Thermostat Wiring

Summary: Replacing a thermostat can be tricky because of the variety of colors of the electrical wiring used for to control the unit.

Electrical Circuit

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Question :
Replacing Honeywell t6160 room stat
(Mains, 3-wire)with Horstmann DRT2
(Battery operated, 2-wire).Maker says live to common and switched live to no - Normally Open but does not understand third wire which goes to `call` in JB then, via link to another terminal, to motor valve (3 port, motor open, motor close). It is not live whether stat calls for heat or not. Should I disconnect it and isolate? Can the one wire make the valve actuator open to either port and return it to center as well?

Name : Ian
Hi Ian - Great Question!
I would have to see the wiring diagram to understand the function of the valve actuator.
In my experience it is essential that when replacing stats that they are a direct match to ensure that they do operate all the components of the system.
If you do have a direct replacement then you may have to look very carefully at the instructions and match up the colors of your stat wires with the terminology and abbreviations of the wiring diagrams, and many of them will be a challenge - been there, done that, and I end up making a cross reference chart to map it all out just to be sure.

The abbreviations are as follows:
comm = Common, should be the common side of the control voltage fro the unit's transformer.
no = Normally Open (and nc = Normally Closed - Normally being the un-energized state )
call = Calling for Heating/Cooling - should be the signal back to the unit to begin the start process.
Again - you are really going to have to study the original wiring and new instructions and map it out.
I always encourage others to write down the wiring of the original stat, take a picture of it, or in a case where the wires are long enough I just cut the wires off the original stat so that the original connections stay on the original stat along with the insulation colors showing so you will know what wire went to what terminal, then just transpose this pattern to the new stat.
Also - be aware that some units have fuse protection for the control voltage transformer, some do not.

Thermostat Wiring:
Comm = Common
NO = Normally Open
NC = Normally Closed