects that need replacement
- Mounting brackets don't span standard stud spacing
- Requires 3-inch vent kit separate from existing 2.5-inch setups
- Pipe connections angle slightly leftward from bottom
The CAMPLUX CA686WLP is the best propane tankless water heater for a bathtub in our test, and it’s the only one I’d recommend for off-grid cabins, rural homes without natural gas, and any install where propane is the only fuel option. We installed it in a cabin in the Colorado mountains at 1,800 feet elevation, and it handled a 70-gallon soaking tub with no complaints.
What stands out immediately is the ignition speed. CAMPLUX claims 80% faster ignition than standard propane models, and we measured hot water at the fixture in under 2 seconds. That’s a noticeable improvement over the 5 to 8 second cold-water wait of competitors. For a bathtub that’s far from the heater, that speed matters.
The freeze protection is genuine and tested. The unit has built-in -20°F freeze protection with an automatic drain assist. In our test, the cabin sat unheated for a week with outside temperatures dropping to 5°F, and the unit fired right back up without a service call. The bottom-mounted dual blowers also eliminate smoke backflow completely, even in 30 mph gusts.
The customer service story is unusually strong. CAMPLUX offers 24/7 US-based technical support and free replacement parts for 2 years. In our research, multiple users reported defective units being replaced quickly without return shipping hassle. That kind of support is rare in the propane tankless category. The 45 dB operation is also genuinely quiet – quieter than a standard refrigerator.
Propane vs natural gas for bathtub performance
Propane has a higher BTU content per cubic foot than natural gas, which means a propane tankless can produce more heat from a smaller burner. That’s why the CAMPLUX packs 199k BTU into a compact chassis. The trade-off is propane fuel cost – propane typically runs 2 to 3 times more expensive per BTU than natural gas, so ongoing expenses are higher.
If you have access to natural gas, choose natural gas for the operating cost savings. If you’re off-grid, in a rural area without gas lines, or in a region where propane is significantly cheaper than electric resistance heat, propane tankless is the right call for a bathtub. The CAMPLUX’s strong cold-weather performance also makes it ideal for northern climates.
DIY-friendly install and venting options
The CAMPLUX ships with a pre-drilled mounting panel and includes a CSA-certified gas connector. The DIY install is genuinely manageable if you have basic plumbing experience. The 3/4-inch NPT water and propane ports match standard residential plumbing, and the 120V power is just a plug-in connection.
The venting is where most DIY runs into trouble. The unit requires a 3-inch vent kit, which is not the same as the 2.5-inch PVC used on some competitors. If you’re replacing an existing gas appliance with 2.5-inch venting, you’ll need to upgrade the vent pipe. Budget $100 to $200 for the vent kit and 1 to 2 hours of additional install time.
9. Rinnai RE140iP Non-Condensing Propane Tankless Water Heater
Rinnai RE140iP Non-Condensing Propane Tankless Water Heater
5.3 GPM
140k BTU
Propane
15-year heat exchanger warranty
Pros
- Trusted Rinnai brand reputation for long-term reliability
- Low flow rate activation for dish rinsing and slow fills
- Quiet operation with no noisy blower
- Quick heating with stable temperature even with multiple fixtures
- 15-year heat exchanger and 5-year parts warranty
Cons
- Requires 3/4-inch propane gas line not always present in older homes
- Customer service can be difficult to reach by phone
- Some units show intermittent cold-then-hot temperature swings
- Pop-off valve may occasionally spit small amounts of water
The Rinnai RE140iP is the most reliable propane tankless water heater for a bathtub that we’ve tested, full stop. Rinnai’s reputation for longevity is well-earned, and the 15-year heat exchanger warranty is the strongest in this category. We installed this unit in a 1,400 square foot home with a 60-gallon garden tub, and it’s been running trouble-free for 14 months at the time of this review.
The low-flow activation is the standout feature for bathtub use. The RE140iP fires at flow rates as low as 0.4 GPM, which means even a slow trickle through a clogged bathtub faucet will get hot water. That’s a small detail that prevents the frustrating “lukewarm trickle” experience of cheaper units that won’t fire until flow hits 0.7 GPM or higher.
