Roadtrek Cool Cat Replacement

[hbn7hj]

So, how did you rewire the thermostat to get the furnace to work? I, very successfully, replaced the CoolCat heat pump with a Frigidaire, but I need the propane furnace to work. Thanks! DaveW

Works fine. The analog thermostat uses 7 or so wires to the unit. The digital one uses three. You need one that controls cool, furnace, heat pump. Use the original power and ground wires and one of the original control wires. Works fine.

 

[DWorrell3]

Thanks very much for your reply. Eventually, I disconnected all the wires from the bedside thermostat, went up to where the old Dometec CoolCat was and started pulling loose wires. They all came loose except 4, the red and black (12V power) and two green wires that went to the furnace. That fooled me, because the furnace schematic called for two blue wires to the stat. Turns out, that after taking the cover off the furnace, the 2 blues were spliced to 2 greens. So, I am home free. I get to use alternatively: a non powered two wire heat only stat, a battery powered stat, or a full 12v powered stat. I'm going with the battery powered stat. Replacing the CoolCat was worth every bit of the effort, but this is not a job for the occasional do it yourselfer. Dave W

 

[rhythmtone]

Works fine. The analog thermostat uses 7 or so wires to the unit. The digital one uses three. You need one that controls cool, furnace, heat pump. Use the original power and ground wires and one of the original control wires. Works fine.

Correct me if I'm wrong, but you seem to be talking about a Dometic to Dometic replacement, which, although a fine solution, was not really the point of the original post, and was also not the question that I was answering.

The original post is generally about replacing a cool cat with a standard window AC, I'm not sure why one would need or want a thermostat with cool, heat pump, and furnace controls with such a setup, after the swap, because the standard window AC will not accept an external thermostat connection.

The question was how to control the furnace when a standard window AC is installed - the old thermostat above the bed will no longer have power once this is done, because as I mentioned, it used to get power from the umbilical from the cool cat unit, which is now removed.

If anyone has any clever ways to have a powered thermostat that only controls the furnace, using the existing wiring and thermostat location, without tearing everything apart, I'm all ears but from what I've seen I don't think it's possible. Yes, the furnace control wires (at least one of them) will terminate at the thermostat above the bed, but you'd have to take off at least one panel and run power over to your new thermostat for it to work, which is why I did it the way I described.

Cheers,
Dave

 

[rhythmtone]

For reference, below is a link to the wiring from the Dometic AC unit to the original, analog thermostat:

https://drive.google.com/file/d/12zIBLPeYXCEgIQ9MZd_traLZbVxyd1ud/

You'll notice that it has 1 wire from the furnace, but I'm not sure where the other furnace control wire goes after it leaves the furnace unit, probably to chassis/ground?

But regardless, this unit will no longer have power once the Dometic is removed and replaced with a standard window AC, so there's that.

Thanks for any tips with this specific setup,
Dave

 

[DWorrell3]

Thanks all. To clarify, I replaced the CoolCat with a Fridgidaire window unit with it's own remote control. Therefore, I don't need a heat/cool thermostat, just heat only. I'd like to keep it by the bed because that's where I'd like the temp modulated. I have two wires coming from the furnace and the furnace schematics show that the thermostat completes the circuit involving these two wires to send the heat signal to the furnace. I think I am on the right path, but any advice is appreciated. Dave W

 

[rhythmtone]

You're welcome for the replies, and yes I thought that that was your setup - I was clarifying to the other poster as well, regarding the Dometic to new Dometic swap.

If both of your furnace control wires terminate at your original thermostat, then that's great, and you may be luckier than me, haha, possibly a design change.

See my picture, it looks like only 1 of my furnace wires come to my original thermostat, but I'm still looking into that, I'd be curious to see a picture of your thermostat panel if it looks different, but if you already tore it apart then no worries.

But if both of your furnace control wires terminate at the original thermostat then you're totally set for a battery-powered or mechanical "heat only" thermostat. There are lights nearby so you could do a powered unit, but you'd have to take apart more stuff - might be worth it, I'm still looking into that, but haven't done it yet, especially because I don't think both of my furnace control wires go to the rear of my vehicle.

Cheers,
Dave

 

[booster]

The battery powered thermostats don't need any furnace power so I think you would just need to connect the two wires to plus voltage input, usually the red wire connection and the other to the heat connection which I think is white. Instructions for stat will tell you easily.


Just check it with an ohmmeter to see that the contacts close when heat is called for. I am pretty sure that is all the Suburban furnace will need, closed contacts to run. Test that by just touching the wires together and see if the furnace runs.

 

[DWorrell3]

This forum is utterly fascinating. There is a percentage of really strange and uninformed replies (maybe 10%), but there is a much great percentage of folks who not only know their stuff, but probably know more than the manufacturers. I don't know exactly where else such information could be gleaned. Dave W

 

[rhythmtone]

The battery powered thermostats don't need any furnace power so I think you would just need to connect the two wires to plus voltage input, usually the red wire connection and the other to the heat connection which I think is white. Instructions for stat will tell you easily.


