12 Best Water Heaters for Radiant Floor Heating (October 2026)

Radiant floor heating does not need a fancy appliance. It needs a heater with enough steady output to warm a sealed loop of tubing, and a control that can hold that loop at a temperature low enough to walk on. The best water heaters for radiant floor heating come down to four families: an indirect-fired storage tank, a condensing tankless heater listed for space heating, a heat pump or hybrid, and a plain gas or electric storage tank. Size by heat loss, never by square footage.

Before any brand names come up, here is the part most roundups skip. Every heater reviewed on this page was built and sold as a domestic hot water appliance. Their product literature describes gallons, flow rate, and first-hour ratings, not radiant duty. A manufacturer’s manual or specification sheet is the only place that tells you whether a given model is approved for space heating, and if it is not, connecting it to a radiant loop can put the warranty out of reach.

That is not a reason to skip radiant floor heating. It is a reason to buy deliberately, and to have your installer pull the specific manual for the exact model number before the day of the install. Over the past several weeks our team read the spec sheets and the owner reviews for twelve of the most commonly recommended heaters in this category, then sorted them by output, temperature control, electrical or gas demands, and warranty length. Here is what we found, plus the sizing math that decides which one you actually need.

If you are still narrowing the field, our picks for the best heat pump water heaters and the best hybrid water heaters are worth a look, since both families can be piped as space heat sources in the right application.

What Type of Water Heater Works Best for Radiant Floor Heating?

Four heater families work for radiant floor heating: an indirect-fired storage tank, a condensing tankless heater approved for space heating, a heat pump or hybrid, and a standard gas or electric storage tank. Each one can deliver the heat. The differences that matter are how evenly the output modulates, how much the appliance must be derated to match a real heat-loss number, and whether the manufacturer lists heating as an approved use at all.

An indirect-fired tank is the most forgiving choice because the heating loop and the domestic hot water loop never touch. A condensing tankless heater rated for space heating is the most efficient, and it needs the most control hardware around it. A standard storage tank is cheap and simple but burns fuel continuously, and a heat pump or hybrid adds a compressor to a job that some climates make expensive to run.

The Warranty Warning: Most Tankless Water Heaters Are Not Rated for Radiant

The single most important thing to check before buying any heater for radiant floor heating is whether the manufacturer lists radiant or space heating as an approved application, because if it is not listed, the warranty does not cover the damage.

This comes up constantly in installer forums, and it is the mistake homeowners discover after the install rather than before. A professional on one long pre-install thread put it plainly, that the major tankless brands do not approve their units for radiant applications and it will affect the warranty. Other installers on the same thread steered toward a condensing boiler or a heating-rated tankless appliance instead.

There is a second failure mode that follows from the same oversight. Radiant loops draw heat in long, low, steady pulls. A heater sized to serve a family of showers is many times larger than that load, so it fires, shuts off, fires again, and the burner and heat exchanger take the damage. Professionals call that short cycling, and they describe it as the fastest way to kill an appliance that was never going to be covered in the first place.

Ask your installer for the model number, then read the approved-uses section of the manual or spec sheet. If radiant is not on it, the fix is not a different brand of the same layout. The fix is a buffer tank, a plate heat exchanger, or a heater that was actually certified for the job.

Top 3 Water Heater Picks for Radiant Floor Heating

Best Water Heaters for Radiant Floor Heating in 2026

The table below lists all twelve heaters we evaluated, with the output figures that actually determine radiant suitability. Flow rate and BTU input matter for domestic hot water, but a radiant loop cares about steady modulated output and control resolution. Read the electrical and venting requirements as carefully as the output numbers, because on this list the install is often the more expensive half of the project.

1. Rheem RTEX-24 24kW Electric Tankless – Best All-Round Starting Point

The Rheem RTEX-24 is the one I keep coming back to for a small or mid-size radiant zone, and it earns that spot on the balance of rating and owner history rather than on a single spec sheet line. It puts 24 kW behind a cabinet measuring 17.63 inches wide by 18.25 inches tall by 3.5 inches deep, which means it mounts on a basement wall next to the manifold instead of claiming a utility room corner.

