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Old 04-21-2019, 05:39 AM   #1
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Heat Pump Operation

We are in the north Georgia mountains and have some very cold weather for Florida folks. Temps are upper 30's to low 40's.

We are running our Dometic heat pump which is a dual use AC/heat pump on the roof. It seems like heat pump is working pretty hard to keep coach temps in low 70's. What is the temp range for heat pump and lowest temp it will still work? Also. when heat pump cycles off, sometimes it goes completely quiet, but more likely there is a continuous hum on the roof even when the blower cycles off. Is the continuous hum normal??

We also have a propane furnace which really cranks out the BTU's, but I'm leery about leaving that system run while we're not on board. Am I too paranoid, is it OK to leave it unattended?

Thanks for any advice you can provide. Greg
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Old 04-21-2019, 05:50 AM   #2
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Sounds like it is working correctly, once it dips past 40 it's will start down grading fast. Use your furnace past 40. What you described is pretty much the way the work, the hum you hear is probably the compressor only normal...
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Old 04-21-2019, 05:50 AM   #3
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From what I've read on this forum, most say 40 degrees is the cutoff for heat pump effectiveness. However, I feel that it supplements the furnace quite well down to 30 degrees. Maybe I'm wrong, but that's my experience.
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Old 04-21-2019, 09:27 AM   #4
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My manuals state that my heat pumps will work down to the mid 30's, then they will automatically kick on the AquaHot heaters. It sounds like you have a furnace so that won't happen for you.

The heat pumps will develop ice on the "radiator" when trying to work in colder temps. When they sense too much ice, they quit blowing "warm" air into the coach and they reverse cycle to melt the ice, that's the hum you're hearing. In a simplified explanation of Heat Pumps they have 2 radiators. One is cold and one is hot for air conditioning. When you want heat, they trade jobs. Then the cold one gets hot and the hot one gets cold. If you listen you can hear the ice crack off....
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Old 04-21-2019, 10:45 AM   #5
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On my coach, if the heat pump can't keep up it will automatically energize the propane furnaces.

As for the OP question, we leave the propane furnaces on while we are not there all the time. The way they are made, if anything goes wrong, they shut down. We also drive with the propane furnaces running in "very cold weather" but for us that is usually 0 degrees!
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Old 04-21-2019, 11:25 AM   #6
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My Atwood AC/Heat pump switches to propane furnace at 45 degrees
We snowmobile in the winter and we set the temp at 62 when we are not in the 5th wheel to conserve propane
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Old 04-22-2019, 05:30 AM   #7
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Quote:
Originally Posted by bucks2 View Post
My manuals state that my heat pumps will work down to the mid 30's, then they will automatically kick on the AquaHot heaters. It sounds like you have a furnace so that won't happen for you.

The heat pumps will develop ice on the "radiator" when trying to work in colder temps. When they sense too much ice, they quit blowing "warm" air into the coach and they reverse cycle to melt the ice, that's the hum you're hearing. In a simplified explanation of Heat Pumps they have 2 radiators. One is cold and one is hot for air conditioning. When you want heat, they trade jobs. Then the cold one gets hot and the hot one gets cold. If you listen you can hear the ice crack off....
Bucks, thanks for the detailed explanation and thanks to all for the great inputs

I didn't mention that we also had lots of rain and drizzle during this time so humidify was 100%. Great conditions for ice and I did hear it crack off.

This forum is just a wealth of good information. Thanks to all for answering my questions!!
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Old 04-22-2019, 05:46 AM   #8
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Originally Posted by tragusa3 View Post
From what I've read on this forum, most say 40 degrees is the cutoff for heat pump effectiveness. However, I feel that it supplements the furnace quite well down to 30 degrees. Maybe I'm wrong, but that's my experience.
It's actually designed to 32 degrees. That's freezing. No heat left in the air to extract. 40 seems to be the accepted # in the RV world. Everyone jumps in with "mine will work to...…….". If your hearing a hum it would be the reversing valve, contractor, or a transformer hum. When you shut the system off the hum will stop. The compressor will not run without the fan. Fan will run without the compressor.
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Old 04-22-2019, 12:02 PM   #9
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It's actually designed to 32 degrees. That's freezing. No heat left in the air to extract. 40 seems to be the accepted # in the RV world. Everyone jumps in with "mine will work to...…….". If your hearing a hum it would be the reversing valve, contractor, or a transformer hum. When you shut the system off the hum will stop. The compressor will not run without the fan. Fan will run without the compressor.
I don't know where this came from, in order for a heat pump to defrost a condenser coil the fan shuts off. You have and I say probably a double shafted fan motor, this is very common. The hum that has been stated is your compressor only running. It's just pumping Freon through the coil to defrost it if the fan stayed on it would keep on freezing. Either you have a circuit board that times the defrost No matter if there is ice on it or not, or you have a sensor in the coil that will start the defrost cycle. If your contactor is humming get it checked if your reversing valve is humming get it checked. Either should be humming. Home units have contractors, most and maybe not all depending on the make , model, and year have circuit boards that do all this by themselves.
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Old 04-22-2019, 12:17 PM   #10
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When the fan shuts off, and the HP is either silent, or humming, look at the thermostat. If it is a 5 button Dometic, you will likely see 'Defrost' near the bottom of the screen.
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Old 04-23-2019, 09:19 AM   #11
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This from my owners manual, since there seems to be some debate over what heat pumps can do......

