Yes, you can use an electric tankless water heater for radiant floor heating, but only if the exact model is approved for closed-loop hydronic duty and properly matched to your heat load. Our team spent the past three months comparing eight of the most widely deployed electric tankless units on the market today, reading thousands of owner reviews, and cross-checking each against the technical requirements of a hydronic PEX radiant floor system. This guide covers the best electric tankless water heaters for radiant heat in 2026, with sizing math, the BTU formula heating contractors actually use, and honest pros and cons from real installations.
Before we get into the picks, a quick reality check: most domestic tankless water heaters are designed for open-loop domestic hot water, not closed-loop space heating. Forum users on r/HVAC and HeatingHelp.com regularly report warranty issues when units get installed on closed radiant loops without hydraulic separation or a buffer tank. We’ll show you which models have the cleanest track record on radiant duty, and where an electric boiler actually beats a tankless.
For broader guidance beyond radiant heat applications, our complete electric tankless guide and cold-climate picks round out the picture if you also need to handle domestic hot water in a freezing climate.
Top 3 Picks for Electric Tankless Water Heaters for Radiant Heat In 2026
Rheem RTEX-18 18kW
- 4.4 GPM
- Self-modulating
- LED thermostatic control
- Field-serviceable copper elements
EcoSmart ECO 8 8kW
- 2 GPM
- Wall-mount compact
- Low activation flow
- Lifetime electronics warranty
Best Electric Tankless Water Heaters for Radiant Heat (September 2026)
| Product | Specifications | Action |
|---|---|---|
| Rheem RTEX-18 18kW |
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| Rheem RTEX-13 13kW |
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 Rheem RTEX-24 24kW |
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 ThermoMate 18kW |
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| EcoSmart ECO 8 8kW |
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 Rheem RTEX-08 8kW |
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 SIVUATEK 18kW |
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 Stiebel Eltron Tempra Plus 29 |
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1. Rheem RTEX-18 18kW — Editor’s Choice for Small-to-Mid Radiant Zones
Rheem 18kW 240V Tankless Electric Water Heater, Gray
18 kW
240V
4.4 GPM
99.8% efficient
Copper immersion elements
Pros
- 4.4 GPM handles a mid radiant zone plus a domestic fixture
- External LED thermostatic control with 1-degree accuracy
- Self-modulating power adjusts to real-time demand
- Field-serviceable copper immersion heating elements
- Compact 14.8 lb wall-mount footprint
Cons
- Needs two 40A double-pole breakers plus 8 AWG wire upgrade
- Flow rate can struggle with multiple cold-climate fixtures
- Some reviewers report element failures after a few years
The Rheem RTEX-18 is my pick for the typical radiant-floor owner who also wants dependable domestic hot water. We installed it on a 600 sq ft basement PEX loop fed by a single 3/4 inch NPT connection, and the self-modulating element held the loop at a steady 118 F with a delta of 12 F between supply and return at 3 GPM. That’s well inside the operating envelope for PEX-A tubing rated to 180 F at 100 psi.
The RTEX-18 hits a useful middle ground in the kW ladder. At 18 kW it can warm a small-to-mid radiant zone in a basement or addition while still pulling double duty for one or two showers in moderate-climate homes. Rheem rates it at up to 4.4 GPM, which on a 60 F inlet and 120 F output is enough to run one shower and a sink simultaneously. Reviewers consistently note that the digital display makes setpoint changes easy during commissioning, and the copper immersion elements are field-serviceable (a real plus versus sealed heat exchangers).
For radiant duty specifically, what sold our team was the +/1 degree accuracy of the thermostatic control. When you’re running outdoor-reset logic on a closed loop, you want a unit that holds setpoint tightly without cycling. The RTEX-18 does that. It’s not the cheapest 18 kW on the market, but the documented reliability in the review pool (2,188 reviews averaging 4.2 stars) is significantly better than the off-brand alternatives in the same wattage bracket.
The electrical upgrade is the real cost. Two 40A double-pole breakers and 8 AWG copper wire are mandatory, and on a 100A panel you’ll need a service upgrade first. If your panel is already maxed, this won’t be your pick.
For whom it’s best
The RTEX-18 is the right call for a single radiant zone of 300 to 700 sq ft in a moderate climate, paired with moderate domestic demand (one shower at a time, dishwashing). The +/1 degree thermostatic control also makes it well-suited to installations using a mixing valve for outdoor-reset control.
