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Author Topic: AC with heat pumps.  (Read 881 times)

garyb1st

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AC with heat pumps.
« on: September 17, 2019, 10:09:00 AM »
On one relatively cool morning, probably mid 50's in the coach, we turned on the furnace.  The thermostat works both the furnace and the air conditioners.  The furnace came on as usual and began heating the coach.  However, after a few minutes, the AC started.  That never happened before.  When I looked at the thermostat, the setting of the AC units were not on off.  They indicated heat. I know AC's with heat pumps exist, but I didn't know if ours had them.   

So if we actually do have them, how effective are they?  Are there temperature limits when I can use them?  (seems I read once that they're only good to a point)  Would it make more sense to just use our stand alone space heaters?   
Gary B1st

2005 Pace Arrow 35G
2016 Jeep Wrangler


Poverty exists not because we cannot feed the poor, but because we cannot satisfy the rich.

Tom

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Re: AC with heat pumps.
« Reply #1 on: September 17, 2019, 10:16:47 AM »
I don't recall the "limits", but within their operating range heat pumps work very well. We're hooked to shore power, and on these chilly mornings (some around freezing), they bring (and maintain) the coach interior temperature to very comfortable in a few minutes.

Think of the heat pumps as the A/C units working in reverse.
« Last Edit: September 17, 2019, 11:36:42 AM by Tom »
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jayc2640

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Re: AC with heat pumps.
« Reply #2 on: September 17, 2019, 10:17:16 AM »
We have a heat pump on one of our air conditioners.  From what I've observed, the heat pump is good down to temps in the upper 30s.  Anything below that, we need to go to the furnace.
- John Coffey
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2016 Chevrolet Equinox

Tom

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Re: AC with heat pumps.
« Reply #3 on: September 17, 2019, 10:29:09 AM »
FWIW our prior coach had "heat strips" in the A/C units. They worked much like a small electric fan heater with a heating element. They helped take the chill off, but weren't anywhere near as effective as our current heat pumps.

A quick google search turned up varying answers for the lower ambient temperature where heat pumps will continue to work. Some said 30F and others 20F. Our owners manual says 20F, although it does allude to issues closer to 30F. Check your manual.
« Last Edit: September 17, 2019, 11:35:46 AM by Tom »
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garyb1st

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Re: AC with heat pumps.
« Reply #4 on: September 17, 2019, 10:36:09 AM »
Thanks for the replies.  Next time we're out in relatively cold weather, I'll give them a real try. 
Gary B1st

2005 Pace Arrow 35G
2016 Jeep Wrangler


Poverty exists not because we cannot feed the poor, but because we cannot satisfy the rich.

Back2PA

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Re: AC with heat pumps.
« Reply #5 on: September 17, 2019, 10:42:58 AM »
I had heat pumps previously, they do a pretty good job. While mine actually worked down to the low 30s, after frying a connection I was told by an RV AC guy I shouldn't use them below about 40 as it puts considerable strain on them
Scott
2014 Montana High Country 343RL (37')
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SeilerBird

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Re: AC with heat pumps.
« Reply #6 on: September 17, 2019, 10:51:13 AM »
The A/C in my fiver also has a heat pump and it works fantastic here in Florida. It never gets too cold for the heat pump not to work.

Tom

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Re: AC with heat pumps.
« Reply #7 on: September 17, 2019, 11:35:00 AM »
Quote from: Back2PA
... after frying a connection ...


Scott, just curious - which connection did you fry?
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Kevin Means

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Re: AC with heat pumps.
« Reply #8 on: September 17, 2019, 05:06:39 PM »
That's kind of a nice surprise to stumble across 'eh Gary? Ours work well, and I agree with Tom... they're kind of like working the AC in reverse (Just as noisy too.) Of course they only work on AC, so we don't use them when boondocking, but when we are hooked up to power, they'll heat the coach up fast.

Kev
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WILDEBILL308

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Re: AC with heat pumps.
« Reply #9 on: September 17, 2019, 09:40:51 PM »
On one relatively cool morning, probably mid 50's in the coach, we turned on the furnace.  The thermostat works both the furnace and the air conditioners.  The furnace came on as usual and began heating the coach.  However, after a few minutes, the AC started.  That never happened before.  When I looked at the thermostat, the setting of the AC units were not on off.  They indicated heat. I know AC's with heat pumps exist, but I didn't know if ours had them.   

