Partially heating a house with a massively thick concrete slab?
January 4, 2019 3:35 AM   Subscribe

Would it be financially viable to provide additional heating to a new house by pouring an abnormal thick/deep concrete basement floor or pad? This is based off of the idea that concrete gives off heat as it cures.

I suppose this will come down to the price of concrete per cubic yard, vs how much heat it gives off, divided by the time it gives off heat... vs. the price of a normal heating solution such as gas or electricity.

This question stems from an article I read about the Hoover dam giving off heat due to the amount of concrete used. I get there are orders of magnitude size difference, but figured this would be a fun one for the hive mind!
posted by thankyoumuchly to Science & Nature (12 answers total) 2 users marked this as a favorite
Would probably make more sense in the longer term if you're also designing your concrete slab so it works as a thermal mass - heated (directly or indirectly) by the sun. Then you're getting cycles of warming, rather than a one-off effect. Take in the heat when there's an excess of it, and release it when it's colder. If your climate is such that there's a large difference between day & night temperatures, it could be worthwhile.
posted by rd45 at 4:01 AM on January 4 [1 favorite]

For the amount of concrete used in roadway paving which would be of similar thickness to a basement slab, it gives off heat for, like, 4 weeks. After that it’s a massive cold sink from the cool earth below.
posted by hwyengr at 4:24 AM on January 4 [23 favorites]

All of the heat that concrete gives off as it cures comes from stored chemical energy in its cement component, and the way that the chemical energy got in there in the first place was in the form of thermal energy from burning furnace fuel.

Since the cost of the concrete includes the cost of the fuel burnt to form the cement, and since much of the released heat will just dissipate into the surrounding soil instead of into the living space, your plan amounts to a very energy- and economically-inefficient way to burn fuel for space heating.

As rd45 notes, though, large volumes of concrete can work well as thermal mass, which functions to stabilize the temperature of attached structures. This allows you to design your heating and cooling systems to move large amounts of energy into and out of the thermal mass without moving its temperature much, which is useful when that energy arrives in relatively uncontrollable forms like sunlight. If you design your structure in such a way as to soak up sunlight when it's available and slowly release that heat into the living space even when it's not, you can get pretty much any stable interior temperature you want by designing with the seasonal average insolation in mind; the day-to-day variation then matters very little.

If you're going to retrofit thermal mass to an existing structure, though, you're better off using tanks of water than huge lumps of concrete. Not only is water substantially cheaper than concrete, volume for volume, it also stores much more heat per cubic metre per degree and much much much more heat per tonne per degree.
posted by flabdablet at 4:55 AM on January 4 [16 favorites]

You can install radiant heat in a concrete floor. It’s slow to warm up but pretty efficient and the effect of the heat rising up through the house is noticeable.
posted by padraigin at 5:37 AM on January 4 [3 favorites]

You wouldn't even have to put in more concrete to install radiant heat. A quick google yielded this article.
posted by mareli at 5:52 AM on January 4

If you're looking at heated concrete flooring (which does indeed make a house feel very comfortable) you should probably be looking at heating it by running hot water through tubing rather than burying electrical elements in it. That way you can get useful amounts of heat for your floor from solar collectors.

Better still would be to arrange the glazing in such a way that incoming sunlight can heat the floor directly in the winter, but gets shaded out in summer when the sun is at a higher angle in the sky. This is passive solar design and it works exceedingly well. You can design passive solar houses where careful glazing, thermal mass, insulation and use of the ground as a heatsink work together to keep the interior comfortable year-round with little to no active heating or cooling.
posted by flabdablet at 6:03 AM on January 4 [4 favorites]

Normally (in colder climates at least) you'd want a damp-proof membrane and a significant thickness of an insulator such as polystyrene, embedded within the concrete. This is standard building practice in much of Europe, where a concrete slab has been the default for new housing for decades. I've just had my old (damp, uninsulated) concrete kitchen floor dug out and replaced. The new insulation and damp-proof membrane have turned a cold, damp kitchen into a warm, dry one. We had considered adding underfloor heating, but our builder thought that the new insulation would be enough, and he was right - it's a big improvement.

As others have said, ignore the heat given out during curing. It's a short-lived exothermic effect that shouldn't have any part in your consideration of how to heat or insulate your home. Unless you plan to pour new concrete every other day.
posted by pipeski at 6:15 AM on January 4 [1 favorite]

Radiant heat can work very well, my parents have a condo where the whole building is radiant heat and they rarely turn on the actual heat. And it does get cold here - not northeast cold but still in teh teens.

probably wouldn't be quite as efficent on a stand alone home but still helpful.

if concrete gave off enough heat on its one to heat your home, sidewalks wouldnt freeze so easy.
posted by domino at 6:18 AM on January 4 [1 favorite]

If you're looking at heated concrete flooring (which does indeed make a house feel very comfortable) you should probably be looking at heating it by running hot water through tubing rather than burying electrical elements in it. That way you can get useful amounts of heat for your floor from solar collectors.

