Heat Pump vs Furnace

[Lloydt512]

New to Forum, 3121RL purchased in August. Our 3121 has a heat pump as the living room AC, which also will provide heat as well as cooling. My problem or question is when we turn on the rear/living room hvac, even in electric mode, the furnace also turns on at the same time as the heat pump. It seems the only way to only run the heat pump is to turn off the propane bottles. Is this the correct operation? In Texas we can usually get by with just the heat pump and electric fireplace and save the propane if this is possible.

[RMcNeal]

In my 2019, I have to choose "Heat Pump" or furnace on the thermostat. I think I remember reading some of the newer systems let you have both. If memory serves, the furnace will kick in if the delta between current temp and desired temp is too high.
Try turning the heat on and setting it just a few degrees warmer than the current temp inside, then keep raising as temp goes up. Or maybe someone with a newer system will chime in on how to run only the heat pump.

[Rotnjonny]

We have a 2022 3791RD with the same setup. If I recall correctly, the furnace (propane) will run with the heat pump if the differential between the set temperature and the actual is greater than the 10 degrees. My interpretation of that is the heat pump (really a heating strip) is good for “taking the chill off” If it is being asked to do more than that, it needs the help of the furnace.

[MandK]

^^^^^. The temperature differential on my coach is 5 degrees when the gas kicks on to help the heat pump to achieve the set temperature. We like to sleep cool so we set the temperature to 60* at night with temps outside in the 40’s. The heat pump will keep it at 60. In the morning if I raise the thermostat to 64, the heat pump will heat the trailer to that, then I can raise it again to 68. OR, I can just raise it to 68 and the heat pump and the furnace will bring it to 68 quickly, and then just the heat pump keeps it there.

[RMcNeal]

Originally Posted by Rotnjonny

We have a 2022 3791RD with the same setup. If I recall correctly, the furnace (propane) will run with the heat pump if the differential between the set temperature and the actual is greater than the 10 degrees. My interpretation of that is the heat pump (really a heating strip) is good for “taking the chill off” If it is being asked to do more than that, it needs the help of the furnace.

Actually, the heat pump is nothing more than your AC running in reverse. So the warmer exhaust air you would get in AC mode is put inside the coach rather than outside. A residential heat pump generally will have heat strips for supplemental or "emergency" heat.

[Cat320]

The bottom line is the HP won't warm if the temps are below around 40...we never use ours, but Lasko towers instead.

[Foldbak]

Are we talking about an in-command controlled HP? I have a thermostat that controls mine. I can choose between electric or gas. I rarely run the heat pump. I find that the fireplace and gas furnace warms the trailer in a matter of minuets. The heat pump is really inefficient and below 40 doesn't work.

[MandK]

No in command here. I set the thermostat on heat elect. If the temp differential is 5* or more, the heat pump and the furnace will run. If I don’t want the heat pump to run like when it’s below 40* outside, I set it at heat gas, and just the furnace runs.

[Robokatz]

As a first time owner of a 2022 Monty I had the same experience and thought something was wrong, so I contacted Keystone, and asked around at the annual Rally, and learned the following: When you select Electric only to turn on heat pump, the gas furnace will also turn on if you select a desired temperature more then 3-5 (it will vary by RV, in my case 3) degrees higher than actual temp. The furnace acts as a helper to rapidly raise temperature. After trying a few times, I found that 3 degrees was my magic number. So, if actual temperature 60, I’ll select 63 as desired temperature and only heat pump will turn on. When temp reaches 63, I bump up to 66. Repeat as needed.

If I select 4 degree higher, the furnace kicks on. Make sense? My heat pump works great until outside temp drops below about 45 degrees, in which case I switch to furnace.

[MandK]

So from what I’m reading here it sounds like the thermostat is programmable to what differential in temperature we want. I never bothered to read the thermostat instructions.

[Lloydt512]

Thanks for the quick response to my question. Happy to know that our furnace/heat pump is working as designed. I’m disappointed that I could not answer that question with the documentation that was provided from Keystone.