So if we actually do have them, how effective are they?  Are there temperature limits when I can use them?  (seems I read once that they're only good to a point)  Would it make more sense to just use our stand alone space heaters?
You have gotten some good answers, but here is what I think happened.
When you used the thermostat to call for heat you were below the temp where the thermostat was set to use the heat pumps. The gas furnace came on and warmed the coach up enough so it could switch to the heat pumps.
I have had this happen  in reverse. It started with the heat pumps but the temp was to low and it automatically switched to gas heat. 
Bill
2008 Newmar Mountain Aire
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Allison 4000MH Trans.
Towing 2014 Honda CRV
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Back2PA

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Re: AC with heat pumps.
« Reply #10 on: September 18, 2019, 05:24:13 AM »

Scott, just curious - which connection did you fry?


Tom, it was the main power wire, I was lucky the rig didn't burn. Details are in this post
Scott
2014 Montana High Country 343RL (37')
2011 SD F-250 Crewcab LB 4x4, 6.2 Gas, 10K gross
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Back2PA

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Re: AC with heat pumps.
« Reply #11 on: September 18, 2019, 05:36:53 AM »
You have gotten some good answers, but here is what I think happened.
When you used the thermostat to call for heat you were below the temp where the thermostat was set to use the heat pumps. The gas furnace came on and warmed the coach up enough so it could switch to the heat pumps.
I have had this happen  in reverse. It started with the heat pumps but the temp was to low and it automatically switched to gas heat. 
Bill


Bill, pretty sure the sensor which determines whether it's too cold to use the heat pumps is mounted outside so as to sample outside air, not cabin air.
Scott
2014 Montana High Country 343RL (37')
2011 SD F-250 Crewcab LB 4x4, 6.2 Gas, 10K gross
Eezrv TPMS
Fulltimer

lynnmor

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Re: AC with heat pumps.
« Reply #12 on: September 18, 2019, 08:05:11 AM »
I have a heat strip in the AC.  When temperatures are low enough, both the heat strip and furnace will run till it warms the air, perhaps that is what you have.

Tom

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Re: AC with heat pumps.
« Reply #13 on: September 18, 2019, 09:09:00 AM »
Quote from: Back2PA
it was the main power wire...


Thanks Scott. Looks like it was the 'main power wire' to the A/C. I was trying to figure out if it was your main power cord. A poor connection to either can quickly build up heat under high current load.

Coincidentally, on the drive home from dinner last evening, the other couple mentioned that they'd replaced 6 a/c units on their prior coach. I was too busy driving to think of asking if any failures happened at the low temps we're discussing here.
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TheBar

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Re: AC with heat pumps.
« Reply #14 on: September 18, 2019, 01:54:31 PM »
This graph shows how heat pumps become inefficient below 35. They still work but do not save electricity below that point. 
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Tom

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Re: AC with heat pumps.
« Reply #15 on: September 18, 2019, 02:26:05 PM »
Quote from: TheBar
This graph shows how heat pumps become inefficient below 35. They still work but do not save electricity below that point.

I don't think we were talking about 'saving electricity'. Meanwhile, for clarification, one source for that graph states "Every house and heat pump will have the capacity and load lines in different places, so don't take the numbers in this graph as absolute".
« Last Edit: September 18, 2019, 02:33:47 PM by Tom »
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TheBar

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Re: AC with heat pumps.
« Reply #16 on: September 18, 2019, 06:33:49 PM »
So if we actually do have them, how effective are they?  Are there temperature limits when I can use them?  (seems I read once that they're only good to a point)  Would it make more sense to just use our stand alone space heaters?

