Dometic Thermostat mod

[popeyemth]

First off I want to say thanks to all those here that advised me on this and to make clear this is not my invention just my variation of someone else's idea.
The problem we were having was waking up at 3 am with cold air blowing on us because the AC fan was a constant on .
We had already replaced the furnace thermostat with a more accurate digital one ($10, on sale at Menards) so we were half done from the start .
This is the control on the AC:

An this is the new furnace thermostat we wanted to use:




This is the relay we added that the new thermostat controlled with a single wire pulled from the thermostat to the relay (plus a ground wire)



The AC is set on coldest,and the fan speed selected.
The wall thermostat provides 12 volt DC to operate the relay.
The 25 amp contacts on the relay (normally open) are wired in series with the AC breaker.





The thermostat on the wall calls for cooling,sends 12 volts to the relay coil, the contacts close and 120 volts is sent to the air conditioner .
The fan speed can still be changed by turning the dial on the air conditioner.
When the preset temperature on the thermostat is reached the relay opens and the AC (and it's fan ) shuts off.
No more waking up at 3 AM with a cold fan blowing on us, and the total cost was less than $25.
The kits sold to do this run $100 plus shipping .

[popeyemth]

The digital Hunter thermostat I used is on sale fairly often at Menards for $10.00

[opnspaces]

I agree nice mod, I now so want to do this on my trailer. 🙂 I guess thought I first have to go to storage and see if my existing digital thermostat can also control the AC.

[SplinterFL]

Nice mod.
Putting it the fuse box is easy and simple. Also easy to undo for warrenty work.

Since mine is well out of warrenty, I'd put the relay in the AC. At least on mine the intake duct has the 3 wires running to the roof unit, a white Neutral, and two hots (blue/black), one for the fan and one for AC compressor. I'd prob put the relay on the compressor hot so the fan could keep running and moving air around. The hard part would be routing the control wire through the roof to the thermostat.

[popeyemth]

Please reply back when you get this mod done and report how you like it.
Good Luck, Mike

[luv2skyski]

Excellent. Thanks, Dave.

[popeyemth]

The relay I used is quite small and should fit inside your breaker box.
I removed the wire from the AC dedicated breaker and put it on one contact terminal and ran a 4 inch long wire from the other terminal to the breaker.
When the relay "makes" or "picks up" you then are connecting the Cooling unit to the breaker.
Set the cooling unit thermostat to max cool and the fan speed where you like-we use low.
Run a wire from the Y terminal on the Hunter thermostat to the coil contact on the relay.
Run another wire from the other coil contact to battery ground to complete the coil circuit.
When set on cool the Hunter thermostat calls for cooling,sends battery voltage to the relay coil (via the Y wire),the relay connects across the contacts and sends power to the cooling unit which then comes on.
When the set temperature on the Hunter thermostat is reached it turns off the Y wire, the relay drops and power is removed from the cooling unit.
Hope this was helpful ,
Mike

[luv2skyski]

Yes. It's a Hunter digital thermostat model 42999B.
Owners manual

[popeyemth]

luv2skyski wrote:
How would you go about wiring the digital thermostat to replace the knob type thermostat on the roof mounted a/c unit? I've had to replace the thermostat on the ceiling twice now and would love to bypass it completely and use the wall mounted digital. Plus it would be nice to have an actual temp reading to go by instead of guessing.
Thanks, Dave.

Do you have a wall mounted furnace thermostat now?

[luv2skyski]

How would you go about wiring the digital thermostat to replace the knob type thermostat on the roof mounted a/c unit? I've had to replace the thermostat on the ceiling twice now and would love to bypass it completely and use the wall mounted digital. Plus it would be nice to have an actual temp reading to go by instead of guessing.
Thanks, Dave.

[popeyemth]

x5rdman wrote:
So the relay is in the breakbox! I thought it was put in the ac unit. Nice idea

Just lazy 🙂
It opens and closes the AC power to the air conditioner to turn it on and off much like the breaker does and it's quite small so it just seemed natural to put it there .

[x5rdman]

So the relay is in the breakbox! I thought it was put in the ac unit. Nice idea

[popeyemth]

Only one wire had to be ran- the positive wire from the Hunter thermostat. I used 22 gauge solid (phone) wire that's pretty stiff so that made it easy . The small relay went right in the breaker box as shown so 12 volt DC ground for the coil was handy and one side of the contacts was the existing positive from the AC breaker,the other side of the contacts that went to the breaker was 4 inches long so the whole thing was quite easy.



This is a blow up off the wiring diagram on the side of the relay.
It has two sets of NO contacts (normally open) that close when the coil picks up so there are spares 🙂

[x5rdman]

Nice, how did you run the wires?

[popeyemth]

Update on using this Hunter thermostat :
Thought I had a problem turning off to leave when returned wouldn't restart the AC
Turned out the thermostat is smarter than me it has a built in 3 minute delay to let the pressure equalize in the AC to save the motor from high amperage starts 🙂