What I like most for radiant work is the control resolution. The digital thermostat adjusts in one-degree increments from 80F to 140F, and the bottom of that range is exactly where a well-insulated slab loop wants to sit. Owners report the setpoint holds steady rather than swinging, which is the whole game when the load is a floor rather than a shower.

The honest limitation is electrical. The spec sheet calls for three 40-amp double-pole breakers, three sets of #8 AWG copper with ground, and a 200-amp household service panel. On a panel that is already close to full, the electrician can easily cost more than the heater, and that is the number most people underestimate when they start planning.

Performance also depends on how cold the incoming water is. Owners in cold-climate regions report reduced output at the far fixture, and a smaller number describe thermostat faults or sediment buildup that required early replacement. A small percentage of the reviews describe inconsistent heating that a thermostat swap fixed, so treat any output shortfall as a control problem before assuming the appliance is undersized.

What heat loss this actually covers

At 24 kW of instantaneous capacity, this is a good match for a single zone such as a main-floor slab or a bathroom-plus-hall run. It is a poor match for a whole-home primary heat source, because the radiant loop will never pull anything close to what the elements can deliver and the unit will spend its life waiting.

Where the radiant approval question stands

Rheem’s published materials for this model describe domestic hot water performance in gallons per minute, not space heating output in BTU per hour. There is no radiant or space heating rating stated in the listing. Confirm it against the installation manual for the RTEX-24 before you sign anything, and plan on a buffer tank and mixing valve as part of the design rather than an afterthought.

2. EcoSmart ECO 36 36kW Electric Tankless – Best Value for Heavier Electrical Work

The ECO 36 is the highest-output electric heater in this group, and that is exactly what you want when the radiant loop is the only thing drawing water. At 36 kW it can hold temperature through two simultaneous showers, and the self-modulating element adjusts power to match real flow rather than running flat out.

Reviewers consistently report three to five years of trouble-free service, and tech support is described as knowledgeable enough to correct wiring mistakes. For a radiant project that matters more than it first seems, because the most common early failures on a hardwired appliance come from the panel side rather than the appliance side.

The electrical requirement is the hard part. Four 40-amp double-pole breakers means eight panel slots, and if your existing service cannot take it, you are looking at a sub-panel or a service upgrade. On some homes the electrical work exceeds the cost of the appliance by a wide margin.

Two other notes from the reviews. Copper supply lines lose heat on the way to the fixture, so a higher setpoint is often needed to deliver a comfortable shower temperature. And savings relative to a conventional tank are frequently reported as negligible, which is worth remembering when the purchase case is framed around the electric bill rather than around the radiant loop.

Whether the output is really needed

Thirty-six kilowatts is generous for a single radiant zone and closer to right if the heater is also serving domestic hot water for a family. A 24 kW model covers the same loop with less electrical infrastructure, and the smaller draw reduces the shock to an older panel.

What to verify before the install

Like every heater here, the ECO 36 is documented for domestic hot water, and no radiant or space heating rating is stated. Warranty coverage on some units also depends on registering the product within 30 days, so get that done the day it is installed rather than the day you remember.

3. Ranein 190000 BTU Gas Tankless – Best for High Radiant Output

Where the electric heaters run out of headroom, the gas side picks up. This Ranein carries a 190,000 BTU input and up to 8.5 GPM of flow, which is enough to serve four to six outlets. For a radiant loop in a larger new build, that kind of margin is the reason gas comes up at all.

The cabinet measures 24.25 inches tall by 15.08 inches wide by 8.07 inches deep and weighs 42 pounds. The blower is mounted underneath the appliance and auto-adjusts its RPM for stable combustion, which is a design choice aimed squarely at backdraft and noise in tight mechanical rooms.

Two constraints are worth reading twice. The unit is not recommended for installation above 2,000 feet of elevation, which is a genuine disqualifier for anyone in the high desert or the Rockies. In cold climates the pressure relief valve has to be opened and the appliance drained if it will sit unused for long stretches.

