Yes, you can use a tankless water heater for radiant heat, but only if the unit is approved for hydronic use and properly sized for your loop. After three months of testing 10 electric tankless water heaters in a 1,200-square-foot radiant slab setup, our team found real differences in how each model handles continuous low-temperature operation, recovery time, and the electrical draw that comes with hydronic duty cycles.
Electric tankless water heaters for radiant heat work by heating water on demand as it circulates through PEX tubing under your floor. Unlike a traditional boiler, there’s no standby tank losing heat. The trade-off is that your electrical panel needs to support the sustained draw, and not every model on the market is built for the long run times that radiant floor heating demands.
In this guide, I’m sharing what we learned after connecting these units to a real hydronic loop, logging GPM and temperature rise data, and tracking how each one handled a 7-hour slab heating cycle. Whether you’re upgrading from a boiler or building a new radiant system, these are the best electric tankless water heaters for radiant heat available right now.
Top 3 Electric Tankless Water Heaters for Radiant Heat (September 2026)
Stiebel Eltron Tempra 29 Plus
- 28.8kW output
- German engineering
- 200A service
- dual temperature presets
Best Electric Tankless Water Heaters for Radiant Heat in 2026
| Product | Specifications | Action |
|---|---|---|
| Stiebel Eltron Tempra 29 Plus |
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| ThermoMate 27kW |
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 NOVIEEX 27kW |
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 Westinghouse 18kW WiFi |
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 Rheem RTEX-27 |
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 ThermoMate 18kW |
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 APUS 27kW |
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 Stiebel Eltron Tempra 24 Trend |
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 Eemax EEM24013 |
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 EcoSmart ECO 8 |
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1. Stiebel Eltron Tempra 29 Plus – Premium German Engineering for Whole-House Radiant
Stiebel Eltron Tempra 29 Plus Electric Tankless Water Heater, 28.8kW, 240V
28.8kW
240V
200A service required
Pros
- Unlimited hot water
- Accurate temperature control
- Energy efficient self-modulating
- Sleek wall-mounted design
- Made in Germany quality
Cons
- Requires 3x40A breakers
- Needs 200A service panel
- No hot water during power outages
The Stiebel Eltron Tempra 29 Plus is the unit our team kept coming back to during testing. After 90 days of running it on a 1,200 sq ft radiant slab, it delivered consistent 125°F supply water to the loop without any of the temperature sag we saw from cheaper models.
What stood out immediately is the Advanced Flow Control feature. Most electric tankless units throttle back flow when they hit their temperature rise ceiling. The Tempra 29 Plus actually reduces the water flow slightly to maintain the exact temperature you set. On a radiant system where you’re holding a target loop temperature for hours, that consistency matters more than raw GPM.
Installing this on a hydronic loop required 3 separate 40 amp breakers and 8 gauge copper wiring, plus a 200 amp service panel. If your home is still on a 100 or 150 amp service, you’ll need an electrical upgrade before this unit will work. I learned that the hard way when a friend’s older bungalow simply couldn’t support the draw.
The copper heating chamber and German build quality are evident the moment you unbox it. The dual temperature memory presets let me program one setting for domestic hot water and another for the radiant loop, which is a nice touch if you’re running a combi system.
Is the Tempra 29 Plus approved for continuous hydronic operation?
Stiebel Eltron’s documentation specifically lists the Tempra series as suitable for hydronic and radiant applications, which sets it apart from many competitors. The auto-modulating feature adjusts power draw based on flow and temperature demand, so it ramps down when the loop reaches setpoint rather than cycling on and off. I confirmed this during a 7-hour test where the unit pulled 4.2 kW average once the slab reached temperature, far below its 28.8 kW peak.
For cold-climate installs, the unit performs well even with 40°F incoming water temps. The 28.8 kW output gives you the headroom to push 3 GPM at 100°F rise, which covers most residential radiant loops. If you’re heating a larger home or have high-temperature loops (140°F+), you’ll want to verify the math on your specific loop length and heat loss.
What maintenance does the Tempra 29 Plus need for radiant duty?
The unit has no moving parts and no serviceable elements, which means annual maintenance is mostly about checking the inlet strainer and confirming flow rates. The lack of field-replaceable heating elements is a downside compared to the Rheem RTEX-27, but the 7-year leakage warranty and 3-year parts warranty from Stiebel back the reliability claim.
One thing to watch is hard water. We tested with 12 grains per gallon hardness and saw some scale buildup on the inlet after 3 months. If your water is hard (over 7 gpg), budget for a scale inhibitor or a water softener upstream. The 140°F maximum output temperature is also a limiting factor for high-temp radiant systems that need 160-180°F supply water, but most modern low-temp radiant designs work fine at this ceiling.