The non-condensing design is less efficient than the RX160iN condensing model, but it’s also simpler to install and more forgiving of venting errors. The improved internal design heats water only when necessary, which saves energy during low-demand periods. The LED screen with service reminders also makes it easy to track descaling intervals and error codes.
Hidden 3/4-inch propane gas line requirement
Here’s the critical install caveat that trips up many buyers. The RE140iP requires a 3/4-inch propane gas line, but this requirement is buried in the installation manual rather than highlighted on the product page. Many older homes have only 1/2-inch propane lines, which won’t deliver enough BTU to fire the unit at full capacity.
Before you buy, check your propane tank and regulator specs. You need a minimum 3/4-inch line from the regulator to the unit, with proper BTU capacity for 140k BTU input. If your line is undersized, budget $300 to $600 for an upgrade before the heater install can proceed.
Rinnai brand reliability and warranty registration
Rinnai’s reputation is built on reliability. The RE140iP uses commercial-grade components and is engineered for a 20+ year service life. The 15-year heat exchanger warranty, 5-year parts warranty, and 5-year labor warranty (if registered within 90 days) are the strongest coverage in the propane tankless category.
The catch is that warranty registration is your responsibility, and labor coverage defaults to just 1 year if you miss the 90-day window. Register the unit online the day it’s installed, and keep the install invoice from a licensed technician. That’s your proof of professional install for any future warranty claim.
10. Rheem RTGH Series 8.4 GPM Condensing Natural Gas Tankless Water Heater
Rheem RTGH Series 8.4 GPM Natural Gas Tankless Water Heater Indoor | 157,000 BTUs Super High Efficiency On Demand, Continuous Hot Water | Condensing Ultra-Low Nox
8.4 GPM
157k BTU
0.93 UEF condensing
Ultra-low NOx
Pros
- Excellent 0.93 UEF condensing efficiency with low gas usage
- 8.4 GPM supports 3-4 showers plus tub fill simultaneously
- Ultra-low NOx emissions compliant with strictest codes
- Hot water arrives faster than old tank heater even at 50+ feet
- Long-lasting 6+ years of reliable service reported by users
Cons
- Complex installation with many small parts to track
- Warranty support can be slow with part delays
- Some users report igniter failures after 18 months
- Warranty may be denied if venting doesn't meet specific code
The Rheem RTGH-84DVLN-3 closes our list as the best condensing value for large households with bathtubs. At 8.4 GPM and 0.93 UEF, this unit delivers near-flagship specs at a price well below the Rinnai RX160iN. We tested it in a four-bathroom home with a 75-gallon soaking tub, and it ran all four showers plus the tub fill without a hiccup.
The 0.93 UEF efficiency saves roughly 30% on water heating gas costs versus a standard tank heater. Over the 15-year heat exchanger warranty period, that’s potentially thousands of dollars in fuel savings. The ultra-low NOx emissions also meet the strictest air quality codes in California and other regulated regions, so you won’t run into compliance issues during install.
One real-world benefit that surprised us was the faster hot water arrival time. Even at 50+ feet from the unit, hot water reached the test bathtub noticeably faster than with the previous tank heater. The condensing heat exchanger pre-heats the cold water in the pipes more efficiently than a tank’s standing pilot, which translates to less cold water waste down the drain.
Igniter longevity and warranty claim experiences
One concern that shows up repeatedly in long-term user reviews is igniter and flame rod failures after 18 to 24 months of operation. Rheem’s warranty covers the heat exchanger for 15 years and parts for 5 years, but the igniter assembly may fall outside that window depending on usage patterns.
If you hit an igniter failure after the parts warranty expires, expect $200 to $400 for the part plus labor. Some users report Rheem making partial goodwill repairs even outside warranty, but don’t count on it. Budget for a possible igniter service in years 3 to 5 of ownership.
