How to Install a Smart Thermostat on My 5th Wheel

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Household wifi thermostat in an RV

• Thread starter freeinwy
• Start date Nov 27, 2019

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• #1

I have a honeywell wifi thermostat i want to install in my 5th wheel rv, The honeywell requires 24 VAC to operate, which is fine in a stick built home, I want hook it up in my 5th wheel 12VDC system, the furnace and air conditioning units are signaled by 12VDV.
I want some ideas on how to go about making it work, I have a 110AC to 24vac door bell transformer to operate the thermostat now i need to convert the 24vac output of the thermostat to 12vdc to operate the furnace and ac units. i tried to find 24vac relays and didnt have much luck, 24vac relays wont care if 12vdc is running through the switch side of the relay but to avail i couldnt find any.
I'm just wanting ideas and thoughts.

• #2

What powers the brains of the thermostat? My "smart" thermostat uses a battery, it does not know what the control voltage is. Some thermostats steel power from the control lines and they need the right voltage.
Is there a relay in the thermostat?

• #4

You might not be aware of this 12 to 24vac converter. Remember that it draws 200 mA, 0.2A doing nothing.

• #5

I dont think you'd have a schematic for this device, if you did we could tell you more, if the device doesnt need ac it might be possible to run if off 12v, the electronics within probably dont run directly off 24v, there will be a rectifier/regulator of some kind.

• #6

In post #3, you posted a diagram. C wasn;t needed in older stats. R and Rc is provided when cooling is fed from a separate 24 VAC transformer. If you have one transformer R and Rc get jumpered. Sometimes they are labeled Rh and Rc which is the better labeling.

R and C get 24 VAC. if you had a battery powered thermostat C would not be needed.

(R & Rc) and C go to a 24 VAC transformer. Think of C as Common.

So a bunch of relays in the furnace/AC use this common terminal and it really is common to a bunch of 24 VAC relays.

R is the other side of the transfomer and it selectively get connected to:

W - Call for heat
Y - call for cool
and G
G - call for fan

Why is there a Y and Y2 on the AC unit? If 2 stages exist. there is also W1, W2 and W3 for the same reason.

The Fan generally may need to be configured.

In heat mode the furnace usually controls the fan. It has to delay onset and make the fan run longer after there is no call for heat.
The thermostat CAN do that role too, but in general, does not.

So, you may need isolated contact closures if the stat needs 24VAC or uses 24VAC to charge the batteries. So, the stat CAN BE "poential free" with a common if battery powered. That COMMON would be R,Rc not C/

To get isolated contacts, you would connect 24VAC relays all connected to the (C) and then a relay to W, another to Y and another to G.

Now you have 3 independent contact closures.

If your missing the 24 VAC because this is a 12V system. 2V VAC is applied between (R,Rc) and C of the thermostat.

If you need the power reduced from the 24 VAC "coils", that can be arranged.

Q

• #7

So I know how to wire up the stat whether 12 Vdc or 24 vac. Multiple Y wires is for fan speed. What it comes down to is, I need to run the stat on 24 vac, my appliances, furnace and 2 roof a/c units run on 12VDC. So I want to run the stat thru relay coils to trigger the 12 vdc switch side to fire up the furnace or a/c units. I'm also trying to use scavenged parts without spending money. It makes it more challenging and fun. I'm a mechanic and I tried heavy 12VDC relays and the ac voltage makes them a buzz box. I have some 120 ac relays and 24 volt ac won't fire the coil enough to close the switch.

• #8

Can you post some pics of the 'stat board, we might be able to advise further.

• #10

Multiple Y wires is for fan speed.

That generally does happen. Two fan speeds, The burner runs at different BTU/h rates. as well.

G is usually FAN - OFF ON AUTO. Auto just means heat or cool have to be on.

Part of your understanding is missing. The heating system in a home runs on a 40 VA energy limiting transformer/ Your stat plus all relays energized at the same time has to be less than 40 VA.

Typically that's the fan contactor, the W1 and W2 contactors. Your 40 VA has to power your new relays.

Can't help with the 12 V issues.

I THINK it might be able to be done with these:
1 Backplane
5V power supply
4 modified BUSIO AC input board for 24 VAC operation
4 Relay boards

If you want to spend more time and make your own, use the BUSIO modified AC input board, a DC supply and an OPTOMS relay from Digikey.com

Heat pump is another animal. Designed/programmed very differently.

ANOTHER WARNING - Check wire colors! The terminals matter. Electrons dont care what color the wire is.

Last edited: Nov 29, 2019

• #11

The "24V AC" thermostats I have used work fine on 24V DC.

I'm only running electronic inputs from the thermostat outputs though, the internal relays may not like switching high current DC loads...

• #12

The "24V AC" thermostats I have used work fine on 24V DC.

That would not surprise me.

• #13

What I was getting at, was that a simple isolated boost converter could power the thermostat and allow all the relays etc. to directly connect in to and control the 12V DC system.
eg. Something like this:
**broken link removed**

Connect the 12V supply to the converter input and connect the outputs to the thermostat, +12 to R and -12 to C; that should run the thermostat electronics and relays etc.

Also connect the 12V feed to R, then take the outputs from W, Y and G or whatever you need, to feed small loads directly or 12V relay coils, with the load or coils commoned to the 12V system ground.
Fit flywheel diodes across the relay coils or any inductive loads.

The common C terminal should not be connected to anything other than the converter, it will be at somewhere around -12V with that setup.

• #14

I have a 2007 RV trailer, and the Dometic thermostat on the wall started to malfunction, meaning I had to constantly raise the temperature setting higher, and higher to get it to work, and to just get 75 degrees I had to raise the slider bar to 95 degrees, so obviously the thermostat was not working properly.
Does anyone know why? Thanks!

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How to Install a Smart Thermostat on My 5th Wheel

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