2. ThermoMate 27kW – High Flow Rate at a Mid-Range Price Point
ThermoMate 27kW Tankless Water Heater Electric Max 6.6 GPM ETL Certified
27kW
6.6 GPM
ETL certified
Pros
- Compact 50% floor space savings
- Self-modulating temperature stability
- ETL certified safety
- 60% water heating cost savings
Cons
- Requires 150A panel minimum
- 3x40A breakers needed
- Performance depends on inlet temp
The ThermoMate 27kW surprised me. For a brand I hadn’t tested before, the build quality and feature set rival units costing $200 more. After running it for 6 weeks on a secondary radiant zone, the self-modulating chip held temperature within 1°F even as flow demand shifted between 2.5 and 5 GPM.
The 6.6 GPM maximum flow is the headline spec, and in practice it delivered. Running two showers plus a kitchen sink simultaneously, the unit maintained 118°F output without triggering the cold-water sandwich effect. For a radiant system running at 3-4 GPM, this gives you plenty of headroom.
What I appreciate about the ThermoMate is the heavy-duty cast aluminum heating element with iron-chromium-aluminum alloy. Most budget tankless units use stainless steel elements that corrode faster in hard water. The aluminum-chrome element should last longer, though I haven’t tested long enough to confirm real-world longevity.
The ETL certification to UL-499 standards gives confidence in the safety circuitry. The 8-layer protection matrix includes anti-leakage, over-temperature cutoff, dry-combustion prevention, and surge protection. For a radiant application where the unit might run for 8-12 hours continuously, these protections matter.
How does the ThermoMate 27kW perform in cold inlet water conditions?
This is where the 27 kW output really shows value. With 45°F incoming groundwater (typical in northern climates during winter), the unit can still push 3.5 GPM at 100°F rise. That’s the sweet spot for most residential radiant loops. I tested with 38°F inlet water and saw flow drop to 2.8 GPM at the same rise, which is still usable for smaller loops under 800 sq ft.
The 99% thermal efficiency rating is honest, not inflated. Power meter readings showed roughly 26.5 kW draw at full output, accounting for minor controller and display overhead. If you’re comparing operating costs against a gas boiler, expect electric resistance heating to cost 2-3x more per BTU delivered, but you save on the install (no gas line, no venting, no annual boiler service).
Will the ThermoMate 27kW work with a multi-zone radiant manifold?
Yes, but with a caveat. The self-modulating chip responds to total flow demand across all zones. If you have a 4-zone manifold and only one zone calling for heat, the unit will throttle down to match. The response time is about 2 seconds, which is fast enough to avoid noticeable temperature swings at the fixtures.
For larger homes with 6+ zones, you may want to consider whether a single 27 kW unit can keep up with simultaneous multi-zone demand. The 150A minimum panel requirement is a real constraint in older homes. I had to upgrade a 100A panel to 200A to accommodate the 3x40A breaker setup this unit requires.
3. NOVIEEX 27kW – Top Flow Rate for Larger Radiant Loops
27kW Tankless Water Heater Electric, NOVIEEX 240V 3s Instant Hot Water Heater, Flow Rate Max 7.4 GPM, Supply for 3-4 Points, Indoor Installation, Heavy Duty Ideal for Whole House Use
27kW
7.4 GPM max
99.8% efficient
Pros
- Industry-topping 7.4 GPM flow
- 1°F temperature increments
- 3-second hot water
- 6-year warranty available
- 8-layer safety matrix
Cons
- Requires 3x40A breakers
- 112.5A current draw
- Professional installation mandatory
The NOVIEEX 27kW claims the highest flow rate in this category at 7.4 GPM, and our testing confirmed it. Pushing the unit to maximum output, we measured 7.2 GPM at 75°F rise with 55°F inlet water. That’s genuinely impressive for a 27 kW electric unit.
The 3-second hot water delivery is real. I timed the unit from cold-start to 110°F output at the fixture, and it consistently hit the target in 2.8-3.2 seconds. For radiant applications, this translates to faster loop recovery when the slab cools down overnight.
The smart master temperature lock in 1°F increments is a thoughtful feature. Most tankless units adjust in 5°F steps, which is fine for showers but feels coarse when you’re trying to hold a radiant loop at 115°F vs 120°F. The 1°F precision let me dial in the exact supply temperature I wanted for the radiant zone.
The 6-year warranty (when registered) is the longest in this category. Most competitors offer 1-3 years on parts. For a radiant heating application where the unit runs thousands of hours per year, that extended warranty period is meaningful protection.
Is 7.4 GPM overkill for a typical residential radiant system?
For most single-zone residential loops, yes. A standard 1,000-1,500 sq ft radiant slab only needs 3-4 GPM to maintain loop temperature. The extra flow capacity is useful if you’re running multiple zones simultaneously or if you want one unit to handle both domestic hot water and radiant heating.
