RV Heat pumps

[Max Hubrich]

I don't get it---

At 24 degrees this morning I had both heat pumps going and running strong.
On 50 amps we were as warm as we could be.

About two weeks ago with the first real cold spell we had, it got down to 29 degrees, my pump quit and the furnace came on, as it should. No big deal.

But last night I left my Pelonis (small electric heater) running all night. The rig stayed nice and warm, all night, with the temp set to 60 degrees. The furnace never came on. The one heat pump I had selected (front), cycled on and off during the night.

Could the heat control system be reading the heat given from the Pelonis and responding to that? The sensor is located above the dry bar. The Pelonis is closeby, maybe 4 feet away (on the floor).

I've read all about heat pumps not being physically able to function below the 30f to 35f degree range.

I'll be able to test it again tonight. We're looking at 21-23f degrees here in Gulfport, MS.

What gives???

[jerichorick]

Did you read 24* or did the weather channel state this temp? 30* is about as low as I have seen my heat pumps work, and that is if the conditions are right. I don't understand the science of it all. What you experienced is either great luck or a misreading of the true temp.

[Steve N Sal]

That's pretty strange Max. What brand units do you have?

[glenda1908]

The heat pump is usually only effective when the temp is above 35 degrees. I am wondering if you had your propane turned on then they are supposed to switch automatically to gas heat so that may be why you were warm.

[Tom-NC]

Hi, Heat pumps will only give heat above 32 degrees and will blow cold air it temp is below that.

What you might have is heat strips in your AC unit rather than a heat pump and in that case you would get heat.

I am in GS, AL. and temp is down to 29 degrees and heat pumps will not give any heat. I set gas furnace to 55 degrees in Zone 1 and Elect heat to off. I have a Pelonis cube working in the LR and a Bionaire tower blowing towards the bedroom and coach is comfortable all night. In the morning I set Zone one temp up to 68 to warm coach and then turn it off. If the sun warms things up to 35 or 37 degrees I turn on heat pumps and stay warm all day.

[TXiceman]

After 40 + years in refrigeration and some in HVAC, I never found a small air source heat pump to be effective below about 38 degF. Yes, some will run, but they produce no real heat. Some will use a supplemental (electric) heat in the outdoor unit to aid in the heat.

Ken

[targaboat]

The heat pump heat output declines with temperature and at some point you will be using more electricity for a BTU of heating they if you used that same amount of electricity on another form of heat. This point varies with the brand of HP. Also significant is that if there is lots of moisture in the air that moisture will collect on the outside coils and if the outside temperature is close to freezing the coils will freeze up so no air will get through rending the inside heat output to zero. In addition, real cold air significantly below freezing has very low moisture and therefore the coils have a low tendency to freeze up so some Hp pumps will continue to work. So there are various reasons for the HP to shut off and a single temperature is not involved in those reasons. Therefore, an explanation of your situation is pure speculation as this point.

[Luv2go]

Max, I believe you have Dometic Penguin heat pumps with a Comfort Control Center (CCC). On page 23 of this manual under Aux Heat it says that the heat pump works down to 24 degrees F, then switches automatically to the furnace until the temperature reaches 34 degrees F.

The 24 degrees is measured using a sensor inside the heat pump, so most likely the heat pump is warmer than the surrounding air when it is off due to heat from inside the coach reaching the sensor. If the heat pump runs less due to the Pelonis heaters the temp of the sensor may not go low enough to trigger the furnace.

Also the rate at which ice builds up on the outside coil may affect the temperature of the sensor, so humidity is a factor.

I suspect these are some of the reasons why the operation is not super consistent. We have experienced this also with the Penguin heat pumps in our coach.

The Dometic heat pumps are effective at less than 35 degrees, they have a defrost cycle like a residential heat pump to remove ice buildup on the outside coil. If the cost of electricity is low enough it may actually be more cost-effective to run the heat pump until the control turns on the furnace as described above.

For example, with our residential heat pump here in Massachusetts it is rarely more economical to run the oil furnace than the heat pump, the economic "balance point" based on the published ASHRE efficiency figures is -9 degrees F.

The thermal "balance point", however, where the heat from the heap pump is simply not enough to keep the house at the correct temperature, is about 17 degrees F. You have to run the oil below that, even though it costs more money. So here we lock out the oil heat above 20-25 degrees F and use the heat pump exclusively.

It definitely feels like it is "blowing cold air" with temps in the 20's but it keeps the house warm enough, and our heating bills show that the efficiency figures are right, the heat pump is actually are cheaper to operate than oil.

BTW, depending again on your electricity cost, it is rarely more cost effective to use resistive electric heat (the Pelonis) than it is to use the heat pump/LP gas furnace combo for heat.

Also, as you may already know, your basement compartments with fresh, gray and black tanks, pumps, plumbing, etc. are heated using your furnace, so to keep them from freezing you may be better off not using the Pelonis and just letting the CCC do it's thing.

Note: Unlike Ken, I am not an HVAC expert, I have lived with heat pumps heating my houses since 1985 (mostly in North Carolina) and, being an engineer, studied them to death before putting one in my house here. I have a slightly different opinion than he does on the effectiveness of them, but it is based on my personal experience and should be taken that way.

[TXiceman]

A residential heat pump is nothing like the small RV heat pumps. They are designed to operate to a lower temp. Then there are ground source heat pumps that will continue to produce more heat right on down to much colder temps.

