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Old 06-11-2019, 11:59 PM   #1
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Dometic T-stat Modifications?

I've had an intense dislike of how the Dometic t-stat operates the A/C system, changing fan speeds, supposedly due to the temperature delta. It's irritating to have the fan start, the compressor start, the compressor stop, the fan stop, the fan restart at a different speed, the compressor come on, and then repeat the process seemingly at random. Another irritant is having the system run for approximately a minute and shut off for maybe two minutes, and repeat ad nauseum at various times throughout the night.

I've been told there is nothing wrong with my system; this is how it is designed. Frankly, that's nothing more than an indication of a poorly designed system, seemingly set up to simply irritate the hell out of anyone having to put up with it.

I could simply set the fan speed to low or high, but that is a waste of money.

It would be an amazing thing if the A/C worked like a typical household system! Does anyone know of a redesign of, or reasonable modifications to, the t-stat or control box, that would allow the system to operate without the speed change?
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Old 06-12-2019, 12:47 AM   #2
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Your post is an interesting read. I have a Dometic 15K A/C unit with the Dometic "automatic" t'stat. I have to say mine does not operate like that, more like a home A/C unit operates. The temp in the TT rises to a point that the A/C comes on and I do think that the compressor comes on about 3-4 seconds before the fan. The unit runs until the temp in the TT is at the set point, or close, on the t'stat and then the compressor cuts out with the fan cutting off 3-4 seconds later. I was one of the lucky, sarcasm there, owners who had the bad Dometic units and had to have my 15K unit replace twice under warranty. I use an independent authorized RV service man about how my unit operates as I mentioned. He told me that that is the way the new (a '15 year model) Dometic systems operate. I'm still on the third unit and it operates the same way.
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Old 06-12-2019, 11:46 AM   #3
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I don't particularly like it either, but I'm not sure what to do about it other than to swap out the control box and thermostat. And that involves pulling new thermostat wiring.

I haven't looked to the communication protocol between the t-stat and controls, so I don't know if the t-stat is making the switch decision or the control board is making that decision. I suspect that it's done in the control board. Either would have to be programmed to eliminate that step and I don't know if there's anyway we could access the programming.
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Old 06-12-2019, 12:09 PM   #4
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Mine is similar but not quite how you describe . if the is more then a 5 degree differential . the unit comes on fan first then compressor . while switching from low to hi for the differential the compressor stays running the fan shuts off for about 2 seconds and then comes on high , once the differential is less the 5 degrees the fan shuts off but compressor stays running and about 2 seconds for the switch to low . then runs till T-stat is satisfied . after that it runs as needed on low without issues . maybe you have a faulty

t-stat wouldn't be the first time . use the AC full time in Florida winters and heat and AC in Montana summers
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Old 06-12-2019, 12:19 PM   #5
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We have the Dometic with two ac units as well as the heater.

Hate it. However it works great. My DW can barely make it work without un ladylike words. Far too complicated for no reason.

Set the rear ac to 71 degrees. Front to 72.
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