Dometic roof A/C: compressor, no fan

[Jim McGarvie]

Our front roof A/C is a Dometic 600315.321. Intermittently (don't we all hate that word?) the compressor will come on without the fan. I can switch the fan from Auto to On on the thermostat and solve the problem temporarily. Or I can turn off the circuit breaker for the front A/C or remove the thermostat from the wall plate, either of which, of course, stops the compressor and the fan. Sometimes when it is acting up I will remove the thermostat for a while, and when I plug it back in the A/C comes on normally.

The thermostat is a residential programmable Honeywell. When this same issue occurred last month my troubleshooting led me to believe the problem was in the thermostat, so I replaced it with a new one of the identical model. That seemed to have solved the problem for a while, then yesterday it returned to its previous behavior.

Any and all suggestions welcome.

Thanks.

Jim

[vito.a]

I have not seen anyone successfully use a residential thermostat on a Dometic RV A/C.  I'm just saying I've not seen one work.

The Dometic thermostats and A/C units use 12v DC signals sent through an RJ11 phone cord to control the A/C.  There are even three different versions of the Dometic A/C control input/output.  

Early 4-button thermostat through about 2002 (these thermostats were very unreliable)

5 button Comfort Control thermostat from 2003 until a few years ago.  

The current CCC2 (Comfort Control Center 2) programmable thermostats.  

The Dometic thermostat must be the same generation as the A/C system, or use a conversion control board.   

I would first call Dometic with your A/C model and serial number and determine what version it is.  Then install the appropriate thermostat.  You will need to program the new thermostat to the A/C.  Switch the bottom thermostat black slide switch off.  Depress both the top and bottom program buttons simultaneously and slide the on/off switch on.  You will see FF in the display.  

Your other alternative is to replace both the A/C and the thermostat with new ones.  

Best of luck!

Edited September 9, 2021 by vito.a

[Ivan K]

I suspect that John's version is the one using individual wire connections instead of the phone cable. I have not seen it since our Class C with a single unit. Never had a problem with it that I remember so no real experience to be of any help. Maybe some intermittent connection up top?

[Gary 05 AMB DST]

Aren't residential thermostats 24v?

Gary 05 AMB DST

[Ivan K]

5 minutes ago, Gary 05 AMB DST said:

Aren't residential thermostats 24v?

Gary 05 AMB DST

I think they make them run off backup batteries,  Jim would probably know best.

[Jim McGarvie]

 

18 hours ago, vito.a said:

I have not seen anyone successfully use a residential thermostat on a Dometic RV A/C.  I'm just saying I've not seen one work.

The Dometic thermostats and A/C units use 12v DC signals sent through an RJ11 phone cord to control the A/C.  There are even three different versions of the Dometic A/C control input/output.  

Early 4-button thermostat through about 2002 (these thermostats were very unreliable)

5 button Comfort Control thermostat from 2003 until a few years ago.  

The current CCC2 (Comfort Control Center 2) programmable thermostats.  

The Dometic thermostat must be the same generation as the A/C system, or use a conversion control board.   

I would first call Dometic with your A/C model and serial number and determine what version it is.  Then install the appropriate thermostat.  You will need to program the new thermostat to the A/C.  Switch the bottom thermostat black slide switch off.  Depress both the top and bottom program buttons simultaneously and slide the on/off switch on.  You will see FF in the display.  

Your other alternative is to replace both the A/C and the thermostat with new ones.  

Best of luck!

Thanks for the reply, Vito, but our 2002 (made in 2001) coach never had any buttons. It was a really old fashioned thermostat, just a couple of switches and a rheostat. Our residential thermostat worked perfectly, for the furnace and for the A/C, for two years.

Edited September 10, 2021 by Jim McGarvie
Deleted excess quotes

[Jim McGarvie]

15 hours ago, Ivan K said:

I think they make them run off backup batteries,  Jim would probably know best.

