Air Conditioner Question

Why aren't home air conditioner units designed to run continuously to keep the air at what ever temperature instead of intermittently? I
guess the same could be asked of heaters too.
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Chris W
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too expensive to run in that mode.

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Dear Chris W:

As "ms" said it is too expensive to run this way. But they do run this way as the temperature difference gets larger, or the refrigerant charge is lost over time.
David A. Smith
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Chris W wrote:

You can do that by controlling the throttling of the expansion valve based on temperature feedback, or even on suction pressure. This affects the amount of torque required from the compressor motor, which would draw less power when not cooling as much. But the amount of energy saved from the lower torque would not make up for running the compressor and fan all the time and the system would not be very efficient.
Don Kansas City
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Some of the new dual stage systems do exactly that. Two compressors, little one runs most of the time big one kicks in as needed. $$$$$$$ They sure are not your track home units.
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SQLit wrote:

From that and other posts, it sounds to me like the real answer to my question is, it is too expensive to make an air conditioner that can run efficiently at a wide range of outputs. An air conditioner that ran continuously would need to efficiently put out a little cold are, a lot of cold air, or anywhere in between. I'm guessing to do that you would need a variable displacement compressor pump, variable speed fan on both the condenser and evaporator. That would certainly raise the cost a lot. If I'm not mistaken, reducing the speed a fan spins, effects it's efficacy too. so you might even need a fan with a variable pitch. Maybe just multiple fans to cover the surface of the condenser and evaporator and turn on as many as needed.
Anyway, I think I would have the same problem on really hot days. I hate having the cold air blow on me. I guess the only solution is to somehow diffuse the air, by having bigger or more vents, so it doesn't move so fast.
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Dear Chris W:
...

Insulate the walls, and run chilled water along the inside of the walls. Then there is no air blowing.
David A. Smith
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N:dlzc D:aol T:com (dlzc) wrote:

Probably wouldn't cool very quickly without any forced convection to help. You would have to rely on the natural convection of the falling cold air along the wall to eventually bring the room to equilibrium.
Don Kansas City
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Dear eromlignod:

It works well with hor water *heating*, but there is a potentially larger temperature difference to do the driving.
David A. Smith
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N:dlzc D:aol T:com (dlzc) wrote:
<snip>

Bad idea! Water would condense on the walls and then run down on to the floor. You would end up a damp, moldy room.
Sorry, Paul D Oosterhout I work for SAIC
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Dear Paul O:

Good point. Actually the whole room wouldn't be moldy, only the "condensor". The dew point in the room would be close to the water temperature...
David A. Smith
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Paul O wrote:

Oops. Not a bad idea except you would have to keep the fluid in the walls above the dewpoint in temp. Which in a house would be impossible. This system is used in europe with chilled beam ceilings.
Air is blown from a main AHU across a chilled beam and diffuses into the room. However we dehumidifi the air coming into the building to around 40 % RH this way the extract dew point temp rarely gets above 12 DegC.
Bye.
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Dear Guido:

It would be possible. I have a "wetwall" in my shower. It would simply be "ugly" to have a huge wall with a floor drain. In every room.

Thanks. In Russia, they do a similar thing, only with heating... so condensation is never a problem.
David A. Smith
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VERY doable in some places.
Bret Cahill
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Bret Cahill wrote:

Very dewable :-) in others.
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No, there is just condensation all over the walls, inside and out. What follows quickly thereafter is mold inside and outside the walls, follwed by sickness and maybe death
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Dear Harry Andreas:
wrote: ...

Yep.
Nope. See where I said to insulate the walls above?

Consider that the condenser coils in a refrigeration system do exactly this. Yet "sickness and maybe death" is a rare thing. The metals of construction probably serve to "poison" most biogrowth.
David A. Smith
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Thermodynamically I assumed that you meant to insulate the outer walls of the building. Insulating the inner walls would make no sense. If you are running the cooling loop inside the wall, then you are depending on conductivity of the inner wall to get a cold surface to the room.

First, the cooling coils of a refrigerator have forced air passing over them, so they remain more or less dry. The inside of the wall would not, which would allow liquid water to form from condensation. Second, non-sterlie liquid water entrapped in a closed space will quickly form mold and mildew on any growth medium present, and there is lots of growth medium available on a house. The metal coils of a refrigerator do not support mold growth even if they are wet. There are lots of sick people around right now because of mold growth in houses, and don't even mention New Orleans. Down there the authorities won't even let workers into some abandoned houses because of toxic mold contamination, unless the workers are wearing full Hazmat suits.
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Dear Harry Andreas:
wrote:

Then there would be no condensation on the outside, nor would there be condensation in the emtpy space between the walls (I've done this in equipment with no outside air exchange).

No they do not. I have moisture condensing and dripping off my evaporator coils.

Again, I have standing water under my evaporator coils, and they are wet. I have no such growth.

Because they are metal, and tend to suppress most biogrowth (especially at cold temps). The collector pan is not metal.

Too bad they don't just blow some air-fed ozone in there for about 30 minutes...
David A. Smith
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To the Group:

So the consensus is that this is a bad idea.
How about a wall-sized waterfall? Chilled water, turned on when the room is to be cooled. The humidity in the room will be near the set point of the water temp. And the moving water will be a driver for air flow. Not the most efficient system perhaps... also only a single-room solution. But the air is constantly cleaned, kept at more-or-less constant humidity, and no directed cold air stream.
David A. Smith
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