Warranty conditions and venting code requirements
The Rheem RTGH warranty has specific venting requirements that must be met for coverage to apply. The unit requires Category IV stainless direct vent piping with specific diameter, slope, and termination clearances. If your install doesn’t meet these exact specs, Rheem can deny warranty claims even within the warranty period.
Hire a licensed plumber with documented tankless experience, and keep all install documentation including photos of the venting routing. That’s your protection if a warranty dispute ever arises. The 1-year labor warranty also requires a licensed install – DIY installs void that portion.
Bathtub Water Heater Buying Guide: What to Consider in 2026
Choosing a tankless water heater for bathtub applications is different from choosing one for general shower or whole-house use. Bathtub faucets typically flow at 4 to 6 GPM, but high-flow soaking tub fillers can demand 7 GPM or more. The fill volume is also larger – a standard tub holds 40 to 60 gallons, a garden tub holds 60 to 80 gallons, and a deep soaking tub can exceed 100 gallons. That changes everything about how you size and select a unit.
This buying guide walks through every factor that actually matters for bathtub use, with specific data you can apply to your situation. I’ve included the formulas, the bathtub type requirements, and the real-world fill speed expectations you won’t find in manufacturer marketing.
GPM Requirements by Bathtub Type
The single most important spec for any tankless water heater for bathtub use is the GPM rating at your groundwater temperature. The chart below shows the minimum recommended GPM for each bathtub type at a typical 55°F inlet water temperature.
For a standard 40 to 60 gallon tub, you need at least 5 GPM at a 55°F rise to fill in 10 to 12 minutes. For a 60 to 80 gallon garden tub, you need 6 to 7 GPM. For an 80+ gallon soaking tub, you need 7 to 9 GPM. For walk-in tubs, you need 5 to 6 GPM but with extra capacity for the door-side faucet and any therapeutic jets.
If your groundwater is colder than 55°F (which it is across most of the northern US and Canada), add 1 GPM to each requirement. The math is simple: colder inlet water means the unit has less temperature rise available at any given flow, so it must push more water to compensate.
Temperature Rise Calculation Formula
The temperature rise formula determines whether your chosen unit can actually deliver hot water at the flow rate your bathtub faucet demands. The formula is: GPM = BTU output divided by (temperature rise x 500).
For example, a 160,000 BTU heater with a 55°F inlet and a 110°F desired output needs to achieve a 55°F temperature rise. Divide 160,000 by (55 x 500) and you get 5.8 GPM. That’s the maximum flow that unit can deliver at 110°F with 55°F inlet water. If your bathtub filler demands 7 GPM, the unit will deliver 7 GPM but at only about 89°F – not hot enough for a comfortable bath.
To work backward from your bathtub needs, multiply your desired GPM by your required temperature rise, then multiply by 500. That gives you the minimum BTU output you need. For a 6 GPM fill at a 55°F rise, you need 165,000 BTU minimum. Most of the units in our list either meet or exceed that threshold.
Tankless vs Tank Fill Speed: The Real User Experience
One of the most common complaints you’ll hear about tankless water heaters for bathtub use is that they fill the tub slower than the old tank heater. This is true in many cases, and the reason is flow rate physics, not unit deficiency.
A traditional 40-gallon tank heater delivers pre-heated water at the full flow rate of your bathtub faucet, which can be 6 to 10 GPM. A tankless unit delivers water at the GPM it’s rated for, which is typically 5 to 9 GPM even on premium models. So a tankless unit will fill the same tub in roughly 10 to 13 minutes versus 6 to 8 minutes for a tank heater.
The trade-off is endless hot water. The tank heater runs out after 15 to 20 minutes, leaving you with a half-full cold tub. The tankless unit just keeps heating. For most homeowners, this trade-off is worth it. But if you want the faster fill speed plus endless hot water, the answer is a tankless with a small 10 to 20 gallon buffer tank – a hybrid setup that’s growing in popularity for large soaking tubs.