A 2-year manufacturer warranty is also the shortest in this group, and for something that will be expected to run for decades behind a floor, that is a real consideration. A 120-volt power supply is required in addition to the gas line.

Why high BTU input is a double-edged number

A 190,000 BTU input sounds like the right answer and often is not. Sized against a real radiant heat loss, it is frequently many times larger than the loop can absorb, which puts you straight into the short-cycling problem that wears out the burner. Size the heater to the heat loss first, then check whether the model modulates down far enough to serve that load.

What to ask the manufacturer

Ask specifically whether the model is listed for space heating and whether the listed warranty terms change when it is used that way. Gas appliances in this class are almost always sold and documented for domestic hot water, and the published literature on this model describes flow in gallons per minute rather than output in BTU per hour for a heating loop.

4. Stiebel Eltron Tempra 29 Plus – Top Rated for Temperature Accuracy

The Tempra 29 Plus has the highest rating of any electric heater in this group at 4.4, and the reason owners keep it is control. It draws 28.8 kW at 240 volts and holds outlet temperature where you set it, with two programmable memory presets that let you store a morning and an evening setting.

Several owners report four to five years of flawless whole-home service while covering two bathrooms at once. The cabinet is 16.63 inches wide by 14.5 inches tall, installs anywhere indoors, needs no venting, and uses no standby power at all.

Flow sensitivity is the recurring complaint, and it matters more for radiant than for showers. Low-flow fixtures can cause the output to fluctuate or the appliance to shut down, which is a different kind of problem when the demand is a circulator pump rather than a faucet.

The electrical spec is three dedicated 40-amp breakers with 8-gauge copper and a 200-amp minimum service, and that often means sub-panel work. Two smaller issues get mentioned repeatedly: the mounting holes are spaced 15 inches on center and do not line up with standard US stud layout, and the hinged front cover is awkward in a tight closet.

Whether 3 GPM is enough for a radiant zone

Three gallons per minute is modest next to the gas appliances in this list. For a bathroom or small in-floor zone that circulates a closed loop, flow is a design question for your installer rather than a fixture-demand question. Confirm the loop pump and tubing sizing line up with the appliance before ordering.

What the ratings do and do not tell you

The 4.4 rating reflects real long-term satisfaction with a domestic hot water appliance, and that is worth having. It does not tell you the model is approved for radiant duty, and the listing carries no such claim. Read the manual for model 239223, check the approved applications, and plan the loop controls accordingly.

5. MIZUDO 199000 BTU Gas Tankless – Premium Pick with App Control

The MIZUDO is the flow leader here at 10.3 GPM, and it also carries a 4.4 rating. For a home combining radiant floor heat with a busy domestic hot water schedule, that combination of flow and temperature accuracy is the reason to look at it first among the gas options.

It modulates to within about 1F and reaches up to 149F, which gives an installer real headroom above the temperature a floor loop wants. It triggers at roughly 0.53 GPM, so low-demand fixtures still get hot water without the unit sitting idle. The sealed concentric direct vent with a bottom-mounted forced-draft fan resists backdraft in wind, which matters for an exterior wall or a detached shop.

Cold-weather reliability is the dominant negative theme in the reviews. Some owners report error codes and hot water lasting only 60 to 90 seconds before going cold when temperatures drop. The manufacturer also enforces an elevation ceiling of 3,000 feet, which one high-desert buyer found disqualifying.

Gas fittings and connections are not included and have to be sourced separately, so budget for that before the install day. The default comfort setting is 107F with adjustment up to 149F. Warranty coverage is 2 years on the full appliance and 5 years on the heat exchanger.

Where a high-flow gas appliance earns its cost

Ten gallons per minute is justified when the same appliance serves two to three bathrooms plus laundry and a dishwasher. It is overkill for a radiant zone that runs at 105 to 130F with a circulator, and at that load the modulation range is what saves it from short cycling rather than the top-end flow figure.

What to confirm about the radiant application

The documentation for this model describes gas fittings, ignition, and flow performance, with no space heating rating stated. Treat the app control as a convenience feature for monitoring, not as a substitute for the outdoor reset control that a radiant loop actually depends on.

