Heat works but not the AC or Fan switch w/ New Thermostat

[Ed Alonso]

I installed a new programmable thermostat that works OK for heat. However the fan doesn't come on when the fan switch is engaged, and the AC doesn't work either.
I have already replaced the thermostat with an identical one so I don't think it is a problem in the thermostat.

Any ideas?

Thanks.

[imported_rlfrazee]

Ed sounds like either a transformer problem or relay problem. Check for 24 volts on the rc terminal at thermostat.

[Ed Alonso]

I do not have 24V at the RC terminal.
I do have 24V at the RH terminal though.
The AC & Fan did work with my previous thermostat.

Would this still indicate a problem with a transformer or relay?

Thanks.

[imported_rlfrazee]

Ed, Im certainly no expert on heating and cooling but the rc terminal should have 24volts on it. This is the control voltage for the relays that turn on your blower fan motor and a/c. when you put your switch in the cooling position this transfers control voltage to the fan and a/c relays causing contacts to energize your fan and a/c. The rc terminal also supplies the control voltage to the fan switch. Check your wiring and trace back to transformer or transformers. I cant help you much with programable thermostats. Seems though common sense tells us if everything worked with the old thermostat you may be short a wire or wired wrong at new thermostat. How many wires does this programmable stat need? I think I would see how many wires I have coming into the thermostat. If you have rc and rh as mentioned I believe you need five wires......RL

[imported_rlfrazee]

Ed here is discription of connections on mercury switch type thermostat. Hope this will help



Wired
This thermostat is designed for a system with five wires -- the wire terminations are marked as follows:

* RH - This wire comes from the 24VAC transformer on the heating system.
* RC - This wire comes from the 24VAC transformer on the air-conditioning system.
* W - This wire comes from the relay that turns on the heating system.
* Y - This wire comes from the relay that turns on the cooling system.
* G - This wire comes from the relay that turns on the fan