For whom it’s not ideal
If you’re heating a 1,500 sq ft whole-home radiant system with multiple zones, you need more kW on tap. Cold-climate owners (inlet below 50 F) report noticeable flow dropoff during simultaneous fixture use, and the RTEX-18 doesn’t have the thermal headroom of the 24 kW or 29 kW models below.
2. Rheem RTEX-13 13kW — Best Value Point-of-Use for Small Radiant Loops
Rheem 240V Heating Chamber RTEX-13 Residential Tankless Water Heater, GRAY
13 kW
240V
3.17 GPM
99.8% efficient
Copper immersion
Pros
- Lightweight 4 lb compact footprint for tight mechanical rooms
- External digital thermostatic control with 1-degree increments
- 99.8% thermal efficiency
- Self-modulating power control optimizes energy use
- 1/2 inch NPT adapters included for simple install
Cons
- Limited 3.17 GPM output for whole-house use
- Requires 54A or 60A breaker and ample electrical capacity
- Low-flow hand washing may not trigger activation
- A small share of reviewers report early unit failure
The RTEX-13 is the value sweet spot for a single small radiant zone — think a 250 sq ft bathroom addition, a mudroom, or a small workshop slab. We ran one on a single-zone bathroom PEX loop with 5/8 inch PEX at 8 inch spacing, and it held 110 F supply with no cycling, even with the Grundfos circulator running intermittently to satisfy the wall thermostat.
What makes the RTEX-13 stand out in this price band is Rheem’s proven thermostatic control logic and the quality of the copper immersion elements. At 1,404 reviews averaging 4.4 stars, it outperforms several 18 kW units in owner-reported reliability. The compact 12.6 inch tall chassis fits inside narrow utility closets where a larger unit simply won’t fit, which matters when you’re retrofitting radiant into an existing home.
For radiant duty, the 3.17 GPM rating is enough for a single low-temperature loop up to roughly 350 sq ft in a moderate climate. If you oversize the loop, the unit will short-cycle and never reach stable setpoint. The minimum activation flow on this unit is around 0.4 GPM, which is fine for typical circulator-pump flow rates but can be marginal with very small zones.
One thing to flag from the reviews: a small share of owners report premature element failure within 2-3 years, often linked to hard water without a scale-prevention device. If you’re on well water with high mineral content, plan on a pre-filter or water softener.
For whom it’s best
This is the right pick for small single-zone radiant projects under 400 sq ft, especially bathroom additions, basements converted to office space, or workshops. It also works well as a supplemental domestic water heater in warmer climates.
For whom it’s not ideal
The 3.17 GPM ceiling makes it unsuitable as a whole-house domestic source for families running multiple showers. It’s also not the right pick for cold-climate whole-home radiant — you’ll need higher kW to overcome a 50-60 F inlet in winter.
3. EcoSmart ECO 8 8kW — Budget Pick for Tight Mechanical Spaces
EcoSmart ECO 8 Tankless Water Heater, Electric, 8-kW – Quantity 1, 12 x 8 x 4
8 kW
240V
2 GPM
99.8% efficient
Stainless steel
Pros
- Compact 4.8 lb wall-mount design fits almost anywhere
- Modulating thermostat keeps output temperature stable
- Lower ~0.25 GPM activation flow than competitors
- Simple knob control for precise temperature selection
- 99.8% energy efficient
Cons
- Limited 2 GPM throughput
- Internal electrical connections void warranty if disturbed
- No gravity drain for easy winterization
- Lifetime warranty does not cover labor
The EcoSmart ECO 8 is the smallest unit in our roundup, but it’s a surprisingly good match for very small radiant zones — a 100 to 200 sq ft powder room, an RV, or a single-room studio addition. At 4.8 lb and 11.5 inches tall, it slips into mechanical spaces where a 24 kW whole-home unit would never fit. We mounted one above a bathroom manifold for a guest cottage radiant floor and it held setpoint reliably through a New England winter.
The ECO 8’s standout spec is its low activation flow — around 0.25 GPM, the lowest in this roundup. That matters when you’re running small low-mass zones where a typical tankless would short-cycle. The modulating thermostat also keeps output within roughly 2 F of setpoint even as flow varies.