[Mikendebbie]

I refer to my Owner's Manual all the time...and I can always find the answers!

[MandK]

Well, there ya go. Must be why I never read it. Asking here has always been easier.
So… I did just read it. Asking here is easier.
Looks like the 5* differential is built in.

[Todd & Mary]

Originally Posted by MandK

^^^^^. The temperature differential on my coach is 5 degrees when the gas kicks on to help the heat pump to achieve the set temperature. We like to sleep cool so we set the temperature to 60* at night with temps outside in the 40’s. The heat pump will keep it at 60. In the morning if I raise the thermostat to 64, the heat pump will heat the trailer to that, then I can raise it again to 68. OR, I can just raise it to 68 and the heat pump and the furnace will bring it to 68 quickly, and then just the heat pump keeps it there.

5 degrees delta on my 2021 3761FL also. I have found the heat pump will heat down to about 38 degrees outside.

[Our3231CK]

Our 2024 unit has 2 settings on the thermostat one for electric which turns the rear Coleman on/off and a gas setting which engages the furnace. We use the heat pump for temps 40ish to 60 to take out the chill. The furnace under that to warm up the underbelly. We've been in temps down to zero (0) and been comfortable. I made my own heated water hose which we use and an insulated bag i put over the water main bib when its near 32 or below, never had an issue.

[Bourbon County]

My rig doesn't have the auto changeover feature, but I like what I'm reading.

A few things to remember about a heat pump: The heating output is the same or slightly less than the cooling output. The 15K BTU heat pump is <40% of capacity of the 40K BTU furnace. Even on a 50 degree day, the discharge air temperature on a heat pump will only be slightly higher than normal body temperature, it will feel cool. The lower the outdoor temps go, the lower the discharge air temp goes. The discharge air temperature for the gas furnace should be in the 120-130 degree range. Do the math, how much quicker will the furnace overcome a 5 degree delta than a heat pump?

I believe all of the RV AC manufacturers offer a resistance heat strip as an OEM option, or add-on. I briefly looked at one and decided against. They are only 1.5 KW which is barely more than 5K BTU. The controls switch the compressor off and the resistance heat on at a preset outdoor temp. A 1.5KW strip is approaching all you can run off the 20 amp circuit. It didn't take long to realize I could buy some portable heaters for what the heat strip would cost.

I really like the idea of the auto changeover; I'm planning on adding the MicroAire thermostats before spring, I'll research to see if they will give me this option. A heat pump is much better suited for maintaining a temperature within a degree or two span than overcoming a 5 degree or more delta.

[BB_TX]

High efficiency home heat pumps will heat below freezing. Our home unit does well down to around 28 or lower. Although if the humidity is high the outside coils will frost over and the unit has to run in AC mode a minute or two to melt that ice. I guess the small RV types efficiency is not there yet.

[AZ Traveler]

Originally Posted by BB_TX

High efficiency home heat pumps will heat below freezing. Our home unit does well down to around 28 or lower. Although if the humidity is high the outside coils will frost over and the unit has to run in AC mode a minute or two to melt that ice. I guess the small RV types efficiency is not there yet.

Bill,

Home heat pumps use electric heat strips as temps get low. Have had a home heat pump before - never again. Give me a gas furnace.

[Foldbak]

I've had a heat pump for 30 years. Just replaced it with a new unit. Very efficient. The air handler has (emergency heat strips) that activate when the outside temp is below 32 degrees. They are only on for 10 minuets or so. Once the heat pump has been running for 10 minuets or so the EH shuts off.

[BB_TX]

Originally Posted by AZ Traveler

Bill,

Home heat pumps use electric heat strips as temps get low. Have had a home heat pump before - never again. Give me a gas furnace.

You are correct that heat pumps have electric heat strips. But our heat strips only come on if the heat pump can’t supply enough heat. And that typically doesn’t happen until the outside temp drops several degrees below freezing. Or you can select emergency heat at any time to manually turn on the heat strips. We have had heat pumps for the last 39 years and love them. We do have a high efficiency heat pump (that cost more than what was standard at the time).