Michael Schonlau is building a house in Omaha, Nebraska, where he can expect 6,000+ heating degree days a year. He’s planning on putting a radiant-floor system in the basement slab of the ranch-style home with a footprint of about 1,600 sq. ft.
In a recent posting on GBA’s page, Schonlau asked whether he’ll have to install radiant loops under the subfloor of the first floor as well as the basement — or will the heat generated in the basement migrate upstairs and keep the house comfortable?
“Please assume the house will be well-insulated and air sealed, with ‘good’ windows,” he wrote. But will that be enough to keep his wife’s toes warm through a long Nebraska winter?
It’s been done
GBA senior editor Martin Holladay tells Schonlau it can be done, but “but only if you aim for near-Passivhaus levels of airtightness, insulation, and window quality.”
Dan Kolbert is more optimistic.
“We’ve built a couple of houses on slabs with no heat distribution on the 2nd floor,” Kolbert says. “They were good, but certainly not Passive House.”
One of the houses had double-glazed windows, “not huge R-value,” very good solar orientation and a sprawling layout. Radiant-floor tubing was extended to the second floor, but after two winters the client had no interest in second-floor heat distribution.
The other house was much tighter. Here again, heat in only the first-floor slab was enough to keep the house toasty.
“I essentially agree with Martin, though, only disagreeing in what the threshold might be,” says Kolbert. “An energy model (and someone who knows what to do with it) would help make the decision.”
Robert Riversong, however, isn’t so sure it’s going to work.
“Unless your house is very energy efficient (32,000-45,000 btu/hour maximum design load), then a single floor won’t be able to comfortably heat an entire house, and you won’t get the radiant comfort advantage on the non-radiant floors,” writes Robert Riversgong. “In fact, a cool concrete/stone/tile floor will be quite uncomfortable.
“All floors should be heated, and the radiant design is critical to effectiveness, efficiency and comfort,” he says. “Get yourself a heating contractor who understands this and who is willing to do the calculations necessary to design an appropriate system.”
Keeping feet warm
People like radiant-floor heat because they think it will keep their feet warm all winter, even when the heat isn’t cranked up.
That, at least, is the perception. But NickJ reports the reality can be a little different.
“My wife wanted warm feet,” Nick writes. “So we installed radiant in our main floor slab and upper wood floors.”
The system keeps the house warm enough, no problem there. But for the heat to be perceptible on the bottom of their feet, Nick adds, “you’d be baked out of the house.”
“There is a widespread misunderstanding that a radiant floor needs to be hot in order for feet to feel warm,” Riversong replies.
“With shod feet, 90% of people report comfort with a floor that is between 66°F and 84°F, and with bare feet a floor between 79°F and 84°F. Your feet (and most of your skin) are typically 91.4°F, so nothing less than that temperature is going to add heat to your feet.”
With bare feet, he adds, the conductivity and specific heat of the flooring material is more important to comfort than the temperature of the floor.
Whole-body temperature is as much dependent on the “mean radiant temperature” of all solid materials in the room as it is on air temperature. That’s why, he says, radiant heat feels comfortable even at lower temperatures.
But the second floor still won’t be comfortable if only the basement slab is heated.
Do the calculations
One bedrock principle of energy-efficient building is not to hire heating contractors who insist on using a seat-of-the-pants method to calculate a home’s heat load. The best way to calculate a home’s heat load — the first step to designing a residential heating system — is to perform a Manual J calculation, a method promulgated by the Air Conditioning Contractors of America.
Scholau’s architect thinks the one-floor heat plan will work, but his wife is skeptical; so Schonlau asked a heating contractor to run a Manual J calculation. The contractor said he could do it, but apparently commented that the calculation might as well be used for toilet paper for all the help it would provide.
“Should I keep looking for HVAC contractors who will do it or is there a better approach?” he asks. “I’d like to know what the heating and cooling loads are up front, so I can better educated about choosing equipment.”
It would probably be best to find a different contractors, says Holladay. “As I’m sure you know, I’ve written that many Manual J analyses are done poorly, and a Manual J calculation is not the be-all and end-all to HVAC system design,” Holladay says. “But to dismiss Manual J entirely shows a remarkable lack of sophistication, so it calls all of the contractors’ promises into doubt.”
Armando Cobo writes that a better assessment would come from “Manual GAMA-H-22.” Presumably, GAMA refers to the Gas Appliance Manufacturers Association, a trade group representing manufacturers of gas-fired water heaters, furnaces, and boilers. Using that standard, Cobo says, a qualified, independent mechanical engineer should be able to analyze the house and recommend a way of heating it. Manual J, he says, is used to calculate heating loads, while H-22 is used to design hydronic systems.
Although David Meiland says he’s rarely seen a third-party mechanical designer on residential jobs, Michael Chandler suggests it’s not so odd.
“Our Energy Star and Green Rating provider (Southern Energy Management) handles all our Manual J and HVAC system design with their in-house mechanical engineer,” Chandler says. “They don’t do it for free, but it’s a great service and gives us really useful data on our assumptions and actual performance and helps us make changes to move our quality in a positive direction.
He suggests Schonlau check with his Energy Star provider. “[There’s] a good chance they can connect you with an engineer who can take it our of the HVAC installers hands,” he writes. “I haven’t had my HVAC installer do his own Manual J for years.”
We asked Peter Yost, GBA’s technical director, for his opinion. Here’s what he had to say:
There are really two distinct issues here:
1. Can you pump enough BTUs through the basement “radiant” floor system to offset the home’s total heating load?
That is the type of question that ACCA Manual J can answer and I don’t doubt that Michael Schonlau can get this calculation done correctly by an energy rater or top-notch HVAC contractor. I also suspect that he can do it without Passivhaus adherence to building envelope performance, but we don’t know any details of the home’s design or siting, so it’s very hard to say with as little information as we have.
2. Can the basement “radiant” floor system maintain thermal comfort throughout the home?
Manual J certainly won’t help us here. And this is an even harder question because it involves real people with lots of individual perceptions of thermal comfort that ASHRAE Standard 55 wraps up into percentages of large numbers of people satisfied and dissatisfied. We heated homes here in New England for many years with “gravity” distribution systems where grates and passive convection accomplished distribution, but accomplished may be a real stretch if you were to ask folks with these systems. Admittedly, none of those old New England farm houses were or are even in the same universe as Passivhaus.
My main point: This is a design question
How heat generated in the basement will effectively make its way up to the second floor is a design question, probably best attempted by computational fluid dynamics (CFD) analysis. But CFD is not in any HVAC contractor’s or energy rater’s tool boxes that I know.
Radiant floor heating may not be best for this situation
And if I were going to heat one floor, I don’t think I would choose a system with such a low and slow rate of heat exchange as a radiant floor system. It’s hard for me to imagine this system not lagging far enough behind changing outdoor conditions that the home would not be swinging between under- and over-heating in all the spaces, but certainly the basement. And forget any setback strategies.
So, can it be done? Absolutely. Will the occupants and their guests be comfortable throughout the heating season? I honestly don’t know but am skeptical. It it were my house, I would be designing in a Plan B to boost the second floor.
Finally, you may be wondering why I have twice put the term radiant in quotation marks when used in the phrase, radiant floor heating. At best, 60% of any hydronic floor heating system is radiant heat transfer. And up on the second floor of this home, it won’t be anywhere near 60%. It will be the air temperature and convection that drive heat distribution with a distinct lack of the cozy “warm toes” phenomenon.