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Q&A Spotlight

Is Radiant Floor Heat Really the Best Option?

A framer weighs his option for both heat and air conditioning

When does in-floor radiant tubing make sense?
Image Credit: Fine Homebuilding

Lukas Smith, a framer by trade, is building a 3,100-sq. ft. house in southern Ontario and plans to install a radiant-floor system in the basement slab as well as the first and second floors. The house will be built with structural insulated panels (SIPs) and have R-values of 33 in the walls and 50 in the roof.

With that as background, Smith poses three questions in his Q&A post: should he pour a layer of gypsum-concrete over the tubing; can he run the system from a hot water heater instead of a boiler; and what is the best way of providing air conditioning?

The discussion is the subject of this week’s Q&A Spotlight.

First, the flooring choices

Above-floor radiant tubing can be installed in aluminized floor panels such as Quik Trak and then covered with finished flooring. Alternately, the tubing can be covered with a layer of concrete, either conventional concrete if the floor framing is designed for the weight, or a lightweight gypsum concrete.

Lucas refers to this second option as an “over-pour,” and says some contractors have told him not to pursue it because the house is so tight. “Others say I would be wasting my money not doing it,” he adds.

But David Meiland thinks Smith is talking apples and oranges: “I don’t see what an ‘over-pour’ has to do with the house being extremely tight,” Meiland says. “You are either going to install tubing using a product like Quik Trak, or you’re going to pour gyp or lightweight over it. In my opinion you ought to review your choices of floor coverings and see which way that pushes you. I personally like gyp but it’s harder or impossible to install some types of flooring over it. “

Will a water heater work?

No matter what kind of flooring ultimately covers the tubing, a more fundamental question is whether a water heater will produce enough hot water for the radiant distribution system.

“You might be able to heat with a water heater,” Meiland adds, “but the real question is, how many BTUs do you need for space heating and how many for water heating? I tend to like boilers a lot more than water heaters for radiant, but ultimately what you need is someone highly skilled to design and install the system, and should probably ask their opinion.

“Anyway,” Meiland says, “it all starts with a heat loss calc.”

Depending on the heating load, a conventional water heater might struggle to handle both heating and domestic hot water needs. Harold Turner points Smith toward the high-efficiency Phoenix water heaters made by Heat Transfer.

Turner says the water heaters have a peak output of 166,000 BTU and come with an optional solar heat exchanger should Smith want to add thermal solar panels down the road.

Damon Lane suggests Smith contact a company called Radiantec, which designs radiant floor systems and recommends a Polaris, another high-efficiency water heater.

Radiantec advocates an “open-direct” system in which domestic hot water is circulated directly through radiant floor tubing.

“They can help you with the installation details,” Lane says, “and I would guess the limited cooling their open direct system offers by pushing your domestic cold water through your floor in summer will be enough for your house.”

What about a ductless minisplit?

Smith does not have gas service, but he reveals that his electric rates are currently about 6 1/2 cents per kWh, and that prompts an entirely different suggestion.

“Your electricity is very cheap,” GBA senior editor Martin Holladay says. “I think that a ductless minisplit system would make sense for you, providing both heating and cooling at a much lower energy cost than propane-fired hydronic heat.”

Ductless minisplits are a type of air-to-air heat pump in which a single outdoor unit serves a number of air handlers in individual rooms, without the need for installing a conventional duct system. Although traditional air-source heat pumps resort to electrical resistance heat when outdoor temperatures drop into the 40s, newer versions are much more efficient in cold weather.

“A good option with your cheap electric rates would be a ductless minisplit, as Martin mentioned,” says Bob Alsop, “which would take care of AC as well as heat during the ‘marginal’ seasons (spring, fall). An electric boiler would take care of the radiant (winter) as well as domestic. Ductless minis are great, but be aware that they are limited when it comes to very cold winter temps. At least here in Vermont.”

Not so, Holladay adds, pointing out that Mitsubishi Electric makes a minisplit that performs at -13°F “without using any electric resistance elements.”

NLehto seconds Holladay’s suggestion. He writes that he has a 1-ton Mitsubishi minisplit in his 1,900-sq. ft home in Connecticut. “As of January 12th I’ve spent $170 heating it this season and that’s with Connecticut’s ridiculously high electricity rates,” NLehto writes. “The coldest outdoor temp I’ve experienced with it so far is -4°F, and it had no problem keeping the house at 70°F.”