Thanks for your confidence in me, but I'm not sure I know how it works. I think the batteries in the thermostat are primarily for display and memory. The thermostat it is replacing is really only a switch, right?

[Guest Ray Davis]

It's non programable but this could probably be made to work.    https://www.homedepot.com/p/Economy-Non-Programmable-Thermostat-with-1H-1C-Single-Stage-Heating-and-Cooling-CT31A/100400017

[Jim McGarvie]

12 hours ago, Ray Davis said:

It's non programable but this could probably be made to work.    https://www.homedepot.com/p/Economy-Non-Programmable-Thermostat-with-1H-1C-Single-Stage-Heating-and-Cooling-CT31A/100400017

Thanks for the input, Ray. But I guess since you are recommending I replace my thermostat you must be assuming there is something wrong with our current one. Can I ask what makes you think that? Because I really don't want to lose the programming capability if I can help it. I don't use it for cooling, but for heating it follows a schedule keeping the interior at 72 degrees until 9 pm, then letting it drop to 55 (which it very seldom does) until about 1/2 hour before I usually get up in the morning, then warming it back up to 72 by the time I get up.

I don't have a reason to believe there is some reason a programmable residential thermostat shouldn't work properly with our particular A/C, primarily because it worked perfectly for two years.

What am I missing?

[Dr4Film]

Jim,

Here is a link to your specific AC, its installation and wiring diagrams. Possibly this may help you.

https://www.manualslib.com/manual/799601/Dometic-57908-321.html#product-600315.321

I along with others are somewhat confused as to how your thermostat is wired seeing that normally these AC's are controlled with a 5 Button Thermostat.

Maybe a photo or two would help clear up the confusion.

Edited September 11, 2021 by Dr4Film

[Guest Ray Davis]

11 hours ago, Jim McGarvie said:

Thanks for the input, Ray. But I guess since you are recommending I replace my thermostat you must be assuming there is something wrong with our current one. Can I ask what makes you think that? Because I really don't want to lose the programming capability if I can help it. I don't use it for cooling, but for heating it follows a schedule keeping the interior at 72 degrees until 9 pm, then letting it drop to 55 (which it very seldom does) until about 1/2 hour before I usually get up in the morning, then warming it back up to 72 by the time I get up.

I don't have a reason to believe there is some reason a programmable residential thermostat shouldn't work properly with our particular A/C, primarily because it worked perfectly for two years.

What am I missing?

Jim,  Sorry if I seemed to imply you needed a new thermostat,  I wasn't meaning to.  However the problem you have could be the thermostat,  or the control board, or in the wiring/connections to those devices.

I was actually addressing the discussion about what residential thermostat could control our Dometic A/Cs.   

The one I pictured is about as simple as they come but is not programable.

I would also like to know the thermostat you have and how it is wired into the A/C.

You mention that you don't use for cooling, will it control cooling?

[Dr4Film]

4 minutes ago, Ray Davis said:

You mention that you don't use for cooling, will it control cooling?

Ray, I took his statement that he doesn't use the "programmable function" to control the cooling, only to control the heat but AFAIK he is controlling the AC & Heat with his current thermostat.

[Guest Ray Davis]

13 hours ago, Jim McGarvie said:

Because I really don't want to lose the programming capability if I can help it. I don't use it for cooling, but for heating it follows a schedule keeping the interior at 72 degrees until 9 pm, then letting it drop to 55 (which it very seldom does) until about 1/2 hour before I usually get up in the morning, then warming it back up to 72 by the time I get up.

Richard,  Apparently he is using the programing feature,  at least for heat.   I still wonder what thermostat he is using.

[Jim McGarvie]

8 hours ago, Ray Davis said:

Richard,  Apparently he is using the programing feature,  at least for heat.   I still wonder what thermostat he is using.

The thermostat is a Honeywell RTH7600D. I use it for both heating and cooling, but I only use the programming feature for heating. I'm sure it would work fine for cooling as well, just haven't found the need.

