Why won't my furnace shut off?

Question

I have a forced hot air oil furnace. There has been a programmable thermostat on it for 5 or 6 years, and recently we had a problem where the furnace cut in and would not stop (it was set for 70 degrees and got up to 78 before I shut off the emergency switch). I replaced the batteries - did not help - so I bought a new programmable thermostat - problem still occurs. I called the thermostat company and they have never heard of this problem - I called the furnace company and they said it must be a thermostat problem. Help?! Most of the posts I have read relate to the blower not shutting off - but in my case the burner keeps running as well. The house just keeps getting hotter and hotter.

Answer 1

Play with the thermostat do you hear a click? That will tell you if it is able to "sense" the temperature.

Answer 2

I would check one thing first. There should be 2 somewhat small wires that go to the thermastat. If these are shorted the furnace won't shut off.

Disconnect them from the thermostat and then touch them together, the furnace should start. Pull them apart, the furnace should shut down. (If it does you need another new thermostat)
If it doesn't look for anywhere that the bare wires might be touching. If you find it either replace the wires or insulate them with electrical tape.

In some furnaces these wires lead to an electricaly operated switch which may need replacing.

If not

There is a pump that feeds the furnace with oil. If it is stuck on the furnace won't turn off. And as long as the furnace is hot the blower will work.

These are different depending on the model and type of furnace. and I'm afraid your going to need to get a repairman for these repairs. sorry.

Answer 3

This Site Might Help You.

RE:
Why won't my furnace shut off?
I have a forced hot air oil furnace. There has been a programmable thermostat on it for 5 or 6 years, and recently we had a problem where the furnace cut in and would not stop (it was set for 70 degrees and got up to 78 before I shut off the emergency switch). I replaced the batteries - did not...

Answer 4

The thermostat connects 2 wires when it turns the furnace on. These 2 wires run back to the furnace. Your furnace will either be the old kind that has one simple relay that turns on, or the new kind with a circuit board. In the case of the relay, the contacts have frozen shut, and the relay needs to be replaced. In the case of the circuit board, there is something called an Optocoupler (or even possibly another relay) that has failed shut. In either case, you will need some knowlege of relays or optocouplers, or even of electronics in general to diagnose the problem. There is also the possibility that the wires have shorted somewhere, most likely near the thermostat or the furnace(the later being the 75% likely).

If you can get to your control panel on your furnace, email me, and I'll describe the problems for you and how to fix them. You'll need an OHMeter to check for continuity. MAKE SURE you cut the main power to the furnace before you start opening it up.

Answer 5

Aquatherm Furnace

Answer 6

Aqua Therm Furnace

Answer 7

Make sure the thermostat is wired correctly to the heater, and is not using the air conditioner controls. Heaters turn off at temperatures higher than the set point. Air conditioners turn off at temperatures lower than the set point.

You can also check the terminals at the thermostat. Figure out which one goes to the heater. If you short the feed and the input to the heater, the heater should turn on. If you disconnect those two, it should turn off. If it doesn't turn off, the problem is with the furnace. If this experiment works, then it's your thermostat.

You also need to make sure that the "span" is correct. The span determines the gap between the temperature at which it is on and off. For example, with a temperature set point at 70 and a span of 6, the thermostat will go on at 67 and off and 73. If your span is set too high, then the thermostat will only turn off at a temperature much higher than the set point.

Answer 8

Okay. I'm having the exact same problem. Disconnected the thermostat...then the thermostat wires at the control transformer. Burner STILL kept running. Put a meter on it and have a dead short between R and W *internal* to the transformer/control board. There's a physical relay on the board, but that checks out okay. I just bought a whole new board. I checked all the wiring for damage. Fingers crossed that this will fix it.

Answer 9

Sounds like:

A whole bunch of people don't know anything and are willing to give their opinions.

Sounds like there is a relay on the furnace that has stuck contacts and this is causing the unit to keep on running. You see the thermostat is simply a low voltage switch that makes a relay (high voltage switch) close its contacts and run the boiler. When these contacts open and close they arc. This arc will eventually cause the relay contacts to stick or weld themselves together.

Call a repair service and they will look for a stuck relay and replace it.

Answer 10

Value the lives of everyone in your home and call a repairman. If that furnace is gas it could do just about anything including putting out carbon monoxide. The silent killer!
If it's electric the same problem except for no carbon monoxide.
Good Luck!

Answer 11

I would call the furnace repair guy.
It is likely to be some sort of switch that is malfunctioning.
But it could also be that the sensor to the thermostat is not working for some reason. It is surely located somewhere in the intake, as this is where the air temp is monitored by the machine unless you know that is located on a wall sensor in the room.