# Air Conditioning and a Separate Dehumidifier

On Sunday, February 8, 2015 at 3:59:02 PM UTC-5, Stormin Mormon wrote:

Well, of course he's right, you can't get something for nothing.
However, we're talking about the difference between adding 6.5 kWthr/day of heat to the room, from running the equipment, and 12.62 kWthr, from runnin g the equipment plus the latent heat. We agree since cold air goes out one hole and warm air out another the sensible heat is a wash, but I tend to t hink the latent heat actually goes into the room.
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On Sunday, February 8, 2015 at 7:04:39 PM UTC-5, TimR wrote:

of heat to the room, from running the equipment, and 12.62 kWthr, from runn ing the equipment plus the latent heat. We agree since cold air goes out o ne hole and warm air out another the sensible heat is a wash, but I tend to think the latent heat actually goes into the room.
Sigh.... But apparently not. There is no however. In an ideal, perfectly insulated room, unless you put some energy from outside into it, the energy content is fixed. The *only* increase in heat from the dehumidifier is the 6.5kwhr from the electrical energy added to the room. The latent heat doesn't go into the room, it's already there.
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On Sunday, February 8, 2015 at 8:05:06 PM UTC-5, trader_4 wrote:

y of heat to the room, from running the equipment, and 12.62 kWthr, from ru nning the equipment plus the latent heat. We agree since cold air goes out one hole and warm air out another the sensible heat is a wash, but I tend to think the latent heat actually goes into the room.

ly

er

Ah. I see what you mean.
Yes, you're right.
But, latent heat has been converted to sensible heat. The air temperature will go up by more than the amount predicted by the 6.5 kWthr of energy add ed; in fact it will go up by double that amount.
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On 2/8/2015 8:18 PM, TimR wrote:

air temperature will go up by more than the amount predicted by the 6.5 kWthr of energy added; in fact it will go up by double that amount.

1) The ideal room has an old man with a cigar who yells at everyone "close the damn door"
2) Most of us humans notice the rise in temperature, rather than the loss of latent heat which was converted away.
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<Sigh.... But apparently not. There is no however. In an ideal, perfectly <insulated room, unless you put some energy from outside into it, the <energy content is fixed. The *only* increase in heat from the dehumidifier <is the 6.5kwhr from the electrical energy added to the room. The latent <heat doesn't go into the room, it's already there.
I guess that Tim will never get it . That latent heat that comes when the vapor is condensed comes from needing more power to the dehumidifier. The more water condensed out , the more power it takes to do it.
Cold air does not go out one hole. Air is drawn in from the side with the cool coil. The vapor condenses and runs out the bottom. The cool air is then heated as it blows by the hot coil.
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On Sunday, February 8, 2015 at 11:37:49 PM UTC-5, Ralph Mowery wrote:

The dehumidifier is in one closed room.
But it's really not, because the cold water is removed from the room.
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On Monday, February 9, 2015 at 6:55:22 AM UTC-5, TimR wrote:

That's a new objection. Yes, you can factor that in, but it's a complication that's then hard to account for. The range of possibilities could range from the cold water going straight down a drain, to the cold water accumulating in the bucket inside the dehumidifier, taking on whatever temp exists inside the area that it sits in, then being dumped down a drain, to the water staying in the room and used to water plants. And if it leavest the room, then you have make-up air coming into the room to replace it. That's why I used the simplified model of an ideal, perfectly insulated, closed room. And it seemed like you were OK with that, because it looked like your argument was that the condensation of water vapor out of the air changed the amount of heat in the room regardless of whether that water stayed in the room or left the room. If it stays in the ideal, closed, perfectly insulated room, then the only heat change in the room is the change from the electrical energy flowing into the room to run the dehumidifier. If you want to add the complication of the condensate leaving the room, then yes, that energy transfer would also have to be factored in.
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On Monday, February 9, 2015 at 7:35:52 AM UTC-5, trader_4 wrote:

Okay, I see where we're off.
The original question was whether the dehumidifier heated up the room, whic h to any common sense person means did the air temperature in the room rise .
And the the original answer was yes, by ONLY the amount of energy added by the mechanical operation of the equipment, which we know is an average 6.5 kWtHr/day. The air temperature in the room would rise exactly the amount p redicted by adding 6.5 kWthr/day.
That was assuming that the air cooled by the cold coil and reheated by the hot coil never left the room and so heat was moved back and forth but never lost or gained.
But as Stormy pointed out that neglected a factor. There is heat recovered by condensing the water vapor into liquid at the same temperature, and it' s a lot of heat. It ends up being an additional 6.1 kWtHr/day.
So I have contended that the temperature in the room will rise by almost do uble the amount of the 6.5kWthr used.
Where did that 6.1 kWthr/day go? It went into the cold coil, circulated th rough the refrigerant, and popped out the hot coil into the room air.
The water it came from can't be reheated and reevaporated, because it went down the drain (at least in my house I have the basement dehumidifier piped to the drain).
By the way, that means a humidifier is roughly 190% efficient at heating yo ur house - you put in 6.5 kWthrs of energy and get back 12.6 kWthrs of heat .
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On Monday, February 9, 2015 at 8:28:28 AM UTC-5, TimR wrote:

The actual question was:
"With a dehumidifier, it's only removing humidity, while adding heat. Does it really add heat? " Words have meaning, especially when you're talking about physics.

That is correct.

No one said that.

Which is, once again, assuming for purposes of discussion, that it's an ideal perfectly insulated, closed room.

