Hi,

When I switch my thermostat to cool, the furnace fan runs as long as I keep setting at cool, but the condenser stop after about 30 minutes. The fan switch on the thermostat set to Auto.

As far as I remember, the whole system should run for a period and then stop for a while (like taking a break), and then back on running (I set the hold temp much lower than the current temp to keep it running). But the furnace keep running, blowing air all the time even the condenser has stopped.

I stop the system, short the terminal R and Y to test the condenser, it will run if it is a fresh start (if didn't run a while ago) but it won't run if it has been running for a while and stopped. I'm not sure how the system is supposed to work here: is that the condenser has a builtin timer that force itself to take a break and won't run again while taking a break, or is it that the thermostat has full control of it, telling it when to stop and when to run?

I think it may be thermostat that having problem but I'm not sure. I don't want to waste a hundred bucks for a new thermostat just to find out that it is not the problem.

Thanks a lot.

It is better to run the blower a little after the heat or cooling shuts off. There are timers or limit switches to do this. Sounds like one if bad. Work back from the blower relay and see if you can find the timer. Also it may use a different relay for the blower for heating and cooling. The cooling relay could be stuck closed.

I did a little experience last week and record its behavior, here is what I know so far:

When I turn the thermostat to cool (desire temp is 6 degrees lower than room temp), the condenser run for 5 minutes, then stop for 15 minutes before running back again, the blower keep running all the time without a stop.

When the condenser stop (after running for 5 minutes), if I turn off the thermostat, short the R and Y terminal, the condenser will also wait about 15 minutes before running, so I think the condenser does have a wait timer itself.


I'm going to test to see if I let the condenser run alone, will it run for at least 10 minutes. I also want to short the terminals so that the whole system will run to see if the behavior will change. If the behavior of the whole system is different when I hard wire it, rather than letting the thermostat controlling it, I can safely conclude that the thermostat is faulty.


My questions this time:
- I remember that shorting R, Y terminals together with either G or W terminal will make the whole system runs (both blower and condenser), but I don't know exactly how to short them (3 of them together?) and which one (G or W).
- If I let the condenser run alone (to see if it does have a run-time timer of 5 minutes or more), will it damage the condenser and/or the whole system?

Thanks a lot for your help.

I am not sure it would hurt, but I would not run the compressor without the blower. Could confuse things more if it is designed to shut off if the blower isn't running.

As for the wiring:

Usually there is a 24 volt AC transformer in the furnace with the secondary winding connected to a red wire running to the thermostat and a blue wire, common, to the gas valve, A/C relay, and fan relay. From the thermostat there will be white wire to the gas valve, yellow to the A/C, and green to the fan. The thermostat is wired to switch the power from the red to the white, yellow, and green as needed with the blue completing the circuit. Most thermostats and furnaces have the contacts labeled R, B or C, W, Y, and G for the corresponding wire colors. It may be wired to have the A/C control wires return to the furnace and its controls and then a second wire goes to the A/C unit. Internal wiring may replace the green wire if the thermostat does not give you the option of fan only or continuous fan. Digital or programmable thermostats may need the blue wire connected to them.