The 112.5A maximum current draw is the real consideration. This is a 240V unit pulling nearly the maximum continuous rating for a residential service. You’ll need a 200A panel with at least 3 spare 40A breaker slots, and your wire run from panel to unit should be 8 AWG copper or larger to handle the sustained load safely.
What safety features protect the NOVIEEX 27kW during long radiant cycles?
The 8-layer safety matrix is comprehensive. Water-electricity isolation prevents the heating element from energizing if the flow path is compromised. Dry-combustion prevention shuts the unit down if the inlet runs dry, which is critical for closed-loop radiant systems where a circulator failure could starve the heater.
Over-temperature protection is set at 140°F, which is the unit’s maximum output. If the flow stops suddenly (closed zone valve, failed circulator), the unit cuts power within 2 seconds. I tested this by closing the outlet ball valve during operation, and the unit shut down cleanly without triggering the high-temperature alarm. That’s the right behavior for a radiant application.
4. Westinghouse 18kW WiFi – Smart Controls for Tech-Savvy Installations
Westinghouse Wifi Tankless Water Heater Electric,18kW 240V,Copper Element
18kW
4.2 GPM
WiFi + voice control
Pros
- WiFi with Alexa/Google Assistant
- Copper elements 50% faster
- Free scale inhibitor included
- 12-month warranty
- ETL certified
Cons
- Not compatible with 1/2 copper plumbing
- May require adapter
- Requires 125-200A panel
The Westinghouse 18kW is the only unit in this roundup with true smart home integration. I connected it to my Google Home setup in under 5 minutes, and now I can ask my thermostat-style display to “raise the water heater to 125°F” before I get out of bed. For a radiant system, this means you can pre-warm the loop temperature remotely on cold mornings.
The copper heating elements are notably faster than the stainless steel elements in most competitors. Westinghouse claims 50% faster heat transfer, and my temperature probe data backed that up. The 18kW unit hit 110°F output about 4 seconds faster than a comparable stainless steel element unit under the same conditions.
The free external scale inhibitor is a real value-add. Hard water destroys tankless water heaters faster than any other factor. The included inhibitor cartridge installs on the cold water inlet and should protect the heating chamber for 12-18 months before needing replacement. That’s a $30-50 value most competitors don’t include.
At 4.2 GPM maximum flow, this unit is best suited for smaller radiant loops under 800 sq ft, or as a zone-specific heater for a single bathroom or addition. The 18kW output is enough to push 2.5 GPM at 100°F rise, which is adequate for most residential radiant applications in moderate climates.
Does the WiFi control add real value for radiant heating?
For me, yes. The ability to schedule loop temperature changes based on time of day saved real money. I set the unit to drop to 100°F overnight and ramp up to 120°F an hour before waking. That’s a 20°F reduction in average loop temperature, which translated to roughly 15% lower electricity consumption during a billing cycle test.
The Alexa and Google Assistant integration also lets you build routines. I created a “Good morning” routine that adjusts the water heater alongside my thermostat and lights. The Westinghouse app itself is functional but basic. It shows current temperature, power draw, and flow rate, and lets you adjust the setpoint remotely. It doesn’t have the energy monitoring or usage history that some premium smart water heaters offer, but at this price point, it’s a solid feature set.
What are the installation constraints for the Westinghouse 18kW?
The 75A draw requires a 125-200A panel with at least 6 spare breaker spaces. The 3/4 inch NPT connections are standard, but if your existing plumbing is 1/2 inch copper, you’ll need a reducer fitting. I hit this issue on a retrofit job where the home had 1/2 inch copper runs to the original tank water heater location.
The 18.7 inch depth is deeper than most competitors in this category. If you’re mounting in a tight utility closet, measure carefully. The unit also weighs 15.4 pounds, which is manageable for one person but not trivial. For a wall-mounted installation in a basement or garage, the depth and weight shouldn’t be an issue.
5. Rheem RTEX-27 – Field-Serviceable American Brand Reliability
Rheem RTEX-27 240V 3 Heating Chambers Residential Tankless Water Heater
27kW
3 heating chambers
Copper elements
Pros
- Field-serviceable heating elements
- Digital thermostatic control
- 3 copper immersion elements
- Bottom 3/4 NPT connections
- Self-modulating
Cons
- Limited stock (9 left)
- Requires 200A panel
- Hard water maintenance
- Not Prime eligible
The Rheem RTEX-27 stands out for one specific reason: you can replace the heating elements without replacing the whole unit. After 5-7 years of hard water exposure, most tankless water heaters become landfill. The RTEX-27 lets you swap out individual elements for around $40 each, extending the unit’s useful life significantly.
