We have a gas furnace with AC. A few years ago our thermostat was "acting up". I don't remember exactly what the problem was, but we wanted a programmable thermostat so I bought one and installed it. I'm sure I installed it correctly - it's pretty simple as long as you put the right wires on the right screws and I did. The new thermostat worked fine for a couple of years, then it started acting up, too. It would not turn the fan on when the fan switch was set to "auto". If the fan switch was set to "on" the furnace would ignite and heat the house and the thermostat would properly regulate the temperature in the house while the fan ran constantly. If the fan switch was set to "auto" you could hear the furnace ignite and the clicking as it warmed up, but the fan would never come on. Eventually, of course, the furnace would get hot and shut itself off because the fan never came on to carry the heat away from it.

This evening I bought a new programmable thermostat and replaced the other one. But the new thermostat has the same problem as the old one. So I am confused. Why can I turn the fan on/off with the fan switch, but the thermostat cannot turn it on/off? Is there a fan relay at the furnace end that might be getting weak? Or something else?

Thanks!

We have a gas furnace with AC. A few years ago our thermostat was "acting up". I don't remember exactly what the problem was, but we wanted a programmable thermostat so I bought one and installed it. I'm sure I installed it correctly - it's pretty simple as long as you put the right wires on the right screws and I did. The new thermostat worked fine for a couple of years, then it started acting up, too. It would not turn the fan on when the fan switch was set to "auto". If the fan switch was set to "on" the furnace would ignite and heat the house and the thermostat would properly regulate the temperature in the house while the fan ran constantly. If the fan switch was set to "auto" you could hear the furnace ignite and the clicking as it warmed up, but the fan would never come on. Eventually, of course, the furnace would get hot and shut itself off because the fan never came on to carry the heat away from it.

This evening I bought a new programmable thermostat and replaced the other one. But the new thermostat has the same problem as the old one. So I am confused. Why can I turn the fan on/off with the fan switch, but the thermostat cannot turn it on/off? Is there a fan relay at the furnace end that might be getting weak? Or something else?

Thanks!
Please provide the model of your fan and the programmable thermostat you have... in your wiring was there a magnetic contactor attached to your fan? It is a device that to turn on your motor fan or maybe there is a thermal relay that is open in your fan?. if thermal really is open you can not turn on the fan no matter what you do... please provide a wiring diagram and how did you install it... dat would give me a picture to your trouble.

There is a fan switch in your furnace. Not knowing what you furnace is I don't know how the fans switch operates, some are heat operated. A bi-metal strip heats and as it get to a certain temp it turns on the fan and then as it cools it opens the switch and turns off the fan. Some are controlled by time and are set to come on so many seconds after the burners have come on, both types can go bad and need replacing, so there is where I would start looking for the trouble. The fan "ON" switch at the thermostat is a direct switch to another fan relay that turns the fan on and off as you use the fan "ON" switch.

Hi,
Thanks, you guys have given me a good start.
Sorry that I left out the make of the furnace, it is a Rheem. I do not know anything about the fan other than that it is inside of the furnace, so is possibly also a Rheem. The old programmable thermostat is Honeywell and the new one is a RiteTemp (I think that's a Home Depot house brand?)
You both seem to be leaning towards a thermal relay and that makes sense. Any hints on how I might find it? Will it be something that is easily accessible or will I have to tear apart half of the furnace to get to it?
Thanks again!

--Brian

Start at the blower motor. The wires from it will either lead to a couple of little boxes with 4 wires, or on newer furnaces, to a control board. The control board is a bad deal, difficult to prove the relay is bad, and expensive when you have to replace the whole thing. Then you discover the problem is actually the blower motor.

In the heating mode, the thermostat only transfers power from the red wire to the white. The blower is controlled by a timer or in older furnaces, a limit switch as already said.

Thanks to those who answered. I've narrowed it down to a couple of thermal switches. It turns out that there is a thermal switch which monitors the temperature of the heated air being blown into the ducting. There is also a thermal switch which monitors the temperature inside of the control area of the furnace. These are both actually wired in series and ensure that the furnace does not overheat. (I figured this out from a schematic which was pasted inside of the door covering the control area.)

I think the problem is probably the switch which monitors the temperature in the control area. The reason is that I took the cover off the control area and started tracing wires, identifying parts and relating them to the schematics. Then I turned on the furnace and the pesky thing worked! I probed around for a while, watching the operation and everything was working fine. So I put the cover back on to go enjoy the heat. After a while the furnace started acting up again. It got into a state where the fan would start up on low speed, then almost immediately shut down. Now it's back to not working at all unless I have the fan switch turned on at the thermostat. I am going to stick a thermometer in the control area to see what the temperature is when the cover is on. I'll probably also try yanking off the cover and measuring voltage drop across the switch, but I don't know how fast acting it is so I'm not sure this will prove anything.