Furnace Temperature Fan switch Problem
holmie
| Posted in Energy, Heating & Insulation on
Have an oil fired furnace with forced air. Fan sometimes runs long after the burner has kicked off blowing cold air out of our ducts. I have looked at the unit and it has a temperature switch that once at the proper temp kicks on the fan and also shuts off the fan after the desired temp has been reached. I have had to lightly tap the temp switch to get the fan to stop running when it won’t shut off automatically. My question is two fold. Is the switch gone bad or is there an adjustment or simple fix I can make to get it functioning right or do I need to install a new one and if so are they available to “Joe Homeowner” at the local furnace supply shop?
Thanks in advance
Replies
I don't know about oil furnaces, but our old gas furnace had one of these and it was adjustable.
If you pulled off the cover there was a dial inside with three adjustment levers. High temp to low they were the burner cutoff temp ("high limit"), the fan on temp, and the fan off temp. You could adjust them (within limits) by holding the dial and moving the levers.
Your unit may not include the "high limit" feature.
Note that it's critical to HOLD THE DIAL while making this adjustment, to prevent damage to the unit.
In your case you'd want to move the lowest lever (fan off) to a higher temp (by 5-10 degrees). If the dial is marked in degrees you probably want fan off setting somewhere above about 105 degrees (though possibly higher, keeping in mind the inaccuracy of the markings). Note that there may be a "bumper" between fan off and fan on settings to keep them some distance apart, so you may end up moving the fan on setting as well.
If adjustment doesn't fix it or the unit isn't adjustable then you likely need to replace it.
Does your control resemble the one in the pic?
If so, it could be a simple case that the mechanicals are hanging up. Happens all the time with that type of fan control. Try removing the cover and locating the sheet metal screws that are attaching the unit to the hot air plenum, so that you can extract the element/probe. Usually you don't have to detach any wiring to do this. Might take all of the slack provided, but should be doable. Then spray the bi-metal element/probe down with some manner of lubricant. Liquid Wrench with teflon works pretty good (lasts a couple years or more) or if you have some aersol fogging oil around, that should work, too. Get a little lube on the backside of that revolving disc, too. Then reinsert, reattach and give it a go. Be careful while you handle the thing that you don't move the current settings as they are likely correct for your unit as they are. Just in case, look where they're set before you even remove it so you can return them to those settings should they get bumped. And as already mentioned, hold the disc steady while/if you move any of the setting tabs back where they were.
If it turns out that you need a new one cause this one doesn't respond to your efforts and run right, make sure you get one with the same length of element/probe.