use portable air conditioner as dehumidifier?
Context:
I’m in an ongoing battle with my wife over the use of air conditioning. For both comfort and health reasons, I want to keep the house below 24C and 65% RH. She finds that too cold, and generally just shuts the AC off and opens doors and windows during the summer. This typically results in conditions in the 24-26C range, which is not horrible, but the RH will be 80-90%. So I’m thinking a compromise for this would be to run a dehumidifier, so we can keep the temperature around 25 while getting the humidity down to the 50-60% range.
So while window shopping for dehumidifers, I noticed that some of those horrible, one-tube portable air conditioners that blow exhaust air out of the house while drawing in unconditioned air, have dehumidifier modes. One I found at a surplus store near me is supposedly rated at 111 pints per day, which is 50% more than any portable dehumidifier I’ve seen before. The other bonus is I’d be able to control it remotely. But are those specs realistic? And is there some factor I’m overlooking that makes this a bad idea?
This is the unit I am looking at:
https://www.honeywellstore.com/store/products/portable-air-conditioner-14000-btu-cooling-hl14ceswk.htm#productManuals
Note: at some point in the recent past, they changed how dehumidifiers are rated, significantly lowering their rated output. Units that used to be 70 pints are now 50 pints. I made the assumption that the 111 pints is based on the old standard, and compared it to the old rating of dedicated portable dehumidifiers.
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Replies
Why not just use a regular dehumidifier?
Just because it appears to have higher capacity.
If the plan is to run the “dehumidifier” with the windows open that is silly as humidity will enter the window as fast as it is removed.
I think all the single hose portable AC are silly. They take conditioned air from the home add some heat to it and expels it from the home. 100% of that air gets replaced with unconditioned air drawn in thru the gaps in the home.
If the plan is to use this unit or a dehumidifier without the AC understand the dehumidifier is a heater and while it removes moisture it adds lots of heat so it will warm the house while it dries it.
If the plan is to use this unit or a dehumidifier with the AC that could work as a band aid but understand what you are describing is the classic complaint for an oversized AC system the is so big it run a small percentage of the time. The run cycles are so short is that little moisture gets removed before the house is to cold.
The classic oversized complaint is the house is cold and clammy.
Walta
No, the plan was to close the windows.
The plan is to run the dehumidifier in conjunction with the AC.
My AC is not oversized, it's pretty much as small as it can be (9k). We just live in a climate where it's humid all the time. The mini split, when I'm allowed to run it, can get the house down to below 60%, but it has to be below 23C to get there. The house never gets above 26C even without active cooling, but the humidity will be like 90%. So there's obviously a lot more moisture removal needed than sensible heat. When I run the mini split in dry mode, it barely ever cycles (not sure it ever does, to be honest).
An 11000BTU ac unit has a much bigger compressor than your typical dehumidifier. Assuming 14seer, that is 800W unit. A typical 50pint unit might only have a 500W compressor.
To get that full humidity removal, the unit would have to run full tilt at low CFM without cycling. These typically don't shut the fan off once setpoint is reached so it would re-evaporte a lot of moisture when cycling. I'm also assuming you would not be venting anything, the exhaust pipe should dump somewhere the intake of your house AC (avoids heating the house plus you get more run-time on the AC unit).
I think a better option would be to wait to fix your main floor mini split and use that to re-heat while the upstairs unit is run in cool mode. You could set the main floor heat to a management friendly temperature and keep cranking the upstairs cooling until you get the house down to say 45% RH.
I inadvertently have this from heat gain of some largish west facing windows and can get away with keeping the house at 25C but there is still enough runtime to keep RH bellow 50%. Because of the low humidity the house always feels cool and fresh even though it is pretty hot by most people's standards.
Running the second mini split in heating mode is probably a good idea. I was actually experimenting with that just before it stopped working. I was just thinking that if I'm actively adding heat it would make sense to put it to use in removing moisture. I'm also concerned that the heated air on the main floor isn't going to mix all that efficiently with the cooled air upstairs, resulting in higher than desired temps on the main floor. Some more experimenting is needed.
