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Why is an air conditioner more efficient in a low-thermal-mass house?

I recently read

To get these efficiency gains it is important to use the air conditioner as it is intended: the unit has to be sized right for the room you are heating/cooling ... and of course - a well insulated, low-thermal mass villa helps too...

-- http://www.penguinaircon.com/index.php/Algarve-Air-Conditioning/inverter.html

which seems to imply that a low-thermal-mass house gives better air-conditioner efficiency than a high-thermal-mass house.

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3 Answers 3

An air conditioner is not, in fact, more efficient in a low thermal mass house than a high-thermal-mass house. In fact, adding thermal mass, everything else held equal, usually makes the air conditioner more efficient.

The article you refer to is engaging in "bench-marketing", picking tests that seem to show their product is better while not mentioning other tests that seem to show their product is not better.

If I were a manufacturer of one kind of air conditioner, I would be tempted to try to attract people's attention to situations where my air conditioner is better than my competitor's air conditioner, no matter how unrealistic those situations may be.

Realistic conditions

In the summertime, I want my house to stay at a nice constant 75 F (297 K), even when the temperature outside is over 100 F (311 K).

Heat energy from outdoors inevitably leaks into my house one way or another. Also heat energy from human bodies, computers, soldering irons, hot-glue guns, etc. also gets into the interior of my house The purpose of the air conditioner is to maintain a comfortable temperature, which it does by pumping all of that that heat energy to the outdoors.

(One could argue that my air conditioner is merely "maintaining a constant temperature" and therefore I'm "not using it for cooling" to a lower temperature.)

Because the thermal mass inside the house is staying very close to a constant temperature, no net heat energy is flowing into or out of that thermal mass. So in theory the air conditioner shouldn't even notice the difference -- an air conditioner would continue pumping heat energy at the same rate, using about the same amount of electric power -- if I double or halve the thermal mass inside my house.

In practice, however, the amount of thermal mass does have a small effect on the amount of energy used by air conditioners.

Two kinds of air conditioners

Some air conditioners are "bang-bang" "on-off" air conditioners that either run at full speed or turn completely off. Some air conditioners are "variable speed" air conditioners that more-or-less continuously run their motors, smoothly changing their speed in response to current conditions.

All on-off air conditioners use energy to spin up the motor every time they turn it on, and practically all on-off air conditioners do not recover that energy (the energy is lost) when they turn the motor off.

Variable-speed air conditioners (sometimes called "inverter air conditioners") run more-or-less continuously, so they don't have the same kinds of losses. At any given instant, heat energy flows into my house at some rate (heat leaking through the walls and windows, chemical potential energy being converted to heat energy, etc.). Assuming the interior temperature is already close to the setpoint (which it almost always is), a variable-speed air conditioner attempts to pump heat energy out of the house at exactly the same rate at every instant.

Unrealistic conditions

In principle, we could attach a on-off air conditioner to a small, leaky box and push the setpoints so close together than it would try to turn on-and-off the motors once a second -- that would be horribly inefficient. Every time you double the thermal mass, that would make the bang-bang air conditioner turn on and off half as often per day, halving the amount of energy wasted on starting-and-stopping the motors. In practice, bang-bang air conditioners run for a long time, and then stay off for a long time, so the amount of energy lost starting-and-stopping the motors is small relative to the amount of energy used to keep me comfortable. (The total energy used by an on-off air conditioner also includes the unavoidable energy that Carnot calculated is required to pump heat energy, and also various other inefficiencies, so doubling the thermal mass won't halve my electricity bill -- it only halves that relatively small amount of energy lost starting-and-stopping the motors).

If you attached a variable-speed air conditioner to such a small box, it would have to rapidly change the speed of its motor every time the flow of heat changed. But that would probably take much less energy than starting-and-stopping the motors of an on-off air conditioner. If you doubled the thermal mass, then the variable-speed air conditioner could take twice as long to ramp up the speed and still keep the temperature in the comfortable zone; slower changes in speed require slightly less energy than fast changes. (The total energy used by a variable-speed air conditioner also includes the unavoidable energy that Carnot calculated is required to pump energy, and also various other inefficiencies).

