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Old 12-09-2003, 07:20 AM   #1
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Heat Pump and Furnace Operation

I was just reading elsewhere in the forum that the heat pump and furnace are "linked" via the Dometic thermostat - i.e. - when the temperature falls below a certain point with the heat pump on, it will automatically switch to the furnace. I have the heat pump option on my 2000 Excella. Is the "linking" new to the system or should my unit function the same way?
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Old 12-09-2003, 07:23 AM   #2
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If you have the Duo-Therm digital (LCD) wall thermostat that controls both furnace and heat pump, it should switch from heat pump to furnace when the outside temperature drops below 40 degrees and switch back to heat pump when it rises above it.
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Old 12-09-2003, 07:34 AM   #3
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Switchover level

I was told that the switchover is at 34 degrees. This seems to be true since we had 2 nights just above freezing this last weekend and the switchover never occurred. Had the switchover been at 40 degrees, we would have been on furnace much of the night.

Unfortunately, my heat pump failed at about 5 am the second night and blew the circuit breaker. From the noise that woke me, I suspect that a compressor failure occurred.

Prior to that, I had used the heat pump on several nights at near freezing on a previous trip without a switchover occurring.
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Old 12-09-2003, 08:03 AM   #4
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From the Duo-Therm Service Manual:

The roof-top heat pump was designed to operate in a MILD
GEOGRAPHICAL AREA for heating where the heat loss is
minimal. The heat pump was designed to operate down to
an outside ambient temperature of 40F. At 40F., the
outdoor thermostat will turn off the heat pump circuit and
start up the coach's main furnace. As long as the temperature
remains below 40 degrees, the main furnace will heat
your home. As the outside temperature increases to 45F.,
the outdoor thermostat switches back to the heat pump
circuit.
NOTE NOTE: Model 59126.501 does not have an outdoor thermostat.
When outdoor temperature is at or below 40F.,
use the main furnace to heat your home.

The advantages of the heat pump are:
1. User friendly; only one unit for both heating and
cooling;
2. Heat pump heating uses campsite electrical hookup
and saves on trips to refill L.P. tanks;
3. When used in mild climates where the outdoor
temperature range is 40 degrees or higher, an LP
furnace is not needed.
NOTE: Geographical location usage should be determined
before omitting a central furnace.
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Old 12-09-2003, 06:39 PM   #5
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Automatic or not?

OK - so I went though everything in the Owner's manual - and read every pamphlet in my Airstream briefcase - and there's nothing that describes the operation of the digital thermostat! The only fact I could come up with is that my A/C (Heat Pump) does not bear the serial number Maurice listed for those units without the outside thermostat. So I assume it probably works as described. Can anyone verify that the 2000 Excella has the automatic switchover?
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Old 12-09-2003, 07:24 PM   #6
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Our manual says that the heat pump will reverse the freon flow when the outside temperature falls into the twenties so it can defrost the outside coils and then reverse back to the heat pump function after a few minutes. The manual says nothing about turning on the furnace while the Duo-Therm is in the heat pump mode. That is for the 2002 model.
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Old 12-10-2003, 08:17 AM   #7
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Re: Automatic or not?

Quote:
Originally posted by Cracker
OK - so I went though everything in the Owner's manual - and read every pamphlet in my Airstream briefcase - and there's nothing that describes the operation of the digital thermostat!
Apparently Airstream doesn't supply anything more than the card which hangs around the thermostat. I called Dometic yesterday and they better explained the operation of the unit and they will be sending me a manual on it. I should get it in the next couple of days so we will see what kind of information it contains.

According to Dometic's tech support the heat pump will cut over to furnace once the outside air temp monitor detects 32 degrees. Supposedly the thermostat will display an "aux" indication when that occurs. At 38 degrees out side air temp the heat pump will then take over the operation.

Unfortunately this is for 2004 and I don't know what changes have been made since 2000. Quite honestly the only way to know for sure is to try it out.

My thermostat is already displaying some quirky tendencies that tech support can't explain. If I walk into a cold trailer at 32 degrees, set the heat pump on and the fan speed to automatic, nothing turns on. If I force the fan on to either low or high speed the compressor runs. If its 38 degrees and I walk into a cold trailer, turn on the heat pump and set the fan speed to automatic, the heat pump turns on. So maybe it will change over but it looks like if you start up cold and you are in that switch over zone, nothing happens.

In some cases its a moot point since the heat pump in my Classic doesn't have enough capacity to hold temperature once outside temps get down below 38 degrees much less bringing a cold trailer up to comfort level. I think the slide out adds too much volume. Its pumping 90 degree air but just not enough of it.

I don't know what you think but that big kitchern Corian counter top is a giant cold conductor. That thing never seems to get warm.

Jack
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Old 12-10-2003, 08:35 AM   #8
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heat/air conditioning question

Pardon my ignorance here. It sounds like these issues are only related to AS classics. Is that due to there being a link between the air conditioning / heat pump and the gas furnace on the Classic models. I have a 2004 Safari which appears to have separate controls for the AC/heat pump unit and the gas furnace. If this is ture, can you change this with a different type thermostate which could linking the units together?

Thanks,

Bob
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Old 12-10-2003, 08:38 AM   #9
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Yes the Safari's have individual controls. At build time you could have opted for the single thermostat. Silvertwinkie's requested that on his new Safari. After the fact would be an issue because your airconditioner has the controls mounted on its face plate.