I'm not sure I follow on how the portable unit in dehumidifier mode would re-evaporate anything. It's got a drain tube so all the liquid water will be going straight down the drain. I made the assumption that the exhaust would be automatically be closed, to simulate a real standalone dehumidifier.
A dehumidifier is a space heater with a COP of about 2.5. So if you take the above 500W dehumidifier, assuming it is removing 50 pints it adds 1250W (~4000 BTU) of heat to house.
I think overall running the mini split as a reheat would consume less energy. The latent heat of condensation and compressor power of your cooling unit is dumped outside so the only heat your main floor unit should be supplying is to push the sensible heat up to get the required SHR on the cooling unit to dehumidify properly without overcooling.
As for the portable units, these typically run the fan at constant speed and cycle the compressor on/off. Each time the compressor cycles off, all the liquid water on the cooling coil is re-evaporated back into the room. This is why you typically find a room cooled by one cold but clammy, the more oversized it it is the more it cycles and the worse the issue is. When running it as a dehumidifier you'll have the same issue but in this case cycling would reduce the amount of dehumidification.
If you have a power meter on the house, you could try running the main floor baseboard heaters in a similar re-heat and monitor power use. Switching the reheat to the minisplit down the road would probably reduce that cost by 4x to 5x.
I've been meaning to calculate this for a while, so lets do it.
A 50 pint (29L) dehumidifier does about 1.9L/kWh. So in a day it consumes 15kWh which translates to 600W (2000BTU) operating. The latent heat load from condensing 50 pints is also around there at 2200BTU. Total heat load is 4200.
Lets take a nice 1 ton wall mount:
https://ashp.neep.org/#!/product/34428/7/25000/95/7500/0///0
To cool that 4200BTU assuming 82F outdoor, the unit runs around COP of 5 so uses ~250W. Total is 850W.
Now for the minisplit re-heat. The SHR on these is not great, so lets use .8 for now.
To remove 2200BTU of latent, the unit needs to also do 8800BTU of sensible. Total is 11000BTU. COP will be around 4.5 so 720W compressor power.
Assuming the heat load from the house is around 5000BTU, we need 3800BTU of reheat.
There is no NEEP data for heat at 82F, but say it is close to the cooling so COP of 5, that means 220W.
Total is 940W. But we area also getting about 1/2 ton of cooling (300W at COP of 5) to the house.
Overall, I think the operating cost for the mini-split re-heat is less but close enough to not matter.
I must make the house colder than I want to control the humidity is the perfect description of an oversized system for the given load.
The fact that someone owns the smallest system on the market does not mean that it is not too large for their load. The fact that a system is sized perfectly for one’s heating load says nothing about how it is sized for the cooling load. Compromises must be made. Do not take it as an insult that the system is oversized for the load it is seeing under the current conditions. Manual J sizes for the 98% worst case so under most conditions the unit is larger than needed.
If the only way to be comfortable is to over cool the air to remove the moisture and then reheat back to a comfortable temp so be it.
Seems to me reheating the air with a dehumidifier or the unvented portable AC unit is more energy efficient than running one mini in cooling and the other in heating. Because the cooling energy it sends outdoors is wasted when it would be useful indoors to remove moisture.
Note I read that Mitsubishi mini split dry mode does not use the temp sensor so it will way over cool in the “dry” mode.
Walta
I'm not offended, but the root of the problem is that the latent load is a lot bigger than the sensible load. It doesn't matter how small the AC unit is, it's going to sensibly cool too much and not remove enough moisture.
Run a dehumidifier to control humidity. If that makes the house warmer than is comfortable, run an AC to remove heat.
This is the situation that the Daikin Quaternity was designed for. It has two coils in the head, it can run with both cooling, both heating, or one heating and one cooling, for dehumidifying. In dehumidify mode it runs like a dehumidifier, in that the heat being rejected from the cooling is used to rewarm the air, so there's no additional energy penalty from reheating.
Unfortunately, I've yet to encounter anyone using this device in the real world.