Robert Riversong argues that at sub-zero temperatures, the heat output of these units is “negligible,” but Holladay adds that Mitsubishi’s Mr. Slim Hyper Heat unit, with a nominal output rating of 38,000 Btu at 47°F, will still produce 30,000 Btu at -13°F — a reduction in heating capacity of only 21%.

Yes, says Riversong, but efficiency suffers. “According to the spec sheet, the [Coefficient of Performance] at 47° is 3.3, at 17° it’s 1.85 and at 5° it’s 1.65. At -13°, I suspect it operates with no more efficiency than resistance heat – COP = 1.

“So, you’re correct that this unit can maintain output at very cold temperatures, but it trades output for efficiency, which makes it inappropriate for very cold climates with little to no AC requirement. “

Holladay disagreed with Riversong, noting that “a COP of 1.65 at 5°F is excellent.”

Minisplits may mean the end for ground-source systems

Kevin Dickson wonders whether the “impressive” performance of the Mitsubishi minisplit, particularly when heating loads are very low, “should make everyone think twice about ever doing a ground source heat pump.”

Ground-source heat pumps have been considered a good cold-climate option because they operate more efficiently than conventional air-source heat pumps. But they are very expensive to install, and some critics think their efficiency numbers are often overstated.

“Marc Rosenbaum has predicted the withering away of GSHPs as a residential option — because the Asian ductless minisplits are getting so good,” Holladay says.

“Martin, good point about the perhaps waning of the GSHP trend,” writes James Morgan. “I was never happy about the extent to which it has been promoted through tax credits.”

Our expert’s opinion

We asked GBA technical director Peter Yost for his thoughts. Here is his response:

Air-tightness and radiant floor heat “base”: I see no connection between radiant floor heating distribution systems and air-tightness, unless this is an oblique reference or connection to the fact that radiant floor distribution, as a non-forced air distribution system, will require a stand-alone ducting for mechanical ventilation system in airtight homes?

Using a water heater to supply radiant floor space and domestic water heating: Absolutely go with a high efficiency tank water heater, such as the Polaris or Phoenix, particularly in a climate like Ontario, CA. And heat calcs are a must.

Real care is required to use radiant floor distribution systems for space cooling because you can’t push very many Btus around for cooling without thermal comfort and condensation issues on the floor. And I would bet that in Ontario the times you need space cooling are the very times you might have higher relative humidity, further constraining the use of radiant floor distribution for space cooling.

Here in our Vermont home in the summer, we only run refrigerant-based cooling for a handful of days in the summer, relying on ceiling and whole-house fans and night-time flushing almost all of the summer. And if we get control of our late afternoon solar gain from the west this year, we won’t need any refrigerant-based cooling at all. I bet in Ontario, the right design and shading strategies can mean no refrigerant-based cooling as well, or at least just one well-placed ductless mini-split.

The main thing here is what he thinks of the thermal comfort of each system. It’s hard to beat the feel and quiet of radiant heat in comparison to the potential noise and cold-blow issues of heat pump-based forced air systems.


  1. Mark | | #1

    Mini-split and ventilation ?
    For Lukas: I wonder about vapour pressure on the joists with the insulation like that - I seem to remember that's a no-no - see

    What I wonder is: If I have a fairly tight house with mini-splits for heat, that's fine, but what do I do to have fresh air coming in to remove toxins and water vapour? If I go HRV for this ventilation (Victoria, BC), I'll need the ducting anyway, so why not go full heat exchanger + HRV? What do you think?

  2. User avater GBA Editor
    Martin Holladay | | #2

    Response to Mark
    Your question is unclear. If you want to ventilate your house with an HRV -- a good choice, by the way -- then dedicated ventilation ducts make sense. Trying to integrate ventilation ducts with heat delivery is never a good idea and always involves compromises.

    If you heat your house with a ductless minisplit, you can use any kind of mechanical ventilation system you want.