6. WINTEMP WN36 36kW Electric Tankless – Best for Larger Electrical Services

The WN36 puts 36 kW behind a cabinet that measures 13.7 inches wide by 21.5 inches tall by 5.2 inches deep. The self-modulating power control is what interests me for radiant, because it trims output as flow changes instead of running the elements flat out against a fixed setpoint.

Owners frequently report no meaningful increase in electricity usage compared with the old tank heater, and the installation template makes mounting straightforward. It is ETL certified with leakage, overheat, and dry-fire protection built in, which is the kind of detail worth having behind a floor you cannot easily reach.

The electrical specification is the hardest in this group. Maximum draw is 150 amps, it calls for four 40-amp double-pole breakers and four 8/2 AWG cables with ground, and 300-amp household service is recommended. That requirement alone rules it out of a large share of existing homes.

It can also shut down if flow runs too high, and several owners install a flow regulator to hold output around 5 GPM. Cold incoming water reduces available hot water volume, the front cover uses snap-in clips that are hard to detach, and one owner raised a security concern about the wireless temperature control.

Whether a 300-amp panel is realistic for you

Ask an electrician to look at your panel before anything else on this list. If the service is already 200 amps, this model is a conversation about a service upgrade, and that changes the economics of the entire radiant project rather than just this purchase.

What the two-year term means for a floor loop

Two years is a modest replacement warranty for a hardwired appliance that will sit behind a slab for the life of the house. The mitigations are the ETL safety certifications and the fact that a self-modulating element is less likely to cook itself on a low, steady radiant load than a fixed-output design would be.

7. SIVUATEK 27kW Electric Tankless – Best for Under-Sink Mounting

The SIVUATEK is the narrowest appliance in this roundup at 2.87 inches deep, which means it can go under a sink or into a shallow wall cavity where nothing else fits. The diagnostic display is unusually informative for this class, showing inlet and outlet temperature, flow rate, and kilowatt draw with plus or minus 1F control.

It is ETL certified at 99.8 percent thermal efficiency, heats in about three seconds, and the water-to-electricity separation design is intended to resist scale buildup. That matters for a hardwired loop where the heating element sits in a closed circuit all winter.

Here is the constraint that decides this heater’s place in a radiant project. The maximum outlet temperature is 131F, and that caps your temperature rise. Any long pipe run, or a loop that needs a higher supply temperature, will not get it from this appliance.

It also needs three 40-amp breakers with 240-volt hardwiring, and it is rated for 4 to 5 usage points, so it can be undersized for a larger home. The temperature adjustment range runs from 86F to 131F in 1-degree steps, which is fine for a loop and short on headroom for anything else.

Whether 131F is a ceiling or a feature

For a well-insulated radiant loop running in the 105 to 130F range, the 131F ceiling is barely a limit and it is arguably a feature, because nothing in the piping gets hot enough to be a hazard. For an uninsulated slab or a cold-climate design that needs 140F supply, it is a hard stop.

What to check on the radiant question

No space heating rating is stated in the product documentation. The 99.8 percent efficiency figure refers to the conversion of electricity to hot water, not to heating system performance, so do not read it as an efficiency claim for a floor loop.

8. APUS 27kW Electric Tankless – Best for Compact and Flexible Installs

The APUS 27kW is a straightforward whole-home electric heater with a 3.5-inch cabinet and standard 3/4-inch NPT connections, which makes it easy to plumb into a tight mechanical space. The self-modulating element adjusts power to real-time flow, and the memory function recalls the last setpoint you used.

There is no storage tank, so there is no standby water reheating. The temperature range is 86F to 140F in 1-degree increments, which gives an installer more headroom than the 131F ceiling on the slimmer unit in this group.

The electrical requirement is three 40-amp breakers and six runs of 8 AWG cable at 240 volts. That is a real electrical project, and it belongs in the budget conversation before the appliance is chosen rather than after.

Actual flow varies with inlet temperature, setpoint, and household demand, and the appliance is suited to roughly 3 to 4 simultaneous use points. For a radiant loop those points are not faucets, but the output derating with cold incoming water is the same physics either way.

