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Duotherm Thermostat Furnace Mode Grief

Yahoo Message Number: 6778
Hi Folks,

Please bear with me through this lengthy description of a problem that comes and goes. I'm hopeful that this great group has some ideas on what the source of this elusive problem might be.
Last April/May, on a cross country trip from Florida, we also went to JC for some service items on our '03 Magna. There, our Duotherm furnace mode problem was NOT fixed despite a lot of diagnostic work and attempts to fix it following technical guidance CC service received from Dometic tech support. We had gone to two CC dealers along the way before JC, and to a Dometic RV service shop after JC, all to no avail.

Briefly, the typical problem behavior is that although the furnace and zone 1 & 2 heat exchangers start up and work correctly after setting the thermostat in the evening before going to bed; hours later, when it gets colder outside, the thermostat stops calling for heat, and we wake up, freezing cold. At this point, the thermostat display is usually blank except for the "zone 1/2" at the bottom of the screen. Then, depressing the mode button repeatedly won't make it display "furnace" mode and doesn't work for any mode. Next, I do a system reset on the thermostat. The thermostat responds showing "FF", then "EE" in the display. I understand that the "EE" result means "communications" error. The thermostat is non-working in that state. However, the problem goes away "by itself" when the outside temp warms up. That is, when it's warmed up outside, I repeat the system reset and then the reset is successful --- furnace mode is available again. After setting the desired temperatures for Zones 1 and 2 the whole system works correctly again.

Soon after the first occurance of this problem, I found that by disconnecting the communications wiring (standard 4 wire telephone wiring) to the 2nd a/c unit, then furnace mode would work correctly for zone #1 (the communications wiring is daisy chain connected from thermostat to a/c #1 to a/c #2). Immediately reconnecting the wiring to a/c #2 would again show that the problem was still in the system, and had not just "gone away".

Dometic tech support advised CC service on what may cause the communications error. After eliminating power as an issue, Dometic said that it must be an RF issue. Upon Dometic's direction, CC ran shielded communications cables to replace the original unshielded communications wiring, but that didn't help. We still get the same behavior when we have cold outside temperatures. Note that over the summer, a/c and fan control always worked without showing any problems like this. Any ideas or suggestions would really be appreciated --- we are headed to Chicago for Thanksgiving.

Thanks for listening,

Paul Thomas

'03 Magna #6239, Mfr'd 10/02

Re: Duotherm Thermostat Furnace Mode Grief

Reply #1 –
Yahoo Message Number: 6779
Paul,

In my 2000 Magna, there are 2 heat sources, one is the "furnace" which is a Webasto diesel heater, the other is the set of a/c's that have electrical heating elements as well (forgot what it's called on the little control unit "heat pump"?).
What you describe (the heater going off when the temperature drops below some level) is what my a/c heaters do. If I use those during the night, and it gets colder, they start blowing unheated outside air.
I understand that this is by design, and one has to live with it. I suspect that when it gets too cold, those elements can't keep the coach heated even when they're on all the time, thus they shut off.

But since you're talking about the furnace, I presume you're not referring to the a/c heater elements? Plus, one would suspect that all the CC engineers know about this and would have told you about it.
Maybe some wiring issue causes the sensor for the a/c heater function to screw up the furnace in your coach. I don't believe that could happen in mine, as each a/c heater element has its own sensor, and shuts off locally (thus one can go off a bit sooner than the other).
Since coaches change from year to year, maybe today there is only one sensor, shutting the elements off through the control unit, thus a wiring issue is possible.

Re: Duotherm Thermostat Furnace Mode Grief

Reply #2 –
Yahoo Message Number: 6788
Hank,

Thanks for your thoughts and time to answer. CC changed the Magna's furnace from Webasto to Hurricane sometime after the 2000 model year and before the '03 models. Like the Webasto furnace, it heats engine coolant and circulates it to the heat exchangers. We have 2 heat exchangers for each of the two control zones. I'm guessing here, but I think control signals initiated by the thermostat "ask" the appropriate set of heat exchangers to provide heat for a zone. If the coolant is hot enough already, they simply turn on a fan in the heat exchanger. If the coolant is not hot enough, the heat exchanger control will fire up the Hurricane furnace.
I've used the heat pump function a little, but prefer to use the furnace.
When I have the problem trying to use furnace mode, nothing works, the thermostat must have a micro-processor that is just hung-up at that point.

Paul

Re: Duotherm Thermostat Furnace Mode Grief

Reply #3 –
Yahoo Message Number: 6790
I don't have a clue what is causing your problem. Their are many unhappy campers with Huricane furnaces tho'. My advice is to keep CCs feet to the fire and not let this slide until it is out of waranty. I would replace the parts one at a time 'til you find the bad one. jerry in NM, '00 Magna.
 

Re: Duotherm Thermostat Furnace Mode Grief

Reply #4 –
Yahoo Message Number: 6818
Quote from: paulkthomas"
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Paul,

From the description of the problem that you have given, I would suspect a cold solder joint or a bad wire connection that is temperature sensitive to the outside temperature. I do not know anything about your system, but I would be looking for something that is exposed to the outside air temperature that can cause something to contract that is associated with the thermostat control on or near the furnace.

Dave G.

2000 Allure, 30491
Millheim, PA