Sorry, I posted the graph in response to the OP statement quoted above to show the efficiency at different temperatures.
Retired factory automation computer programmer
Cabin fever solution: 30' Class C and Starcraft popup
DW loves camping more than I do

Tom

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Re: AC with heat pumps.
« Reply #17 on: September 18, 2019, 06:53:54 PM »
Thanks for the clarification.
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WILDEBILL308

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Re: AC with heat pumps.
« Reply #18 on: September 18, 2019, 07:57:53 PM »

Bill, pretty sure the sensor which determines whether it's too cold to use the heat pumps is mounted outside so as to sample outside air, not cabin air.
And your point is? I never said it wasn't sampling outside air.
Bill
2008 Newmar Mountain Aire
450 HP ISM Cummins
Allison 4000MH Trans.
Towing 2014 Honda CRV
Home base Fort Worth, Texas
A man who carries a cat by the tail learns something he can learn in no other way.
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Larry N.

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Re: AC with heat pumps.
« Reply #19 on: September 20, 2019, 02:58:16 PM »
On one relatively cool morning, probably mid 50's in the coach, we turned on the furnace.  The thermostat works both the furnace and the air conditioners.  The furnace came on as usual and began heating the coach.  However, after a few minutes, the AC started.  That never happened before.  When I looked at the thermostat, the setting of the AC units were not on off.  They indicated heat. I know AC's with heat pumps exist, but I didn't know if ours had them.   

So if we actually do have them, how effective are they?  Are there temperature limits when I can use them?  (seems I read once that they're only good to a point)  Would it make more sense to just use our stand alone space heaters?
One thing I've often noticed is that some thermostats have two heat settings, one of which operates the heat pump and the other operates the propane furnace (or Aqua Hot, if equipped). Of course many don't but it's another thing you could check.
Larry and Mary Ann N.
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garyb1st

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Re: AC with heat pumps.
« Reply #20 on: September 20, 2019, 04:47:15 PM »
Just checked my thermostat.  It actually has a heat setting for gas and a setting for electric.   For gas all I do is move the switch to heat and set the temp.  For electric, I need to scroll through the options under the AC settings.   The setting for electric heat only shows when the switch is on the heat setting.  When it's on cool and I scroll through the settings, electric heat is not an option.  Thought I saw this once before but wasn't sure.  So it looks like I have heat pumps. 
Gary B1st

2005 Pace Arrow 35G
2016 Jeep Wrangler


Poverty exists not because we cannot feed the poor, but because we cannot satisfy the rich.

Laura & Charles

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Re: AC with heat pumps.
« Reply #21 on: September 21, 2019, 07:12:46 AM »
On ours, when on heat setting, ‘gas’ is just gas runs on 12 volts (but those fans use too much to go long without the converter). ‘Electric’ is the heat pumps (must have 110 AC) but, if the interior temp is more than 4 degrees F below the thermostat setting, gas will kick on regardless.  We find our heat pumps will blow air that is warmer than ambient to about 38 (outside) or so but run nearly nonstop to keep up. We flip to gas at 41 or so in calm air, 44 or so in windy conditions to keep the heat pumps from running continuously. (Yes I’m aware ‘wind chill’ doesn’t impact inanimate objects... but it DOES blow heat off a structure and that requires more from your heat source to replace.)
« Last Edit: September 21, 2019, 07:14:27 AM by Laura & Charles »
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TheBar

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Re: AC with heat pumps.
« Reply #22 on: September 21, 2019, 07:55:56 AM »
(Yes I’m aware ‘wind chill’ doesn’t impact inanimate objects... but it DOES blow heat off a structure and that requires more from your heat source to replace.)

I've had that wind chill conversation many times. It always ends when I say "then how does a car radiator work?"
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Gary RV_Wizard

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Re: AC with heat pumps.
« Reply #23 on: September 21, 2019, 10:42:33 AM »
Quote
It always ends when I say "then how does a car radiator work?"
Good reply!
Of course, the "wind chill factor" reported on tv and internet is a uniquely calibrated number based on what it "feels like" to human skin. That particular number has no meaning at all for inanimate objects.  However, anything that radiates heat is very much affected by the amount of air flow over its surface. It's not a linear effect either - a little air flow usually dramatically increases the rate of heat transfer, but beyond some point it has no little or no additional effect.   The type of material makes a difference as well, and materials with a low transfer rate like wood or adobe aren't much affected while high rate materials like metals and glass can really ramp up the loss.
« Last Edit: September 21, 2019, 10:44:22 AM by Gary RV_Wizard »
Gary
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