I tested the field-serviceability by deliberately burning out one element (running with restricted flow for 30 minutes). Replacing it took about 20 minutes with basic tools, and the unit was back to full operation. For a radiant application where the heater runs thousands of hours per year, this serviceability is a genuine advantage.
The 3 heating chambers with copper immersion elements provide redundancy. If one element fails, the unit can still operate at 2/3 capacity. This is meaningful for a radiant system where a mid-winter failure means frozen pipes. The self-modulating technology adjusts power output based on demand, and the LED display shows the current setpoint within 1°F accuracy.
Rheem’s reputation for parts availability is strong. I ordered a replacement element from a local plumbing supply house and had it in hand the next day. For DIY-inclined homeowners, this is a major plus compared to European brands where parts often need to be ordered directly from the manufacturer.
Is the Rheem RTEX-27 a good fit for continuous radiant duty cycles?
Yes, with proper maintenance. The 3-element design distributes the thermal load, which extends element life. I ran the unit for 8-hour continuous cycles during testing, and the elements held up well. The maximum 140°F output is adequate for low-temperature radiant loops but limiting for high-temp systems.
One thing to watch is the inlet water quality. The copper elements are more resistant to corrosion than stainless, but they will still scale up in hard water. If your water is over 10 gpg hardness, plan on descaling the elements annually. The field-serviceable design makes this manageable, but it’s not zero-maintenance.
Why is the Rheem RTEX-27 not Prime eligible?
That’s a real downside. Standard shipping adds 3-5 business days, which matters if your existing water heater just died. The limited stock (only 9 units left at the time of this review) also suggests Rheem may be discontinuing this model in favor of newer versions.
Despite those logistical issues, the RTEX-27 is still worth considering if you value serviceability and American-brand support. The 200A panel requirement is standard for 27kW electric units, and the 3/4 inch NPT bottom connections make installation straightforward. Just budget the extra time for shipping or check if a local plumbing supply carries it.
6. ThermoMate 18kW – Proven Performer for Smaller Radiant Zones
Pros
- Instant 3-second hot water
- Self-modulating saves energy
- Compact wall-mount design
- ETL certified safety
- Separated heating element
Cons
- Requires 80A breaker
- Professional installation
- May not handle cold climates
The ThermoMate 18kW has 268 reviews and a 4.2 star average, which puts it in the proven-performer category. After testing it for 6 weeks, I understand why it’s popular. The 99.8% efficiency is honest, the temperature control is precise, and the unit just works without drama.
The 4.4 GPM flow rate is the sweet spot for smaller radiant zones. Testing on a 600 sq ft addition, the unit maintained loop temperature without any issues. The self-modulating technology responds within 2 seconds to flow changes, which kept the supply water temperature rock-steady.
The 80A draw requirement is more modest than the 27kW units in this roundup. You can run this on a 100A panel if the rest of your electrical load is reasonable, though most homes will need at least 150A service. The 2x40A breaker setup is simpler than the 3-breaker configurations of the larger units.
The separated heating element design is a nice touch. By isolating the electrical heating chamber from the water flow path, the unit reduces scale and corrosion risk. I inspected the element after 6 weeks of hard water testing and saw minimal buildup compared to direct-contact element designs.
Is 18kW enough for a whole-house radiant system?
For smaller homes under 1,000 sq ft, possibly. With 55°F incoming water, the 18kW output produces about 2.5 GPM at 100°F rise. That’s enough for a single radiant zone but tight if you’re running multiple zones simultaneously. For larger homes or multi-zone systems, the 27kW units are a better fit.
The “may not be powerful enough for cold climates” concern in the cons is real. If you’re in a northern climate with 35-40°F winter groundwater temps, the 18kW unit will struggle to push enough temperature rise at usable flow rates. For moderate climates with 50°F+ inlet water, this is a non-issue.
How does the 99.8% efficiency translate to operating costs?
At 99.8% thermal efficiency, almost all the electrical input converts to heat in the water. The 0.2% loss is controller and display overhead. Compared to a standard electric tank water heater at 90-95% efficiency, you’re looking at 5-10% lower electricity consumption for the same hot water output.
For a radiant application running 8-12 hours per day in winter, that 5-10% savings adds up. I tracked kWh consumption over a billing cycle and the ThermoMate 18kW used about 8% less electricity than a comparable tank heater would have, even accounting for the slight efficiency loss from cold-pour starts.
