On Trombe Walls and Heat Sinks
I’m going to be building a single familyhome soon, and I have been researching passive solar, trombe walls and heat sinks. My home will have a sourthern exposure with significant glass on this facing wall. I’m looking for information on the advantages and disadvantages on trombe walls with very little success, but I have really found very little on heat sinks. I’m wondering if there is a material that would act as a heat sink yet at the same time not be too heavy which would require structural modifications as I plan on having a full basement under this possible heat sink. From the little I have read, the way to go would be a concrete floor of about 4 inches in thickness. However since I am having a basement, I would have to have some significant support to hold such a large slab. I’ve thought about some of the lightweight concrete the Europeans have used, but I really don’t know if it would give me the thermal mass that I would need for an efficient heat sink. I’ve also considered the possibility of air entrenched concrete, but again I’m not sure that would give me sufficient thermal mass to make a viable heat sink for passive solar applications. Does anyone have any pertinent information or even some links that might be able to steer me in the right direction?
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Replies
John,
1. You need to tell us your location or climate.
2. In almost all U.S. climates, Trombe walls lose more heat than they gain compared to a very well insulated wall with less glass. Direct-gain double or triple glazing makes more sense than a Trombe wall. Get a good energy simulation program and run different iterations of your south wall to confirm this.
3. Concrete floors are routinely poured over wood framing. Hire an engineer if you don't know how to calculate the required framing.
John,
Trombe walls are often used in NM. Most of them have heating failures due to not providing active ventilation to move the hot air form the wall surround to the “other” side of the house. It totally overheats one room while the rest of the house is cold and it is specially affected with the thermostat location.
If you choose Trumbe walls, make sure you dedicate 1 or 2 return air, up high on a wall or ceiling, going directly to the plenum to circulate the warm air to the rest of the house. You can use a coffer ceiling also if you want to keep the return ducts in the condition space or insulate under the roof decking and use the attic.
As Martin said, use an energy simulation program to coordinate with the HVAC system you are planning to use.
I should add that you should use an Air Cycler or an IAQ Thermostat to "move" that ir when the HVAC system is not. It works better with variable speed systems too.
John,
By "heat sink" you mean thermal mass, and ideally diurnal thermal mass that can absorb, store and release solar gain on a 24-hour cycle.
As you're aware, this is best accomplished with 4" of dense (not lightweight or air entrained) concrete with a medium to dark colored surface (earth tones work well) and low specular reflectivity (not polished).
Trombe walls are a more massive point load than a uniformly distributed thermal mass floor, and are less efficient as passive solar collectors than good windows (no more than 12% of floor area) and direct-gain mass. You can also use a thermal mass wall that receives light reflected off a polished floor. It is easier to support a center wall to ground than a concrete floor.
I cover thermal mass issues in my Home Power Magazine article on "Passive Solar Slabs" in the April/May edition. Email me at HouseWright at Ponds-Edge dot net and I can send you a pdf copy if you can't access it on-line.
Hi everyone: Thanks for the quick replies. The area is located on the border of Iowa and MO just directly south of Des Moines on Interstate 35. That is still considered Great Lakes weather (it breaks down toward Kansas City into the next climate zone). I'll certainly consider running past an Engineer, but other other question is 1) how practical is it? 2) is it worth the effort (with the increased structural support and will there be a commensurate Return on the Investment?) And Robert I will definitely go to Home Power Magazine and bone up.
John,
1. "How practical is it?" Since a Trombe wall will lose more heat than better alternatives, the answer is, "Not very."
2. "Is it worth the effort?" -- No.
If you are in an area with others that have a Trombe Wall installations.
I might suggest looking into putting some PEX or other tubing snaked into the wall to allow you to run a small pump to pump the heat to other sections of your house. It might be un-conventional, but if you are talking Trombe walls, you are already thinking out of the box for most folks.
Please do have a good backup heat system as well.
But you are doing well to have a HVAC engineer in your area to review it. You might even have one engineer it for you and review it with their BLAST program (Building Load Analysis SysTem . is one a friend used to use in his consulting, I am sure there are others by now).