Why the setpoint range matters for a floor loop

A 140F ceiling gives more design room than most of the compact electric heaters here. If your heat-loss calculation lands on a higher supply temperature because of a poorly insulated slab, this is the kind of unit that can serve it without modification.

What the two-year warranty covers

The warranty is a 2-year limited term with 24/7 customer service listed as a feature. As with the rest of this list, no radiant or space heating approval is stated in the documentation, so the manual check before purchase is not optional.

9. WINTEMP 27kW Electric Tankless – Best for Mid-Size Homes

This is the middle of the electric range: 27 kW at 240 volts with up to 6.5 GPM, in a cabinet that measures 11.1 inches wide by 19.1 inches tall by 4 inches deep. The self-regulating technology reduces power as flow drops and draws nothing at idle, which is a sensible behaviour for a loop that idles for long stretches between heating calls.

It is ETL certified with leakage, overheat, and dry-fire protection, and the listed application is two to four whole-home usage points. For a mid-size radiant zone, that capacity class lines up well with a small circulator-fed loop.

It still asks for three 40-amp double-pole breakers and 8/2 AWG wiring with ground, and 250-amp household supply is recommended. That is a smaller ask than the 36 kW appliances on this list but a larger one than a 200-amp panel can comfortably absorb.

One detail to note: the listed flow of 6.5 GPM is slightly higher than the figure in the product title, and real output falls as inlet water gets colder. The warranty is a 2-year return and exchange term, which is modest for a hardwired appliance you intend to leave in place for decades.

Whether 27 kW covers a slab loop

For a single main-floor slab with a real heat-loss calculation in the low tens of thousands of BTU per hour, 27 kW of instantaneous capacity is workable. The key is that the control has to be allowed to fire continuously rather than pulse, which is a control-design question more than an appliance question.

What to confirm on the radiant rating

Nothing in the product documentation states a space heating rating. Where a listing does not say, treat the heater as domestic hot water equipment and verify the approved applications in the installation manual for the specific model number.

10. CAMPLUX 199000 BTU Gas Tankless – Best for Quiet Operation

The CAMPLUX carries the highest average rating in this roundup at 4.5, and the reason is not subtle: it runs quietly. At roughly 45 decibels it is quieter than a refrigerator hum, which matters when the appliance ends up in a space adjacent to bedrooms rather than in a basement corner.

It runs a 199,000 BTU input for a 170,000 BTU output across four flame settings, and the dual undercarriage blowers adjust RPM to hold a 16 to 1 air-fuel ratio. That four-stage modulation is the part that matters for radiant, because a loop that wants a steady low flame is exactly what staged combustion is built to deliver.

Listed capacity is up to 7.6 GPM across six simultaneous points, and the cabinet measures 24.7 inches tall by 14.3 inches wide by 7.3 inches deep, which fits a closet or a laundry room.

What the four flame settings do for a floor loop

Staged combustion is the closest thing on this list to a modulating burner, and it lets the appliance match a low, steady radiant demand without pulsing. Combined with 45 dB operation, it is the profile I would look for when the heater shares a wall with living space.

The maintenance commitments are specific

Annual descaling is required to maintain longevity, and the unit must be winterized below 20F using the included quick-drain valve. Altitude compensation stops at 2,000 feet. Some owners also had to source a separate vent kit or different fittings because of the 3-inch vent and 3/4-inch connection, and the mounting brackets may not span two studs without added blocking.

11. APUS 140000 BTU Gas Tankless – Best for Low-Demand Fixtures

The APUS 140,000 BTU gas heater uses a built-in booster pump to raise flow by up to 130 percent to a maximum of 6.8 GPM. Owners report running two showers and a sink together without a temperature drop, and the zero cold water single-circulation system delivers hot water in about a second.

Control is handled through a Tuya app from 95F to 140F with plus or minus 1F accuracy, and the bottom turbo fan design is reported to run about 30 percent quieter than a standard tankless appliance. The heat exchanger is oxygen-free copper with a UEF of 0.81, and eight protection systems are listed including freeze and overheat protection.

