Question about Honeywell CT87B ROUND HEATING&COOLING MECHANICAL THERMOSTAT - WHITE Heater

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I have installed a Honeywell CT8775C, when I put it on the heat the A/C goes on!! I have checked my wiring and everything looks right? what should I do to correct it.

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You should have a minimum of four thermostet wires. The red wire goes on the R terminal. White wire on the W terminal. Green wire on the G terminal. Place the yellow wire on the Y terminal. In most cases NO other terminals are used...... IF YOU DO NOT HAVE A YELLOW WIRE...... place the blue wire on the Y terminal. This should hopefully fix it. IF NOT....See if the thermostat has a RH and a RC terminal. If so I'll bet that you put the red wire on RC ternimal. this is for airconditioning ONLY. The RH terminal is for heating ONLY. Put a small jumper wire that will connect RH & RC. Then this should solve your problem.

Posted on Mar 08, 2009

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Had new heating installed an got honeywell thermastat .weet to put heating on today for firsf time heating wont come on


If it was changed during the cooling season, the first thing I'd check would be for a jumper between the Rc and Rh terminals on the thermostat sub base. If not there, install a short jumper to power the heat side of the system. If there IS a separate wire on each of the Rc and Rh terminals, you have to check to make sure there are TWO transformers in the circuit, one for heating, one for cooling. Most new systems are NOT set up this way, but there a few dinosaurs out there. If there ARE two transformers, do NOT connect the jumper or one of the transformers will immediately burn out. If there ARE two, proceed to troubleshoot the heat side of the control circuit. Look for a secondary fuse or tiny circuit breaker, loose or broken wire on Rh, W1 on t-stat or in furnace. Then call a service tech....If you are not capable of performing the above, call the tech first.

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I printed out the instructions for your thermostat. It looks like you should have the following.

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