Atwood ducted heat pump fan cycling

[Xgary]

I have a model AC-1511 (15033) 15KBTU ducted heat pump with remote thermostat.  When there is no heat (or cooling I presume) demand, the circulating fan runs for about 5 seconds every minute.  Other than that it seems to be working okay. I contacted Atwood (Dometic) they told me is not operating properly and I should take it to a service center to be checked.  Unfortunately even though it is less than a year old there is no warranty so does anybody have any suggestions on how I can determine where the fault might be?

[Kevin Means]

Hi Xgary, and welcome to the RVForum. That is odd behavior, but I'd suspect a problem with the thermostat, because that's the component that tells the AC/heat pump what to do. You might try disconnecting the thermostat's wiring harness, see if the problem continues, then reconnecting it.

Kev

[Gary RV_Wizard]

I know this is going to be confusing, but my guess is the opposite of Kevin's. The thermostat simply signals the a/c "cooling [or heating] needed". It doesn't do anything else. If the a/c started up and ran a normal (cooling or heating) cycle, I would say its the thermostat, but if only the fan runs, and just briefly, I would suspect either the zone controller (if a multi heat/cool zone system) or the heat pumps own controller board.

if you have two (or more) heat pumps, you might try swapping the zone control and unit control board with the other unit (assuming it does not misbehave) and see if the problem moves with the electronics. Beyond that, you may need skilled, professional a/c tech help (probably NOT the usual guy at the rv dealer shop).

Maybe there are some clues in this Atwood service booklet:
http://www.bdub.net/manuals/AtwoodRoofACRepair.pdf

[Xgary]

Kev-
Since the stat has a fan only mode your suggestion makes sense, I'll check into that.

Gary-
I didn't spell it out properly, the unit runs normally under heat demand (I haven't tried cooling).  The fan cycles normally for both furnace and heat pump operation.  It's only when there is no demand that it runs every 60 seconds. When the coach cools below the setpoint both the fan and compressor run for a few minutes until the space warms up then shut off.  I appreciate the link to the service manual very much and am now confident that I can track it down.

Thanks to both for the great information.

[Gary RV_Wizard]

I got that that it works ok in normal conditions and the problem is extra fan cycles when they should not occur. It could be the t-stat giving a fan-only signal, but regular and repeated fan on/off at 60 second intervals sounds a bit beyond what failing thermostat might do.

The a/c control board has all the possibilities of the thermostat (afterall, it is what the thermostat talks to), plus some more of its own.  In addition to doing fan-only mode, it can cycle the fan without running the compressor if the internal compressor freeze-up sensor trips, and it also does it as a part of normal time delay after any heat or cool cycle.

[Xgary]

That certainly makes sense too.

[Charlie 5320]

If it is a digital stat, and it should be for a heatpump, it could be the stat. They normally energize the g and w terminal electronically.  Pull the wire off the g terminal and see if it still does it. Does this same stat work the furnace too?