# Air Conditioning and a Separate Dehumidifier

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• posted on May 22, 2013, 1:26 am
Anyone done any sort of calculations or theories to see if running a separate dehumidifier in a basement cuts down any central air conditioner run time on the rest of the house?
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<%-name%>
• posted on May 22, 2013, 1:38 am
Increases, as the dehum puts out heat. . Christopher A. Young Learn more about Jesus www.lds.org . .
Anyone done any sort of calculations or theories to see if running a separate dehumidifier in a basement cuts down any central air conditioner run time on the rest of the house?
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<%-name%>
• posted on May 22, 2013, 1:52 am
On 5/21/2013 9:38 PM, Stormin Mormon wrote:

but how much heat? If I have a 10 seer 1.5 ton a/c that's 18 000 btu removed per hour, if my memory is correct.
But if the air is drier, then the human body can handle a higher temperature, thus the a/c would be run less?
Or so the theory goes...
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<%-name%>
• posted on May 22, 2013, 2:38 am
You'd have to check the watts of the dehum. IIRC, 1500 watts an hour is 5200 BTU per hour. . Christopher A. Young Learn more about Jesus www.lds.org . .
On 5/21/2013 9:38 PM, Stormin Mormon wrote:

but how much heat? If I have a 10 seer 1.5 ton a/c that's 18 000 btu removed per hour, if my memory is correct.
But if the air is drier, then the human body can handle a higher temperature, thus the a/c would be run less?
Or so the theory goes...
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<%-name%>
• posted on May 22, 2013, 1:57 am

This heat can be good if air conditioner is over sized, and get proper dehumidification.
Greg
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<%-name%>
• posted on May 22, 2013, 9:08 am
wrote:

That's for sure. If you use the AC to take out the humidity, it's dumping the heat generated by the compressor outside and also taking heat from the house and moving it outside too. With a dehumidifier, it's only removing humidity, while adding heat.
Only reason I can see for using the dehumidifier would be if the system is so oversized that the house gets cooled off without taking out enough humidity.
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<%-name%>
• posted on February 4, 2015, 1:14 pm
On Wednesday, May 22, 2013 at 5:08:48 AM UTC-4, trader_4 wrote:

Well, sure, a little from the operation of the motor and compressor.
But in the sense of an AC unit, not really. An AC unit has a small amount of heat generated by the operation of the compressor, and a huge amount of heat MOVED from inside the house to outside the house. (A heat pump does t he reverse in cold weather).
A dehumidifier doesn't do that. It operates the same way an AC does, but i t only moves the heat from the front of the unit to the back of the unit. So that heat movement is a wash.
I would think a whole house AC with reheat would be more efficient than a n ormal AC plus a separate dehumidifier though.
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<%-name%>
• posted on February 4, 2015, 1:34 pm
On Wednesday, February 4, 2015 at 8:14:14 AM UTC-5, TimR wrote:

Note that this is an old thread, which has just been revived. But yes, that's the heat I was referring to. How much is a little depends on your perspective. It's not unusual for dehumidifiers to cost \$30 a month to run. Some folks probably have small electric heaters that they use that use a similar amount. And that \$30 produces the same amount of heat via the dehumidifier. Is it going to significantly raise the temp of a house? No, but it's still added heat.

t of heat generated by the operation of the compressor, and a huge amount o f heat MOVED from inside the house to outside the house. (A heat pump does the reverse in cold weather).

it only moves the heat from the front of the unit to the back of the unit. So that heat movement is a wash.
Yes, that portion is. But you still have \$30 worth, or whatever the monthly bill is, of electric energy being dumped inside the house. With the central AC, only the blower portion is contributing heat. The much larger compressor load is losing it's heat outside.

normal AC plus a separate dehumidifier though.
IDK, but I say it's a moot point, because whenever I've had a humidity issue in the house, putting on the AC, dropping the temperature 2F, took care of it. In other words, the humidity only becomes a problem when some AC is also desirable, or at least not objectionable.
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<%-name%>
• posted on February 4, 2015, 5:41 pm
On Wednesday, February 4, 2015 at 8:34:08 AM UTC-5, trader_4 wrote:

Okay, I see your point, and agree. Note that I was in error saying a dehum idifier moves heat from the front to the back of the unit. It does, but it also dumps heat down the drain in the form of condensed water. Some of th at water may absorb heat from the dehumidifier as it warms up after it come s off the coil, so it's probably not \$30. Might be \$29.50.

a normal AC plus a separate dehumidifier though.

An AC reduces humidity but doesn't control it. Whether that reduction is e nough depends mostly on luck, a little bit on getting the size of the unit correct. You can control it with a humidistat and reheat, that's going to take more electricity use though.
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<%-name%>
• posted on February 4, 2015, 7:34 pm
On 2/4/2015 12:41 PM, TimR wrote:

error saying a dehumidifier moves heat from the front to the back of the unit. It does, but it also dumps heat down the drain in the form of condensed water. Some of that water may absorb heat from the dehumidifier as it warms up after it comes off the coil, so it's probably not \$30. Might be \$29.50.