Ralph and I always explicity accounted for the 6.1kwhr of incoming electrical energy. In fact, I said it's the *only* increase in heat. There is no extra 6.1 kwh per day of heat.

No, only now are you starting to talk about temperature. I previously pointed out that I thought you were mixing up heat and temperature.

No shit Sherlock. At least that's where most of it went, assuming you're talking here about the actual 6.1 of electrical energy used.

We were talking about an ideal, closed, perfectly insulated room, not your house.

No idea where than number came from. And once again, you're conflating *heat* and *temperature*. You could get an increase in temperature, but not an increase in heat. Good grief.
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On Monday, February 9, 2015 at 8:47:58 AM UTC-5, trader_4 wrote:

ent down the drain (at least in my house I have the basement dehumidifier p iped to the drain).

Doesn't matter. The water can sit in the humidifier reservoir in the room forever. Re-evaporating that water is the ONLY way to lose the 6.1 kWthr h eat gained from condensing that water. That heat was gained from the phase change of the water, not from changing the temperature of the air or water . That's why it is so large, roughly equal to the mechanical energy added. That heat stays in the room but is transferred from the water to the air. So technically yes, it was in the room all along, but it was trapped in t he roughly 1% of air that is water vapor not in the 99% rest of the air.
It is a good thing it is not an ideal closed perfectly insulated room. If it were, assuming a 20x30x8 room, and if I did the math right (you are welc ome to check it) the temperature rise from the electrical energy used would be 160 degrees and from the water condensed, 150 degrees.
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On Monday, February 9, 2015 at 10:17:51 AM UTC-5, TimR wrote:

went down the drain (at least in my house I have the basement dehumidifier piped to the drain).

m forever.
Well it does matter. If the water sits in the humififier reservoir, you have a *closed* system. Then, the only heat added to the room is from the electricity consumed. If the water goes out of the room, then you have an array of complex situations to consider.

ed from condensing that water. That heat was gained from the phase change of the water, not from changing the temperature of the air or water. That' s why it is so large, roughly equal to the mechanical energy added. That h eat stays in the room but is transferred from the water to the air.
There you go again, conflating heat with temperature.
So technically yes, it was in the room all along, but it was trapped in t he roughly 1% of air that is water vapor not in the 99% rest of the air.

f it were, assuming a 20x30x8 room, and if I did the math right (you are we lcome to check it) the temperature rise from the electrical energy used wou ld be 160 degrees and from the water condensed, 150 degrees.
Temp rise and heat are two different things, capiche?
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<Well, of course he's right, you can't get something for nothing.
<However, we're talking about the difference between adding 6.5 kWthr/day of heat to the room, from running the <equipment, and 12.62 kWthr, from running the equipment plus the latent heat. We agree since cold air goes out <one hole and warm air out another the sensible heat is a wash, but I tend to think the latent heat actually goes into <the room.
You are still not accounting for the enegery to condense the water out of the air. The more humidity in the room ,the more enegery it will take to cool the room if water is being condensed out of the room.
Here are some more calculations to support that. https://www.physicsforums.com/threads/air-conditioner-efficiency-humidity-precooling.405361/
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On Sunday, February 8, 2015 at 7:31:54 PM UTC-5, Ralph Mowery wrote:

The energy to condense the water doesn't change the amount of energy that's in the room. It's just moving the energy around, ie the energy goes from the water vapor to the other air gases in the room. The only net change in heat in an ideal, perfectly insulated room, is the from the addition of the electricity used to run the dehumidifier. It's basic conservation of energy.
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On 2/4/2015 8:14 AM, TimR wrote:

Dehum adds both the heat from the motor, and also the heat which is called "latent heat of vaporization" which comes into play when a liquid changes to a vapor. Or, in this case a vapor to a liquid. When water vapor condenses, it releases a LOT of heat.
Free standing dehum release a lot of heat.
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On Tue, 21 May 2013 21:38:07 -0400, "Stormin Mormon"
I would agree, though it's not that simple. If all of the water that was taken out in the basement were to end up in the upper floors (doubtful) much of that energy would have to be used by the AC. The AC, being larger, will likely be more efficient but it's not just adding the two together.
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Hench wrote:

Since the basement is cooler, you'd be better off putting the dehumidifier in the warmest, most humid place in the house - usually upstairs. Remember, warm air can carry more water vs cool air.
But the efficiency of small portable dehumidifiers is crap, so the heat they put out is more of a load on your A/C vs the good they do at removing a quart or so of water an hour.
Clean your A/C evap and condenser coils - that'll do way more good vs putzing with the dehumidifier.
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Another way to get a bit more humidity out of your indoor air is to run the melt water drain line on the back of your frost free fridge into a drain.
When a frost free fridge goes into it's defrost cycle, it has an electric heater that melts the frost off the evaporator coils. That melt water then flows down a tube into a receiving pan which is typically around or above the warm compressor. Thus, that melt water just gets re-evaporated into the ambient air in your kitchen. Directing that melt water into a drain helps dehumidify your house a bit by preventing the melt water from being re-evaporated into your ambient air.
The melt water drain tube will typically be a black rubber tube running under the condensor tubing on the back of the fridge from the back of the freezer compartment to the bottom of the fridge where the compressor is. You can just drill a hole through the floor and add an additional length of tubing to run the melt water into a floor drain in your basement or whatever. We're not talking about much water, but we're not talking about much expense for 20 feet of vinyl tubing either.
--
nestork

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