EcoHome is in the process of building our next demonstration low-energy house, and to do so we have partnered with a company that manufactures frost-protected shallow foundations (slab-on-grade) that are heated with air rather than liquid. There are many advantages to air-heated radiant floors over hydronic radiant floors, but none greater than the ability to inject solar-heated air into the thermal battery which is the concrete slab floor.
Solar air heating panels are a growing trend in green building online, with one notable shortfall – the heat that is generated is difficult to store or control. Many people take advantage of the relatively simple and affordable nature of air-heated solar panels (compared to photovoltaic panels) to harness the energy of the sun to heat garages and workshops, but solar radiant air-heating has yet to be a viable and reliable option for well-balanced home heating. Until now ….
When looking for a heat-storage solution that would allow us to incorporate solar-air heating panels into our design in an "engineered" and reliable way, we approached Legalett, a manufacturer of slab-on-grade insulating concrete form kits for problem soils and air-heated radiant floors, to see if high efficiency solar air panels could feed air directly into their existing in-floor duct work. They welcomed the challenge and engineered the system that will be the main heat source of our new (as close as we can get it to ) Zero Energy home. Needless to say that we will be incorporating high efficiency insulation and air tight building envelope just like Passive House design.
Legalett's existing radiant floor heating process consists of a closed-loop network of 4-inch tubes running through the concrete with a heater box embedded directly in the floor. In this case, added to that system are 6-inch tubes (seen in video) that will feed air to air solar panels on the south wall of the house. Air heated by those solar panels in winter, when the sun's angle is lower, will be fed into the floor heating radiant system, which will reduce the energy demand on the heater coil. Conversely and cleverly, in summer if the heat isn't needed in the house, the heat is diverted to an air/water heat exchanger for heating the hot water tank instead. What this adds up to is Energy savings; ideal for Passive Solar Homes, ZNE, PNE, LEED & Passive House projects.
If you are interested in getting a quote for a Legalett air heated slab floor forming system, see here.
Generating solar domestic hot water when heating is not required also serves to protect the panels from damage that can occur due to overheating in summer.
In the springtime, our pre-fabricated house with dense-packed cellulose walls will be delivered and installed on the slab. Below is a short video of the solar heated floor slab construction, stay tuned for more on the house that will be seated on it!
Back breaking work...been there done that!! Good job guys!!
Hi Fred,
Glad you liked it! Every slab we've done in the past involved building wooden forms first, so the fact that we didn't need to drive stakes into the crushed stone with a sledge hammer made this one seem like a picnic!
How big is the footprint of this house? Single floor?
Hi Igor,
It's a 2 story house, the main floor is about 1,000 sq ft with a bit less on the second story. Here is a link to more info on this high performance prefab home, and you can see some images of the interior.
Can you speak to the advantages of forced air radiant floor heating vs water? Also, in another article you mentioned how photovoltaic ends up being more useful/versatile for water heating than direct solar water heaters - is that how this is working? I'm confused as to how the solar air heater panels will also provide hot water.
Hi Alex
In the mechanical room there will be a heat exchanger that in summer months will generate hot water instead of hot air, and frankly I haven't seen the inside of that machine yet myself. The engineers at Legalett designed it, and this being the first one there is limited information on it to date. As we get to that stage we will most definitely feature something on our pages about it when that time comes.
And yes, there is an article where we speak of possibly increased benefits of solar water heating with PV solar panels powering standard water heaters instead of setting up a dedicated thermal solar system. The theory being, that when no water heating is neccessary, your solar investment is working for you to provide power. But, that theory that did predate a few things, first of all this particular system - which doesn't have actual 'liquid' panels, so it avoids the maintenance issues and other hassles of liquid systems, but as this is still experimental we don't have anything we can report yet on total costs and benefits. The second innovation that may rebut that PV / water heater theory is an innovative new panel system by Viessman which you can read about here.
And here is our page on air vs hydronic heating. I was a hard sell myself at first but having done it both ways I am a total convert to air now.
