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Old 10-15-2018, 01:03 AM   #1
Bierp
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Heat pump issue

Hey gang,

I'm not seeing anything like this in my search, so I'll post a new thread.

I have a 2018 3950BR. 4 months old, so still under factory warranty. The living room AC is the heat pump unit, controlled by the primary thermostat on the wall outside the bonus room. The unit is Dometic, but I don't have the model number as I write this.

We're in the Pacific Northwest. Outside temps have been mid to upper 40's at night. The heat pump has been doing an adequate job of keeping the edge off.

However, several times in the past few days we've woken to a 60 degree rig and the HP wasn't running, despite the thermostat being set to 68 and not showing an error. Today I started paying a bit more attention to it and it seems that sometimes (not every time, yet) it's kicking on, making a low electric/mechanical hum, then turning off.

I'm no engineer, but that seems bad.

The thermostat still doesn't show an error and the other features seem to be fine. (Fan works on the AC unit, furnace controls work. I didn't try the AC, cuz brrr.)

We're full-timing, so getting it into the shop is less than ideal. Before I start hassling with that I was hoping someone here knew a magical fix for this. I can use the furnace, but with the mild weather we usually have up here, I was really hoping to use the heat pump quite a bit.

Thanks,
Jeremy
 
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Old 10-15-2018, 06:55 AM   #2
Tow Infinity
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Hi Jeremy,
Thanks for posting this question. Did you get any answers? I'm having a similar problem. My furnace fans kicks on and it sounds like it fires up for several seconds and then stops. It seems as if the furnace isn't getting enough propane. Let me know if you learned anything.
Thanks again,
Tom
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Old 10-15-2018, 08:50 AM   #3
Kyle and Lisa
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Our heat pump is good to about 47 degrees ambient temp, the thermostats are about 5 degrees off. I turn the a/c setting up when running for heat and just the opposite with the heat for cold, seems to work better for us.
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Old 10-16-2018, 03:53 PM   #4
Bierp
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Quote:
Originally Posted by Tow Infinity View Post
Hi Jeremy,
Thanks for posting this question. Did you get any answers? I'm having a similar problem. My furnace fans kicks on and it sounds like it fires up for several seconds and then stops. It seems as if the furnace isn't getting enough propane. Let me know if you learned anything.
Thanks again,
Tom
This is different than my issue, which is specific to the heat pump, not the furnace. That being said, I had a similar issue with a previous SOB unit. Turns out my control board was getting wet and was essentially shorting out. I had to dry it with a hair dryer (actually, used my truck window defrosters) to make it work correctly. Not saying this will help you, but it might be worth a look.



Quote:
Originally Posted by Kyle and Lisa View Post
Our heat pump is good to about 47 degrees ambient temp, the thermostats are about 5 degrees off. I turn the a/c setting up when running for heat and just the opposite with the heat for cold, seems to work better for us.
I recognize that the heat pump isn't as effective as a furnace, specifically at lower temps, and I recognize that the thermostat isn't exactly precision hardware, but this seems like something else.

I was fully 8 degrees below the target temp and the unit wasn't even running.

Correct me if I'm wrong here, but when a heat pump stops being effective due to a low outside temp, it doesn't shut down, right? It keeps trying to pump heat, but it isn't effective. My unit was powered down. I think it's a different issue.

That being said, I ran it last night and it worked perfectly. The ambient temperature wasn't much different than the nights where it failed.

So something is amiss, just not sure what. Still hoping to run into an answer here rather than having to track it down through service.


J
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Old 10-17-2018, 07:09 AM   #5
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i am confused about your rig having both a heat pump and a furnace. I have installed heat pumps in homes and they work fine. Not sure why they would have both in a RV. As for your problem you can call a service tech from a heating and air company to come out and look at it. They are all the same. A heat pump is a interesting unit. it acts like a AC in the summer than reverses itself and acts like a heater in the winter. It reverses the Freon in the unit and uses the heat from the Freon to heat the RV. There isn't no gas hooked up to a heat pump they are all electric. It has a closed system like a residential refrigerator with a compressor and motor inside. Your thermostat may be bad or the circuit board in the unit may be bad. Very hard to say. Something isn't telling something to come on.
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Old 10-17-2018, 07:28 AM   #6
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We have a heat pump in our unit. It is separate from the furnace, as it is part of the a/c unit. We have used it a few times, and it seems to work well. Of course, as everyone knows, air source heat pumps like this are only efficient to a certain temperature.


