Gas and Oil Home Heating Furnaces - Gas furnace blower issue

I have a gas furnace. Once in a while I would notice the furnace blower come on for 30 seconds then shut off. After a short time (1 min) the furnace would come on again for a normal heat cycle. I read through a ton of posts, and the problem seemed to be with my fan/limit switch. So I started to watch what happens.

When the blower came on for the 30 seconds, it was without any burners going. (I don't recall if the pilot was on. - electric start). So I decided that IK would adjust the limit settings. When I did this, I think I turned the dial, and I'm not sure if it went back to it's original position. I got nervous then and saw the white plunger. I tried it in and out see what the difference was. (I don't recall it's original position).

My situation now is this. When the plunger is set to Auto, things seem to be ok. When I set to manual, the fan comes right on and I see the dial for the limit switch rotate. Way past the off position.

If limit switches are not to expensive, I'd be inclined to replace it if i screwed this one up. If they cost a lot, I'll just leave it on Auto.

Can I re-calibrate the switch?

Thanks for your help!

Dan

danieljanderson:

It sounds like it could be the limit switch.
The setting should be on @ 160 off @ 120 and limit @ 180.

Regardless of how much it costs it is a safety device.
If your not sure on this you would do well to call a pro.

It does not make sense to risk you and your families safety for a couple of bucks.

I feel I could do it myself.

How much would I expect to pay?

What good is Auto if I shouldn't trust it?

danieljanderson:

I have already given instructions on setting the fan/limit control.

Your correct in that you must be able to trust the auto setting because not only does it control the fan, it will also be the safety cut-out for the burner .

Cost?

What brand is the control? If it's Honeywell, you can download literature at http://hbctechlit.honeywell .com Type in either the model # which starts with an L or Fan Limit Control. When you pull out the white button the fan will run full time. At www.grainger.com you can look for prices.

Save your money!,,,your switch is most likely fine.
As long as you have a hair dryer and a temperature gauge of some sort, you should be able to check the switch. pull the switch after you write the wiring hookup down, and sit at the kitchen table. The little slides on the face of the dial move for adjusting the set points, not the dial. The dial is hooked up to a helical spring, as the spring gets warm and lengthens, the dial rotates. use the meat thermometer and a hair dryer and see if its within 15 or 20F, if it is, your fine. The reason the dial continues to rotate down is the room air is 70 or so and that is about where the dial is before it will stop rotating because its cooling off. Use an ohm meter to see if the switch opens at the set points. If in manual, the fan never stops turning. The fan switch and limit switch can be separated schematically speaking, see Honeywell for the wiring schematic. Note if you do decide to replace the part, measure the probe and note if the jumper is in place at the bottom of the V shape black plastic / or if the jumper has been
cut.

the fan combo limit switch is a relatively cheap part, and contains a safety to turn off the gas overtemp. if is giving you trouble replace it is my advice.