In Grand Rapids, Michigan, Andy Zbojniewicz is planning a large, high-performance home that ultimately will be powered by a 15-kW solar array. The house will have more than 3,000 square feet above grade, and another 1,633 square feet in the basement.
His immediate concern is how to heat and cool the house.
“I was told the home was too large to heat and cool with air-source heat pumps and be comfortable,” Zbojniewicz writes in a Q&A post, “so [I] was primarily looking at geothermal.”
To that end, Zbojniewicz sought quotes from two HVAC contractors for ground-source heat pumps. One of the contractors said the heating load would be 50,797 Btu per hour at an outside temperature (design temperature) of 7°F, and recommended a 5-ton (60,000 Btu/h) heat pump. The second company didn’t provide its load calculations, but suggested that Zbojniewicz would need two 3-ton heat pumps in order to stay comfortable.
“I have a friend who is in HVAC and he feels strongly that I should have radiant tubing run in the basement to help with heating in winter,” Zbojniewicz adds, “and while I was at it I was going to run it in the mudroom, master bath, and three-season porch.”
After doing some reading at GBA, Zbojniewicz realizes there are plenty of critics of both ground-source heat pumps and radiant-floor distribution systems.
“Is there a better alternative to geothermal if I want to have the potential for net zero?” he asks. “If I stick with geothermal, are two 3- ton heat pumps overkill (they’re certainly more expensive)? Should I abort the radiant altogether?”
That’s the topic for this Q&A Spotlight.
There is nothing wrong with air-source heat pumps
The outside design temperature for Grand Rapids, Michigan, is 5° or 6°F, GBA Editor Martin Holladay says, so Zbojniewicz can certainly consider air-source heat pumps. A heat pump from a U.S. manufacturer paired with a conventional forced-air duct system would be fine. Holladay suggests that Zbojniewicz read a GBA article, “Ducted Air-Source Heat Pumps from American Manufacturers.”
Dana Dorsett adds, “I’m aware of a house in Vermont slightly larger than yours and slightly less insulated, in a location with a 99% outside design temp of -12°F that is heated with four ductless mini-splits. The notion that your house is ‘too large’ to heat with a heat pump is just silly. It’s the size of the heat load, not the house, that matters.”
There are some pluses to a ground-source system
Jon R contends that radiant heat paired with a ground-source heat pump is great — as long as Zbojniewicz doesn’t care about how much the system costs. Such a system not only provides space heating but also provides efficient water heating.
“If you are looking at air-to-water geo-exchange, then you can add a water tank (thermal mass), resolving any heat pump over-sizing issue other than cost,” Jon R adds. “If very large, it can enable some use of solar PV that doesn’t involve net metering. Multi-stage or multiple heat pumps also mitigate over-sizing.”
Those load calculations are probably wrong
The capacity of any system that Zbojniewicz installs should be based on two factors: the climate, and how well the house is air sealed and insulated. And Dorsett doesn’t have much confidence in the heat load estimates that one of Zbojniewicz’s contractors has provided.
“Don’t believe the 50K heat loss number unless it was done by a qualified third party,” Dorsett writes, “an engineer, RESNET rater etc., somebody who makes their living and reputation on the accuracy of their numbers rather than installing and maintaining HVAC equipment.”
Even a code-minimum house of that size with a measured air tightness of 3 ach50 would have a heat load of less than 40,000 Btu per hour, he says, assuming an outdoor temperature of 0°.
“The only way to stretch it to 50K would be to have excessive air leakage or excessive expanses of window area,” Dorsett says. “Your house is likely to come in closer 30K @ 0F, 36K tops and even less at +7F.”
With that in mind, a heat pump with a capacity of 60,000 Btu/hour is about twice the size that Zbojniewicz really needs. The place to start, Dorsett says, is a Manual J heat load calculation, and it’s his bet that two modulating Fujitsu minisplits or possibly a 3-ton Carrier Infinity Greenspeed air-source heat pump with heating strips would keep the family comfortable.
Steve Theinalienable’s experience bears that out.
“My house insulation, climate, windows, and foundation insulation are almost exactly the same as yours, just a bit smaller (2200 square feet),” he says. “My design heat load at -18°C [roughly 0°F] is 18K Btu, but we had a day like that this week, and my air-source heat pump didn’t have to work that hard to keep up, so, I think it’s probably a bit conservative.”
Get your house rated by a pro, Theinalienable says.