We have had no issues at all with it. I would suggest a mobile RV tech.
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Old 10-17-2018, 09:15 AM   #7
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Quote:
Originally Posted by Bierp View Post
Correct me if I'm wrong here, but when a heat pump stops being effective due to a low outside temp, it doesn't shut down, right? It keeps trying to pump heat, but it isn't effective. My unit was powered down. I think it's a different issue.
I don't know if they shut off when they are ineffective. I do know when I tested mine a couple days ago when the temp was around 20°F, they wouldn't turn start at all. A little later when it got around 35°F, they ran fine.
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Old 10-17-2018, 01:06 PM   #8
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question. I do not know it the answer will help, but it can't hurt. When you are running the heatpump. is the fan on auto?
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Old 10-17-2018, 02:11 PM   #9
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Originally Posted by jeffba View Post
question. I do not know it the answer will help, but it can't hurt. When you are running the heatpump. is the fan on auto?
Yes. .
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Old 10-17-2018, 03:32 PM   #10
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Okay try running the fan on either high or low.
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Old 10-17-2018, 07:27 PM   #11
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Quote:
Originally Posted by The Bone View Post
i am confused about your rig having both a heat pump and a furnace. I have installed heat pumps in homes and they work fine. Not sure why they would have both in a RV.

I think all the more recent Montana's all have both a heat pump (as a function of the primary AC) and a furnace.

Why both? Several reasons that come quickly to mind. The HP only outputs about 15k BTU of heat, which isn't really enough to keep a 40' 5th wheel warm in winter. (Furnace puts out 50k BTU.) Also, it seems, that the HP stops being effective as it gets colder outside, which would be an issue.

But probably the biggest reason is that you can't run the AC (and therefore the HP) on the 12v system. Or, you can't do that 'out of the box'. With enough batteries and a powerful inverter perhaps or a generator, but those aren't standard items from the factory. In order to run it you need shore power. This would make for some very cold boondockers.
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Old 10-17-2018, 07:30 PM   #12
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Originally Posted by jetskier View Post
I don't know if they shut off when they are ineffective. I do know when I tested mine a couple days ago when the temp was around 20°F, they wouldn't turn start at all. A little later when it got around 35°F, they ran fine.
It didn't start at all when it was cold? That's interesting. I'd love to hear if others are getting the same behavior.

That being said, it hasn't dropped below 45°F here yet and that's the range where I expect this thing to get the heaviest use.

Quote:
Originally Posted by jeffba View Post
question. I do not know it the answer will help, but it can't hurt. When you are running the heatpump. is the fan on auto?
I've gone with both HIGH and AUTO on the fan, but I haven't found a consistent pattern yet.
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Old 10-17-2018, 08:02 PM   #13
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It didn't start at all when it was cold? That's interesting.

I'd imagine there's a thermistor on the unit that it uses to detect freeze ups on AC mode. That same thermistor probably says no-go for the heat pump too. That's just a guess. I vaguely remember a copper tube and a temp probe on the front of the unit when I had the covers off.
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Old 10-21-2018, 03:56 PM   #14
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My heat pump also has a low fan or humming when it initially comes on but then the fan kicks up to full speed and works effectively.

The reason for the dual system is to provide heating opinions. Below 40 degrees LP is more effective, where above the hp can maintain proper temps..A lot of home have dual systems as well.
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Old 10-21-2018, 05:26 PM   #15
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My heat pump doesn't run after it gets cold or after sun down. We run the hp during the day to use less propane then switch to the furnace for the night.
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Old 10-21-2018, 07:52 PM   #16
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If I remember correctly, the HEAT pumps in our rigs are really a heat strip located in the primary AC unit. The owners manual does say the the heat pump will not work at anything less than 40 degrees. And we all know how accurate our thermostats are.
It sounds like your heat pump is working just fine.
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Old 10-21-2018, 07:58 PM   #17
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My heat pump brought the 5er to 74 a few minutes ago and shut down normally. I switched over to furnace and have the fireplace on. Outside temp is 34. I usually switch the 1st time it reaches temp and quits when the outside temp hits 36. Love the HP as the furnace is affectionately called our Propane Pig.
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Old 10-21-2018, 09:16 PM   #18
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Two things. First our heat pump works well right down to 32 and it is a real heat pump not heat strips. Second, the heat pump does nothing to protect your pipes and tanks. Only the furnace can do that, so I've never tried to run the heat pump at lower temps than that.
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Old 10-22-2018, 05:23 AM   #19
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I have had very good experiences with the telephone support from different manufacturers. Try calling them to see if they can give you any insights on the problem. Make sure you have all your model number and owners manual when you call, as well as lots of data on inside and outside temps when the problem occurred.
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Old 10-22-2018, 05:41 AM   #20
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I have a heat pump on front unit also, my question can you run the furnace of the bedroom stat. Just thought about this cant try trailer has been stored. I do no the heat pump warms the front nice but back bedroom is chilly. So I would think it should work
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