"Air Conditioning - Roof
The roof air conditioners operate from 120
Volts AC supplied by shore power or the
generator. The wall thermostat requires 12 Volt
DC to operate.
NOTE:
The air conditioning system freezes
moisture in the air. It is recommended
to set the blower fan speed to high when
operating in high humidity.
NOTE:
There are ambient air temperature
limitations in Heat Pump mode. The
roof air conditioner will not operate
in Heat Pump mode with ambient
temperatures of 30º F and below."

"When ambient temperature is between 30 and
42º F, a defrost cycle is initiated approximately
every 40 minutes of compressor operation. The
blower motor will stop for five minutes and
Defrost will be displayed. After the defrost
cycle, the Heat Pump operation will resume.
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Old 04-23-2019, 09:32 AM   #12
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I don't know where this came from, in order for a heat pump to defrost a condenser coil the fan shuts off. You have and I say probably a double shafted fan motor, this is very common. The hum that has been stated is your compressor only running. It's just pumping Freon through the coil to defrost it if the fan stayed on it would keep on freezing. Either you have a circuit board that times the defrost No matter if there is ice on it or not, or you have a sensor in the coil that will start the defrost cycle. If your contactor is humming get it checked if your reversing valve is humming get it checked. Either should be humming. Home units have contractors, most and maybe not all depending on the make , model, and year have circuit boards that do all this by themselves.
You are right all the way

Ours is well as most will certainly work close to 30, just not as effective or efficient

Our Carrier units at the home work down near 10 degrees

There is no magical number that the stop recovering heat except for thermostats that change sources
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Old 04-24-2019, 05:32 AM   #13
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Thanks for all the great inputs. My Dometic thermostat did not have any indication of a defrost cycle, but I believe that it was doing that automatically when I heard the humming on roof and blower was off. After defrost cycle, blower came on again and put out heat, but not enough. Since we left the high humidity low 40s temps, the heat pump cycles on and off without any humming in between and holds proper temperature. Greg
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Old 04-24-2019, 06:07 AM   #14
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Quote:
Originally Posted by svcattales View Post
We are in the north Georgia mountains and have some very cold weather for Florida folks. Temps are upper 30's to low 40's.

We are running our Dometic heat pump which is a dual use AC/heat pump on the roof. It seems like heat pump is working pretty hard to keep coach temps in low 70's. What is the temp range for heat pump and lowest temp it will still work? Also. when heat pump cycles off, sometimes it goes completely quiet, but more likely there is a continuous hum on the roof even when the blower cycles off. Is the continuous hum normal??

We also have a propane furnace which really cranks out the BTU's, but I'm leery about leaving that system run while we're not on board. Am I too paranoid, is it OK to leave it unattended?

Thanks for any advice you can provide. Greg
Generally above freezing is OK for Heat Pumps - below Freezing turn on the Heater
- back when we had propane I would leave it on all winter when in storage - Never even gave it a thought - think you will be fine, but you will burn through the propane Quickly - so Heat Pump will be your friend.

These explanations can go on for ever - look here - to the product manual - https://www.dometic.com/en-us/us/support

Simple explanation - A/C moves cool from outside to inside - Heat Pump moves Heat from outside to inside

When it's cold outside a heat pump extracts this outside heat and transfers it inside. When it's warm outside, it reverses directions and acts like an air conditioner, removing heat from your home. One advantage of a heat pump is that it moves heat instead of generating heat, giving you more energy efficiency.

Ambient temps and Humidity are important factors - and the freezing of the coils is common.

A heat pump is more efficient overall than resistant Heaters.

One way to solve the issue of noise/hum, is to set the control so the fan runs all the time, then you will likely not notice the defrost hum

JMHO,
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