The weak point is capacity at the top end. Some owners report insufficient output to fill a bathtub to a comfortable temperature, and a 21 percent share of one-star ratings is the most polarized distribution of any gas appliance in this group. That spread tells you the unit is very good in the right house and disappointing in the wrong one.

You will need to source extra fittings and adapters before the install. The appliance must be fully drained if ambient temperature approaches 41F or lower for extended periods, and one owner reported an initial ignition failure caused by insufficient downstream flow to trip the sensor.

Whether a booster pump helps a radiant loop

Not much. The pump is built to solve a low-flow problem at a faucet, and a radiant loop fed by a circulator runs at steady flow by design. Judge this appliance on its BTU input and its burner staging, not on its GPM figure.

What the warranty covers

Coverage is a 2-year manufacturer warranty, and the documentation describes domestic hot water specifications only. Before committing, have your installer confirm the approved applications for model AP-AWH-Q20NG in writing, so the radiant question is settled before the gas line is run.

12. FOGATTI 120000 BTU Gas Tankless – Best for Low Water Pressure Homes

The FOGATTI is the one to look at if your incoming water pressure is marginal. It runs from 15 to 150 PSI, which is a low floor, and it uses a water servo with temperature sensors to hold output within about 1F. Owners confirm two simultaneous showers without trouble on homes where pressure is not generous.

The four-stage combustion system is built for seasonal stability, and the 120,000 BTU copper heat exchanger is paired with a 304 stainless steel burner. At 23.9 by 15.5 by 7.3 inches it takes up roughly a third the space of a conventional tank heater.

Two cautions. The minimum flow activation can let outlet temperature spike to scalding levels on partial faucet openings, which is a real safety point to design around in a system that includes a mixing valve anyway. And some owners report recurring E1, E2, and E8 error codes that required repeated resets or outright replacement.

The box also contains only the heater and instructions, so vent pipe and fittings are an unplanned cost. At 5.1 GPM, shower pressure can fall if household flow demand exceeds the appliance capacity, and customer service responsiveness is criticized by at least one long-term owner.

Whether 120000 BTU is enough for a floor loop

This is the smallest gas input in the group, and for radiant that is not automatically a disadvantage. A small zone with a modest heat-loss number may find that a 120,000 BTU appliance is a better match than a 199,000 BTU one, because the smaller unit spends more of its time near the middle of its firing range instead of pulsing.

What the 15 PSI floor really buys you

Most homes sit well above 15 PSI, so this specification matters mainly in older homes, in rural properties with long service runs, and in buildings where a pressure-reducing valve is already in place. Check your actual static pressure before paying for a feature you do not need.

How to Size a Water Heater for Radiant Floor Heating

Size by heat loss, not by square footage. Square footage assumes a specific envelope quality, insulation depth, and window area, and a slab-on-grade basement in a cold climate can lose heat through the ground fast enough that any rule of thumb will be wrong. A proper heat loss calculation is a small piece of work and it is the difference between a system that runs for decades and one that cycles itself apart.

Three worked examples will make the numbers concrete. A single bathroom zone in a well-insulated interior space commonly lands in the low thousands of BTU per hour, and a small electric tankless in the 24 to 27 kW class covers it with room to spare. A main-floor slab-on-grade zone in a moderate climate often lands somewhere between 25,000 and 35,000 BTU per hour, which is where the 120,000 BTU class gas appliances start to look proportionate. A whole-home primary heat source can exceed 100,000 BTU per hour, and at that point you are sizing a boiler rather than a domestic water heater, with an indirect-fired storage tank as the sensible bridge.

Next, settle the water temperature question, because it changes the appliance you need. A well-insulated radiant loop does not need 180F supply. Running the loop in the 105 to 130F range is more efficient and it removes the scald hazard entirely, which is why one builder we spoke with sets a standard water heater to 105F to 110F and avoids mixing valves and the extra tubing that come with them. Loop temperature should be set with a thermostatic mixing valve, and the control should vary it with outdoor conditions if you can afford outdoor reset.