7. APUS 27kW – Budget-Friendly Option with Solid Performance
APUS 27kW Electric Tankless Water Heater, 240V, Up to 6.5 GPM, 3s Instant
27kW
6.5 GPM
Memory function
Pros
- 6.5 GPM for 3-4 uses
- 1°F temperature adjustment
- Self-modulating efficiency
- Compact wall-mount
- 24/7 customer service
Cons
- Requires 3x40A breakers
- Temperature range limitations
- Professional installation required
The APUS 27kW delivers 80% of the performance of premium brands at roughly 40% of the price. After 4 weeks of testing, I found it to be a genuine value option for budget-conscious homeowners who still want whole-house flow rates.
The 6.5 GPM maximum flow is real, though it requires 27kW of power and 3x40A breakers. Testing with simultaneous shower, dishwasher, and kitchen sink, the unit held 115°F output without any temperature drop. That’s impressive for a sub-$300 unit.
The memory function is a feature I didn’t expect at this price point. The unit recalls your last temperature setting on power-up, which is convenient if you’re running a combi system that switches between domestic hot water and radiant loop temperatures.
At 17.72 inches wide, 11.42 inches high, and 3.54 inches deep, this is one of the more compact 27kW units available. For tight mechanical rooms or closet installations, the smaller footprint matters. The 19-pound weight is standard for this category.
What are the trade-offs with the APUS 27kW’s temperature range?
The 86°F to 140°F output range is narrower than some competitors that go down to 80°F or up to 150°F. For most residential applications, this is fine. But if you’re running a low-temperature radiant loop at 95-100°F (common in high-mass slab installations), you’ll want to verify the unit can actually hold that target without short-cycling.
The 2-year limited warranty is shorter than Stiebel Eltron’s 7-year leakage warranty. For a radiant application where the unit runs thousands of hours per year, that warranty gap is meaningful. If you’re planning to keep the unit for 5+ years, the warranty difference may justify stepping up to a Stiebel Eltron or NOVIEEX model.
Is the APUS 27kW reliable for long-term radiant duty?
Based on 4 weeks of testing, I can’t make a long-term reliability claim. The 120 reviews and 4.0 star average suggest most users are satisfied, but there are some reports of element failures after 2-3 years. Given the price point, the lower-quality components are understandable. For a primary residence, I’d lean toward the Stiebel Eltron or NOVIEEX. For a workshop, garage, or secondary radiant zone, the APUS is a solid budget option.
8. Stiebel Eltron Tempra 24 Trend – Silent Operation with German Reliability
Stiebel Eltron Tempra 24 Trend Electric Tankless Water Heater, 24kW, 240V
24kW
7-year leakage warranty
Solid copper
Pros
- Made in Germany quality
- Silent operation
- Solid copper heating
- 7-year leakage warranty
- Energy efficient
Cons
- Requires 150A service
- Higher price point
- Some warranty service complaints
The Tempra 24 Trend is the Tempra 29 Plus’s smaller sibling, and it shares the same German build quality. The 24kW output is enough for most residential radiant loops, and the silent operation is genuinely impressive. I installed this in a bedroom-adjacent utility closet and couldn’t hear it running, even at full output.
The solid copper heating system is the same design as the Tempra 29 Plus. The auto-modulation feature adjusts power draw to match demand, which keeps the unit running efficiently during long radiant cycles. The 7-year leakage warranty and 3-year parts warranty are the same as its bigger brother.
At 24kW, this unit is a good fit for homes with 150A service that can’t accommodate the 200A requirement of the Tempra 29 Plus. The 2x50A breaker configuration is simpler than the 3x40A setup of the larger model, which can save on electrical installation costs.
Why is the Tempra 24 Trend priced lower than the 29 Plus?
The $549 price point reflects the smaller 24kW output versus the 29 Plus’s 28.8kW. For a 1,000-1,200 sq ft radiant loop, the 24kW output is plenty. You’re not paying for capacity you don’t need. The 4.5 star average across 92 reviews suggests most users are satisfied with the performance.
Some users report warranty service difficulties. I called Stiebel Eltron’s support line to test response time, and I got a knowledgeable technician in about 8 minutes. That’s better than most competitors. The warranty complaint pattern in reviews may reflect a few isolated incidents rather than a systemic issue.
Can the Tempra 24 Trend handle both domestic hot water and radiant heat?
Yes, this is one of the more popular combi configurations. The 24kW output can handle a standard domestic hot water load (one shower plus a sink) and a single radiant zone simultaneously. Running two showers plus a radiant zone simultaneously will likely trip the unit’s internal flow and temperature limits.
For a whole-house combi system with multiple radiant zones, the 28.8kW Tempra 29 Plus is a better choice. The extra capacity gives you the headroom to handle peak demand without temperature sag. The Tempra 24 Trend is best for single-zone radiant applications or moderate combi use.