CY: The dehum moves heaat from the back (cold) to the front (blows out warm). Water releases heat as it condenses, so as the water turns from vapor to liquid, it released more heat into the room. Which heat is blown out the front of the dehum.

that reduction is enough depends mostly on luck, a little bit on getting the size of the unit correct. You can control it with a humidistat and reheat, that's going to take more electricity use though.

CY: That makes sense. - . Christopher A. Young learn more about Jesus . www.lds.org . .
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<%-name%>
• posted on February 4, 2015, 8:46 pm
On Wednesday, February 4, 2015 at 2:34:16 PM UTC-5, Stormin Mormon wrote:

Much as I hate to admit it, you're right. I had brain cramp, obviously.
The heat transfer to cool the air down to dewpoint is a wash, just moves from back to front. The heat transfer to make the water change state from gas to liquid is a net gain to the room.
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<%-name%>
• posted on February 4, 2015, 9:28 pm
On 2/4/2015 3:46 PM, TimR wrote:

is a wash, just moves from back to front. The heat transfer to make the water change state from gas to liquid is a net gain to the room.

Just to be OCD, here. Please include the electric motor in the compressor, which releases heat.
Brain cramp. No big deal, some times I forget the
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<%-name%>
• posted on February 5, 2015, 1:09 pm
On Wednesday, February 4, 2015 at 4:27:57 PM UTC-5, Stormin Mormon wrote:

Nope. Not gonna do that. If I did that, I'd have to include the heat the motor in the ceiling fan adds, and that means ceiling fans are a bad idea, and nobody wants to hear that.
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<%-name%>
• posted on February 5, 2015, 1:48 pm
On Thursday, February 5, 2015 at 8:09:42 AM UTC-5, TimR wrote:

You already did include it. We were talking about a dehumidifier that uses say \$30 worth of electricity a month. That \$30 includes the heat that's generated by the compressor motor. Essentially all that \$30 winds up as heat in the room.
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<%-name%>
• posted on February 5, 2015, 3:47 pm
On Thursday, February 5, 2015 at 8:48:45 AM UTC-5, trader_4 wrote:

I'm sure you know but it may not be obvious to all that the \$30 for operation of the equipment does not include the heat generated by phase change of the water being condensed.
Air conditioners use a given amount of electricity to produce a given amount of work.
But they move about 5 times as much heat as the work that is input. Some of that heat is latent heat.
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<%-name%>
• posted on February 5, 2015, 4:44 pm
On Thursday, February 5, 2015 at 10:47:41 AM UTC-5, TimR wrote:

The \$30 includes all the heat energy added to the room period. Draw a closed box around the room with the dehumidifier in it. Whatever energy, ie the \$30 worth of electricity, comes into that closed room, is heat that is added to the room.

It's a dehumidifier. It can move 5x, but it's moving it from one side of the unit to the other. The moving doesn't add to or subtract from the heat in the room. What does add to the heat in the room is the \$30 worth of electrical energy that is added over the course of a month and which winds up as heat.
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<%-name%>
• posted on February 5, 2015, 4:59 pm
On Thursday, February 5, 2015 at 11:44:56 AM UTC-5, trader_4 wrote:

mount of work.

me of that heat is latent heat.

m

No, Stormy is right on this one.
Put 10 watthours (or Btus or whatever you want) of electric energy into it.
Get 10 watthours of heat energy dumped to the room as waste.
Also get 50 watthours of heat energy moved from place to place. BUT! That 50 watthours of energy includes the heat in the air moving from front to b ack AND the conversion of water vapor to liquid. The moving of the air doe sn't add to the heat in the room but the condensing of the water does.
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<%-name%>
• posted on February 6, 2015, 12:43 pm
On Thursday, February 5, 2015 at 12:00:00 PM UTC-5, TimR wrote:

amount of work.

Some of that heat is latent heat.

rom

t.

at 50 watthours of energy includes the heat in the air moving from front to back AND the conversion of water vapor to liquid. The moving of the air d oesn't add to >the heat in the room but the condensing of the water does.
Wrong. It would be a gross violation of the principle of conservaton of energy if somehow heat was miraculously added to the room. The dehumidifier moves heat from one place to another. The conversion of water vapor to liquid is exactly that, the movement of heat within the room. It does not add or remove heat from the room. Phase changes don't affect the total energy within the assumed perfectly insulated room. The only total heat change in the room in your example is the addition of heat from 10 watthours of electric energy. If it were possible to run that dehumdifier system without putting the 10 watthours of electricity into it, there would be zero net change to the heat in the room by the dehundifier. It moves heat, it doesn't create it.
You may be conflating heat with temperature.
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<%-name%>
• posted on February 8, 2015, 7:59 pm
On Friday, February 6, 2015 at 7:43:39 AM UTC-5, trader_4 wrote:

Okay, Stormy, over to you to defend this one. It was your idea. I think trader is wrong but I'm not 100%.
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<%-name%>
• posted on February 8, 2015, 8:59 pm
On 2/8/2015 2:59 PM, TimR wrote:

> idea. I think trader is wrong but I'm not 100%.