Consider the all-electric option
With a tight building envelope and a relative large photovoltaic system, why not consider an all-electric house? asks Armando Cobo.
“I would highly advise you to consider going all-electric with an air-to-air heat pump, and hopefully installing [Energy Star] appliances and lighting, electric fireplaces and an induction cooktop,” he says. “That’s what I spec on all my houses, which are [zero energy ready homes]. Just a thought!”
In fact, Zbojniewicz is trying to convince his wife that an electric fireplace and induction cooktop are good choices.
Zephyr7 suggests he invest $50 or $60 in a portable, one-burner induction cooktop and experiment with it.
“Have your wife play around with it,” he says. “It’s an inexpensive way to test out induction cooking. Note that not all pots are compatible with induction cooking.”
Insulating the rim joist
One footnote to Zbojniewicz’s efforts to obtain more accurate heat load estimates involves insulation. He’s found a company that will do Manual J calculations, but he’s not sure how to specify the insulation he will use at the rim joist.
What’s the best approach?
“Closed-cell [spray foam] is pretty standard for use on rim joists,” replies Zephyr7. “Lots of people use kits like those from Foam it Green and the Dow Froth-Pak. What you want to avoid is keeping the wood wet. Your assembly sounds like it won’t be a problem, but make sure you have a capillary break between the rim joist and the top of the foundation wall.”
Cobo says open-cell foam is best at the rim joist when rigid foam will be applied to the outside of the wall. Because Zbojniewicz is planning to use Rockwool on the outside of the house, either open- or closed-cell foam can be used, he adds.
Holladay suggests he choose closed-cell foam, not open-cell foam. “In a cold climate, closed-cell spray foam would be a better choice for the interior of a rim joist,” he says. “You’re right that either type of spray foam is an air barrier. The reason that closed-cell spray foam is better is because it is a vapor barrier, unlike open-cell spray foam. The closed-cell spray foam prevents outward diffusion of water vapor, and therefore keeps the rim joist dryer.”
Our expert weighs in
Peter Yost, GBA’s technical director, adds this:
Cold-climate heat pumps: It is interesting to hear the misgivings that air-source heat pumps will be unable to manage either larger homes and/or cold climates. I was just in Wisconsin recently and heard the same litany. Dana Dorsett, as usual, is right on: Heating loads are assessed using U-factors and building enclosure surface areas (and other metrics, including airtightness), not square feet of living or conditioned space.
In the Northeast, heat pumps designed to wring out BTUs at air temperatures well below 0°F have proven efficient and reliable. Marc Rosenbaum, a leading and inherently skeptical mechanical engineer, has helped spread the word by recounting his own great success with cold-climate air-source heat pumps. We did have problematic cold-climate heat pumps in the past (both Nyle and the Hallowell cold climate heat pumps had problems and went out of business some 10 years ago) and that news seems to persist. The technology of today’s cold-climate heat pumps is very different and has proven to be robust. (See numerous GBA resources on the topic, including this one.)
Radiant-floor heating. We just can’t seem to get our arms around the pros and cons of this method of heat distribution.
It is not more efficient. There is nothing magical about radiant-floor heat. Its efficiency is driven by the same physics that governs all other distribution systems. And part of the physics is the mass to which the distribution system is connected. It’s very difficult if not impossible to gain efficiency by way of thermostat setback.
It is more comfortable. It’s hard to argue with this, either empirically or based on heat transfer. In my experience, well-designed and installed radiant floor heating systems deliver superior thermal comfort.
Radiant-floor distribution can be used for space cooling. As Robert Bean has said more times than he cares to count: We keep blaming condensation during cooling on the radiant system when, if the latent load were properly managed, radiant cooling can work just fine (for more, see this).
Insulating the rim joist: In my experience, rim joists get wet from backsplash at grade from the exterior or, less commonly, wicking because there is no capillary break between the mudsill (or sill beam) and the foundation wall.
In cold climates, I don’t see rim joists getting wet from air leakage because the stack effect is pulling cold dry air in during the winter at the rim joist. So, the main reason for me to choose open- or closed-cell spray foam at the rim joist — since both can provide a good air seal at this location — is based on how much drying potential I need to the interior (open-cell spray foam being more vapor-permeable than closed-cell).
Manual J: It’s really heartbreaking to hear how often heat-load calculations are not done. Can you imagine a car company designing a transmission system without knowing the horsepower and torque the engine can generate? On the other hand, it’s heartening to see how often the general public ends up on GBA’s Q&A pages to work with leading building professionals on home performance.