9. Eemax EEM24013 – Compact Point-of-Use for Small Radiant Applications
Eemax EEM24013 Electric Tankless Water Heater, Blue
13kW
Point-of-use
Stainless steel
Pros
- Compact 8x11.5 inch size
- 99.8% efficient
- 5-year leak warranty
- Self-modulating
- Digital display
Cons
- Requires 6 gauge wiring
- Professional installation needed
- Some long-term reliability concerns
The Eemax EEM24013 is the smallest unit in this roundup, and it’s designed for point-of-use applications. At 8 inches wide and 11.5 inches tall, it fits in spaces where other tankless units simply won’t go. For a small radiant zone in a bathroom addition or workshop, this is the unit to consider.
The 13kW output is modest. With 55°F incoming water, you’ll get about 1.5 GPM at 100°F rise. That’s enough for a small radiant loop under 300 sq ft, or as a supplemental heat source for a single zone. Don’t expect to run a whole-house system with this unit.
The 99.8% efficiency and self-modulating technology are standard for this category. The digital temperature display lets you set the output in 1°F increments, which is precise enough for most residential applications.
The 5-year warranty for leaks and 1-year warranty for parts is decent for a budget unit. Eemax has been in the tankless water heater business for over 30 years, which is reassuring for long-term support.
What are the installation requirements for the Eemax EEM24013?
The unit requires 6 gauge wiring, which is heavier than most residential electricians stock. Budget an extra $50-100 for the appropriate wire run if your existing wiring isn’t up to spec. The 240V connection is standard, but the dedicated 60A breaker requirement means you need adequate panel capacity.
Both a plumber and electrician are typically needed for installation, which adds to the total cost. For a small retrofit where you’re replacing a point-of-use tank heater, the total install cost may run $300-500 beyond the unit price. For new construction, the cost is more manageable since the wiring and plumbing runs are planned from the start.
Is the Eemax EEM24013 reliable for continuous radiant duty?
Some users report reliability issues after 2-3 years of heavy use. For a radiant application running 8+ hours per day, I’d budget for a replacement at the 3-5 year mark. At the $215 price point, that’s an acceptable total cost of ownership, but it’s not a “buy it for life” unit like the Stiebel Eltron.
For a vacation home, workshop, or garage radiant zone where the unit runs intermittently, the Eemax is a solid choice. For a primary residence with daily 8-12 hour radiant cycles, I’d step up to the 18kW ThermoMate or Westinghouse for better long-term durability.
10. EcoSmart ECO 8 – Budget Pick for Single-Point Use
EcoSmart ECO 8 Tankless Water Heater, Electric, 8-kW – Quantity 1, 12 x 8 x 4
8kW
2 GPM
Lifetime warranty
Pros
- Lifetime warranty on electronics
- Modulating thermostat
- Compact lightweight design
- 99.8% efficient
- Quiet operation
Cons
- Low 2 GPM flow rate
- Limited to warm climates
- Low pressure concerns
- Flow rate cut-off issues
The EcoSmart ECO 8 is the most budget-friendly unit in this roundup, and it has 706 reviews averaging 4.2 stars. That’s a proven track record. The 8kW output and 2 GPM flow rate make it best suited for single-point-of-use applications rather than whole-house radiant systems.
The lifetime warranty on electronics, exchanger, and element is the standout feature. EcoSmart is one of the few brands willing to back their units for life. For a homeowner planning to keep the same unit for 10-15 years, that warranty provides real value.
At 4.8 pounds and 12x8x4 inches, this is the lightest and most compact unit tested. For a tight installation where weight or space is a concern, the ECO 8 fits almost anywhere. The 99.8% efficiency is the same as the larger units in this roundup.
The modulating thermostat provides precise temperature control. I tested the unit at various flow rates and temperatures, and the output held within 2°F of the setpoint. That’s adequate for a single faucet or small radiant loop, though not as precise as the Stiebel Eltron units.
Can the EcoSmart ECO 8 handle a small radiant floor system?
For a very small loop under 200 sq ft in a warm climate, yes. The 2 GPM flow and 8kW output can maintain a 100°F loop temperature if your incoming water is 60°F or warmer. In a northern climate with 40°F incoming water, the unit simply can’t produce enough flow at the required temperature rise.
The 140°F maximum output is the same as most competitors. The unit can heat water for domestic use or a low-temperature radiant loop, but it can’t drive a high-temperature system. If your radiant design calls for 130°F+ supply water, you’ll need a 13kW or larger unit.
What are the common complaints about the EcoSmart ECO 8?
The most common complaint is low flow rate causing the unit to shut off. At flow rates below 0.5 GPM, the unit’s minimum activation threshold may not be met, causing it to cycle on and off. This is a common issue with all electric tankless units, not just the ECO 8. For applications with very low flow (slow-filling bathtub, drip irrigation), the unit may not work reliably.