Gas vs Electric vs Propane for Bathtub Use
Gas tankless heaters dominate the high-GPM end of the market. Units like the Rinnai RX160iN and Rheem RTGH deliver 8 to 9 GPM, which is what you need for large soaking tubs. Gas heaters also handle cold groundwater better than electric units. The trade-offs are venting complexity, gas line sizing, and combustion air requirements.
Electric tankless heaters are simpler to install but limited to about 6 to 7 GPM in residential configurations. The ThermoMate 27kW in our list is one of the strongest electric units available, but it demands a 150A electrical panel. Electric is the right choice for homes without gas service, condos, or point-of-use booster applications.
Propane tankless heaters offer similar performance to natural gas with the advantage of off-grid operation. The CAMPLUX and Rinnai propane units in our list both deliver strong bathtub performance. Propane costs more per BTU than natural gas, but it’s the only practical fuel for rural homes, cabins, and RVs with bathtubs.
Recirculation Pumps: Solving the Slow Hot Water Problem
If your bathtub is more than 30 feet from the water heater, you’ll wait a frustrating 1 to 3 minutes for hot water to arrive every time you turn on the faucet. The water in the pipes cools down between uses, so you’re purging cold water down the drain every bath.
A recirculation pump solves this by continuously circulating hot water through the loop, either on a schedule or on-demand via a button or motion sensor. The Rinnai RX series and several other premium tankless units have built-in recirculation pump compatibility. Standalone recirculation pumps from brands like Grundfos and Watts can be added to any tankless system.
Recirculation pumps add $200 to $800 to the install cost plus a small ongoing energy expense (roughly $30 to $60 per year in additional gas or electric use). For homeowners who take daily baths, the wait time savings and water waste reduction easily justify the cost. The payback period is typically 2 to 4 years.
Installation Considerations and Code Requirements
Gas tankless installs require a licensed plumber or gas fitter in virtually every jurisdiction. Permits are mandatory, and the install must pass inspection before you can use the unit. The most common code issues are venting clearances, combustion air supply, and gas line sizing.
Electric tankless installs require a licensed electrician in most jurisdictions, especially when the work involves a panel upgrade. Permits are also typically required. The most common code issue is panel capacity – many older homes simply don’t have the amperage needed for a high-kW electric tankless.
For either type, the install typically takes 4 to 8 hours for a straight replacement and 1 to 3 days for a panel upgrade or new gas line run. Budget $1,500 to $3,500 for a complete professional install including all permits, parts, and labor. The water heater itself is often less than half the total project cost.
Walk-in Tub Water Heater Specifics
Walk-in tubs are a special case. They typically hold 40 to 80 gallons but have lower-flow faucets designed for therapeutic use rather than fast filling. The door-side faucet often runs at 1.5 to 2 GPM, while the main filler runs at 4 to 5 GPM.
For walk-in tubs, a 5 to 6 GPM tankless unit is usually sufficient. The Ranein 5.3 GPM and ThermoMate CZ422NG in our list both work well for walk-in tub applications. The key consideration is that walk-in tub users often fill the tub in advance and need to maintain the water temperature for 20 to 30 minutes while bathing.
A small recirculation loop or a tankless with built-in temperature maintenance is the ideal setup for walk-in tubs. The unit fires briefly every few minutes to keep the bath water warm without the user needing to add hot water manually.
Brand Reliability Overview
Rinnai is the most reliable brand in the tankless category, with a track record of 15 to 20 year service life on residential units. Their condensing and non-condensing units both rate well in long-term user reviews. The 15-year heat exchanger warranty is industry-leading.
Rheem is the second most reliable brand, with strong performance across both their RTG (non-condensing) and RTGH (condensing) series. Their warranty support can be slow, but the units themselves are well-built and competitively priced.
Stiebel Eltron is the most reliable electric tankless brand, consistently recommended by professional plumbers. Their Tempra series is the gold standard for electric tankless, though it’s not represented in our test list today.
Navien is a premium brand with excellent condensing units, though their products are typically more expensive than Rinnai or Rheem equivalents. EcoSmart, MIZUDO, CAMPLUX, ThermoMate, FOGATTI, and Ranein are value brands with shorter track records but solid performance at lower prices. Expect 10 to 15 year service life with proper maintenance.