  3. Luke Morton | | #3

    Domestic Hot Water Radiant, and Response to Mark
    First-- forgive me if this didn't already appear in the original post, but at one point when I was discussing open-loop with some colleagues, we discussed the threat (albeit minor) of legionnaires disease. The problem is defined thusly-- after the summer season, one *might have 3 month old water sitting stagnant in the hydronic tubing in the floor. This *might be water stagnant conditions which don't discourage growth of Legionella. That water is then recirculated back to the water heater, and if the water heater doesn't have a chance to reheat back up to 130-140 deg F, the Legionella might be dispersed as an airborne infectious agent to the infirm in the house.
    There are a lot of ifs there, but it is conceivable. I haven't found any documented cases where this has been system has been the culprit.
    A way to eliminate this is to circulate the water in the hydronic on a regular basis. If there is a mechanism to do so, then great! This problem should be solved (hopefully without creating other significant problems.).

    Lastly, to Mark:
    Yes, HRV's/ERV's need ductwork. But while forced-air heating systems in a minimally code-compliant envelope usually need 1000's of CFM's to effectively counterbalance heat loss, HRV's generally operate at less than 100 CFM for a house. And the ductwork can be downsized accordingly-- 3-4" ducts from 6" trunks.
    I'd much rather have the problem of installing small ventilation ductwork in an efficient, tight envelope, than opting for a leaky inefficient envelope that needs large ducts.

  4. TJ Elder | | #4

    Radiant heat and finish floor material
    If you want to install a radiant system and can accommodate the dead load of a concrete topping, that can double as finish floor. As discussed, some flooring materials don't work well over radiant heat, and most will compromise thermal performance to some extent by insulating the radiant tubing. Exposed concrete makes a durable floor and distributes the heat effectively.

  5. Chris K | | #5

    The practicality of in-floor heat in a SIP home
    I've experienced an inverse relationship between tight homes (particularly SIP homes, as this one will be) and the practicality of in-floor heat. The tighter, more energy efficient the home is, the cooler the floor can be to maintain a comfortable indoor air temp. In fact, a 72 degree F floor feels pretty cold in bare feet. If folks are looking for a warm floor (and in my experience that is what most expect) they will be disappointed. Unless they have a leaky or otherwise energy inefficient home.
    That said, if you're pouring a basement slab anyway, and there's any chance you'd like heat down there, in-floor can be a good choice. I've done this with a dedicated water heater to good effect.

  6. Mark | | #6

    Thanks to Martin, Lucas
    Ah - I see. Thanks a bunch.

  7. Bob Coleman | | #7

    mini split
    i've researched the mini-split systems for the northern states and it is still not ready for primetime

    only mitsubishi offers a low outside temp model, and the COP around 1.5 for outside temps in teens to single digits which are experienced for several months in this region is not good at all. you could save the $2000 and spend it on the .5 of electricity lost buying and running cheap electric heaters

    mitsubishi appears to just be rolling out the low outside temp tech. it is only available in single unit wall mounted models, which are not ideal to cool/heat an entire home. the multi output unit models and ones with various output possibilities, like concealed, are not yet low outside temp compatible

    it would be nice to have a way to recoup some of the compressor heat or summer heat rejection for hot water heating, like GSHP are able to do.

    the cooling efficiency is impressive though

  8. Joe | | #8

    Ductless Mini Split
    For this this house (low loads, cheap electricity), a ductless mini split is a no brainer. Too many homes like this screw things up by putting in overkill HVAC systems (geothermal) that are just not necessary, I have one in Connecticut and have been doing some detail data logging for the past two winters alternating between electric baseboard and the DHP. Based on this, it is averaging around a 3.0 COP. I will be publishing a paper on my findings in the near future.

  9. NLehto | | #9

    Joe I am also data logging a
    Joe I am also data logging a mini split for my home in CT. It would be interesting to compare notes.

  10. Kevin Dickson P.E., MSME | | #10

    Cold Floors
    I put in radiant in my similar house before I learned it's not necessary. The concrete living room floor of my house is cold 98% of the time.

    The extra $10k-$13k spent on the radiant heat just might be worth it in resale value, however.

    My basement insulation was detailed pretty well, and most of it is on the outside. I have never needed any heat in the basement for it to stay above 63F. Still, installing the pipe in the basement slab is not expensive.

