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Old 12-26-2012, 10:32 AM   #1
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Question on heat pump

I have looked through the literature and didn't find the answer.

Does the Dometric heat pump not work in temps below freezing?

That is what I have surmised, can anyone confirm?

Thanks.
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Old 12-26-2012, 11:30 AM   #2
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Heat pumps usually work at 40 degrees or better
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Old 12-26-2012, 12:01 PM   #3
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Originally Posted by dpkelly3 View Post
I have looked through the literature and didn't find the answer.

Does the Dometric heat pump not work in temps below freezing?

That is what I have surmised, can anyone confirm?

Thanks.
The heat pump will still work but at a very low register temp. It would then have to depend on the strip heat to kick on. Depending on what BTU your heat strips are to supplement the HP. will determent the temp at the register with the out door temp below 40. If you only have a 1500 watt heat strip, you would be better off adding an using a separate space heater in combination being they would be on separate circuits. About every 45 mins. it will go into the defrost cycle to get the ice build up off the condenser coil automatically.
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Old 12-27-2012, 05:46 AM   #4
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The heat pump in our Sunseeker 3100 (Class C) does not have heat strips. It works fine down to about 40 F. At any temperature, it spends more time defrosting when the humidity is higher. One time the temperature unexpectedly dropped to the mid-30’s with light rain. At this temperature and 100% humidity it was defrosting as much as it was heating. For that reason, I switch to the LP gas furnace when it drops below 40 F. I sometimes supplement it with an electric space heater, running on low (I don’t know the wattage at that setting). I use it as much to warm the floor as for heat.
Our Sunseeker has a 30 amp service. The heat pump takes about 12 amps when running, and more when starting. A space heater on high (1500 watts) takes 12 amps. These two plus other odds and ends max out the service. If the water heater (also 12 amps) happens to come on with the heat pump running and a space heater on high, either the breaker at the service post or my main breaker will kick out.
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Old 12-27-2012, 11:23 PM   #5
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I have been told that heat pumps are not efficient when around 32°. I want to add one when we install a second unit on our 5er to help on cooler mornings w/o kicking on the furnace.
Our house has 2 heat pumps and we usually switch them over to emergency( heat strips- all electric) when it gets in lower 30°s at night. Seems that they run all the time and makes you a little stuffy if you leave heat pumps on when it gets colder then say 30-34° outside at night.
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Old 12-28-2012, 04:33 AM   #6
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MillerTime, You are correct, I would make sure you get the strip package included when you switch over and add a hp.
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Old 12-28-2012, 05:18 AM   #7
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MillerTime, Also you can check a older thread that was called CHEAP HEAT WARNING, you will find the cost to add just a strip heat of 5600 btu's to your exiting ac unit without changing to a HP system. The only problem I found was that you need to add an extra stat wirier. I do not ever run my ac fan on high it makes to much noise and really will not produce that many more CFM. You would in that case use that wire for the strip heat installation and not have to pull new stat wirier....
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Old 12-28-2012, 08:23 AM   #8
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I've been installing Heat Pumps in homes since 1976. Heat Pumps get their source of heat from the outside air. The colder it is outside the less heat they produce. Why anyone would build a heat pump without electric strip heat is beyond me. In cold weather the heat pump will frost up on the outside coil and need to go into defrost in order to clear the frost. How this is accomplished is the reversing valve switches into AC Mode. In a residential application the outdoor fan stops running, the compressor is now sending the heat to the outdoor coil to melt the frost but inside it's blowing cold air. The strip heat comes on to offset the cold air when the unit is in defrost. The other function of auxilary heat strips is to run with the heat pump when the compressor alone can't keep up to the temperature requested. It will also come on when the temperature is raised by more than 3 degrees on the thermostat. Simple explanation of a Heat Pump is It's AC running in reverse. AC - Inside warm air is transferred to outside through the refrigerant to the outside coil, cool air is sent to inside. The cool air coming in contact with the warm/hot humid air inside produces condensate which drains out of the coil. Heat Pump - Same function as AC only in reverse. The condensate now gathers on the outside coil but turns to frost / ice and needs to be cleared. This is accomplished by the system running in AC Mode to melt the outside frost / ice on the outside coil.
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Old 12-28-2012, 10:12 AM   #9
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MillerTime, Also you can check a older thread that was called CHEAP HEAT WARNING, you will find the cost to add just a strip heat of 5600 btu's to your exiting ac unit without changing to a HP system. The only problem I found was that you need to add an extra stat wirier. I do not ever run my ac fan on high it makes to much noise and really will not produce that many more CFM. You would in that case use that wire for the strip heat installation and not have to pull new stat wirier....
I'm actually adding a 2nd ac, so I was just going to get a hp for it. Probably get the heat strip too.
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Old 12-28-2012, 10:13 AM   #10
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Continuing Charlie’s (cfositman’s) discussion of heat pumps, when I had a new system installed in my house about 30 years ago, I studied the heat pump and gas furnace performance sheets. I’m an engineer – I can’t resist digging into stuff like that. (I chose a gas furnace then.) Heat pumps are rated by Coefficient of Performance rather than efficiency. COP is the amount of BTU’s a heat pump transfers compared to the BTU’s it consumes. Heat pumps have been improved, but 30 years ago at 70 F outside the COP was about 3. In the 30’s, it was about 1. In other words, in the 30’s you are only getting the heat into your home (or RV) from the energy that the compressor motor is consuming, which at a COP of 1 is the same as with strip heaters. So why run it and wear out your system when you are getting no more heat than you would with strip heaters? In your RV, just shut the heat pump off when it gets in the low 40’s and run an electric space heater or your LP gas furnace.
This is off topic, but I had my 28 year old home natural gas furnace and A/C system replaced 3 years ago. I had a heat pump installed with a natural gas furnace instead of electric strips as auxiliary heat. Gas is much cheaper to run than strip heaters, it heats up the house very quickly after we have been away, and I can run the gas furnace on a generator during power outages.
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