replacing manual thermostat with digital on ac

[grumbolt]

i have a dometic briskair ac unit in my bedroom that is controlled by manual controls on the unit itself.



my problem with it is the fan never shuts off when you have the ac on, and it has a very wide range between the cycle on and off using the manual thermostat.


i am thinking of using a hunter digital thermostat on the wall to control it, and realize i will have to get creative with relays, as the hunter unit is not designed to take the current that the old mechanical switches and thermostats control.



question is, has this been done here already (i did a quick search and didnt see anything on it) and,

is there any interest in a write up about retrofitting such a control to the mechanical ac units?

[DQDick]

Could be wrong, but can't remember anyone doing that successfully before. So yes, if you can figure it out, please share.

[jcurtis934]

I would have figured that the brisk air has a control box built in that allows for a dometic thermostat to control it...just like every dometic ac unit used in rvs.

[grumbolt]

they have the 6 wire plug on them, the control comes from the airbox side of things.. to get the dometic thermostat to control it, you have to buy the CCC box that goes in the roof unit.



i am a little too thrifty for that, i now have a pinout, and the pinout of the inexpensive thermostats from the box stores, now to out together a relay control box for the thermostat to control.



basically the thermostat will provide low power control to the relays,

the relays will switch the higher amperage loads providing control over the actual ac unit.



i havent decided if i want to use 24vac control voltage, 12vdc control voltage, or 120vac control voltage.



i am leaning heavily towards 12vdc control voltage being our rigs have 12vdc built in, and i think (who cares what ya think, just what ya know) most all of our units have 12v up at the ac units which is basically a 14"vent until they put an ac unit on it.


from what i have been told most are prewired with power to run a fantastic fan, so 12v shouldn't be hard to find.. worse case scenario a inexpensive 12vdc power supply could be used, or a 24vac control transformer either would be fine.


the other problem i am gonna have is routing the wires from the ac unit in the ceiling over to the wall. putting it inside the wall and ceiling will be problematic so i will more than likely use surface mount wiremold to keep it looking tidy.


once i get it worked out and installed and am satisfied with how it works, ill post diagrams, part numbers, and photos.

[Mark7]

I like your way of thinking... and your idea. I have the same issue and complaints about my bedroom A/C. I did research a kit made by the manufacturer of the unit (I think) or someone making it for that unit that was a plug and play deal for a LV thermostat, but it was expensive and I don't full time so I turned my attention to other mods and let that one go.

One observation... since you will be building something to house the relays with LV coils, why don't you just include a 24VAC transformer and use 24 VAC coils (Relays) since that is exactly what most LV thermostats are looking for, of you use a digital or programmable model.

I did change my main A/C to a residential / programmable thermostat to get away from the temperature swings, but the one I used has a couple of AA batteries in it that run the board and the relays that all for heat, cool, or fan. It really worked out well after I figured the fan only thing out.

Looking forward to seeing your eventual setup!

[grumbolt]

i am leaning on going with a 12 volt wall wart style power supply i will be able to use a voltage regulator to drop down to 3 volts to power the thermostat and eliminate the batteries, and then use the 12 volts to power the relays. i have looked around my unit and didnt see a 12 volt circuit there already so power supply it is lol...



the above is half the reason i want to use 12v. the other half is that the 12v bosch style relays i plan on using will more than handle the current, and their biggest benefit is that they are easily and cheaply sourced at absolutely any auto parts store.



typically the hunter home thermostats have dry contacts that simply open and close to provide switching it doesnt really care what flavor the voltage running through the switches is, as long as it is low current.

i just gotta figure out how to package everything up plug and play so that it looks and operates like it was meant to be there from the beginning.



i will be ordering the receptacles and hope they are the right ones being the originals dont have part numbers on them.

[Mark7]

Originally Posted by grumbolt

i am leaning on going with a 12 volt wall wart style power supply i will be able to use a voltage regulator to drop down to 3 volts to power the thermostat and eliminate the batteries, and then use the 12 volts to power the relays. i have looked around my unit and didnt see a 12 volt circuit there already so power supply it is lol...



the above is half the reason i want to use 12v. the other half is that the 12v bosch style relays i plan on using will more than handle the current, and their biggest benefit is that they are easily and cheaply sourced at absolutely any auto parts store.



typically the hunter home thermostats have dry contacts that simply open and close to provide switching it doesnt really care what flavor the voltage running through the switches is, as long as it is low current.

i just gotta figure out how to package everything up plug and play so that it looks and operates like it was meant to be there from the beginning.



i will be ordering the receptacles and hope they are the right ones being the originals dont have part numbers on them.