  11. Kevin Dickson, MSME | | #11

    Bacteria in radiant systems
    For more information about the Legionella bacteria problem, see

  12. User avater
    Ted Clifton | | #12

    You are all missing one of the main points here...
    There are many reasons to use thermal mass in your floors, an in-floor radiant heat system is only one of them. (A damned good one, I might add!) Assuming the house in question has any southern exposure at all, a few well placed windows can provide up to half of the annual heating requirement for the house, depending on your climate zone. Furthermore, the additional mass in the in-floor radiant system allows the floor to store up to 400 times more energy than if the floor were framed in wood, with no extra thermal mass. This allows an air-source heat pump, like the UniChiller we have been using (air-to-water heat pump) to be turned off on the coldest nights, when it is below -5F, and come back on during the day when it is much warmer, with only a couple of degrees of temperature loss during the night. The COPs are very similar to those of the ductless mini-split, but because of the thermal mass factor, it doesn't have to fight the lowest outside temperatures.

    Then there is the cooling season. The added thermal mass allows the home to remain cool, as it can absorb copious amounts of heat, before the room actually warms up noticeably. By using a combination of range hood for exhaust, coupled with a powered HEPA filter for fresh air supply (balanced air pressure), yesterday's stale air can be replaced each morning with cool morning air, bringing the house down to a cool 68F in the cool of the morning each day. The well insulated house Lucas Smith was describing above, as long as windows are correctly placed, and overhangs are the correct size for the latitude, will not heat up more than five degrees during the day, to a very modest 73F even on a 90F degree day.
    How do I know this stuff? Because I do it all the time. This is not theory, this is our daily routine.

  13. Michael Schonlau | | #13

    Follow-up Questions
    Would tile over gypcrete work as a thermal mass floor or would it be too thin? Can I space my floor trusses at 24" oc with a gypcrete + tile floor over them?

    If the cost is similar - what is the difference between using a super-insulated water heater (Polaris) vs a boiler for heating the hydronic radiant floor loops?

    Are there some rules of thumb for how much area the mini-splits will heat/cool? I've had local HVAC folks tell me I'd need six or seven of the inside units in a 3000 sq ft home.

  14. User avater GBA Editor
    Martin Holladay | | #14

    Response to Michael Schonlau
    I'll tackle two of your three questions:

    Q. "If the cost is similar, what is the difference between using a super-insulated water heater (Polaris) vs a boiler for heating the hydronic radiant floor loops?"

    A. The difference is that the boiler is likely to be more dependable and require less maintenance than the Polaris.

    Q. "Are there some rules of thumb for how much area the mini-splits will heat/cool?"

    A. Yes, that information is available from the minisplit manufacturer. Assuming you have done a heat loss calculation for your home, simply compare the heat loss with the heat output of the minisplit at your design temperature.

    Perhaps your question is more complicated -- for example, does a bedroom need an indoor unit or will it be adequately heated by an indoor unit located in a common room? The answer to that question is, "it depends." In general, the tighter your home, and the better insulated your home, the more likely that temperatures will be fairly uniform from room to room. Some designers put (rarely used) resistance baseboard heaters in bedrooms without an indoor unit for very cold nights.

  15. Garth Sproule 7B | | #15

    You say that a boiler

    You say that a boiler is likely to be more dependable and require less maintenance than a condensing water heater. Just curious as to why you would say this.

  16. Keith Bickford | | #16

    Published Paper and Energy Model
    The following paper may be helpful:
    I recommend an energy model to determine the size of system required. With this tight SIPS home and the R values specified, you might be able to save if you 'right size' whatever system you are using.

  17. User avater GBA Editor
    Martin Holladay | | #17

    Response to Garth Sproule
    For years, the reputation of Polaris water heaters has been that they require frequent maintenance; a common complaint is that they have igniter problems. Some Polaris owners keep spare igniters on hand and have learned how to replace failed igniters.

    Many boilers, including Weil-McLain boilers, have a good reputation for dependability.

    Of course, your mileage may vary.

  18. Horst Fiedler | | #18

    Water heater
    I have radiant heat for about 500 sq-ft of basement run off the home's water heater. The 50 gallon water heater has an output of around 36,000BTU and the radiant heat consumes about 10,000 BTU when running. I used 1/4" fanfold and 3/4" plywood strips and home made heat transfer plates on top of the uninsulated slab because pre-made systems were not readily available at the time. The room is very comfortable and our pets like to sleep on the floor down there. It's been trouble-free for 10 years now. The system runs once in a while in the summer, so stagnant water is not an issue. The effect on the gas heating bill was negligible.

