# Cooling thought experiment

| Posted in Energy Efficiency and Durability on

My house works like this, century old balloon framed poorly insulated home. I have calculated that each degree uses about 5kWh of energy, 5kW to heat the house by 1 degree Celsius with the natural gas furnace and 5Kw of energy removed per degree with central air. I have done these experiments as best i can to control for time lag in heating and cooling of the mass of the building and have taken efficiencies and outputs into account. So for this thought experiment lets assume its correct. If its not the magnitude will change but not the concept.

Also i have found that a heatwave of 35ºC outdoor temperature will add about 5-6ºC a day to the indoor temp or about 25-30kWh of heat. This scales roughly linearly, less temp will add less per day and even on days its the same temp outside as inside will add some heat because of solar gains through the windows and walls and ceiling. Obviously i can reduce the heat gain with better insulation, another question will be about my attic insulating problems so i don’t want to address the house’s shortcomings here.

As an aside what i currently do is run exhaust fans at night when its below the indoor temp cool the house down to a goal of 22ºC, let the house temp rise during the day to 25-27ºC and the next night repeat, but use the central air on nights when its not cool enough outside to cool the house appreciably. I reduce my central air use maybe 50-75% this way.

Lets say the goal is a comfortable 23ºC but some oscillation is acceptable, say 2ºC so 21-25C is acceptable.

Central air will keep the house cool during the day at high energy cost. Then at night the temp often falls below the indoor temp.

At night the outdoor temperature often runs to 15-20C. Not always of course.
Would it work to have a thermal mass that can absorb say 30kWh of heat and then discard it outdoors at night? Say a concrete block in the basement that has water or air tubes running through it so during the day the internal gain is dumped into the thermal mass and at night cold outdoor air “recharges” it with coldness?

On nights where it does not go below the indoor temp no recharge will be possible and the central air would have to cool the house and wait until a night when its cool enough to recharge and the earth could also provide some nice cooling to the mass. Probably best to thermally separate this mass from the house as well because in winter it will suck heat out of the home. Say R20 of foam on sides that face indoors.

Thoughts?

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### Replies

1. | | #1

I can't speak to the finer technical points, but I will link:
(and related articles)

and I will add that your experiment sounds fun, but impractical if your goal is simply to lower energy usage. Between procuring the thermal mass, insulating it (?), and running duct work or piping through it, it feels as though simple insulation and air-sealing will get you a lot further for a lot less fuss. This is mostly a feeling.

It may also be helpful to consider that it is HEAT we are managing, not 'coldness.' Potato/potatO perhaps -- but remember that heat is energy, cold is the absence of heat, and energy is what will be stored and moved in this system.

1. | | #2

I don't plan on retrofitting my house with this, the cost would be enormous. I meant as an idea for experimentation on a new build if the numbers make sense.

2. Expert Member
| | #3

Thermal mass that is a natural part of the building construction can certainly even out diurnal temperature swings, and can supplement or even eliminate the need for heat and/or AC in some seasons and in areas with large diurnal swings. However, Adding thermal mass is rarely cost effective when compared to the alternative of insulation and air sealing. Also, thermal mass doesn't address the issue of latent heat stored as moisture in the air and building materials (and thermal mass). Areas with large diurnal temperature swings also tend to be very dry areas, and latent heat is not so much of an issue.

If your outside temperature only barely gets below room temperature at night, it is often not energy efficient to open it up to outside air, because that air is also very humid. The humidity gets soaked up by the building materials inside, and when the place heats up and A/C is turned on, the A/C system must deal with all of that stored moisture as well as stored heat plus the heat baking in from outside. You might reduce the hours that the A/C is turned on with this method, but actually increase the A/C runtime. Or, you might end up with a house that is cool, but clammy because of stored interior moisture.

1. | | #4

"However, Adding thermal mass is rarely cost effective when compared to the alternative of insulation and air sealing."
No argument here

"If your outside temperature only barely gets below room temperature at night, it is often not energy efficient to open it up to outside air, because that air is also very humid. The humidity gets soaked up by the building materials inside, and when the place heats up and A/C is turned on, the A/C system must deal with all of that stored moisture as well as stored heat plus the heat baking in from outside. You might reduce the hours that the A/C is turned on with this method, but actually increase the A/C runtime. Or, you might end up with a house that is cool, but clammy because of stored interior moisture."

I have been doing this manually for a couple years now, it works okay except that finding high cfm fans is difficult so i am using many fans at the moment. As mentioned i have cut my AC use maybe 50-75%.

1. | | #7

I think you would find that reduction of AC usage would not scale over to an efficient house. You've got an inefficient house with very modest comfort parameters. An efficient house would already use 50-75% less AC than your house to achieve the same or better comfort, so the low hanging fruit you're collecting right now wouldn't be available.

1. | | #10

But 50-75% less AC would make it even easier to get by with 50-75% less thermal mass :D

3. | | #5

You didn't fully explain how the heat is dumped into and extracted from this concrete block. You say air or water tubes, but how are those tubes a)collecting the heat from the house during the day and b)sending the heat outside at night? To effectively collect heat from the house is going to require a very elaborate series of heat exchangers connected via pex pipes in the case of water, and a lot of registers and ducts and a huge central fan in the case of air. In both cases you need a heat exchanger outside. Not forgetting the concrete block itself, in both scenarios there are going to be large energy and capital cost inputs during manufacture and install, and non trivial amounts of electrical energy to do the transfer (relative to how much you're going to benefit vs just opening the windows at night).