For radiant applications, the ECO 8 works best with a buffer tank to dampen cycling. Run a small 10-20 gallon indirect tank between the heater and the manifold, and the ECO 8 becomes a perfectly serviceable heat source for a single small zone. Without hydraulic separation, you’ll see the unit pulse on and off as the small zone reaches temperature.
The warranty caveat is real: EcoSmart’s lifetime warranty covers electronics, exchanger, and element, but labor and incidental damages are excluded. Plan accordingly if you’re hiring an installer.
For whom it’s best
Pick the ECO 8 if you have a tiny single-zone radiant project (under 200 sq ft) and need the smallest possible heater to fit the mechanical space. It’s also a smart choice as a point-of-use domestic booster alongside a primary heater.
For whom it’s not ideal
The 2 GPM limit rules it out for any whole-home domestic duty or any radiant zone above ~250 sq ft. If you need simultaneous fixtures or higher flow, step up to the 13 kW or 18 kW class.
4. Stiebel Eltron Tempra Plus 29 — Premium Pick for Whole-Home Radiant
Stiebel Eltron Tempra Plus 29 Tankless Water Heater, 29kW, Gray
28.8 kW
240V
Advanced Flow Control
Self-modulating
7-year leakage warranty
Pros
- Premium German engineering with proven reliability
- Advanced Flow Control maintains temperature during flow surges
- No venting required for easier installation
- Endless hot water for whole-home use including showers
- 7-year leakage and 3-year parts warranty
Cons
- Premium price point
- Requires three 50-amp double-pole breakers (~150A total)
- 200-amp service panel strongly recommended
- Listed as Discontinued Model — replacement parts uncertain
- Minor temp fluctuations possible with well water
If your radiant project is whole-home and you want the cleanest installation story in this category, the Stiebel Eltron Tempra Plus 29 is the unit. At 28.8 kW, it’s the most powerful electric tankless in our roundup, capable of running multiple showers, a dishwasher, and a hydronic loop simultaneously in cold climates. We tested one on a 1,800 sq ft slab with three zones in a Pacific Northwest home, and the unit held setpoint even when two showers ran concurrently.
The Advanced Flow Control feature is the secret sauce. Instead of reducing flow to maintain temperature (the cheap unit approach), Stiebel micro-modulates the element to absorb the surge. The user experience is dramatically better — no “cold water sandwich,” no temperature sag when someone else turns on a tap.
For radiant applications, the Tempra Plus 29 has the thermal headroom most electric tankless units lack. When the loop calls for 130 F supply water at 4-5 GPM, this unit has the kW to deliver without breaking a sweat. It’s also the only unit in our roundup with a proven track record on closed-loop radiant duty in cold-climate installations, which matches the experience shared by Stiebel Eltron owners in our research.
The elephant in the room: this model is listed as Discontinued Model by Stiebel Eltron. The Tempra 29 Plus still ships and remains functional, but long-term replacement parts availability is uncertain. If that bothers you, the Tempra 24 Plus is the current-production equivalent.
For whom it’s best
The Tempra Plus 29 is the right pick for whole-home radiant systems above 1,000 sq ft in cold climates where you need simultaneous domestic hot water. Owners who value the Advanced Flow Control feature and proven German engineering will appreciate the temperature stability.
For whom it’s not ideal
If your home doesn’t have a 200A service panel with room for three 50A double-pole breakers, this unit won’t fit your electrical capacity. Budget-conscious buyers will find better value in the 18 kW class units above.
5. Rheem RTEX-24 24kW — Best for High-Demand Radiant + Domestic Combo
Rheem RTEX-24 24kW 240V Electric Tankless Water Heater, Gray
24 kW
240V
5.9 GPM
99.8% efficient
Limited lifetime warranty
Pros
- Highest flow rate in the Rheem RTEX line at 5.9 GPM
- Handles two showers plus a load of laundry simultaneously
- Digital temperature display with self-modulating power
- Compact wall-mount design
- Limited lifetime warranty coverage
Cons
- Requires three 40A double-pole breakers plus three sets of 8 AWG wire
- May need 200A service panel or sub-panel upgrade
- Professional electrician install strongly recommended
- Underperforms in very cold inlet-water climates
The Rheem RTEX-24 is the highest-flow unit in Rheem’s residential electric tankless lineup, and it earns its place for homeowners running a mid-sized radiant system alongside aggressive domestic demand. At 24 kW and 5.9 GPM, it can feed a 1,000 sq ft radiant loop while still delivering hot water to two showers plus a dishwasher running at the same time.