I like this a lot. I am only familiar with building forms first, so this is a much friendlier process. I had a look at the link to see the house that went up on the slab. I like it very much, what would be the approximate cost for this house. I am in Ottawa, Ontario. Thank you.
I really like this approach to off grid electrical and heating.
What is the benefit/ tradeoff to simply running the Legalet system with electric heater units off of a typical solar system as opposed to the solar air panels? Is the idea that the solar air panels could be mounted on the southern wall, leaving the roof space to accomodate electricity generating panels?
Also, how is the build going? Very interested to see updates on construction, but I don't think any have been posted recently, unless I've missed them!
The Legalett heater is designed to run on its own, this is just to inject some free heat into the system. A normal solar air heating panel doesn't store the heat, so as great as they are, they aren't easy to regulate. So you will over heat, then cool off quick and need additional heating. That's still better than not having a solar heater, but the purpose of this was to be able to use solar air heat and still maintain a consistent ambient temperature like in any newly built home. When the solar panels don't provide enough heating then the electric coil just kicks in. The panels still aren't installed at this point but we should have them on by winter. This is a very efficient home, and though we don't have data yet, it is likely that the entire year's heating will be somewhere in the area of $150 - $200 , and no there is no zero missing :)
And as for putting them on the wall - they've been designed that way, there is very minimal loss in output due to being on the walls rather than the roof, and it is best to keep them as close as possible to the point where they are injected into the slab. There won't be rooftop solar production either, Since Quebec is powered by renewable energy its already 'clean' power, so adding solar panels would add a lot of cost and not really provide any ecological benefit. Quebec Power is pretty cheap, so solar doesn't have as great of a return on investment.
At 3:30 in the video, does Mike say "wrap slab"? ("meaning that the weight is distributed evenly") If that is possible, why is it not standard for slab-on-grade construction?
What was done to prepare the ground?
Sorry for the pronunciation Jonathan, that's RAFT slab.
Building code has standards to ensure a house won't shift, but you can build many ways - thickened edge slabs (footings) raft slabs, full foundations or screw piles. Things become standard out out of familiarity and just remain is probably the reason why it isn't standard. Like a basement is the 'standard' way to build a foundation even though they cost more, they flood and they get moldy. and buidling basements with poly vapor barriers on the inside is also 'standard' even though its the worst thing you can do to a basement wall. All that to say, 'standard' isn't always a good thing.
Raft slabs are stronger so they can reduce the infiltration of Radon Gas into homes, they are less prone to shifting and you don't see structural cracking they way you do with slabs built with footings.
What was done to prepare the ground here is quite simple because it is a raft slab with no footing - you just remove the organics layer and replace it with clear crushed stone and then you build up. Here is our page about how a raft slab works so you can get a better understanding of the differences.
Hi Mike, I am also a fan of using air (living both mixed and off grid as well). I always felt uncomfortable with building on top of styrofoam or any soft foam ... because of mices and rats ! Do you have some data/feedback on this, is there something special to prevent them from digging extremely comfortable tunnels in them ;) ? Pretty cool blog btw, thanks !
Thanks. I always wondered as they once dug a short "conduit" in the XPS ceiling in my crawlspace... It brought cold air next to my floor but to be honest, I spotted it with a thermal camera and it happended only once -- and I am unsure it had any impacted on the overall isolation of the house (the noise they made was more of a concern) ! Thanks for your reply :)
Could you blow cold air through the system in the summer?
Cooling a concrete floor is not recommended. The primary action of an air conditioner is really about removing moisture, and were you to cool a floor it would not provide any reduction in humidity, at least no 'desired' dehumidification. What you would get, would be a cold floor acting as a de facto dehumidifier with hot humid air that will likely condense on the floor, so it would be cold and clammy with a risk of mold and mildew forming. The other issue is that it would not be very efficient since col air doesn't rise, it falls, so it would be very cold at floor level rather balanced cool air within the home.