Replacing a Dometic RV Thermostat With a Household Unit

[RicTrim]

Ok, as may others have noted in many forum posts the Dometic thermostats in the RV are deficient. I looked at the Micro-aire replacements (and got sticker shock). Is it possible to just replace these with a Honeywell, or equivalent from Home Depot? What am I missing?

 

[Mikendebbie]

I dont know the full answer, but i will bet the answer is starts with “12 volt system”. Low voltage for a house system is something other than 12 volts i think. I know this topic comes up often - have you tried a forum search?

 

[jsb5717]

A quick internet search says "no". Residential tstats work on 24v. One site suggested that if you could find a battery-operated unit then perhaps it would work. Also consider that RV systems work differently than residential systems. Your RV furnace is triggered via a relay in the AC unit. Honestly, I don't know enough about how all the electrics of the system work but I would want to make sure that a tstat was compatible.

Generally, though, you need a tstat that works on 12v. FWIW, the Micro Aire has been a worthwhile investment for us. It works great and is easy to use with the app on your phone. They do go on sale from time to time.

 

[Todd & Mary]

While the Microair is expensive it gives you the ability to monitor your rig from anywhere and adjust your settings. If you have pets you can set alarm limits to send you emails if the temp goes outside the settings. I think it is well worth the money for what it does.

 

[scootsk]

I used one on my last FW. Worked very well. A Google search will give you the schematic

 

[Mark77]

I’ve got all the hardware and a plan. I am going to have to use a 24VAC transformer to power the Honeywell smart thermostat, but it can be done.

I’ve done a few with battery powered smart thermostats, but this will be my first venture into 24 VAC models.

I’ll post info when I get around to doing it here in the next few weeks.

 

[ChuckS]

If it were mine i would either replace with like item Tstat or upgrade to MicroAir Tstat... Yes a house Tstat could be used but then you have something that is wired and works differently than a typical RV Tstat does.. This in turn makes it difficult for the next owner or RV repair tech to try and decipher what was done.... Either stock replacement or MicroAir for a top notch.. no issue job is how I would do it

 

[Mark77]

If it were mine i would either replace with like item Tstat or upgrade to MicroAir Tstat... Yes a house Tstat could be used but then you have something that is wired and works differently than a typical RV Tstat does.. This in turn makes it difficult for the next owner or RV repair tech to try and decipher what was done.... Either stock replacement or MicroAir for a top notch.. no issue job is how I would do it

For sure... It will also involve a relay module since (in the furnace case) the ground is switched vs the +12V and I have to isolate furnace from the thermostat.

A good share of the reason I do these projects is for the challenge. As far as the next owner, the ones I have sold in the past I always take a day and undo what I have modified, unless the purchaser wants the mod... or I want to save it for the next rig.

One of the big reasons I am doing the stat is because I plan on having on-board internet 90% of the time because of the Victron ecosystem, also providing me the needed 120 VAC for the transformer and I have an extra Honeywell wifi thermostat that I will put on the same account as the 4 I have in my home and shop now!

I'll do a new thread when I get around to doing it, fully understanding that most will opt for an OTC option as it will be a little complicated for many DIY'ers.

 

[kilch123]

Yes, house thermostats work on 24V AC. I installed the Micro-Air and feel that it has paid for itself by allowing me to set schedules and turn down the heat/turn up the A/C when I'm not there. I do full-time, so the savings comes quicker for me. I really like being able to control my HVAC from my phone.

 

[Daryles]

I have read of someone using a battery powered house thermostat to control the AC unit. I can't find the article. I don't recall what model AC unit it was.

 

[Mark77]

I have read of someone using a battery powered house thermostat to control the AC unit. I can't find the article. I don't recall what model AC unit it was.

That is what I have in the rig now, a Lux brand I had in my house before I started modernizing. I have had that same thermostat in 3 rigs.. 2 Excel's that I had and now the Montana. Works great because the 2 AA batteries power the relays. Pretty easy to rig that up, but it doesn't give me Wifi connectivity.

 

[laverdur]

I’ve got all the hardware and a plan. I am going to have to use a 24VAC transformer to power the Honeywell smart thermostat, but it can be done.

I’ve done a few with battery powered smart thermostats, but this will be my first venture into 24 VAC models.

I’ll post info when I get around to doing it here in the next few weeks.

 

[laverdur]

RV thermostats and the signaling voltage from the furnace and A/C all operate on 12VDC. That said, the operating voltage for the thermostat, signaling voltage for the furnace and signaling voltage for the A/C(s), would require some kind of voltage step-up (12VDC to 24VDC) and inversion (DC to AC) would be required to use a residential thermostat. A small step-up transformer (12VAC to 24VAC) could be used after the 12VDC was inverted to 12VAC. Voltage loss could be a problem if the inverter or step-up transformer are not operating at ideal efficiency. If you ever expect to need to run the furnace or A/C while on the road, I'd recommend sourcing the 12VDC from the house batteries vs. from the converter which does not normally operate unless connected to shore power/running a generator while driving. The alternative may be to use a battery operated thermostat but that doesn't solve the signaling voltage problem.

 

[pyoung47]

At least on a 07 Dometic I did, there is a relay box in the AC unit, remember it also controls heat. In that case, it isn’t just an on/off therm. Good luck, but you may be disappointed.

 

[RichV]

Retired HVAC contractor here. Based on the lack of agreement, residential versus RV, compatibility may depend on the age/model of your HVAC system. The following applies to my 2018 3120RL, and to probably any existing thermostat that uses only three wires. The residential and RV thermostats are not interchangeable. While voltage is an issue, there is an even greater issue. Residential thermostats, with the exception of some specialized systems, operate analog. You need one wire for power and an additional wire for each function; heat, cool, and fan, four wires. My Dometic and its replacement Micro Air RV thermostats are digital. They have only three wires; power, neutral/ground and one wire over which all the digital signals are sent. I LOVE Micro Air and it is worth the cost. My Dometic had an intermittent problem, the worst kind of problem to troubleshoot.

 

[Mark77]

You guys are going to force me to do this project sooner rather than later .

Got to finish my radon remediation project on our home first!

 

[Mark77]

Got a Honeywell wifi thermostat installed (kind of) in my Monte today. Works perfectly. The only reason I went with Honeywell is because I already have 4 on them in and around my home.

The thermostat is hanging by the wires now, and the relay module where all of the magic happens is laying on the floor... and I don't like my choice of wire so I am going to clean it up tomorrow and then I'll post a thread about the install.

 

[Daryles]

Looking forward to your write up.

 

[DutchmenSport]

If replacing the thermostat for the AC, make sure it still works for the furnace too. And make sure you can still run your AC on "Fan only" mode.

 

[Mark77]

If replacing the thermostat for the AC, make sure it still works for the furnace too. And make sure you can still run your AC on "Fan only" mode.

Got it all covered Dutch... Going to start my thread this afternoon/ evening.

See it HERE