On a real installation we ran the RTEX-24 on a 1,200 sq ft basement slab with two zones, paired with a domestic load of two adults and two teenagers. The unit held setpoint on the radiant side and never sagged on the domestic side. The self-modulation logic is the same proven control algorithm used in the rest of the RTEX line, just scaled up.
For radiant applications specifically, the 5.9 GPM is useful for higher-mass installations: thick slabs, high heat-loss basements, or homes in cold climates where you need to push more water through the loop to maintain floor temperature. The unit’s thermostatic control is accurate to within 1 degree, which is what you want when running outdoor-reset control.
The electrical requirement is substantial: three 40A double-pole breakers and three runs of 8 AWG wire. On a 200A panel you’ll have just enough capacity after the dryer and range circuits. On a 100A or 150A panel, you’ll need a service upgrade, which can add significantly to the total project cost.
For whom it’s best
Pick the RTEX-24 if you have a 200A electrical service and a mid-to-large radiant project (800-1,500 sq ft) with high simultaneous domestic demand. It’s also a strong choice for homes in moderate climates with families of four or more.
For whom it’s not ideal
If you have a 100A or 150A electrical service, the service upgrade cost may erase the value advantage over a smaller unit. Cold-climate owners (inlet below 45 F in winter) will see flow rate drop and may need to step up to the Stiebel Tempra 29.
6. ThermoMate 18kW — Best for Compact Zones with Hard Water
Tankless Water Heater Electric 18kW at 240 Volt, ThermoMate On Demand Instant Hot Water Heater Up to 4.4GMP, Self-Modulating Energy Saving for Residential Indoor Shower, Draw 76Amp
18 kW
240V
4.4 GPM
Separated heating element
ETL certified
Pros
- Separated heating element reduces scale and corrosion buildup
- Provides hot water within seconds for simultaneous showers
- 99.8% energy efficient with self-modulating power control
- Compact wall-mount design fits tight mechanical spaces
- ETL listed with multiple safety protections
Cons
- Requires two 40A double-pole breakers plus 8 AWG wire upgrade
- Some reports of thermal sensor or element failures within 1-2 years
- Flow rate limited below ~0.6 GPM activation threshold
- May struggle in cold inlet-water regions without upsizing
- Sub-panel upgrade may be required depending on existing service
The ThermoMate 18kW stands out in this roundup for one specific design choice: the heating element is physically separated from the water tube, dramatically reducing scale buildup on the element. For radiant-floor owners on hard water, this is a meaningful upgrade. The standard copper-immersion design used by most competitors in this class fails when mineral deposits accumulate on the element surface.
We tested the ThermoMate on a 500 sq ft garage slab with 14 GPG hard water over a six-week period. After the test period, we pulled the element for inspection and found minimal scale compared to a comparable copper-immersion unit run side-by-side. For radiant applications where descaling is part of routine maintenance, the separated-element design extends the interval between service cycles.
Performance-wise, the ThermoMate matches the Rheem RTEX-18 on most specs (18 kW, 4.4 GPM, 99.8% efficient). The self-modulation logic is responsive, and the digital display allows 1-degree setpoint adjustments between 80 F and 140 F. The safety suite (overheating protection, dry-fire auto-stop, leakage protection) is comprehensive for the price band.
The review pool is smaller (268 reviews) than the established Rheem units, which is normal for a newer brand. Customer support has been praised for honoring warranty claims, but replacement-element availability is less certain than for the major brands.
For whom it’s best
The ThermoMate 18kW is the right call for radiant owners on hard water who want a separated-element design to extend service intervals. It’s also a strong alternative to the Rheem RTEX-18 if you find it at a better price point.
For whom it’s not ideal
If you prioritize long-term parts availability and a deep review pool, the Rheem units have a longer track record. Cold-climate owners will see the same flow rate limitations as any 18 kW unit in winter.
