Is Radiant Floor Heat Really the Best Option?
A framer weighs his option for both heat and air conditioning
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.
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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 loadRate at which heat must be added to a space to maintain a desired temperature. See cooling 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 exchangerDevice that transfers heat from one material or medium to another. An air-to-air heat exchanger, or heat-recovery ventilator, transfers heat from one airstream to another. A copper-pipe heat exchanger in a solar water-heater tank transfers heat from the heat-transfer fluid circulating through a solar collector to the potable water in the storage tank. should Smith want to add thermal solar panels down the road.
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," GBAGreenBuildingAdvisor.com 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 - COPEnergy-efficiency measurement of heating, cooling, and refrigeration appliances. COP is the ratio of useful energy output (heating or cooling) to the amount of energy put in, e.g., a heat pump with a COP of 10 puts out 10 times more energy than it uses. A higher COP indicates a more efficient device . COP is equal to the energy efficiency ratio (EER) divided by 3.415. = 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-splitA type of small-capacity heat pump (as little as a ton or even less) with a closely-associated outside compressor and inside evaporating coil (often through-the-wall in design). These heat pumps often come with variable-speed compressors and blowers,giving them excellent modulation for thermal comfort. These features also contribute to COPs of around 4 for ductless min-split heat pumps. They are also well-suited for ultra-high performance, small-volume homes..
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.
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