Gas and Oil Home Heating Furnaces - Blower motor turns on......

I have an old forced air oil furnace in my house. After running a burner/blower cycle the blower motor starts agian after about 1 minute. I watched the stack control ( I think thats what is called ). After the blower shuts off the heat exchanger warms up ( the burner is off ) enough to tell the blower to turn on again. It only runs for maybe 20 seconds but it wear & tear on the motor. The stack control is new this year. This is a really old/simple furnace not alot of control boards. So do I move the tab that turns the blower on & off to a higher position of a lower psoitioin? Or is there another fix? Thanks Tim

So do I move the tab that turns the blower on & off to a higher position of a lower psoitioin? Or is there another fix? Thanks Tim

Yes. The control is called a fan switch, and it controls the operation of the fan. One that has a dial that rotates to measure the temperature would be common, and you can adjust those just as you suggest.

Commonly there are three tabs. The one for the highest teperature turns off the burner if the furnace overheats.

The middle tab sets the temperature at which the fan turns on, and the lowest temperature tab is the one that shuts the fan off. So you probably want to set that last tab to a slightly higher temperature.

SP....set the tab to a higher temp? Wouldn't it shut off earlier and reach the on setting even faster? Doesn't he want to set it just a bit lower to extract more heat from the exchanger, so it will not hit the on setting?

Hey gunguy---

Having cool air drafts is annoying. Increasing the temperature at which the fan shuts off can be expected to eliminate the annoyance.

SP...no I understand what you are saying, but maybe I'm not thinking correctly. I had the same issue on my rooftop package unit and IIRC, I adjusted the lower limit down just 5 degrees or so. It removed the last of the risidual heat so that the fan wouldn't turn on again after the burners shut down.

Thats what I read as the OP's concern, that the air handler fan would come on again after the initial shutdown. Seemed like he wanted the standard cycle...burner on, fan on, burner off, fan off. Repeat as needed.

Not argueing at all, I just read the posts differently I think.

<<SP...no I understand what you are saying, but maybe I'm not thinking correctly. I had the same issue on my rooftop package unit and IIRC, I adjusted the lower limit down just 5 degrees or so. It removed the last of the risidual heat so that the fan wouldn't turn on again after the burners shut down.

>>


Old oil furnaces are famous for retaining a fair amount of heat in the heat exchanger. Mechanical fan switches and blower can cool off the air, allowing shutting off the fan. But the heat exchanger can then warm the air enough to turn the fan on again, which is what Tim didn't like. Often that creates drafts and having the fan turn on again can be annoying.

A small amount of heat may be lost. Not enough to matter much.

The poor guy probably got confused and left.

The bottom line is his "off"-"on" settings are too close together, no matter what they are set at. That means that when the blower stops (at "off"), the heat that rebuilds in the heat exchanger, all by itself, will trigger the "on" again, since it was so close to the "off" setting. If he sets the "off" lower, AND the "on" higher, causing more separation, it will stop his problem. OR - if the fan "off" was too high anyway, and he could benefit to recover more heat without windchill effect, just set the "off" lower. OR - if the "on" setting was too low, set that higher.

Ok ecman you got what I was talking about. The on and off needed to be further apart, one way or another. Closer together would only make it worse as I understand them.

Anyway, hope Tim got it the way he like it.

I'm still here. ;) Thanks for all the input.

This is a pretty old furnace. Technically the Blower on & Blower off tabs are the same tab. Or at least one tab performs both functions. Burner comes on & warms heat exchanger, blower kicks in, burner turns off, blower turns off, heat exchanger warms up again from left over heat, blower comes back on for 20 sec then goes off again. I was having some problems with the switch at the beginning of the heating season So I bought a new one & installed it. There are 4 wire connections on this switch. 2 are labeled "Fan" one for "Load" one for "Line" and 2 are labeled "Limit" one for "Load" one for "Line". Wires are hooked to Fan Load, Limit Load & Limit line. Fan line has no wire. There are 3 movable tabs The tab at the lower end doesn't seem to do anything. The middle tab controls fan on AND off. The upper tab I'm not sure it never gets there but I'm guessing that turns off the burner if it gets too hot. There is a chance that I put a wire on the wrong terminal If I put the Fan Line wire on the Limit Line terminal what would happen? I don't think I did but anything is possible.

Sounds like a typical Honeywell fan/limit control, or one that works like one. You say the 2 lower tabs cause both?, yet you say there indeed are 3 tabs. And yes, the top one is for high limit. The bottom one is supposed to be turning it off. Something is haywire. Are you sure you cannot seperate those 2 bottom ones some?

Sounds like a typical Honeywell fan/limit control, or one that works like one. You say the 2 lower tabs cause both?, yet you say there indeed are 3 tabs. And yes, the top one is for high limit. The bottom one is supposed to be turning it off. Something is haywire. Are you sure you cannot seperate those 2 bottom ones some?

Tenically the middle tab turn the blower on & off. It may be possible that they are so close together that it looks like the one tab is doing both funtions but maybe the lower tab is actually doing the off function

Oh BTW yes it is a Honeywell Fan Limit control.

Yes. That is why you are having that problem. Separate them. Hold the round dial. Do not allow it to turn!, as you part those 2 lower tabs, by say a quarter inch for starters. May be varying opinions on where to actually set. But you may try 90 off, 110-120 on, and the limit where it is at at either 180 or 200/just leave it alone.

Thanks everyone. I lowered the fan off limit and everything seems fine. Seems the 2 tabs where too close togehter. Normally I would have had the furnace reapir guy do the repair but $ is a bit tight. Thanks Again

Glad you got it working the way you like...