Why Won't Your Furnace Turn Off?
Usually, when something is wrong with a furnace, it does not turn on. So if your furnace is stuck in the "on" mode and won't turn off, it's normal to be perplexed. Rest assured — this does mean something is wrong with your furnace. The good news is that most problems that cause a furnace to get stuck "on" are easily repaired by an HVAC contractor. Here's a look at a few of them.
Disconnected Thermostat Wiring
Your thermostat is wired to your furnace. One of those wires is meant to communicate with your furnace as to when it should turn on and off. If the thermostat wiring gets pulled out, shorts out, or otherwise becomes damaged, the thermostat may stop telling your furnace to turn off — even once your home reaches the set temperature. An HVAC contractor can replace the wiring, or if needed, replace the entire thermostat.
There are a couple of sensors in your furnace. One senses when the thermostat is in "on" mode. The other senses when there is heat coming from the burner and tells the fan to turn on in response. If either of these sensors is broken, then your furnace may get stuck on. Or, it may keep blowing air through your ducts, even if the burner itself is not producing any heat.
These sensors are small parts. An HVAC contractor can undo a few screws to remove the current sensor. Then, they can screw a new one into place.
Broken Limit Switch
The limit switch is a switch that is responsible for turning the fan on when the furnace burner is on. It then turns the blower motor off when the burner is off. (This is a different part from the sensor that detects whether the burner is on or off. The sensor detects the heat, and the switch manually controls the fan.) If this switch breaks, then your furnace can either get stuck on or off, depending on the mode it was in when the switch broke. A furnace repair contractor can easily replace this part as it is small and inexpensive. Broken limit switches are often covered under warranty.
If your furnace is stuck in "on" mode and you need to turn it off, you can switch off the breaker it is connected to. Then, call an HVAC contractor. They'll figure out what part has broken and make the necessary repairs.
For more info, contact a local AC contractor.