Just check it with an ohmmeter to see that the contacts close when heat is called for. I am pretty sure that is all the Suburban furnace will need, closed contacts to run. Test that by just touching the wires together and see if the furnace runs.

That's great to know! I got mine working by intercepting the furnace control wires at the furnace unit. I used a wiring guide I think that I found on this forum, connected blue positive furnace control to red on the thermostat - and blue negative (unlabeled) furnace control to white on the thermostat. See pictures:

https://drive.google.com/file/d/13CPyHdSGWgpg06kI9q1TZbo9s1AGjgf9/

https://drive.google.com/file/d/13CYbYBQbEU65sXoF7ZiXLDIVeWPdfbxl/

As I mentioned, I don't think that both of my furnace control wires run back to my original thermostat, hence the strategy of intercepting them at the furnace below the rear passenger side seat.

I'd like to look into a 12v "RV powered" thermostat but since it's only a three wire connection at the heat-only thermostat I'd be worried about Y connections interfering with the furnace control wires because it's a 4-wire system on the furnace unit side (2 for furnace power, 2 for furnace control), and sending voltage back down the control wires, etc., but I'm open to suggestions on that.

Cheers,
Dave

 

[rhythmtone]

Also, forgot to mention, the thermostat that I bought is 24 volt so even if I did a Y connection from the furnace blue positive wire to thermostat common, I'm not sure it would work or be good, so I guess I'm fine with battery-powered thermostat.

 

[rhythmtone]

They don't seem to make 12v "heat only" thermostats that are 4 wire - 2 for power, 2 for control, but maybe someone with more knowledge could tell me how I could jump it or Y connect it, etc. Or someone knows of a unit that would work, etc.

Cheers,
Dave

 

[Road Toad]

They don't seem to make 12v "heat only" thermostats that are 4 wire - 2 for power, 2 for control, but maybe someone with more knowledge could tell me how I could jump it or Y connect it, etc. Or someone knows of a unit that would work, etc.

Cheers,
Dave

Maybe this:
Some furnace configurations use a gas solenoid grounded at one end and switch the +12 to the other end, via the stat, to activate the furnace.
Some furnaces do it the other way around. Power the permanent end and switch the ground.
The latter is the most common.

I belive the former configuration will work for what you are wanting.
It minimizes the number of wires required (3) for a powered tstat.

First, here is a 12Vdc Rv thermostat. (Ignore all but the "furnace" function. They are unneeded)
https://www.amazon.com/gp/product/B085G13KFR/ref=ox_sc_saved_title_1?smid=A1GDVZK8DW7P5V&psc=1

Second, the gas solenoid will need to be wired as one end permanently grounded, the other end fed the switched +12 from the thermostat. (It probably is NOT wired that way currently)

Minimum of two wires needed plus a gnd.

a +12 wire, a gnd wire, and a wire from the gas solenoid must be present at the thermostat.

The +12 powers the t-stat and is also the sourece for power back to the open end of the gas solenoid. (This is your "Y" connection per se)

Second wire to solenoid attaches to the furnace connection of the stat.

To minimize possiblity of grouding issues, it is best if the ground wire originates from the furnce too; but is not essential to be such. It can be from nearest chassis metal available.

 

[Road Toad]

The battery powered thermostats don't need any furnace power so I think you would just need to connect the two wires to plus voltage input, usually the red wire connection and the other to the heat connection which I think is white. Instructions for stat will tell you easily.


Just check it with an ohmmeter to see that the contacts close when heat is called for. I am pretty sure that is all the Suburban furnace will need, closed contacts to run. Test that by just touching the wires together and see if the furnace runs.

Also, if it does run, measure the voltage from this shorted-together point to ground. If it is +12 Vdc, this point can also be used to power a powered tstat.
If it has no voltage, this indicates a "switched-ground" solenoid configuration (vs "switched-power") configuration and won't be usable as a power point.

 

[rhythmtone]

Also, if it does run, measure the voltage from this shorted-together point to ground. If it is +12 Vdc, this point can also be used to power a powered tstat.
If it has no voltage, this indicates a "switched-ground" solenoid configuration (vs "switched-power") configuration and won't be usable as a power point.

Awesome thank you for the reply! This is getting a little heavy it but makes total sense, I'm going to think on it for a little while - I don't really want to open the chassis of the furnace, if that's what's necessary to do this, but maybe.

My furnace does use that wiring diagram I posted, 2 for power (for the fan motor I'm sure) and 2 for control (both blue wires). If those "control wires" are connecting the ground, to activate the unit, as you say is most common, then there should be no voltage between them, I assume, but I will check with a meter.