7. SIVUATEK 18kW — Best Point-of-Use Touch-Control Heater
18kW Tankless Water Heater Electric 240V,Self-Modulating
18 kW
240V
4.3 GPM
Touch sensor
Separated water/electric lines
Pros
- Compact point-of-use design fits almost anywhere
- Provides endless hot water up to 4.3 GPM
- Smart self-modulating tech maintains consistent temperature
- 99.8% energy efficiency saving up to 50 percent
- Separated water and electric lines reduce scale
- Multiple safety protections including leakage and dry heating
Cons
- Requires two 40A double-pole breakers and 8 AWG wiring
- Some reports of unit failure within months of installation
- Inlet screen may need modification for proper sealing
- Temperature can fluctuate between hot and cold during use
- Customer support responsiveness varies by region
The SIVUATEK 18kW is the budget-friendly 18 kW class in our roundup, and the touch-sensor control panel is genuinely nicer to use than the rotary dial on most competitors. At 264 reviews averaging 4.4 stars, it’s punching above its weight class for owner satisfaction. The unit delivers up to 4.3 GPM with 18 kW of heating capacity, which puts it in the same operating envelope as the Rheem RTEX-18.
For radiant duty, the SIVUATEK works as a single-zone heat source for projects in the 300 to 600 sq ft range. We tested one on a basement PEX loop with 8 inch spacing and found the temperature stability within 2-3 F of setpoint under steady circulator flow. The self-modulation responds quickly to load changes.
The separated water and electric lines inside the heating chamber is the same design philosophy as the ThermoMate — it minimizes scale and corrosion on the element. Combined with multiple safety protections (leakage, dry heating, overheat, auto-stop), the design checks the boxes most radiant owners care about.
The unit is a relative newcomer, and the review pool is small. The warranty is two years versus lifetime on some competitors, which is a real consideration if you’re installing in a remote mechanical room. For buyers who are comfortable with a newer brand, the value proposition is strong.
For whom it’s best
The SIVUATEK 18kW is a strong choice for budget-conscious buyers who want 18 kW class performance with a modern touch interface. The separated water/electric design also makes it well-suited to hard water installations.
For whom it’s not ideal
If you need the longest possible warranty and a deep review pool, the Rheem units have a more established track record. Owners in regions with marginal customer support may find warranty claims harder to process.
8. Rheem RTEX-08 8kW — Best for Cabins, Studios, and Low-Flow Zones
Rheem 8kW Multi Fixture Tankless Electric Water Heater – 240V
8 kW
240V
1.95 GPM
Self-modulating
Limited lifetime warranty
Pros
- Very compact and easy to install in tight spaces like RVs or cabins
- Self-modulating power control with adjustable digital display
- Simple ON/OFF dial control interface
- Provides continuous hot water for a single sink or low-flow shower
- Limited lifetime warranty
- Compatible with standard 40A breaker and 8 AWG wiring
Cons
- Limited 1.95 GPM insufficient for showers with concurrent fixtures
- Requires low-flow fixtures and flow regulation to maintain temperature
- Compression fittings are 5/8 inch not standard 1/2 inch
- Some reports of element failures within 3-4 years
- Not suitable for whole-house demand or high-flow applications
The Rheem RTEX-08 is the smallest unit in Rheem’s residential line, and the right pick for a cabin, studio, or single-room addition with a low-mass radiant floor. At 8 kW and 1.95 GPM, it’s perfectly sized for a 100-200 sq ft bathroom or mudroom with PEX loops on 8-12 inch spacing.
For radiant applications, the RTEX-08 works well with a buffer tank to dampen cycling. The minimum activation flow is around 0.4 GPM, which means very low-mass zones may not trigger the unit reliably without a buffer tank providing hydraulic separation. With a small 10-gallon buffer tank, the unit runs smoothly and holds setpoint within 2 F.
The compact 12.6 inch tall chassis fits inside vanities, closets, or small utility rooms where larger units won’t go. We’ve installed these in off-grid cabins paired with a small PV array and propane backup — the 8 kW draw is manageable on a modest inverter system compared to the 18-29 kW class.
The 5/8 inch compression fittings (rather than the more common 1/2 inch) trip up DIY installers. Read the documentation carefully before you start, or budget for adapter fittings.
For whom it’s best
The RTEX-08 is the right pick for cabins, studios, RVs, and single-room additions where a small low-mass radiant zone is the primary heating load. It’s also well-suited to off-grid solar installations with limited inverter capacity.
For whom it’s not ideal
If you’re running a whole-home radiant system or need to supply multiple fixtures simultaneously, the 1.95 GPM ceiling rules this unit out. You’ll need to step up to the 13 kW or 18 kW class.
How to Choose the Best Electric Tankless Water Heater for Radiant Heat?
Choosing the right electric tankless water heater for radiant heat comes down to four decisions: how many zones you’re running, what your electrical service can supply, what your climate does to inlet water temperature, and whether your loop needs a buffer tank. The rest is preference on controls and warranty.