Just a little gentle pushback on this concept. I see a lot of pre-existing solar thermal installations in my job as a solar site visit technician, and they mostly have issues. Solar thermal has always been a niche product and it can be very hard to find people who will work on it when it breaks. The collectors and the pipes are pretty fragile compared to most residential systems, especially after a decade or so of UV exposure and thermal cycling. I've visited multiple customers who have pre-existing solar thermal installations that have been broken for years but are still up there on the roof in derelict form. The one customer I saw whose system was pristine was someone whose entire personal life basically revolved around maintaining and renovating their gorgeous, 19th-century Queen Anne-style home. They were also an anesthesiologist making anesthesiologist money.

Meanwhile, photovoltaic solar has come down in price by 75% in the last ten years and is getting cheaper and more mainstream by the day. We have reached the point where servicing PV systems is routine and can be done by many different companies. The industry, and the product, is maturing and becoming standardized.

My own company used to do both solar thermal and solar PV, but we are effectively 100% PV at this point. Most of the customers with pre-existing solar thermal whose houses I visit are having it removed and replaced with PV as part of the project we are doing for them. It has reached the point where overall it is generally more cost effective and convenient to use the sun to make electricity and then use the electricity to make heat, rather than using the sun to make heat directly. That's counterintuitive, but that's where we're at right now.
posted by Anticipation Of A New Lover's Arrival, The at 7:55 AM on January 4 [8 favorites]

We're off in the weeds here, but AOANLOA,T's experience above is bound to be highly location specific. Solar hot water (I.e. thermal, not PV) is completely standard here and still vastly more efficient than PV. It's in no way a dead end and a very run of the mill thing to install.
posted by deadwax at 12:55 PM on January 4 [1 favorite]

If you are interested in some numbers regarding concrete and heat production, here is a good PDF.

The short answer to your question is that concrete cures fairly quickly and most of the heat is generated in just the first few days after mixing. So why do they talk about the Hoover Dam needing cooling tubes to cool? That is not because of continued heat production -- most of that takes place in a few days. It is because concrete is a fairly poor conductor compared to metal and the dam is hundreds of feet thick. It takes a very long time for the heat to travel to the surface -- decades.

If you look at the PDF above, Figure 2, you will see a graph of the cumulative heat production over time. Note that the horizontal scale is logarithmic. So it generates 60 calories per gram in the first day, only 20 more calories by day 10, and 20 more calories by day 100. So you can see that the heat production is dropping off very rapidly. It starts off at 60 calories in the first day but by day 100 it down to only an additional fraction of a calorie per day.

So the other part of the picture is the thickness of the concrete. For that look at Figure 1 in the PDF. There are a series of curves for the temperature over time at the center thickness of a wall slab. The labels in parentheses are inches thickness of the wall. You can see that wall thickness significantly increases the maximum temperature at the center of the wall when you compare a 12-inch thick wall to a 40-inch thick wall. Secondly you can see that the temperature declines more slowly the thicker the wall and that is due to the poor conductivity of concrete, but in each case the temperature peaks at about 2 days.

So you can see that a 40-inch wall has a peak temperature of 48 degrees C and cools down to ambient at about 2 weeks. Now you can imagine what Hoover Dam would be like with walls hundreds of feet thick if there were no cooling. It would get over 100 degrees C and take many years to cool down. The high temperatures and later contraction with cooling would cause the concrete to be fractured and weak.

What all this means to you is that using a concrete slab in more human scale to heat a home isn't going to work. In Figure 1 you can see that even a 2-foot thick slab is going to cool off in about a week.
posted by JackFlash at 6:36 PM on January 4

Getting a bit deraily here perhaps but:

If you're going to retrofit thermal mass to an existing structure the best way is to fit PCM panels.
PCM (Phase Change Material) panels are the same size as standard drywall but have encapsulated microbeads of wax.
When you get too hot the wax melts, storing heat. When you get too cold the wax solidifies, releasing energy.

With the concrete, you get one phase change. With PCM boards you get the benefit both ways.
A normal drywall thickness of PCM has the same thermal mass as a foot thick wall of concrete,
posted by Just this guy, y'know at 1:06 AM on January 5 [1 favorite]

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