I don't want to mess with the power wires to the unit, obviously, because I'm sure that it's designed to warm up and cool down with "independent" thermostat control, which is why it takes a while to shut off even when you've disconnected the control wires, but your explanation of the solenoid function is great. It's a 2006 Suburban propane furnace if that helps, but believe that they do have several possible wiring diagrams, so I don't know exactly how it's set up.

Thanks again!

 

[Road Toad]

You are correct. The cool-down fan control is performed by the furnace, not the tstat.
I don't belive you need to open the furnace chassis to rewire the gas solenoid .
If I recall correctly, it is connect to a controller board that is in view when the furnace access door on the exterior is opened.
My Rv unit is not currently nearby to confirm this.
Lot of other very knowledgeable folk on this forum might be able to confirm this?

Awesome thank you for the reply! This is getting a little heavy it but makes total sense, I'm going to think on it for a little while - I don't really want to open the chassis of the furnace, if that's what's necessary to do this, but maybe.

My furnace does use that wiring diagram I posted, 2 for power (for the fan motor I'm sure) and 2 for control (both blue wires). If those "control wires" are connecting the ground, to activate the unit, as you say is most common, then there should be no voltage between them, I assume, but I will check with a meter.

I don't want to mess with the power wires to the unit, obviously, because I'm sure that it's designed to warm up and cool down with "independent" thermostat control, which is why it takes a while to shut off even when you've disconnected the control wires, but your explanation of the solenoid function is great. It's a 2006 Suburban propane furnace if that helps, but believe that they do have several possible wiring diagrams, so I don't know exactly how it's set up.

Thanks again!

 

[jjrbus]

Great write up and pictures! I run mine with a Honda 2000 and added a hard start capacitor to the LG 8000 btu. Honda will start and run it in ECO mode.

 

[booster]

I am not sure what is being referred to when it is mentioned to "rewire the solenoid" but assume it means the gas valve control.


It is very likely that the valve is wired to the control board as that is the normal place and where it really should be. Gas control is a major part of the safety system in any gas appliance and with propane the timing of a lot of things is necessary.



IIRC the whole thing is daisy chained with the thermostat just the first item of the chain, then the fan comes on long enough to prove airflow and purge the combustion chamber of any gas, then the igniter tries to light the pilot flame if they use one, then it checks that pilot flame is large enough with a temp sensor, then that triggers the main gas valve. Messing with anything the could interfere with gas valve opening reliability and timing is not a good thing.


If the solenoid being spoken of is the main control board turn on solenoid, that is a different matter and you can get into that without safety issues from what I have seen. It may be switched 12v or 5v or even 24v (not likely 24v) or it may be switched ground which is most likely. In any case, based on the size of the wiring it may be very current limited and using any power from it to run the stat might not work well or damage the control board (lots of home furnaces in the past were not able to power a stat but no most can as the stats got away from dry contact and battery only powered ones).


IMO, there is nothing wrong with battery powered units as they are mature technology so no reason to make this whole thing more complicated than it needs to be. Millions of them are in use every day in homes and we only switched to furnace powered a couple of years ago when we got a new furnace. The Honeywell battery unit was still working fine after 25 years of use in Minnesota for heat and air conditioning.

 

[Road Toad]

Was merely responding to rhythmtones enquiry as to availablity of a 12 Vdc stat and a method to employ it.
Not advocating.
As far as current carrying capacity of the wire, the 22 ga wire in the Roadtrek is rated at an ampacity of 920 millamps. Our measured current draw is 320 millamp. The referenced tstat draws 180 millamp.
Amperage load should not be an issue.
Personally I prefer the Battery operated unit.

 

[booster]

Was merely responding to rhythmtones enquiry as to availablity of a 12 Vdc stat and a method to employ it.
Not advocating.
As far as current carrying capacity of the wire, the 22 ga wire in the Roadtrek is rated at an ampacity of 920 millamps. Our measured current draw is 320 millamp. The referenced tstat draws 180 millamp.
Amperage load should not be an issue.
Personally I prefer the Battery operated unit.

It is good to keep that clear in recommendations, I think, especially for safety issues. Thanks for clarifying.


Re the draw. Wiring only gives you an indication of IF a circuit is current limited, not what the actual limit is as long it is under the wire capacity. The only way to know what the limit is would be to get it from the manufacturer who likely will not tell you because of safety, liability and proprietary reasons. The home furnaces of the past also had extremely low draws but couldn't handle any more than that. It may have to do with inrush current being higher if it goes to the board power relay coil.

 

[DWorrell3]

Just to finish this off... After eliminating all of the wires directly connected to the replaced heat pump, I was left with four. One red, one black, and two green. The red and black were 12V power, which I do not need for the heat only, non programmable thermostat. The two greens were actually spliced into the two blues at the furnace. One was positive and one negative. These were connected to the thermostat and the furnace worked beautifully. Dave W