[Jim McGarvie]

12 hours ago, Dr4Film said:

Jim,

Here is a link to your specific AC, its installation and wiring diagrams. Possibly this may help you.

https://www.manualslib.com/manual/799601/Dometic-57908-321.html#product-600315.321

I along with others are somewhat confused as to how your thermostat is wired seeing that normally these AC's are controlled with a 5 Button Thermostat.

Maybe a photo or two would help clear up the confusion.

Thanks for the link, Richard. That should be helpful.

I can't speak for "normally", but I can tell you unequivocally that when we picked up this 2002 coach at the Monaco factory in August of 2001, it had a thermostat with no buttons whatsoever. I threw it away when it died a couple of years ago so I can't send a picture of it, but I have attached one of the bedroom A/C thermostat. The original front thermostat was identical but for a switch for heat for the furnace. Nary a button in sight.

12 hours ago, Dr4Film said:

I along with others are somewhat confused as to how your thermostat is wired seeing that normally these AC's are controlled with a 5 Button Thermostat.

Maybe a photo or two would help clear up the confusion.

For those of you who have asked how I wired this thermostat, I have attached a diagram I found somewhere online. I think it is pretty clear, and worked fine. The only way in which I varied from this is I did not add a switch for hi fan. I just wired it to always use hi fan.

[hitechpete]

It’s quite easy to use a domestic thermostat to control the furnace, when we suffered from a problem that wouldn’t allow our CCC2 to see the zone with our furnace (water in RJ11), I used a simple room thermostat with isolated contacts to turn the furnace on. The furnace is switched on by simply connecting the 2 blue wires that run from the A/C unit PCB together, this is easily accomplished at the furnace end.

[Dr4Film]

23 minutes ago, hitechpete said:

It’s quite easy to use a domestic thermostat to control the furnace, when we suffered from a problem that wouldn’t allow our CCC2 to see the zone with our furnace (water in RJ11), I used a simple room thermostat with isolated contacts to turn the furnace on. The furnace is switched on by simply connecting the 2 blue wires that run from the A/C unit PCB together, this is easily accomplished at the furnace end.

Peter,

Controlling a heater or an Aqua-Hot Heat Exchanger is not the concern or confusion posted in this thread. In fact I do that myself with three different thermostats on my Aqua-Hot for the three different zones.

What is confusing is controlling the AC's with a residential thermostat. It appears that Jim is possible an isolated case where he is using one to control his AC's and we ALL know that the Dometic Penguin AC's come with four wire RJ-11 connectors to connect the AC's to the normal Dometic thermostats.

We still don't know how those four wires are connected to his thermostat unless they are using some sort of adapter that goes from a four wire RJ-11 plug to residential wiring. There would have to be two wires for the 12 VDC + & - and two more wires to turn the AC's on/off.

12 hours ago, Jim McGarvie said:

The thermostat is a Honeywell RTH7600D. I use it for both heating and cooling, but I only use the programming feature for heating. I'm sure it would work fine for cooling as well, just haven't found the need.

Jim,

Here is the manual for the Honeywell RTH7600D. Possibly it may help resolve your problem.

https://honeywellstore.com/store/images/pdf/rth7600d-7-day-programmable-touch-screen-thermostat-installation-manual.pdf

[Jim McGarvie]

9 hours ago, Dr4Film said:

What is confusing is controlling the AC's with a residential thermostat. It appears that Jim is possible an isolated case where he is using one to control his AC's and we ALL know that the Dometic Penguin AC's come with four wire RJ-11 connectors to connect the AC's to the normal Dometic thermostats.

We still don't know how those four wires are connected to his thermostat unless they are using some sort of adapter that goes from a four wire RJ-11 plug to residential wiring. There would have to be two wires for the 12 VDC + & - and two more wires to turn the AC's on/off.

