It’s cold outside and getting colder in the house. The thermostat reads 65°F and the temperature is set at 70. But the furnace won’t kick on. What could be the problem?
As much as we take our home furnace for granted, there are a number of reasons why it may not be turning on when we think it should. Here are several possible situations that you can look for, and perhaps take care of the problem yourself before having to call a furnace service technician to diagnose the problem.
Why My Furnace Won't Turn On
Here are the most common reasons why a furnace won’t kick on - and their solutions. Some are easy; others might require a call to a qualified furnace tech.
Gas Valve Off
Check the gas valve shutoff on the gas line leading to the heater / furnace. Perhaps it was shut off to service the furnace? Or turned off at the end of the last heating season? The handle should run parallel with the gas line for it to be on. Something like the image on the right. If it’s perpendicular, it is off. Rotate it to on, and see if your furnace fires.
Power Switch to Furnace Turned Off
Many traditional furnaces have a switch mounted on the side of the cabinet. It will often look just like a standard light switch in a metal box. If it is turned to the “OFF” position, the furnace will not run.
Your particular furnace may be a heat pump that is located outdoors. These usually have a disconnect box mounted on the house near the heat pump. If is turned off, or if the fuse block is out of it, the furnace, which becomes the air handler when a heat pump is running, will not run.
Pro Tip: It wouldn’t hurt to check the electrical panel to make sure the furnace circuit is turned on too. It might have been turned off for the reasons above – or might have been tripped off during a recent storm or electrical surge? The issue should be fixed by turning on the breaker switch.
Gas Has Been Shut Off/ Propane Tank Empty
It can happen. Maybe you missed that shutoff notice that got buried in with some junk mail. Now the gas company has turned off your natural gas. If you use propane gas, perhaps you have used up all that was in the tank. Some people have a “keep filled” clause in their contract, but not everyone does.
If either of these two sources of fuel have been depleted and your furnace needs them to run, this would be the reason why your home heater won’t turn on.
The only way to solve this issue is to contact your gas supplier and make whatever arrangements are necessary to get fueled up again.
Thermostat Set to “Cool”
Almost every thermostat that has been manufactured in the last 20 years has a way to select between “Heat” and “Cool”. This is because so many homes have central air conditioning that works along with the air handler in the furnace.
If your thermostat is a manual model with a dial to set the temperature - or perhaps buttons to raise or lower it – it probably has a separate switch to choose between “Heat” and “Cool”. If it is either a digital or smart thermostat, there will probably be a program setting to choose between these two modes. Whatever type you have, if it is set to “Cool”, the furnace will not come on.
The simple way to fix this is by setting the thermostat to “Heat” and then setting the temperature control to whatever you desire. If the setting is higher than the room temperature, the furnace should automatically kick on.
Thermostat Set Too Low
This may sound like a no-brainer, but there are many ways a thermostat can be set lower than what you might normally want during the heating season. Kids playing in the house and someone playing a prank are two that immediately come to mind.
If your furnace won’t turn on, be sure to look to see that it is set to a number higher than the ambient temperature of the room. If it isn’t and you want more heat, simply dial it up to what you want, and the furnace should automatically fire.
Loose or Broken Thermostat Wire
Now we’re getting into the area where you might not feel comfortable or qualified to even diagnose the problem, let alone fix it. But if you are a little bit handy with simple hand tools, you can try taking a look inside the thermostat for a loose or broken wire.
There is no dangerous voltage inside your thermostat, but it’s always a good idea to go to your furnace and turn off the power switch that is normally mounted on the side of the cabinet. Take the cover off the thermostat by pulling straight away from the wall, with maybe a little twisting/wiggling at the same time. Inside, you should see several wires of various colors – usually blue, red, green, yellow and white.
Without getting into the details of what each of these wires do, what you can easily do is look to see if any of them are loose or broken – that is, they are not securely fastened into the terminal strip alongside the others.
Pro Tip: The red and white wires are especially important to this issue, so you might just wiggle them gently to make sure they aren’t loose.
If you find a loose wire, you should be able to put it back into its corresponding terminal and tighten the small screw. The terminal should be clearly marked with a letter for each color. “R” for red, “W” for white, and so on.
If a wire is broken, it will need to have a small amount (1/2”) of insulation stripped off the end, then put back into the terminal. Loosen the screw, gently slip the wire into the terminal, then retighten the screw. Replace the thermostat cover and turn the power switch on the side of the furnace on.
Any thermostat can go bad, especially if it has been in service for many years. If you have already removed the cover according to the above item, you can quite easily check to see if the problem is a defective thermostat. For this test, however, the power switch on the furnace must be “ON”.
As mentioned above, the red and white wires inside the thermostat are the ones that turn on the furnace. The red wire has 24 volts on it, and the white is the signal wire to tell the furnace to turn on. If you “short” these two terminals together, the furnace should come on, which would tell you that the thermostat is bad – assuming you have already checked the previous items.
A good way to do this test is to strip a small amount of insulation from both ends of a short piece of wire. Touch one end to the Red terminal and the other end to the White terminal. You could also use the two ends of a pair of needle-nose pliers to short the screwheads in the two terminals. A paper clip bent so that it will reach the two terminals would also work. Just be careful to not short any other terminals, as this could cause an additional and costly problem.
If you have determined that the thermostat is defective, or if it has been on the wall for many years, you can purchase another one at any big box store or local hardware store.
Pro Tip: Maybe the thermostat just needs cleaning? If it doesn’t change temperature - turn high enough to kick on the furnace, see our Thermostat Won’t Change Temperature troubleshooting guide.
Time to Call an HVAC Technician?
The above possible reasons why your furnace won’t turn on are about the limit to which most people are qualified or willing to go. Even some of these may be more than you want to attempt to check. There are many things inside the furnace itself that can keep it from kicking on, but it really should be determined by a competent HVAC repair company.
Go back over the list from top to bottom and double check to see if one of those simpler reasons could be what is causing your furnace to not run. When you are confident that you have done all you can, call your local HVAC technician and let them take it from there. We can make this fast and convenient by using the toll-free number on this page