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Old 10-29-2014, 04:00 PM   #1
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Does Heat Pump Dehumidify?

So, I might be exposing my "stupid" here, but I was wondering. We all know that running the A/C dehumidifies the inside air in the RV. Since the heat pump is some flavor of the A/C running backwards (as I sort of understand it) does it then ALSO dehumidify inside when it's running?

I looked for an answer and Dometic lists a heat pump/A/C that dehumidifies the RV.

We have some smart folks on this forum and I thought I would ask. If SO, then what a great benefit!! Winter moisture in the RV is one of the most damaging things there is.

Thanks! ~Theresa
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Old 10-29-2014, 04:17 PM   #2
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Not 100% sure myself, but our heat pumps in the house really sap the moisture out of the air. I would think that the ones on our rigs are no different.
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Old 10-29-2014, 04:20 PM   #3
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I think they do. We NEVER have foggy windows in the mornings (even in the corners) and it has been pretty chilly at night, low 40's. This MUST be because of the heatpumps. It's easy to humidify the air, that's for sure, if needed, but a real pain to get it out when it keeps building. We struggle with it in our truck camper when we use it in chillier weather. I am now considering upgrading the A/C unit on the camper to a dual heatpump model. What a bonus, if this is the case. ~Theresa
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Old 10-29-2014, 04:53 PM   #4
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Given it is a reverse flow and the condenser becomes the evaporator, I would think it would not de-humidify. It may actually humidify a little.

But I don't actually know.
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Old 10-29-2014, 06:52 PM   #5
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A heat pump works by taking heat out of the cool outside air and using that heat inside, I would expect that you wouldn't get any de-humidification from the unit itself, unlike the A/C function where the air passing the cooling coils causes water to condense because the cooling coils are cooler than the dewpoint of the air. But, like a furnace in your house there is a drying effect as the humid air escapes the house (air leakage) assuming that you don't dump excess humidity into the space. That may be difficult given the small size of RV's compared to homes.

I actually wish we had the heat pump option in our Redwood as it would be less expensive than propane for heating the trailer (especially at RV parks where the electricity is included!).
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Old 10-29-2014, 10:50 PM   #6
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Quote:
Originally Posted by TNTBaker View Post
So, I might be exposing my "stupid" here, but I was wondering. We all know that running the A/C dehumidifies the inside air in the RV. Since the heat pump is some flavor of the A/C running backwards (as I sort of understand it) does it then ALSO dehumidify inside when it's running?

I looked for an answer and Dometic lists a heat pump/A/C that dehumidifies the RV.

We have some smart folks on this forum and I thought I would ask. If SO, then what a great benefit!! Winter moisture in the RV is one of the most damaging things there is.

Thanks! ~Theresa

Just a touch of thermodynamics here.............

As the air is heated, it can hold more moisture. If you take air with a given moisture content and raise the air temperature, the relative humidity actually decreases. But, you are not removing moisture from the air. In a reverse sort of way, when the heat pump is raising the air temperature, you do perceive lower humidity.

When in air conditioning mode, running the air across the cool coils causes condensation of moisture in the air on the coils and actually removes moisture from the air resulting in lower relative humidity.

Hope this helps.
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Old 10-29-2014, 11:01 PM   #7
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Helped me!
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Old 10-30-2014, 03:07 AM   #8
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Thanks Guy! I knew I could count on some good help!!
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Old 10-30-2014, 04:09 AM   #9
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FWIW, we don't have a heat pump, but our dual pane windows don't seem to fog when it's cold outside and we're running the propane heater.
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Old 10-30-2014, 12:00 PM   #10
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This is our 1st 5th Wheel with a Heat Pump, I was not sure about how well the RV Heat Pumps worked, now with the cooler weather here in the Southeast (40s and 50s), we are loving the Heat Pump option, it works great.
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Old 10-30-2014, 04:43 PM   #11
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Yes we love the heat pumps too. 40s and 50s and only running one and its working GREAT.
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Old 11-03-2014, 04:11 PM   #12
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This weekend we still had water running from the top while using the heat pump only...
'ASSumed' it was coming from the heat pump since it was stunningly blue skies and gee-or-geous @ Mill Creek in Canton, but may have been just the humidity condensing on the coach...
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Old 11-03-2014, 04:48 PM   #13
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It seems our unit looses efficency when we drop below about 38 degrees. They run but don't seem to do much heating.
We do enjoy the heat pumps when they are used to knock the chill out of the unit.
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Old 11-03-2014, 04:55 PM   #14
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Originally Posted by johnboytoo View Post
This weekend we still had water running from the top while using the heat pump only...
'ASSumed' it was coming from the heat pump since it was stunningly blue skies and gee-or-geous @ Mill Creek in Canton, but may have been just the humidity condensing on the coach...
Yes, we have noticed this too, on dry, chilly days. This is what caused me to wonder if they pulled moisture out of the air. Interesting.

Well, they work great... we LOVE them. Haven't had to use the furnace yet, temps are usually above 45 degrees, but we shall see.
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Old 11-03-2014, 05:22 PM   #15
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Originally Posted by Gegallant View Post
It seems our unit looses efficency when we drop below about 38 degrees. They run but don't seem to do much heating.
We do enjoy the heat pumps when they are used to knock the chill out of the unit.
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Any heat pump (RV or Stick and Bricks Central System) will loose efficiency at about 40 degrees.
The heat pump works like a A/C unit running backwards, when the outside air gets as cold or colder than the outside coils, there is no transfer and the heat pump will stop heating.
That is the main reason Heat Pumps do not work up North during the winter, they are made for milder climates.
When the outside temperature gets below 40 degrees, it will not work, it is time to turn the Heat Pump off and switch to the fireplace or LP Furnace.
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Old 11-03-2014, 05:28 PM   #16
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Had just the heat pump on Friday night in East Texus !
and our furnace cranked up on it's own during the night when it dipped into the 30's...

Had it set on 72 iirc since the wife and my visiting daughter said they were cold...

heard the furnace crank up about 2 am and got up to check and had nice warm, toasty floors
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Old 11-03-2014, 05:30 PM   #17
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When in heat pump mode, the compressor should actually shut off around 35-40, so it may just be your fan running if you feel cold air.

Ours sticks and bricks actually has defrosters around the base that melts the snow around the outside coils before it runs, as we can have snow on the ground even though it is 50 outside. Kind of weird hearing that thing run with snow on the ground.
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Old 11-03-2014, 06:33 PM   #18
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I always set ours on elec. & use heat pump as long as possible. Like JB2 said furnace will take over after about 6-10 degree drop inside.
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