Finally, budget for the control hardware. A radiant loop needs a circulator pump sized to the loop, a manifold or zone valves, a mixing valve, an air separator, an expansion tank, and somewhere to put them. On a primary and secondary loop design with a hydraulic separator, the appliance stays isolated from the loop pump, which is how you keep a low, variable radiant demand from short cycling the burner.

Our guides to space heaters cover the other end of the comfort spectrum, and are useful if you are weighing radiant against point heat room by room.

What Plumbers and HVAC Installers Actually Recommend

These are the rules that come up again and again in the installer forums we read while putting this together. They are not opinion pieces. They are the things professionals say when someone asks whether a given appliance can be the heat source for a floor.

1. Confirm the approved applications in the manual for the exact model number, before anything is ordered, not after it is installed.
2. Size to the heat-loss calculation. If the calculated load is 25,000 to 35,000 BTU per hour, a 190,000 BTU appliance is many times oversized and will short cycle itself to failure.
3. Add a buffer tank or a plate heat exchanger between the appliance and the loop to cut cycling and create hydraulic separation.
4. Use a primary and secondary loop with a hydraulic separator or low-loss header, and keep valves out of the closely spaced tees.
5. Use oxygen barrier PEX and bronze or stainless circulators. Non-barrier tubing lets oxygen diffuse into the loop and eats the circulator bearings.
6. Leave nothing between the heat source and the point where the pressure relief valve drains. No ball valve, no restriction of any kind on that path.
7. Size the mixing valve for the temperature range you actually run. Some common mixing valves require a minimum supply temperature and give a fixed output with no weather compensation.
8. Insulate the slab, and do not rely on the soil beneath it as a heat store. A concrete slab has a time constant of more than four hours, which is why night setback on a slab is a mistake.

One contradiction in the forum is worth resolving explicitly, because readers hit it constantly. Some builders recommend a standard hot water heater over a boiler because the water heater can be set to whatever temperature the loop needs, which avoids the overheated water that has to be cooled before entering the floor. Others say the same appliance is a mistake. Both are right under different conditions. A modulating or staged appliance set to a low loop temperature works. A fixed high-temperature appliance firing continuously does not, and neither does any appliance used outside its listed applications.

Five Mistakes That Ruin a Radiant Heating System

Buying an appliance that is not listed for radiant use. This is the first mistake and the most expensive one, because it is discovered at the worst possible moment. The manual, not the marketing copy, is the document that settles it.

Oversizing the heat source. An appliance several times larger than the calculated load will fire, shut off, and fire again. Professionals describe that as killing the appliance before its time, and the efficiency curve at low firing rates is not generous enough to compensate.

Using non-barrier PEX. Oxygen diffuses through non-barrier tubing into the loop water and attacks the circulator. Installing half-inch non-barrier tubing because a supplier specified it is a mistake the forums call out repeatedly, and the replacement cost lands years later on the circulator.

Restricting the pressure relief discharge. There must be no ball valve or any other restriction between the boiler and the place where the relief valve drains. This is a code issue and a safety issue, and it is a five-second check during install.

Skipping slab insulation and running a low loop temperature anyway. A well-insulated slab lets the loop run in the 105 to 130F range, which is efficient and safe. An uninsulated slab pushes you toward higher temperatures and higher fuel use, and the downward heat loss can exceed what you are paying for above the tile.

Frequently Asked Questions

Conclusion

Start with the Rheem RTEX-24 if you have a single small or mid-size radiant zone and a 200-amp panel, because it is the best balance of rating, owner history, and control resolution in this group. Look at the Stiebel Eltron Tempra 29 Plus if temperature accuracy matters more to you than electrical capacity. Go to the gas side, the MIZUDO or the CAMPLUX, when the radiant loop is large enough to need real BTU input and staged combustion.

Whichever one you choose, do three things before the purchase: get a real heat-loss calculation, confirm radiant approval in the manual for that exact model number, and budget for a mixing valve, a circulator, and a buffer tank. That last habit is what separates a system that runs quietly for twenty years from one that cycles itself apart in three. As we noted in our hybrid water heater picks, efficiency claims only hold up when the control hardware around the appliance is designed for the load, and the same rule applies to radiant floor heating in 2026.