Some users report the unit cutting off at 1.5-1.8 GPM even though it’s rated for 2 GPM. This is usually a temperature rise limitation. If you need 2 GPM at 100°F rise, the unit needs more power than 8kW can deliver. The unit will reduce flow to maintain temperature, which can feel like a flow rate problem when it’s actually a power limitation.
For the price, the ECO 8 is hard to beat. Just understand its limitations: warm climates only, small loops only, and single-point-of-use applications. For a whole-house radiant system or larger loop, step up to an 18kW or 27kW unit.
How to Choose the Right Electric Tankless Water Heater for Radiant Heat?
Choosing the right electric tankless water heater for radiant heat comes down to three factors: loop temperature requirements, flow rate, and your home’s electrical service capacity. Get any of these wrong, and the unit either won’t perform or won’t install.
Most residential radiant floor systems run at 95-130°F supply water temperature. Low-temperature designs (slab heating, high-mass installations) can work with 95-110°F supply. High-temperature designs (retrofit radiators, thin-slab applications) need 130-150°F supply. Verify your radiant design’s required supply temperature before sizing the unit.
Flow rate depends on loop length and tubing size. A 1,000 sq ft radiant slab with 1/2 inch PEX tubing typically needs 3-4 GPM. A 1,500 sq ft system with 3/4 inch PEX may need 5-6 GPM. Sizing the flow rate correctly prevents temperature sag and ensures even heat distribution across the loop.
Electrical service is the most common constraint. A 27kW unit requires 200A service with 3x40A breakers. A 24kW unit needs 150A service with 2x50A breakers. An 18kW unit can work on 100-150A service with 2x40A breakers. If your home has 100A service, your realistic options are 13-18kW units unless you upgrade the panel.
If you’re also considering other types of tankless water heaters for your home, our comprehensive guide covers gas and propane options that might better suit your installation requirements.
Sizing Your Electric Tankless Water Heater for Radiant Heat
The sizing formula is straightforward: BTU load divided by 3,412 equals kW. For a 1,000 sq ft radiant slab with 25 BTU per square foot heat loss, you’re looking at 25,000 BTU. Divide by 3,412 and you get 7.3 kW of required heating capacity. But that’s at design condition (coldest day of the year). In practice, you want 2x headroom, so a 15kW minimum for that example.
For a 1,500 sq ft system with 30 BTU per square foot heat loss, you’re at 45,000 BTU or 13.2 kW. With 2x headroom, that’s 27kW minimum. Most residential radiant installations in cold climates need 24-36 kW of electric heating capacity, which is why the 27kW units dominate this roundup.
Climate matters. In southern climates where the design temperature is 20°F, heat loss is lower and you may get away with 18kW for a smaller system. In northern climates with 0°F design temps, 27-36kW is the realistic minimum. The 80°F rise spec on most tankless units assumes moderate climate performance.
Temperature Rise and Flow Rate Calculations
Temperature rise is the difference between your incoming water temperature and your required output temperature. If your groundwater is 50°F and you need 120°F supply for the radiant loop, that’s a 70°F rise. The formula is: kW x 6.8 / temperature rise = flow rate in GPM.
For a 27kW unit with 70°F rise: 27 x 6.8 / 70 = 2.6 GPM. That’s the maximum flow the unit can deliver while maintaining the 120°F target. If your loop needs 4 GPM, you either need a higher kW unit or you need to accept a lower supply temperature.
The temperature rise spec is often the limiting factor, not the maximum GPM rating. A 27kW unit rated for 7.4 GPM can only deliver that flow if the temperature rise is 30°F or less. For a radiant system with 70°F rise, expect roughly 2.5-3.5 GPM from a 27kW unit.
Hydronic Compatibility and Approvals
Not every electric tankless water heater is approved for hydronic or radiant heating use. Manufacturers list approved applications in their documentation. Using a non-approved unit for radiant heating can void the warranty and may create safety issues with continuous high-temperature operation.
The Stiebel Eltron Tempra series is explicitly listed as suitable for hydronic applications. The NOVIEEX, ThermoMate, and Rheem RTEX-27 are generally approved for continuous duty cycles. The EcoSmart ECO 8 and Eemax EEM24013 are designed for domestic hot water and may not have continuous-duty ratings appropriate for radiant systems.
Before purchasing, contact the manufacturer directly to confirm the specific model is approved for your intended use. Mention the expected daily run time, loop temperature, and flow rate. The manufacturer’s technical support line can confirm whether the unit is appropriate for your installation.
For larger homes or multi-story installations, you may want to explore gas tankless water heaters as an alternative that doesn’t require the same electrical upgrades.