I just checked the manual on my Coleman Mach HP unit. They claim it will operate down to freezing ambient. I know for a fact that you stop getting heat out of of the unit by 35 degF. The coach temps continues to drop until the inside temp is 5 degrees below the set point of the t'stat and it will start the furnace. If it does this 3 times in two hours, it locks out the heat pump.

The COP (coefficient of performance) starts dropping like a rock below about 40 degF ambient. When ice builds up on the coil, the performance really drops. Ice make a tremendous insulator on the coil. Higher humidity will ice the coils faster.

Ken

[Rotten-Red]

Originally Posted by Luv2go

Max, I believe you have Dometic Penguin heat pumps with a Comfort Control Center (CCC). On page 23 of this manual under Aux Heat it says that the heat pump works down to 24 degrees F, then switches automatically to the furnace until the temperature reaches 34 degrees F.

The 24 degrees is measured using a sensor inside the heat pump, so most likely the heat pump is warmer than the surrounding air when it is off due to heat from inside the coach reaching the sensor. If the heat pump runs less due to the Pelonis heaters the temp of the sensor may not go low enough to trigger the furnace.

Also the rate at which ice builds up on the outside coil may affect the temperature of the sensor, so humidity is a factor.

I suspect these are some of the reasons why the operation is not super consistent. We have experienced this also with the Penguin heat pumps in our coach.

The Dometic heat pumps are effective at less than 35 degrees, they have a defrost cycle like a residential heat pump to remove ice buildup on the outside coil. If the cost of electricity is low enough it may actually be more cost-effective to run the heat pump until the control turns on the furnace as described above.

For example, with our residential heat pump here in Massachusetts it is rarely more economical to run the oil furnace than the heat pump, the economic "balance point" based on the published ASHRE efficiency figures is -9 degrees F.

The thermal "balance point", however, where the heat from the heap pump is simply not enough to keep the house at the correct temperature, is about 17 degrees F. You have to run the oil below that, even though it costs more money. So here we lock out the oil heat above 20-25 degrees F and use the heat pump exclusively.

It definitely feels like it is "blowing cold air" with temps in the 20's but it keeps the house warm enough, and our heating bills show that the efficiency figures are right, the heat pump is actually are cheaper to operate than oil.

BTW, depending again on your electricity cost, it is rarely more cost effective to use resistive electric heat (the Pelonis) than it is to use the heat pump/LP gas furnace combo for heat.

Also, as you may already know, your basement compartments with fresh, gray and black tanks, pumps, plumbing, etc. are heated using your furnace, so to keep them from freezing you may be better off not using the Pelonis and just letting the CCC do it's thing.

Note: Unlike Ken, I am not an HVAC expert, I have lived with heat pumps heating my houses since 1985 (mostly in North Carolina) and, being an engineer, studied them to death before putting one in my house here. I have a slightly different opinion than he does on the effectiveness of them, but it is based on my personal experience and should be taken that way.

Luv2go,
Your explanation is spot on. Both my roof heat pumps produce heat down to about 24 degrees @ which time the thermostats in the outside units switch HP's off and turn gas furnace on. When ambient temp. rises sufficiently it will automatically turn gas furnace off and restart the HP's.
As for residential HP's, I also heat my home with one. Maybe many years ago they were only usable above 30 degrees or so. That's not the case anymore. I have my outside unit set to turn off at 7 degrees. At which time (just like the MH) it switches to my LPG furnace. When the ambient temp reaches about 13 degrees it will revert back to the HP.
I'm in Michigan where the current temp is 12 degrees, and the HP is keeping the house @ a toasty 74 degrees.

[targaboat]

On my Penguin the manual says HP off at 40F and back on at 45F so there must be a difference in the age of the Penguin.

[jerichorick]

Originally Posted by TXiceman

After 40 + years in refrigeration and some in HVAC, I never found a small air source heat pump to be effective below about 38 degF. Yes, some will run, but they produce no real heat. Some will use a supplemental (electric) heat in the outdoor unit to aid in the heat.

Ken

It was 34* this morning and the two zone 1 pumps heated fine. No heat strips. I have gotten heat from them as low as 32*, under certain conditions. I was very surprised. I have not had the coach long enough to have a greater knowledge of the variables. All I know is that these HP's are far more efficient than my old '05 basement air Vectra coach. They are very noisy, but we are staying warm this winter and cool in the summer.

[Max Hubrich]

Okay--

It was 26f at 10pm, 23f at 3am, and 22f at 6am (CST). The front heat pump was working all night putting out heat at 6am (22f). I then added the rear pump and it kicked in for about 2 minutes and then the furnance took over.

My heat pumps are producing heat down to 23- 24f.

I have two digital outside therometers. One is in the coach rear view mirror, the second ones sensor sits on top of my RR outer dual. They read witin one degree of each other.

Saying all this, my heat pumps used to peter out around 30f over the past 8 years. For some reason this year they are operating down to lower temps- Something must be different.

I have 2, original equipment, "Dometic duo therm" units.

Everything- schools, businesses, military bases, have been closed for two days now in Gulfport MS. Heck, even one of the 12 Casinos in this area closed (Boom Town)- EEK.

Thanks for your inputs-- It must just be an, unexpected and welcomed, anomaly- .

[Max Hubrich]

The free air temp is now 27f

I turned off the furnace and put both heat pumps back on and "walla" both came on- and they are putting out heat.