    I found 10,000BTU is about the limit to ensure enough water for the floor and domestic use. For a larger system I feel a dedicated water heater is a must. A conventional water heater is much less expensive than a boiler, and the plumbing is simplified.

  19. Debra Kay | | #19

    Understanding Radiant Heat
    Just a couple of additional points here. Radiant heat warns the entire floor space and all the furniture in the space. The heat is continuous rising past the occupants and keeping them warm too. Any other heating system heats the air and air does not conduct and store heat well, so the temperature drops and the system starts heating the air again. To maintain the same level of comfort the radiant system could be set 2 - 3 degree lower than the forced air or zonal system.
    Compacted sand will store heat about a foot under the slab. Set the tubing in the sand, then pour 4" standard concrete mix for your area over a foot of sand. No rigid insulation or plastic sheeting.
    The tubing will heat the sand, drying it out over about 3 weeks. The concrete will warm and there will be no condensation either.

  20. User avater GBA Editor
    Martin Holladay | | #20

    Response to Debra Kay
    1. In fact, in-floor hydronic heating systems are not "radiant" heating systems. They heat by all three heat transfer mechanisms: conduction, convection, and radiation.

    2. Forced-air heating systems don't just heat the air -- they also heat furniture and floors.

    3. Your suggestion that someone should install in-floor radiant heating system in a slab-on-grade home without insulation under the floor is irresponsible. Every such floor needs to be very well insulated to prevent heat loss to the soil.

    4. There is no evidence that people in homes with radiant-floor heat set their thermostats lower than people living in homes with other types of heating systems. For more information, see ‘Walls Need to Breathe’ and 9 Other Green Building Myths, in which I wrote:

    "Proponents of in-floor radiant heating systems often claim that such systems save energy compared to conventional heating systems. The idea is that people living in homes with warm floors are so comfortable that they voluntarily lower their thermostats, thereby saving heat. The only problem with the theory is that no reputable study has ever shown it to be true, while at least one study has disproved it. Canadian researchers visited 75 homes during the winter to note where the homeowners set their thermostats. The 50 houses with in-floor radiant heating systems had thermostats set at an average of 68.7°F, which was a little bit higher than the thermostat setting at the 25 homes with other types of heat delivery (either forced air or hydronic baseboard), which averaged 67.6°F. Since homes with radiant floors don't have lower thermostat settings, the researchers concluded that “there will generally be no energy savings [attributable] to lower thermostat settings with in-floor heating systems.” "

  21. John Richardson | | #21

    Response to Debra Kay and Martin Holladay on Radiant Heat
    Martin's comments here are appropriate and accurate. An additional consideration with radiant heating, that I do not see mentioned, is its thermal inertia problem. Radiant heating systems work best when set to a constant temperature -- very efficient for a 24 x 7 warehouse that you want to keep at a constant 55 degrees.

    However, with radiant, there's no viable way to set back the temperature at night -- or for a few hours (e.g., if you go out for dinner and a movie). It seems to be most challenged during shoulder seasons when heating demands are extremely variable.

    During the winter, everyone loves warm tile floors in their kitchens and baths. There have been many good articles in Fine Homebuilding on supplemental radiant heating -- both electric and hydronic.

    After living with several heating options including radiant, that's what I would recommend.

  22. Monty Worthington | | #22

    Suplemental Radiant Heating
    John Richardson said: "supplemental radiant heating -- both electric and hydronic.
    After living with several heating options including radiant, that's what I would recommend."

    What would your primary heat source be then? I am looking for options to radiant in floor hydronic heat due to the high costs ($25k). I would love a low cost ducted or ductless minisplit type system, but we don't need cooling, and don't like forced air...

  23. User avater GBA Editor
    Martin Holladay | | #23

    Response to Monty
    You can use a ductless minisplit for heating, even if you don't need cooling.

    There are many ways to heat a house that are cheaper than in-floor hydronic. Gas-fired space heaters are inexpensive. (Rinnai, Monitor, etc.)

  24. John Scime | | #24

    Why not consider tankless heaters?
    If the house is going to be super-insulated, requiring less than 30,000btu's for heating, then why not consider a tankless system?

  25. User avater GBA Editor
    Martin Holladay | | #25

    Response to John Scime
    There are a lot of reasons why it is tricky to use a tankless water heater to provide space heating. Read about some of the issues here: Stuff I Learned at Joe Lstiburek’s House, Part 1.

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