It's already been pointed out, but I will reiterate that you haven't addressed the latent heat load much with this proposed system.

A better system would be to dispense with the concrete block, and use a geothermal loop under the house instead. Then you have a fairly consistent 15degC or so heat sink, available all the time instead of just on select nights. You could run it during the day when it's needed the most, and you will get a significant amount of latent heat removal when it really matters (dew point >18degC). Then if it cools down and night, shut it off and open the windows. If you had 15degC ground temps, you could quite easily extract 3000BTU/h of sensible heat 16 hours a day, plus some amount of latent heat. I know this because of measurements I've taken with my geothermal preconditioning loop for my HRV. You still need a way to re-distribute air around the house. Note, I am not advocating anyone do this, even this system is very likely impractical.

1. | | #11

"You didn't fully explain how the heat is dumped into and extracted from this concrete block. You say air or water tubes, "
I did not say water tubes, that would get expensive fast.

"You say air or water tubes, but how are those tubes a)collecting the heat from the house during the day and b)sending the heat outside at night? To effectively collect heat from the house is going to require a very elaborate series of heat exchangers connected via pex pipes in the case of water, and a lot of registers and ducts and a huge central fan in the case of air."
I would prefer to avoid all this complexity.
I thought about tubing in the thermal mass but something like PEX/ABS/PVC etc would have poor thermal conductivity meaning inefficient.
Copper would have excellent conductivity but would be expensive and probably corrode quickly.
Air channels are probably the best choice
As for how to get air through them forced air system are already in common use, for the daytime redistribution having a channel through the mass and using the recirc would work well. As for recharge at night a separate system that draws air form outside through the mass (and also into the house) would be excellent.
I have not yet mapped this out as the concept is still in the conceptual phase.

"In both cases you need a heat exchanger outside. "
No way, scratch that

"Not forgetting the concrete block itself, in both scenarios there are going to be large energy and capital cost inputs during manufacture and install"
Yes, adding extra concrete and placement will have to be decided at the construction phase, retrofitting is probably stupidly cost prohibitive.

"and non trivial amounts of electrical energy to do the transfer (relative to how much you're going to benefit vs just opening the windows at night). "
That is to be determined
Just opening the windows at night is not doing the job, i use fans with a good deal of forced air movement, about 500cfm but would prefer about 1500cfm if i could find suitable fans that are not very expensive (i might start a question on this)

"It's already been pointed out, but I will reiterate that you haven't addressed the latent heat load much with this proposed system. "
You don't.

"A better system would be to dispense with the concrete block, and use a geothermal loop under the house instead. Then you have a fairly consistent 15degC or so heat sink, available all the time instead of just on select nights. You could run it during the day when it's needed the most, and you will get a significant amount of latent heat removal when it really matters (dew point >18degC). Then if it cools down and night, shut it off and open the windows. If you had 15degC ground temps, you could quite easily extract 3000BTU/h of sensible heat 16 hours a day, plus some amount of latent heat. I know this because of measurements I've taken with my geothermal preconditioning loop for my HRV. You still need a way to re-distribute air around the house. Note, I am not advocating anyone do this, even this system is very likely impractical."
As i understand geothermal is not cost effective. Even though several companies such as Google are working on reducing the cost and it has a cult like following.

4. Expert Member
| | #6

The original people of New Mexico have experimented with thermal mass for over 1,000 years, but recent "pueblo style" houses don't work properly for a myriad of reasons. Today's builders can surpass home performance due to better building techniques for less money.

1. | | #12

We are more advanced in knowledge then they were. That said i am not familiar with their techniques so cannot critique them.
I am reminded of a GBA article, about mass and glass vs super insulation and that the research was not done till the 1970s. 1000 years has nothing on what we know now.

5. Expert Member
| | #8

Concrete is only 0.3 Wh/kgC. Since for comfort you want small temperature swings, plus to get heat in/out of a the material you need a deltaT, you would need a very large mass to store enough energy. The cost of that mass is much more than insulating the house.

Concrete is not a great insulator, but when you are working with small temperature differences, even small R value matters. This means you can't use a solid block of concrete, needs to be some kind of channel through it. You can't use water for moving the heat as the water to air HX would add too much delta T, so you are stuck with flowing house air. This means you have your entire house air going through a porous surface that could potentially be near dew point. Trouble.

Finally there is the cost of running the fans. Moving air is not free, less than AC but you are moving a lot of CFM, so I don't think it would be that low.

6. | | #9

Your experiment will work better in some locations than other.

Comfort is not just about temperature, you must consider the humidity.

When you open the windows at night in a swamp the outdoor air will not cool as much at night even when the incoming air is cooler than the indoor air. It is likely AC has been running and the indoor air is less humid.

When you open the windows at night in a desert the outdoor air will cool much more at night and is still likely to still be dry.

Walta

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