Thanks for your time. I live in Fredericksburg, VA and this will be my first winter here. With the weather getting cooler, I am finding out the hard way that I have a problem with my gas furnace. Once I switch the thermostat over to heat and with it set around 70-72, the heat will come out of the vents but after a while it will turn to ambient temp. The fan *seems* like it never kicks off but it is hard to say since this problem occurs in the wee hours of the night. IF I turn the heat off at the thermostat, wait a few minutes, then turn it back on, the blowing air out of the vents will be heated. When I crank the thermostat up to 74-76, it is difficult to determine if I have the same problem since it gets too darn hot to wait to find out!
The furnace is the original, I think, and pray that I don’t have to replace it. I have someone coming Fri to look at the system but I would appreciate any input.
Thanks in advance!!! 🙂
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You don't say how old the house/furnace is, but I'm guessing from the 70s.
The typical gas furnace of 30 years ago is designed so that the burner comes on first, and, once the heat exchanger has warmed up, the fan comes on. (The fan doesn't come on immediately because it would be blowing cold air.) After the thermostat setpoint is reached the burner turns off. The fan continues to run for awhile to "use up" the remaining heat in the heat exchanger.
Usually this is controlled by a "fan/limit switch" that is a box with an electrical connection that sits behind the top panel of your standard updraft furnace. (Note that a furnace that blows down -- downdraft -- will usually have a timer instead of the fan switch.)
This box effectively contains two thermostats. One, the limit switch, will turn off the main burner if the furnace overheats (roughly 180F). The other thermostat is the fan switch. It turns the fan on when the heat exchanger temperature reaches, say, 140F, and turns it off when the temperature drops to, say, 120. (The exact settings are a trade-off between comfort and efficiency.)
This switch can fail, be out of adjustment, or be damaged by someone adjusting it improperly.
You can remove the top panel of the furnace, find the switch, and check the adjustments. Usually the cover pulls straight off. IMPORTANT: The electrical connections SHOULD be covered somehow, even with the cover off, but someone may have left off a protective cover, so check that no bare wires are exposed, or trip the furnace breaker off before poking inside.
You should see a disk with a temperature dial on it and three adjusting levers. Check the levers -- the top (limit) should be set between roughly 170 and 190, the middle (fan on) should be between 120 and 150, and the bottom (fan off) should be between about 100 and 130. Generally it's impossible to set the fan on and fan off levers closer together than 10-15 degrees apart, since they will bump into each other.
Note that usually the disk itself will turn as temperature changes -- it is part of the actual thermostat mechanism. For this reason, if you apply force to the disk you can DAMAGE the unit. So, if you try to adjust the unit you must CAREFULLY hold the disk steady as you slide the levers.
From your description, it's possible that the fan off lever is simply set too low -- to 85-90 degrees, eg. If this low (and given the limited accuracy of the switch) it may never get cold enough to turn off the fan.
Another possibility is that the limit switch is triggering and turning off the burner, either because it's set too low, or because the filter is clogged or the fan speed to slow.
Of course, this could be an early high-efficiency unit, and you could be experiencing a pressure imbalance in the burner or some such. However, if you can see the flame in the unit (except through a glass window) it's probably not high efficiency.
Note that if this furnace is from the 70s (and is not a high-efficiency unit) it's basically a hole in your roof through which you burn money. You should budget for a replacement within the next couple of years.
No electrons were harmed in the making of this post.