Check eBay for the Bosch style relays. You can buy them in bulk (5 or 10) WITH sockets for cheap! I always have a few around.

You’re comfortable switching 120 VAC with them? Guess 30 amp is 30 amp, but I’ve never used one for line voltage. I guess I never checked the ratings.

I could also point you to a 120 / 240 VAC 2 pole contractor with a coil voltage of you’r choice for about 30 bucks brand new. Not positive about 12 volt tho... but for sure 24 V either A/C or D/C.

[grumbolt]

Originally Posted by Mark7

Check eBay for the Bosch style relays. You can buy them in bulk (5 or 10) WITH sockets for cheap! I always have a few around.

You’re comfortable switching 120 VAC with them? Guess 30 amp is 30 amp, but I’ve never used one for line voltage. I guess I never checked the ratings.

I could also point you to a 120 / 240 VAC 2 pole contractor with a coil voltage of you’r choice for about 30 bucks brand new. Not positive about 12 volt tho... but for sure 24 V either A/C or D/C.

i actually have about 40 sets laying around. i have to do some more research into the specs of them before i put them into service on line voltage.

could use some omron relays in either the 12v 24vdc or 24vac flavors, i have used them for many years in industrial control systems with great success.


if the specs allow, i plan on using the bosch style simply because if you have one go bad, an inexpensive replacement can be had at any auto parts store.


i have the molex connectors on the way, so when they come in, depending on what else i am working on ill be getting started on it .

[grumbolt]

well i currently have the digital thermostat controlling my roof air...

i originally used 30 amp bosch relays and after several camping trips it seems that the compressor was a little too much for them. (i wanted a year of testing with no issues before i posted diagrams etc)


i then put 50 amp bosch style relays in and have had no issues with it at all.. it regulates the temp very nicely, i have the option of running the fan only, or fan on with compressor only, or fan on all the time and compressor cycling..



so first the parts list....



i ordered extras of the molex stuff only because i like having spares and for the cost, it doesn't make sense to not have extra..



i ordered:
4- https://www.mouser.com/ProductDetail/538-36644-0003
4- https://www.mouser.com/ProductDetail/538-36644-0005
100- https://www.mouser.com/ProductDetail/538-02-08-1002
100- https://www.mouser.com/ProductDetail/538-02-08-2004


1- https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1




1- https://www.amazon.com/Honeywell-TH5...8-d89d95c82244




the thermostat uses AA batteries for power (they last about a year and a half)


you will also need to either get a 12 volt 1 amp dc power supply (wall wart style will work fine) to power the relay coils.




attached are the pinouts my unit had. the box in red is the actual thermostat you will mount on the wall. if you want variable speed fan, you will need to connect the wires through a switch like pictured. personally i haven't used anything other than high speed since i put the unit in. the only reason i can see to use a slower speed is if you put in a heat strip and wanted lower airflow to allow the strip to heat the air more.



as always, your mileage may vary, and i am not responsible if you damage your unit, or your camper.


if you choose to use 30 amp relays, i suggest using 2 of them wired in parallel to control the compressor. this will allow you to find replacements pretty much anywhere auto parts are sold.


if you use 50 amp relays then you shouldnt have any issue. i have been running the 50 amp relays for about a year now (1.5 months of actual usage)


btw the great thing about this setup is that it should be plug and play so that if it malfunctions, you can simply unplug it and plug the original controls back in if needed.


before wiring this up, i strongly suggest checking the function of the wires on your unit with a meter(what wire is hot when the fan is on, which is hot when the compressor turns on etc) and comparing them to the diagram in the pdf.

[Mark7]

I like the way you think! I am always finding new uses for Bosch style relays... this is one I have not seen. I have never ran 120VAC through the relays, though I know they should handle it.

Where did you mount the relay box you got on Amazon? Ive never seen one of those before either but sure could have used it yesterday when I mounted 3 relays (for isolation) on an Nh3 applicator. They will be out in some pretty harsh, dusty weather and we will have to keep an eye on them.

Might give your project a try next season!

[grumbolt]

I mounted this box inside the ac unit. None of the wiring in the ac unit was altered at all. I also installed the optional heat strip and it does a decent job of taking the chill out of the air. Word to the wise... mount the thermostat with the slide pulled in, that way it is easy to adjust when you are traveling and use it at the rest area. This box is the same one I use on my little geo tracker off road toy and it performs well. I am including a pic of the tracker.. the samurai on the left looks small, but it is on 38" tall tires..