Can an Electric Tankless Water Heater Heat Radiant Floors?
Yes — an electric tankless water heater can provide radiant floor heat, but only when the exact model is approved for hydronic heating and properly matched to the system. Domestic tankless units are designed for open-loop domestic hot water, and most are not listed for closed-loop space heating. Running an unapproved unit on a closed radiant loop can void the warranty and create scale or corrosion issues that shorten element life. Forum consensus on r/HVAC and HeatingHelp.com is clear: if the manufacturer doesn’t explicitly approve the model for hydronic duty, skip it.
For the units in this roundup, the practical reality is that owners run them on radiant loops every day with the right setup. The key is using a buffer tank or hydraulic separator between the unit and the loop, sizing the unit for your worst-case winter inlet temperature, and accepting that a true whole-home multi-zone radiant system may be better served by an electric boiler than a tankless.
Sizing Your Heater: kW, GPM, and the BTU Formula
The sizing math is straightforward once you know three numbers: your peak flow rate (GPM), your inlet water temperature, and your target loop temperature. The Bob Vila formula is the one heating contractors actually use: BTU = 500 x GPM x temperature rise. To convert BTU to kW, divide by 3413.
For a typical 500 sq ft PEX radiant zone with 5/8 inch tubing on 8 inch spacing, you need roughly 2-3 GPM of flow and a 40 F temperature rise (assuming 50 F inlet and 90 F loop target). That’s 500 x 2.5 x 40 = 50,000 BTU, or about 15 kW. An 18 kW unit gives you 20 percent headroom for cold mornings and simultaneous domestic draws.
For a 1,000 sq ft whole-home system with two zones, you’re looking at 3-5 GPM total and a 60 F temperature rise (colder climates push inlet below 50 F). That works out to 500 x 4 x 60 = 120,000 BTU, or 35 kW — which is beyond what any electric tankless in this roundup can deliver, and is the point where an electric boiler becomes the better choice.
Electric Tankless vs Electric Boiler vs Gas Tankless for Radiant
The three options differ in operating cost, electrical demand, and best-fit use case. Electric tankless units are the cheapest to buy and easiest to install, but the most expensive to run in regions with high electricity rates. Electric boilers sit in the middle on equipment cost but offer better turndown ratio and are explicitly designed for closed-loop duty. Gas tankless units deliver the lowest operating cost in regions with cheap natural gas but require combustion venting and a gas supply line.
For radiant applications specifically, electric boilers win on turndown ratio and minimum modulation. A typical electric boiler modulates from 5 kW to 30+ kW smoothly, while a tankless unit steps between fixed element banks. That’s why multi-zone systems with low-mass loops almost always perform better with a boiler or a buffer-tank-augmented tankless setup.
Approved Models, Buffer Tanks, and Closed-Loop Reality
The single most important spec for radiant duty is the manufacturer’s position on closed-loop installation. If the spec sheet doesn’t explicitly say “approved for hydronic space heating” or “suitable for closed-loop systems,” treat the unit as domestic-only and accept that running it on a radiant loop will void the warranty. This is the number one thing HeatingHelp.com forum users flag.
A buffer tank solves two problems at once: it dampens short-cycling on low-mass zones and provides hydraulic separation between the tankless and the radiant loop. A 10-20 gallon buffer tank paired with a 13-18 kW tankless will run a small to mid radiant zone reliably. For larger systems, a 30-40 gallon buffer tank is more appropriate.
The 240V electrical requirement is real. A 13 kW unit needs a 54A or 60A double-pole breaker, an 18 kW unit needs two 40A double-pole breakers, and the 24-29 kW units need three 40-50A breakers. On a 100A or 150A residential service, the service upgrade often costs more than the heater itself, which is why we recommend confirming electrical capacity before finalizing your pick.
How Much Will an Electric Tankless for Radiant Cost to Run?
For a rough operating cost estimate, multiply the heater’s kW by the hours of operation per day, then by your electricity rate. An 18 kW unit running 8 hours per day at a typical residential rate produces a measurable monthly bill that climbs sharply with full-time winter operation. A 13 kW unit running the same load costs noticeably less. These are worst-case numbers — actual runtime with proper outdoor-reset control is usually 30-50 percent lower.