Jim,

Here is the manual for the Honeywell RTH7600D. Possibly it may help resolve your problem.

https://honeywellstore.com/store/images/pdf/rth7600d-7-day-programmable-touch-screen-thermostat-installation-manual.pdf

Thanks, Richard. I already have that manual.

You may all know that the Dometic Penguin AC's come with four wire RJ-11 connectors to connect the AC's to the normal Dometic thermostats, but our Dometic AC doesn't know that. It did NOT come with four-wire RJ-11 connectors, but with a cable containing six conductors. There is no adapter.

I know you guys are all very knowledgeable and experienced. The only thing I can think of to explain this apparent contradiction is that maybe our AC is NOT a 600315.321. I was relying upon the parts list that came with the coach from the factory. Maybe it is some other Dometic DuoTherm?

I have attached a picture showing the baseplate of the Honeywell thermostat showing the wiring connections. Only four of the six wires are used; the other two are taped and stuffed back into the wall.

[Dr4Film]

Jim,

My 2002 Monaco delivered in 2001 came with two Dometic Peguin Low Profile AC's with a 5 Button Thermostat. The coach was wired with 4 wire DATA cables and RJ-11 plugs, adapters and receptacles.

According to the 2002 Monaco Knight Brochure your 30 PBD came with only one 13.5 BTU AC. So do you only have one AC and one thermostat?

Without actually seeing a photo of the AC there is no telling what type of AC your coach has and how it is all wired together. Based on the what is shown above on the baseplate of your thermostat, if in fact your AC is a Dometic then the factory did some custom wiring job to make it work.

https://www.monacocoach.com/resources/media/user/1469212215_brc_pdf.pdf

Edited September 12, 2021 by Dr4Film

[Jim McGarvie]

11 minutes ago, Dr4Film said:

Jim,

My 2002 Monaco delivered in 2001 came with two Dometic Peguin Low Profile AC's with a 5 Button Thermostat. The coach was wired with 4 wire DATA cables and RJ-11 plugs, adapters and receptacles.

According to the 2002 Monaco Knight Brochure your 30 PBD came with only one 13.5 BTU AC. So do you only have one AC and one thermostat?

Without actually seeing a photo of the AC there is no telling what type of AC your coach has and how it is all wired together. Based on the what is shown above on the baseplate of your thermostat, if in fact your AC is a Dometic then the factory did some custom wiring job to make it work.

https://www.monacocoach.com/resources/media/user/1469212215_brc_pdf.pdf

This is getting curiouser and curiouser. Our Knight came with two Duo-Therm ACs: a 13.5 in the front and a 600312.32 (11.0 BTU maybe?) in the back. That may have been an option. Each has its own thermostat, and the front thermostat also controls the furnace.

While this discussion is interesting, in my uneducated opinion it is moot. I am pretty sure that, while it is possible the Honeywell might have an issue, my choice of that thermostat is not the problem since it worked fine for so long. All indications seem to point toward a stuck relay or a faulty control board in the AC.

[Dr4Film]

Jim,

You are correct, after looking at the brochure in detail it was an option to have the second smaller AC in the rear on it's own thermostat.

I would take you own advice and start digging into the rooftop AC components to see which one has failed that is causing your problem. Hopefully you may be lucky and find parts for such an old unit or you will have to upgrade to a new AC.

Good luck!

[Jim McGarvie]

33 minutes ago, Dr4Film said:

Jim,

You are correct, after looking at the brochure in detail it was an option to have the second smaller AC in the rear on it's own thermostat.

I would take you own advice and start digging into the rooftop AC components to see which one has failed that is causing your problem. Hopefully you may be lucky and find parts for such an old unit or you will have to upgrade to a new AC.

Good luck!

Thanks Richard. I will keep my fingers crossed. I was able to find all the replaceable parts to rebuild our equally-old Atwood furnace, so maybe I'll get lucky. If not, it probably wouldn't hurt (except the pocketbook) to replace the 20-year-old AC.