Jack
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Old 12-10-2003, 09:00 AM   #10
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Heat Pump

I am wondering if my 69 Overlander has a heat pump or is it a heat strip? What is the difference?

Thanks,

Gloria Pace
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Old 12-10-2003, 09:49 AM   #11
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The air conditioner compresser runs when you have a heat pump. Normally it is thermostaticly controlled. The heat strip uses a heating element to warm the air. On my Safai I had a heat strip and it was all or nothing. When the trailer got warm I had to turn it off. There was no thermostat control of it.

Normally you can hear the extra noise from the heat pump running (sounds just like the air conditioner in the summer). The heat strip is quieter with the only noise being the fan running.

Jack
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Old 12-10-2003, 10:14 AM   #12
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Quote:
According to Dometic's tech support the heat pump will cut over to furnace once the outside air temp monitor detects 32 degrees. Supposedly the thermostat will display an "aux" indication when that occurs. At 38 degrees out side air temp the heat pump will then take over the operation.
This description fits what I have observed for my Classic which was built in April of this year. Maurice's info says the changeover is at 40 degrees, but my unit has definitely stayed in heat pump mode over a few nights use at just above 32 degrees. So far, it hasn't dipped below 32 degrees here to cause the changeover.

Since my AC compressor has apparently failed, I won't get to test it this weekend. I'll use my two Pelonus ceramic heaters to get us past the weekend and then take the trailer in to the dealer to get fixed.

So long as it lasted, the heat pump was great. Better than being at home since I have to burn my own propane at home and with the heat pump I'm spinning someone else's electric meter.
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Old 12-10-2003, 10:40 AM   #13
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I haven't checked it for myself. The info I have comes from the service manual techs use for installation and repair. It doesn't have the latest electronic control that's in our trailers, so things could've changed. It'll be interesting to see what Jack gets from them.
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Old 12-10-2003, 10:48 AM   #14
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Heat Pump and Furnace Operation

Thanks Jack,

I now know we have a heat strip.

Gloria & Don
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Old 12-10-2003, 10:58 AM   #15
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Readability

My only quarrel with the current electronic thermostat is that the LCD display is very hard to read without a lot of ambient light. The temperature is in nice, big, easy to read digits, but the mode is in tiny unreadable letters. In dim light, my eyes sure aren't what they used to be.

At first, I fished out the flashlight to better read the mode, but now that I have memorized the sequence, I can get into the right mode a lot more easily. I simply remember the order of the modes and that air cond and heat pump are two words and the rest of the modes are single words.

It is neat that there is a built-in delay so that one can step through without things coming on or turning off as you go.
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Old 12-10-2003, 11:36 AM   #16
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My big gripe with it is that it doesn't display the actual interior temperature in addition to the setting.
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Old 12-10-2003, 12:34 PM   #17
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Quote:
Originally posted by RoadKingMoe
My big gripe with it is that it doesn't display the actual interior temperature in addition to the setting.
I hadn't thought of that, but it would be very nice, especially if the numbers were nice and large like the temperature setting.
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Old 12-10-2003, 12:45 PM   #18
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Our '99 Excella's manuals say that the heat pump will cut over at 40F. But I haven't been in it at anywhere near that temp yet to see what actually happens.
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Old 12-10-2003, 01:21 PM   #19
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The Mystery Continues!

I just got an E-mail back from Airstream's Customer Service Representative in answer to my question. Airstream says that the heat pump on my 2000 Excella will turn off at 24 degrees and switch to the furnace ---- then at 34 degrees it will change back to the heat pump. Not that it really matters a whole lot - but us bedazzeled engineering types really puzzle over different "factoids" like we've encountered so far in this string! The telephone number for Dometic, to request a manual, is:
1-800-544-4881.
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Old 12-10-2003, 02:46 PM   #20
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Re: The Mystery Continues!

Quote:
Originally posted by Cracker
I just got an E-mail back from Airstream's Customer Service Representative in answer to my question. Airstream says that the heat pump on my 2000 Excella will turn off at 24 degrees and switch to the furnace ---- then at 34 degrees
I love this forum. I bet we have generated a dozen calls or emails to them and Dometic on this exact same topic. I got the same answer as above in an email response from Airstream. Obviously Airstream's tech support on the cutoff temperatures differs greatly from the manufacturer of the equipment, which also differs from what my dealer advised. I guess we will see what the manual says and go from there.

I'm putting an indoor/outdoor wireless thermometer in the Classic to watch temperatures. First because it has a large display and is easily read. Secondly its because of the positioning of the thermostat. Unfortunately the wall they mounted the thermostat on is one side of the compartment that envelops the refrigerator. I remember this with the Safari that when the fridge was in use the actual temperature reading displayed on my Hunter digital thermostat was about 2 degrees higher than the inside of the trailer. Its just about impossible to keep some of that ambient heat within the fridge compartment from affecting the thermostat operation. Normally I had to make my heat set point 2 degrees warmer that what would have been expected to keep the inside comfortable. It wasn't a big deal but it was somewhat disconcerning in the summer to see the inside temperature display higher that it actually was in the trailer. I wonder if that is why they don't display the actual temperature on this unit?

Jack
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