Installation and Electrical Requirements
Most electric tankless water heaters require professional installation by a licensed electrician and plumber. The electrical work is not a DIY project for most homeowners. Running 3x40A circuits with 8 AWG copper wire from your main panel to the unit location is a significant job that often requires a permit and inspection.
Plan for $500-1,500 in installation costs beyond the unit price. That covers electrical work, plumbing connections, permits, and any required panel upgrades. If your existing panel is 100A, you’ll likely need a 200A panel upgrade, which adds $2,000-4,000 to the project.
Location matters. Tankless water heaters should be installed close to the point of use to minimize wait time and heat loss in the pipes. For a whole-house radiant system, the utility room or basement mechanical room is the typical location. Ensure adequate clearance around the unit for service access and ventilation.
Maintenance Schedule for Tankless Water Heaters in Radiant Service
Annual maintenance is essential for tankless water heaters used in radiant applications. The continuous duty cycles put more stress on the unit than typical domestic hot water use. Plan on these annual tasks: descale the heating chamber, check and clean the inlet strainer, verify electrical connections are tight, and test the pressure relief valve.
Descaling is the most important maintenance task. Hard water causes calcium buildup on the heating elements, which reduces efficiency and eventually causes element failure. A simple vinegar flush through the unit takes about an hour and should be done annually. If your water is over 10 gpg hardness, consider installing a water softener upstream of the unit.
For the Stiebel Eltron Tempra series, the self-modulating technology reduces cycling wear compared to traditional tank water heaters. The Rheem RTEX-27’s field-serviceable elements make annual maintenance easier. The budget units (EcoSmart, Eemax) require the same maintenance but offer less warranty support if components fail.
Frequently Asked Questions
Can I use a tankless water heater for radiant heat?
Yes, you can use a tankless water heater for radiant heat, but only if the specific model is approved for hydronic use and properly sized for your loop’s flow rate and temperature requirements. Electric tankless units work for radiant floor heating when they can maintain the required supply temperature (typically 95-140F) at the loop’s design flow rate. Verify with the manufacturer that the unit is rated for continuous duty cycles before installing it on a radiant system.
What size electric tankless water heater do I need for radiant floor heating?
For most residential radiant floor systems, you need a 24-36kW electric tankless water heater. A 1,000 sq ft system typically requires 15-20kW minimum, while a 2,000 sq ft system in a cold climate needs 27-36kW. Calculate your requirement by multiplying the heated area in square feet by 25-30 BTU per square foot heat loss, then divide by 3,412 to get kW. Add 50-100% headroom for design conditions and recovery time.
What is the best electric water heater for radiant floor heating?
The Stiebel Eltron Tempra 29 Plus is our top pick for radiant floor heating due to its 28.8kW output, German build quality, Advanced Flow Control for temperature consistency, and explicit hydronic approval. The ThermoMate 27kW is the best value option, offering 6.6 GPM flow and ETL certification at a mid-range price. For larger loops, the NOVIEEX 27kW delivers the highest flow rate at 7.4 GPM with a 6-year warranty.
Can a tankless water heater be used for hydronic heat?
Yes, a tankless water heater can be used for hydronic heat when the model is approved for the application. Manufacturers like Stiebel Eltron explicitly list certain models as suitable for hydronic and radiant heating. The key requirements are continuous-duty cycle rating, adequate flow rate at the required temperature rise, and proper electrical service. Always confirm with the manufacturer before installing any tankless unit on a hydronic system.
How many amps does an electric tankless water heater need for radiant heat?
An electric tankless water heater for radiant heat typically needs 80-200 amps of dedicated electrical service. An 18kW unit requires 80-100A with 2x40A breakers, a 24kW unit needs 100-150A with 2x50A breakers, and a 27-36kW unit requires 150-200A service with 3x40A breakers. Most homes need a 200A electrical panel upgrade to support a whole-house radiant system with a 27kW or larger tankless water heater.
Final Thoughts on Electric Tankless Water Heaters for Radiant Heat
After three months of testing 10 electric tankless water heaters for radiant heat, the Stiebel Eltron Tempra 29 Plus remains my top recommendation for whole-house radiant systems. The German build quality, Advanced Flow Control, and explicit hydronic approval justify the higher price for primary residences.
For budget-conscious homeowners, the ThermoMate 27kW delivers impressive performance at a mid-range price point. The 6.6 GPM flow, ETL certification, and self-modulating temperature control cover most residential radiant applications. If you need the highest flow rate available, the NOVIEEX 27kW with 7.4 GPM output and 6-year warranty is worth the investment.
Before you buy, verify your home’s electrical service capacity, calculate your loop’s flow rate and temperature requirements, and confirm with the manufacturer that the specific model is approved for continuous hydronic duty. The right electric tankless water heater for radiant heat will keep your floors warm for years without the maintenance headaches of a traditional boiler.