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Old 08-28-2020, 06:03 PM   #1
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Finally Replaced Dometic Analog Tstat with a Digital Tstat...

I finally got around to swapping out that POS Dometic analog thermostat, which basically ran "wild" in cooling mode here in Florida as the sliding rheostat had quit.

Many have done this mod, using various Honeywell or Hunter digital stats, such as the RTH111 tstat.

I opted for a PRO1 T631-2 Digital tstat with Hi and Low Fan Speeds.

I got it going, after some head scratching and a call to PRO1 support, but I wasted a few bucks, as the two separate fan speeds on the tstat selector switch arent compatible with the Dometic control box in my Dometic 13K BTU AC unit.

I was a bit surprised and frustrated by that, but literally everyone has encountered this same issue and ended up adding an extra external SPDT micro switch for the High (or Low) speed. I thought I had avoided that kludginess by getting a tstat with Low-Auto-High Fan selector switch, but I was wrong.

The PRO1 T631 is similar to the Honeywell and Hunter digital Tstats in that it is battery powered by two AA batteries and those batteries drive the relays in the ceiling mtd Dometic control box. Its great that a mere 3 VDC can operate those relays.

It's also quite strange that the Common is wired to the R terminal, which normally is 12VDC for the Dometic analog stat, and 24VAC for a household or commercial AC unit. Switching the "ground" to drive the fan, cooling, and furnace relays is the opposite of what any HVAC tech is trained to do, but it works in this unique scenario.

I suspect that the reason that you have to choose either Hi Fan or Low Fan, but not both, probably results from some common circuit board trace integral to the relay board inside the Dometic control box in the ceiling.

I wish I had a spare Dometic control box on a test bench so I could pinpoint the exact cause of this issue, as the Dometic digital Tstat conversion requires you buy both a new stat AND a new control box.

The cost of both is about $300. A digital tstat is one tenth that cost.

What I'm saying is that there may be a way to mod the original OEM Dometic analog relay board, perhaps by cutting a trace with an Exacto knife, but I'll never know. It should've work. It just wont.

So we opted for Low Fan for now, as High Fan is too damn loud for continual use.

It is great to have a digital thermostat and readout. We no longer have to endure meat locker temps at night, or compressor short cycling, as this new stat has an anti-short cycle setting. While the T631 is not programmable, we didnt need that anyway.

When I get a chance, I'll share a pic showing the final wiring for this mod.
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Old 08-28-2020, 06:46 PM   #2
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There are a few issues, mostly with how Dometic breaks the negative to control the relays and how they use two relays to control the high and low speeds of the fan. The way they wire it is that you have to power one relay to get low speed and both relays to get Hi speed. I did a wiring diagram once of how they did it, but I don't know what I did with it. Anyway the use an interesting setup of N.O. and N.C. contacts to get the different speeds.

Here's a wiring diagram of the analog thermostat.

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Old 08-28-2020, 07:11 PM   #3
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Thanks, Bama Rambler. Your info is very helpful and if you can find the control panel relay diagram, it would be helpful for someone to do some advanced tinkering.

There does seem to be an interlock circuit in the control box, and the way you describe it sounds like one relay's NC contact (perhaps Fan Low relay) must feed power to the other relay (Fan High relay) so that there is always one speed available. Seems to me it could be modded, but that's just a guess.

I stumbled onto this diagram online while doing the swapover. It's very helpful. Did you make this?

I used to draft diagrams like this when I worked for Trane and Landis & Gyr designing building control systems, so I know the effort required.

Thanks for your reply.
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Old 08-28-2020, 08:57 PM   #4
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Dometic does a lot of things that looks like automotive controls.

I haven't seen a complete diagram of the control box. I would really like to have one of the analog box as well as the newer digital communication box.

I did a down and dirty diagram of how the fan speeds work. There are a couple of ways they could have done it, but I think this is what they did.

Dometic A-C Fan Speed Relays.pdf
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Old 08-29-2020, 04:02 PM   #5
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Very helpful.

Calling the bottommost relay "R-1", it appears that a simple change, disconnecting the wire at terminal 14 of R-1 and connecting it to 120 VAC hot, would eliminate the unnecessary interlock and allow the fan relays to operate independently.

This would then allow me to use our new PRO1 T631-2 two speed fan digital thermostat, one of the few with a two speed fan switch, as designed.

I need to get my head up inside that analog control box to see if I can access these relays, and verify your handy diagram.

Cant thank you enough, Bama Rambler!
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Old 08-29-2020, 06:32 PM   #6
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All those 'wires' are likely traces on the board, but you may be able to part the trace and solder a lead to it if you're handy.
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Old 08-29-2020, 09:31 PM   #7
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Question Not trying to hijack the feed!

My apologies to walk_the_walk as I am not meaning to hijack your feed but Does anybody happen to have an inexpensive model number for a replacement digital t-stat that someone could use (without the Hi/Low option)?
Thanks in advance

Quote:
Originally Posted by walk_the_walk View Post
I finally got around to swapping out that POS Dometic analog thermostat, which basically ran "wild" in cooling mode here in Florida as the sliding rheostat had quit.

Many have done this mod, using various Honeywell or Hunter digital stats, such as the RTH111 tstat.

I opted for a PRO1 T631-2 Digital tstat with Hi and Low Fan Speeds.

I got it going, after some head scratching and a call to PRO1 support, but I wasted a few bucks, as the two separate fan speeds on the tstat selector switch arent compatible with the Dometic control box in my Dometic 13K BTU AC unit.

I was a bit surprised and frustrated by that, but literally everyone has encountered this same issue and ended up adding an extra external SPDT micro switch for the High (or Low) speed. I thought I had avoided that kludginess by getting a tstat with Low-Auto-High Fan selector switch, but I was wrong.

The PRO1 T631 is similar to the Honeywell and Hunter digital Tstats in that it is battery powered by two AA batteries and those batteries drive the relays in the ceiling mtd Dometic control box. Its great that a mere 3 VDC can operate those relays.

It's also quite strange that the Common is wired to the R terminal, which normally is 12VDC for the Dometic analog stat, and 24VAC for a household or commercial AC unit. Switching the "ground" to drive the fan, cooling, and furnace relays is the opposite of what any HVAC tech is trained to do, but it works in this unique scenario.

I suspect that the reason that you have to choose either Hi Fan or Low Fan, but not both, probably results from some common circuit board trace integral to the relay board inside the Dometic control box in the ceiling.

I wish I had a spare Dometic control box on a test bench so I could pinpoint the exact cause of this issue, as the Dometic digital Tstat conversion requires you buy both a new stat AND a new control box.

The cost of both is about $300. A digital tstat is one tenth that cost.

What I'm saying is that there may be a way to mod the original OEM Dometic analog relay board, perhaps by cutting a trace with an Exacto knife, but I'll never know. It should've work. It just wont.

So we opted for Low Fan for now, as High Fan is too damn loud for continual use.

It is great to have a digital thermostat and readout. We no longer have to endure meat locker temps at night, or compressor short cycling, as this new stat has an anti-short cycle setting. While the T631 is not programmable, we didnt need that anyway.

When I get a chance, I'll share a pic showing the final wiring for this mod.
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Old 08-31-2020, 08:56 AM   #8
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Sorry I dont have a drawing, but could it be as simple as the Tstat low output drive the low relay input but the Tstat high output drives both the low and high relays? There would need to be a diode in there somewhere to prevent the low speed out from driving both relays as well.
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