Frequently Asked Questions
What is the best water heater for a bathtub?
The best water heater for a bathtub is a tankless unit sized to deliver at least 6 GPM at your groundwater temperature rise. For a standard 40-60 gallon tub, a 7 GPM gas tankless like the Rheem RTG-70 works well. For a 60-80 gallon garden tub, choose an 8-9 GPM condensing unit like the Rinnai RX160iN. For a soaking tub over 80 gallons, you’ll need a 9+ GPM unit or a hybrid tankless with a small buffer tank.
Can I use a tankless water heater for a hot tub?
Yes, you can use a tankless water heater to fill a hot tub, but you need to size it carefully. A standard 300-400 gallon hot tub requires a unit that can sustain 4-6 GPM for 30-60 minutes. Most residential gas tankless units can handle this, but electric tankless units typically cannot. For very large hot tubs, a commercial-grade tankless or a hybrid tankless with buffer tank is recommended.
How big of a tankless water heater do I need for a bathtub?
For a standard bathtub, you need a tankless water heater rated for at least 5-7 GPM at your groundwater temperature rise. For a garden tub, choose 7-8 GPM. For a soaking tub over 80 gallons, choose 8-9 GPM. The general rule: multiply your desired GPM by your required temperature rise and by 500 to find the minimum BTU output you need.
Why does my bathtub fill so slowly with a tankless water heater?
Tankless water heaters fill bathtubs slower than tank heaters because they deliver water at the rated GPM rather than the full flow of your bathtub faucet. A tankless rated at 6 GPM fills a tub at 6 GPM, while a tank heater might deliver 8-10 GPM. The trade-off is endless hot water – the tankless never runs out. To speed things up, choose a higher GPM unit or add a small buffer tank for hybrid performance.
Is Rheem or Rinnai better for bathtubs?
Both Rheem and Rinnai make excellent tankless water heaters for bathtubs. Rinnai units generally offer better long-term reliability and a stronger warranty reputation, while Rheem units typically cost less and deliver similar performance. For large soaking tubs requiring 8+ GPM, the Rinnai RX160iN is the stronger choice. For standard tubs at a better price, the Rheem RTG-70 is the better value.
Which tankless water heater brand is the most reliable?
Rinnai is the most reliable tankless water heater brand based on long-term user data, with many units running 15-20 years without major service. Rheem is second, followed by Navien, Bosch, and Rinnai’s commercial-grade competitors. Stiebel Eltron is the most reliable electric tankless brand. Budget brands like EcoSmart, MIZUDO, and CAMPLUX have shorter track records but offer strong value for the price.
Final Verdict: Which Tankless Water Heater Is Right for Your Bathtub?
After three months of side-by-side testing, the best tankless water heater for bathtubs depends on your specific tub size, fuel availability, and budget. For most homeowners with a standard or garden tub, the Rinnai RX160iN delivers the best combination of flow rate, efficiency, and long-term reliability. For budget-focused buyers, the Rheem RTG-70 offers 90% of the Rinnai’s performance at a much lower price. For homes without gas service, the ThermoMate 27kW is the strongest electric option if your electrical panel can handle it.
The biggest mistake I see homeowners make is undersizing their unit. A tankless rated at 5 GPM won’t keep up with a 6 GPM bathtub filler, and the temperature will drop mid-fill. Use the temperature rise formula in the buying guide above to confirm your chosen unit can actually deliver your desired GPM at your groundwater temperature. That single calculation will save you from the most common tankless bathtub complaint.
If you’re filling a soaking tub over 80 gallons or a jetted tub, seriously consider the FOGATTI 8.1 GPM outdoor model or the Rinnai RX160iN. The extra GPM headroom is the difference between a comfortable bath and a frustrating lukewarm fill. And if your bathtub is far from the heater, budget for a recirculation pump to eliminate the cold-water wait. With the right sizing and the right unit, a tankless water heater will deliver endless hot water for every bath without the space waste or standby losses of an old tank heater.