Ask Me Help Desk - Blower won't run in auto for heat with oil furnace

Blower won't run in auto for heat with oil furnace

I have an oil furnace.

A/C works fine.

Heat last season worked fine.

Started up furnace this year just fine. But blower would not start on auto. The blower will run in heat mode when set to On.

I replaced the Limit Switch in the Plenum. No change.

I replaced the Fan Relay. No change.

Not sure what to do next.

Any ideas?

Thanks in advance

Did you check your thermastat?

You might check the wiring going to the fan motor if this is a 2 speed motor. Sounds like the low speed portion of the motor might have a problem. Check voltage at relay and go from there

Quote:

Originally Posted by eschuen

you might check the wiring going to the fan motor if this is a 2 speed motor. Sounds like the low speed portion of the motor might have a problem. Check voltage at relay and go from there

If there was an issue with the wiring to the fan motor, wouldn't this also be a problem in the A/C mode? In any event, unless I'm misunderstanding you, I only have one speed other than off.

Thanks.

In most systems the blower uses a slower speed from a different winding controlled by a different relay than in A/C or fan on. So it is perfectly possible for the blower to work fine in A/C andfan on, but not heat. Look at the wires to the blwer motor. A single speed motor would only have a hot and a neutral. It is has more than 2 wires, it is a multiple speed motor.

Note, in heat, the thermostat has nothing to do with the blower. The blower is controlled by a limit switch or a timer on the control board.

Quote:

Originally Posted by labman

In most systems the blower uses a slower speed from a different winding controled by a different relay than in A/C or fan on. So it is perfectly possible for the blower to work fine in A/C andfan on, but not heat. Look at the wires to the blwer motor. a single speed motor would only have a hot and a neutral. It is has more than 2 wires, it is a multiple speed motor.

Note, in heat, the thermostat has nothing to do with the blower. The blower is controlled by a limit switch or a timer on the control board.

Thanks for the feedback.

There are only two wires going to the motor. The motor has several wires that give you the option of what speed to run at depending on which one is connected. The red wire on the motor is the one that is connected and sets the speed at low.

Also, my furnace is of a vintage that does not use a control board. Other than the limit switch in the plenum (replaced), the fan relay (replaced), there is one additional switching component. It has 3 low voltage wires that run back to the relay and has one hot wire. There is a "dial" on the switch that sets a time adjustment between 150 and 200 sec. I assume it is to set the length of time the blower runs after the furnace switches off.

Thanks again.

Quote:

Originally Posted by blueskies4ever

Thanks for the feedback.

There are only two wires going to the motor. The motor has several wires that give you the option of what speed to run at depending on which one is connected. The red wire on the motor is the one that is connected and sets the speed at low.

Also, my furnace is of a vintage that does not use a control board. Other than the limit switch in the plenum (replaced), the fan relay (replaced), there is one additional switching component. It has 3 low voltage wires that run back to the relay and has one hot wire. There is a "dial" on the switch that sets a time adjustment between 150 and 200 sec. I assume it is to set the length of time the blower runs after the furnace switches off.

Thanks again.

I'm still having the same issue as originally described and as per my follow up. I thought I'd try and put this out there one last time in the hope that someone might have an idea of what my problem might be.

Thanks.

I frequently leave questions hoping somebody that knows more than me answers. Often as not, it is somebody that knows less.

Try jumping the hot wire to the timing switch to the terminal on the relay that the other wire goes to. If the motor then runs, the switch is bad. Remove it and take it to a HVAC supply and let them match it.

Quote:

Originally Posted by labman

I frequently leave questions hoping somebody that knows more than me answers. Often as not, it is somebody that knows less.

Try jumping the hot wire to the timing switch to the terminal on the relay that the other wire goes to. If the motor then runs, the switch is bad. Remove it and take it to a HVAC supply and let them match it.

I'm afraid I must be dense. There is one hot wire going to the timing switch. When you are saying to jump this to the terminal on the relay that the other wire goes to, I'm afraid I don't understand how are you distinguishing the other wire. I also assume the switch you are suggesting I fault isolate is the one with the time dial on it, is that right?

Thanks again.

Say blueskies4ever, how did you fix this relay problem? I have a very simlar problem.