Summary

If our goal is to keep the house at a comfortable temperature, adding thermal mass makes both kinds of the air conditioner more efficient.

In the "unrealistic conditions", the on-off air conditioner is horribly inefficient and so the other kind of air conditioner looks vastly superior.

In more realistic conditions, the superiority of either one over the other not at all obvious.

(Why am I posting an answer to my own question? Because it is "explicitly encouraged" by an article linked to in our FAQ. Does this mean I think my answer is the One True Answer? No. In fact, this answer is jaded and cynical, and I'm kind of hoping some other answer will bring up some point I'm overlooking.)

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Your own (David Cary's) answer is good, but there is one thing you've overlooked, which might make a big difference in some situations. This is simply that there might periods of the day in which you're not in the building, and ideally you'd like to turn the air conditioning off during those times.

The advantage of a low thermal mass in this case should be fairly obvious. If the house has a low thermal mass then you can leave the air conditioning off all day. When you get home it will be sweltering hot, but you can just put the air-con on and it'll be nice and cool in a few minutes. If the thermal mass is a bit larger then you can still do this, but the system will have to do that bit more work to remove the heat that's built up during the day, since the amount of heat the system has to remove is proportional to the thermal mass.

If the thermal mass is even larger then it will take ages to cool down once you turn the unit on, and you'll be tempted just to leave it running all day. With a very, very large thermal mass, the house's temperature will stay more or less constant regardless of what time of day you have the air conditioning running, and in this case the unit is effectively doing all the work needed to keep the temperature constant 24 hours per day, instead of just when you need it.

There is another factor that you overlooked. By itself I think it mostly works against the idea that a low thermal mass is good. This is that the outside temperature typically varies quite a lot over a 24 hour cycle. Let's imagine an example where the outdoor temperature at night is close to the target indoor temperature, but the daytime temperature is much hotter. In the very-high thermal mass case, the air conditioning will run more or less continuously at the same rate, whereas in the low-thermal mass case it won't run much at all during the night, but will have to run twice as fast during the day. Which of these is more efficient depends on the properties of the air conditioning unit. My intuition is that running all day at a moderate speed is probably more efficient than running half the time at twice the speed, but I'm not sure.

However, these two effects can also combine. If the nights are cool and you're mostly at home during the night then a low thermal mass definitely seems like a good idea, since then you just don't need to worry about how hot it gets during the day, and you only need to run the system for a short time when you get home in the evening before switching it off for the night. This is obviously a lot more efficient than running it all day in the high thermal mass case.

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An air-conditioner tends to have quite limited cooling power - they're relatively expensive bits of kit, compared to say a simple resistance heater of equivalent heating power.

Power is energy per unit time. So for any given time, an air conditioner can only remove a limited amount of energy.

Thermal mass is, roughly speaking, a measure of how much energy needs to be removed (/added) , to decrease (/increase) the temperature.

Having a lower thermal mass means that less energy needs to be removed, to reduce the temperature by a given amount.

So, the air-conditioner's low power requires a low thermal mass room to operate in, in order to reduce the temperature by an appreciable amount.

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In principle, that would be relevant if I ever wanted to "reduce the temperature (of the room) by an appreciable amount". However, in practice, every thermostat I've ever seen attempts to keep the temperature (of the room) the same. Or is there some connection between the temperature of the outside air (that the air conditioner cools off and blows into the room) and the thermal mass that I am missing? –  David Cary Dec 26 '12 at 19:22
    
@DavidCary an air conditioner is a cooling device. If you're not using it for cooling (for example, if it's just a door stop or a dehumidifier) then thermal mass is irrelevant. –  EnergyNumbers Dec 26 '12 at 21:53
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