Forced Hot Air Heating System Blower Running Frequently
I’m not sure of the correct terminology but it was explained to me that there is a thermostat inside the plenum that causes the blower fan to turn on/off depending on the temperature inside the plenum. The blower fan has been turning on and off even when the furnace motor has not run. The HVAC tech said he knew exactly what the problem was when I explained the symptoms over the phone. He said that the 2 temperature limit switches were too close together. I interpreted this to mean that the temperature settings at which the blower would start and stop were too close. This thermostat was replace a couple of years ago. My questions are (1) should this thermostat require additional adjustments after its initial installation and (2) should it be checked as part of the annual cleaning/tune-up?
Replies
A little unclear. A standard old-fashioned gas furnace has a burner and a fan motor, and no separate "furnace motor". Most units that have a motor associated with the burner don't use an old-fashioned plenum thermostat.
But assuming you have a conventional plenum thermostat, there are three settings: Fan-on, fan-off, and overtemp. The fan-off temp is the lowest, the fan-on something like 10-20 degees warmer, and the overtemp 20-30 degrees warmer than that (just pulling numbers out of my hat).
When the burner comes on, at first the fan doesn't start. But then, when the temp reaches the fan-on temp (the middle setting) the fan starts to run. The fan cools the plenum, so the plenum temperature may, in theory, drop after the fan comes on. If components are properly mated & adjusted, though, the plenum won't get so cold that it would blow cold air, nor will it get so cold that the fan-off temp is reached. But when the wall thermostat turns off the burner, the plenum temperature will drop below the fan-off temp and the fan will turn off "normally". If something bad happens (eg, the fan fails or the ducts are completely blocked) the the overtemp limit may be reached, turning the burner off (and hopefully keeping the house from burning down).
If the fan is cycling on/off while the burner runs this would normally indicate that the fan-off setting is too high. But there could be other reasons for this: The burner may not be putting out full heat for some reason, or the ductwork may be pulling in cold air from outside. And probably a few others.
Normally these thermostats are pretty reliable and will last 20-30 years, but they do fail on occasion. And, if the fan-on adjusment was on the "hairy edge" when first adjusted, some minor change (eg, removing register grills that were restricting airflow) might cause that edge to be crossed.
Note that there's no great harm in having the fan cycle on/off occasionally. It's annoying and very slightly increases motor wear, but the world won't end. However, I kind of favor setting the fan-off setting several degrees lower than "stock" to "milk" the last bit of heat out of the plenum during the furnace turn-off sequence.
Thank You
Thank you for the detailed response. FWIW it's an oil burner. My electrician was the one that noticed the problem. I'll try to re-explain. Furnace runs and then the blower fan runs blowing hot air. Furnace stops running and blower fan stops running. A few minutes later the blower fan runs and then stops. A few minutes later the blower fan runs and then stops. This blower fan run/stop sequence continues several more times but the furnace has not run during this time.
By "furnace runs" I assume
By "furnace runs" I assume you mean the oil burner fires. I'm remembering it's something that looked vaguely like a leaf blower in the furnaces from 50 years ago.
The heat exchanger of an oil burner is much heaver than for a gas burner, and retains more heat after the burner shuts down. The fan carries away the heat on the air side of the heat exchanger, but there is still heat "buried" in the exchanger that takes more time to work its way out, so it's conceiveable that the fan could run several times after the burner shuts down.
If the new plenum thermostat is a generic one for both gas and oil then likely it came pre-adjusted for gas. For oil one probably needs to lower the fan-off setting a few degrees to minimize the effect you see. (But it may be impossible to eliminate entirely.)
Yes, Oil Burner Fires
Yes, when I say 'furnace runs' I mean the oil burner fires. A few years ago this thermostat was replaced. I'm wondering why after 2 years it needs adjusting? The HVAC tech knew immediately what the problem was. The furnace had its annual cleaning/tuneup a few months ago. Could the tech have mis-adjusted it during the tuneup? Is there any reason to touch this thermostat during a tune-up if customer hasn't mentioned a problem? I just received a bill for the service call and I'm wondering if it is justified....am I being charged to fix a tech's mistake?
Well, hard to say what might have caused it to start behaving differently. As I indicated earlier, some change in ductwork could cause this, or possibly a change in the burner output (the tech may have adjusted this during the previous service call). I'd not generally expect him to adjust the fan-off setting, but he might have tweaked the fan-on value, and, with some thermostats, the two are kind of linked together.
Thank You for taking the time to respond to this. I'll talk to the company owner....I've been a customer for over 25 years.