Can You Heat a House with Air Ducts in a Concrete Floor?
These uncommon designs have loyal followers but no real performance data: Is it just a throwback to the '70s?
Concrete floors with high thermal massHeavy, high-heat-capacity material that can absorb and store a significant amount of heat; used in passive solar heating to keep the house warm at night. are often at the heart of passive solar designs. The density of concrete helps it store thermal energy and helps to reduce uncomfortable swings in indoor temperatures.
Slabs collect some heat from the sun through south-facing windows, often supplemented by radiant-floor heating systems that use a network of embedded plastic tubing to circulate hot water.
Nothing unusual here. But in a recent Green Building Advisor Q&A, Jay asks about the possibility of inserting air ducts directly in concrete to keep it warm.
"I'm interested if there is any added efficiency in passive solar applications where air is circulated through the slab," Jay writes. "My impression is that this technique opens you up to a lot of potential moisture/mold scenarios. Does anyone know of any research that looks at the implications of this technique?"
As usual, there's no lack of opinion on the topic, although scientific studies that might answer Jay's question definitively don't seem to exist.
Some builders use this approach
AJ Builder points to the work of Bruce Brownell, who, he says, has "spent a lifetime designing, refining and building" this type of house.
"I have been in two of them," AJ adds, and they are very livable environments. The owners almost heat alone with very small heat sources, hot water tanks, woodstoves or just the sun at many times...I think ducted slabs are a great choice among many."
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AJ includes a link to Adirondack Alternate Energy, presumably Brownell's Edinburg, N.Y., company, which describes how the houses are heated. A small fan gathers heat at the peak of the house and distributes it through "a heavy (70-100 ton) mass storage system under the lowest floor, and back to the house interior perimeter."
Houses built this way will never freeze in winter, maintain a healthy indoor humidity level of between 45% and 50% in winter, and indoor temperatures that never fluctuate more than 6 degrees, the company claims. Moreover, indoor air is filtered three times per hour to eliminate most dust.
The company says it's built more than 300 such houses.
Chris R says a Canadian builder by the name of Don Roscoe has built a number of similar passive solar homes in Nova Scotia in which 5-in. air ducts are incorporated in an 8-in. slab.
So widely used or not, the technique is not unheard of.
But where's the science?
Daniel Ernst was among the first to jump in with concerns about a lack of data that would support claims for how well these buildings perform, and in particular how much heat would be contributed by windows.
"But do you know of any controlled studies that compare House A (with ducted slab) to House B (without ducted slab)? Where are the BTUBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. measurements? The hygothermal models? The documentation?" Ernst asks.
"But no matter what climate or window or house, you can't create a system that uses passive solar gain to heat air, and then use that air to heat 70-100 tons of masonry to any meaningful extent," he adds. "The physics and calculations don't support the concept. There is no evidence that it works."
One of Brownell's houses is described in more detail in a link Ernst supplies. But, he adds, maybe Brownell should be encouraged to monitor and post actual energy data to make his case?
Two other points: the minute a design incorporates a blower, you're no longer talking about a simple passive design, says Kevin Dickson. Plus, a slab as thick as Brownell suggests may crack where it changes in thickness around ducts.
Do alternate views get a fair shake?
Brownell's designs have been steadily improved over the years, replies AJ, and besides, what's wrong with considering alternate views?
"Seems like every time I mention Bruce there are 'attitudes' here at GBA," he adds. "What's with that? Is this a little cliquish group of greenies or something? Bruce started down the road of green before I bet many, many here, and for sure before most of us in this country. He should be applauded for all his efforts over all these decades."
Hang on, says James Morgan.
"It's always been my assumption that the fundamental premise of green building is mindful and appropriate use of the materials and processes we employ for our shelter," he says.
"Whatever the norm may be in your area, suggesting there might be data to justify a triple-thickness slab is jumping down no one's throat. Environmental benefit returned for environmental cost invested is not a difficult concept."
Morgan, who works in North Carolina, says he's built many more than 350 energy efficient projects and says the value of high thermal mass is overrated.
"I would be intrigued to know why so many architects, designers and builders in the green building movement do seem to worship at the altar of generic high thermal mass," he says. "The theory seems to make sense only for a few very specific climates (hot dry days, cool dry nights, most days of the year). "
Building science has moved on
Sorry, but this approach really is typical of early mistakes that many passive solar designers made in the 1970s, says Robert Riversong.
"Some of the early experimentation, including air ducting in the floor and between double walls and roofs (the envelope house) or excessive or inadequate thermal mass, has been rendered obsolete by better design and a more comprehensive understanding of thermodynamics and building science," Riversong writes.
In addition, claims that the building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials. with walls of R-26 are far better than conventional houses is "absurd," he says.
Finally, a house built that tight requires heat- or energy-recovery ventilation. "Such a non-breathable house also likely violates hygro-thermal engineering principles that are essential to incorporate into a healthy and durable home, including a breathable envelope and hygric buffering," Riversong writes.
The discussion might go in different directions with the participation of Bruce Brownell himself. AJ writes he's extended the invitation.
- The Passive Solar House by James Kachadorian
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