750mv vs 12v Thermostat Question UPDATE 2
Updated 29 Nov-- Update 2--29 Nov: got it to work. Not clear on the thermostat voltage for these things whether I can use the cheaper version or what. I want it for my furnace only set-up in the R...
Forum Discussion
BFL13 wrote:
I got that RTH111 thermostat and the wiring instructions made no sense so I called Honeywell and was told this thermostat will not work with my 12v system etc etc.
So I told him other guys use it and they say it works, so where do I attach my two 12v wires--pos and neg?
He said you can try using just W and R and makes no difference which for pos and neg. He says if it does not work, it will not fry the thermostat, it just won't work. ( He said the CT53K is the model I should have for this job.)
Anybody who has posted above that theirs works want to comment on how you got it to work with your wiring? I will wait till later to hear about that before I try it out using the W and R. (He said never mind the jumper between R and Rc, just leave it there)
W and R are the terminals you want to connect to. on heating cooling thermostats Rh is call for heat, Rc is call for cooling but doesn't switch power.
but you will connect +12VDC to the W terminal and you will connect the wire that went to the furnace to R. the 12V Common or ground isn't connected to the thermostat in this case. and in fact If what you had was a typical RV thermostat, it has a +12VDC wire and a wire to the furnace but no ground connected to the thermostat. didn't need one. basically it is acting as a switch to the furnace switching the +12VDC to the furnace control board call for heat.
DO NOT CONNECT +12VDC TO THE W TERMINAL AND GROUND TO THE R TERMINAL (OR VICE VERSA). THAT WILL SHORT 12VDC TO GROUND WHEN THE THERMSTAT CALLS FOR HEAT.
W normally comes from one side of the 24VAC and R goes to the call for heat which triggers the furnance to start.
In the case of 750mv systems, W comes from one side of the thermopile and R goes to the gas valve, so when a call for heat comes on 750mv goes to the gas valve. ground is via the common between the two.
In the case of an RV hookup, +12V goes to W and the wire going to the furnace goes to R. When there is a call for heat, then +12VDC goes to the furnace controller to turn on the heat.
here is a link to a picture
https://xtronics.com/wiki/images/Thermostat.gif