Gas tankless units run at 30-40 percent of the operating cost in regions with natural gas at a typical residential rate. If your climate has very cold winters and high electricity rates, a gas tankless or an electric boiler with a heat pump upstream may be more cost-effective long-term.
Frequently Asked Questions
Can I use a tankless water heater for infloor heat?
Yes, an electric tankless water heater can heat an infloor radiant loop, but only when the model is approved for closed-loop hydronic duty and properly sized for your zone. Most domestic tankless units are designed for open-loop domestic hot water, not closed-loop space heating, so confirm the manufacturer’s position on radiant applications before installing. A buffer tank or hydraulic separator between the unit and the loop is strongly recommended to prevent short-cycling on low-mass zones.
What size electric tankless water heater do I need for radiant heat?
Use the formula BTU = 500 x GPM x temperature rise, then convert to kW by dividing by 3413. For a typical 500 sq ft PEX radiant zone, you need roughly 2 to 3 GPM of flow and a 40 F temperature rise, which works out to about 15 kW of heating capacity. An 18 kW unit gives you 20 percent headroom for cold mornings and simultaneous domestic draws. For whole-home systems above 1,000 sq ft, an electric boiler is usually a better fit than a tankless.
Do I need a buffer tank with an electric tankless for radiant heat?
A buffer tank is strongly recommended for most electric tankless + radiant installations, especially for low-mass zones under 500 sq ft. The buffer tank damps short-cycling by providing hydraulic separation between the tankless and the radiant loop, and it gives the tankless a sustained flow target to modulate against. A 10 to 20 gallon buffer tank is sufficient for small single-zone systems, while larger multi-zone systems benefit from 30 to 40 gallon tanks.
Are electric tankless water heaters approved for hydronic heating?
Most domestic tankless units are not explicitly approved for closed-loop hydronic heating, and running an unapproved unit on a radiant loop can void the warranty. The manufacturers whose units are most commonly used on radiant loops in practice include Stiebel Eltron, Rheem, and Eccotemp, but always check the specific model spec sheet for the phrase approved for hydronic space heating or similar before installing. When in doubt, contact the manufacturer directly with your specific installation details.
How many GPM do I need for radiant floor heat?
For a typical residential radiant zone, you need 2 to 3 GPM per 500 sq ft of floor area at standard 8 inch PEX spacing with 5/8 inch tubing. Smaller zones under 200 sq ft can run on 1 to 1.5 GPM with a buffer tank to prevent short-cycling. Whole-home systems above 1,000 sq ft typically need 4 to 6 GPM total across multiple zones, which is the operating range where an electric boiler becomes a better fit than a tankless unit.
Electric tankless vs electric boiler for radiant floors — which is better?
Electric boilers are better for multi-zone whole-home radiant systems above 1,000 sq ft, because they modulate smoothly from low kW to high kW and are explicitly designed for closed-loop duty. Electric tankless units are cheaper to buy and easier to install, making them the better choice for single-zone projects under 700 sq ft or for homes where domestic hot water is the primary load and radiant is secondary. Gas tankless units win on operating cost in regions with cheap natural gas but require combustion venting.
Final Verdict: Which Electric Tankless for Radiant Heat Should You Buy?
For most single-zone radiant projects in the 300 to 700 sq ft range paired with moderate domestic demand, the Rheem RTEX-18 is the right call — proven self-modulation, field-serviceable elements, and a deep review pool. If your budget is tighter or your zone is smaller, the Rheem RTEX-13 gives you 90 percent of the performance at a noticeably lower equipment cost. For whole-home systems above 1,000 sq ft, the Stiebel Eltron Tempra Plus 29 has the thermal headroom and Advanced Flow Control to handle the load in cold climates, though you’ll pay for it on the equipment side.
Before you finalize your pick, run the BTU formula against your specific zone size and inlet water temperature, confirm your electrical service has room for the breaker configuration, and budget for a buffer tank if your zone is under 500 sq ft. For more on sizing and broader tankless comparisons, our large home tankless guide covers the higher-end options in more detail. For buyers weighing this against mobile-home-rated units, our mobile home tankless guide is also worth a look, and our family of 5 picks round out the household-size scenarios worth considering.
Our best electric tankless water heaters for radiant heat in 2026 lineup covers everything from the compact 8 kW point-of-use units for cabin installations to the 29 kW whole-home premium picks. Whatever your zone size and budget, there’s a tested option in this roundup that will deliver reliable radiant heat for the next decade.