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Old 05-22-2005, 02:41 PM   #1
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Heat pump switched on in AC mode

We were running the AC last week as a line of thunderstorms approached. We could see lightning and hear thunder in the area. Suddenly the AC switched to heat pump although the thermostat (Comfort Control Center) was still displaying "air cond".

It was obvious that it would have run forever in that mode since the thermostat was constantly calling for AC. That scares me since we leave the cats in the trailer with AC running and the Fantastic Fans set to start up if the AC fails. The fans probably could not have kept up with the heat pump and the Texas sun.

I suspect that a line transient caused the heat pump valve to switch on and I suspect that it would stay on as long as the AC kept running. I am currently downloading wiring diagrams to see if I can install a toggle switch that would break (ground?) the line that switches the AC to heat pump mode so that the AC could not get into heat pump mode in the summer.

Has anyone else ever had this happen?
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Old 05-22-2005, 03:12 PM   #2
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Should have mentioned that I switched the Comfort Control Center off, wated a minute, and turned it on again. After the usual delay, the AC came back on correctly and worked properly the rest of the rally.
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Old 05-22-2005, 03:19 PM   #3
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Didn't you have a problem with another ac unit that you had to replace because the valve was stuck on heat? Or was that someone else.
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Old 05-22-2005, 04:53 PM   #4
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Quote:
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Pahaka
Didn't you have a problem with another ac unit that you had to replace because the valve was stuck on heat? Or was that someone else.
Yes, that was me. In that case, though, the reversal valve had failed in the AC unit. Don't know whether there might be a connection. I watched the tech cut the wire to the reversal valve and it was still stuck in heat pump mode. When they replaced the AC, the electronic unit was replaced, also.

According to schematic I just downloaded, there is a dedicated wire for the heat pump valve in the four-wire telephone cable that goes into an electronic unit in the AC. I may be able to interrupt that wire with a switch for some peace of mind.
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Old 05-22-2005, 04:59 PM   #5
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Quote:
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I watched the tech cut the wire to the reversal valve and it was still stuck in heat pump mode. When they replaced the AC, the electronic unit was replaced, also.
Can you tell from the schematic if the valve has a default position or if it flip flops with a signal. With it failing in heat both times I wonder.

John
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Old 05-22-2005, 05:29 PM   #6
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Can you tell from the schematic if the valve has a default position or if it flip flops with a signal. With it failing in heat both times I wonder.

John
It has a single wire and appears to default to AC. I want to talk to the tech at the local dealer tomorrow with schematic in hand before I go any further. I may call Dometic tomorrow, also.
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Old 05-22-2005, 05:43 PM   #7
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I would talk to them also if AC is default and it keeps going to heat. Pretty strange, but it sounds like your switch will work if that is the case.

John
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Old 05-22-2005, 05:46 PM   #8
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Correction!

Quote:
Originally Posted by Pahaska
According to schematic I just downloaded, there is a dedicated wire for the heat pump valve in the four-wire telephone cable that goes into an electronic unit in the AC. I may be able to interrupt that wire with a switch for some peace of mind.
Wrong! On further reading of the pdf I downloaded, there could not be a dedicated wire for the heat pump solenoid between the CCC and the roof unit. There are only four wires in the connecting cable that is able to control two daisy-chained roof ACs in two different zones. That tells me that the commands must be coded on those 4 wires, maybe by coded pulses.

One thing just occurred to me. I had just adjusted the temperature downward and I may have erroniously hit the zone button which is supposed to be non-functional in a single AC installation. I wonder whether that possibly have caused a timing glitch in a coded command.

Now, I'm sure I will call Dometic in the morning.
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Old 05-22-2005, 06:14 PM   #9
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What is the url for the pdf, I'd like to look.

John
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Old 05-22-2005, 07:42 PM   #10
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pdf

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Originally Posted by 74Argosy24MH
What is the url for the pdf, I'd like to look.

John
I found it here. The info is for a 4-button CCC, but the hookup to the AC is the same. The 5th button is for a second zone, but is supposed to be inactive due to the DIP switch settings in the AC electronics.

Don't know why A/S uses the 5-button since they ordinarily have no second ACs.
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Old 05-22-2005, 08:11 PM   #11
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Reverse valve

I just found on a second pdf that the reverse valve is actuated in cooling mode. That makes the cheese a little more binding since the wire to the valve can not be simply opened by a switch.

I may have to rig up a second thermostat that would kill the AC if it goes hot again.
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Old 05-22-2005, 09:09 PM   #12
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When I was troubleshooting my ac I remember that the reverse valve is in the cool mode by default. The control board must actuate the valve for heating. You may have a control board problem. My trouble was that the thermistor in the evaporator fins was turning off the compressor as it was sending a signal that the evaporator coil was to cold and went into a defrost mode / turn off compressor. I may have had a faulty thermistor so I just removed it from the evaporator fins and let it dangle, the ac works fine now.
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