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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 6022
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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Need help with a few thermostats.

Customer Question

Need help with a few thermostats.
Submitted: 1 year ago.
Category: HVAC
Expert:  BillyHvac replied 1 year ago.


My name is ***** ***** are your questions?

Customer: replied 1 year ago.
I am trying to replace two thermostats in a Masterol mini-zone TrolATemp system I have. I successfully replaced one of the thermostats which was a 6 wire thermostat and subbase (O, G, B, Y, R, W) out of 4 that I purchased but the wiring was not what I expected on the other thermostats. To explain my system I have a two zone system with the older mercury gold Honeywell thermostats. The basement has a air handler with dampers etc. My main floor has one thermostat with subbase this was the 6 wire I successfully replaced. The upstairs of my house has a second air handler behind a closet in one of the upstairs rooms there is another Thermostat with Subbase Heat/Cool, On/Off levers on the top. This one however was not the same it was a 5 wire setup (RC, RH, G, Y, W). My first question is can I use the 6 wire thermostat I purchased and what would I do? Would I take out the little gold jumper flat connector between the R and RC and connect the RH to the R and RC to RC and leave the Blue and Orange screws blank as is or do I need something else etc? That is one issue I have. The 2nd one I ran into today was on the main floor where I successfully connected the 6 wire there is another round mercury thermostat in our master bedroom but it does not have the Heat/Cool and On/Off levers. I know that just calls the system to run that room. That thermostat has "3 wires" (R, W, Y) I believe from doing some research it is a Series 20 "spdt" thermostat also says TrolATemp on the front. What I did was purchased Heat only manual ones that I thought was right but those only have R and W. What can I use to replace the 3 wire thermostat?
Expert:  BillyHvac replied 1 year ago.

first..the thermostat with rc, rc, y, w, g...does rc and rh each have a separate to it?

Expert:  BillyHvac replied 1 year ago.

and do you have boiler/radiator heat and ducted ac?

Customer: replied 1 year ago.
In the old thermostat on the wall yes the RC and RH have separate screws each. So that one has RC, RH, G, Y, W. The new one bought that worked downstairs has the R, RC, G, Y, W and B. So wondering first if I need to return the 2nd one I got because of the 5 wire one upstairs. No idea why they are different. I can send images of everything needed if that helps. So my house is on oil with a boiler and burner system. The TrolATemp system I can send the wiring as well diagram but not sure why the top floor Heat/Cool one has 5 wires versus the same 6 as downstairs. I will take some pics and send them to you to help.
Expert:  BillyHvac replied 1 year ago.

This is going to be a challenge. You have a 2 transformer system, zones, master thermostat and then secondary thermostats.

The 3 wire stats can only do the mode the mast thermostat is in...

Where you have heat/cool the jumper removes and rc and rh are separate.

The thermostat with B attached is feeding another thermostat via the B terminal.

Your 3 wire stats will need to be a standard 4 wire thermostat but you will only utilize 3.

I am afraid this type of project would require a complete drawing per zone or an onsite fact finding call.

Your system is the "perfect storm" of thermostat wiring...

Customer: replied 1 year ago.
Here are the images. I am attaching two images of the system and two images of the thermostats for problem 1 the 5 wire and 6 wire new one I bought.
Customer: replied 1 year ago.
Attached images that hopefully will help. Can I use the 6 wire thermostat for the existing 5 wire and not use the Blue and Orange somehow or do I need a different thermostat for that one? Also when you say that the 3 wire will only do what the master for that zone that is correct. In fact the one in the main floor has to be on in order for the master bedroom when set to a temp to work properly.
Expert:  BillyHvac replied 1 year ago.

Ok been looking it over.

Unfortunately this is not a quick one and would take a bit of time to draw out.

This means more likely tomorrow for info. I will open to all experts to see if someone can get started on it.


Customer: replied 1 year ago.
Okay thanks. I mean the goal is to figure out what can I replace the 3 wire stats (R, W, Y) with. Looking for a comparable white round digital thermostat or at least what to look for. Then also what to replace the 5 wire one with round stat. I was hoping I could use the 6 wire or I will have to return it.
Customer: replied 1 year ago.
Oh Billy one more thing actually when you mentioned the B terminal feeding another stat totally right. Then I didn't mention the top floor Heat/Cool lever stat the 5 wire one does "not" have another manual actually working with it. The top floor is just that one stat Heat/Cool levers. My main floor the 6 wire one that I was able to replace with the B and O wires that has the secondary stat in my master bedroom no levers just manual setting one and also one in the basement. So actually I think the basement one works off the main floor 6 wire stat like my bedroom. Hope that makes sense. So maybe that is why the top floor only had the 5 wires no secondary stat to communicate with. The system would then be like you said two zones:- Zone 1 - Main floor 6 wire stat (R, G, Y, W, B, O) with two secondary stats that work off this main (master bedroom and basement manual stats). Air handler in the basement- Zone 2 - Top floor 5 wire stat controls whole top floor has its own air handler behind a closet on the top floor no secondary stats.
Customer: replied 1 year ago.
There are two Central Air fans as well outside one goes on when the main floor cool is on and the top floor cool setting turns on the second fan outside
Customer: replied 1 year ago.
Wondering if this one would satisfy the 3 wire stat. It seems to be a R, W, Y. I was hoping they would have a digital front 8775 series one but cant it does then I would be down to just the 5 wire stat.