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This is a perfect troubleshooting guide if your Furnace is not kicking on when the temperature drops. There can be a few reasons your furnace is not kicking on when the temperature drops, but these mainly depend on your system type.
If your thermostat is not kicking on when the temperature drops, this could be due to a bad ignitor, depleting fuel from a gas tank or thermostat wiring and setting issues. To fix the problem, ensure that the thermostat is set to heat mode and use a multimeter to measure continuity across the ignitor terminals.
Before you start troubleshooting any problems on your furnace, check for any error codes on the furnace. If there are error codes, refer them to the user manual for your equipment or check the label on the inside of the panel of your furnace. However, if no error code is displayed, this troubleshooting guide should be good enough to help you fix this problem.
Reasons A Furnace Won’t Automatically Turn On
If you use a central heating and cooling system, the problem of your furnace failing to kick on when temperature drops could be attributed to several factors. These include the following:
1) A defective circuit control board.
When the temperature in your home falls below the set temperature, the thermostat will call for heating by sending 24 volts to the furnace. And if the circuit control board is not defective, it should relay the heating signal to the ignitor and the blower for the furnace to start heating.
However, your furnace won’t kick on when the circuit board is defective, and each time there is calling for heating from the thermostat. So how do you know if there is a problem with the circuit board? You can check for 24 volts across the w and c terminals on the terminal block of your furnace.
Your thermostat is good if there are 24 volts on c and w. However, if the furnace does not kick on, the control board is defective when there is a call for heating.
In cases where you don’t have 24 volts across terminals C or W on the terminals, the problem could be with your thermostat wiring (I will be talking about this shortly).
The c wire ensures a continuous power supply to your thermostat, and the w terminal controls heating.
So, when you have a defective circuit control board, it cannot send power to the gas valve to open. Another solution is to replace your circuit control board. You may need help from a licensed HVAC technician with this.
2) Bad ignitor
A furnace kicks on with the help of an igniter. When the ignitor is defective or is not getting hot enough, it will not be able to light up your furnace. But how do you know if your ignitor is defective? Check for spots on the ignitor. But the most effective way to check whether or not your ignitor is defective is by using a multimeter.
Disconnect the ignitor and put the multimeter across its terminals. If no continuity means you probably have a defective multimeter.
A reliable solution to a bad ignitor problem is a replacement. Ignitors are relatively cheap; you can buy one from a local hardware store or online.
3) Problems with the gas supply.
Its rare for a gas valve to become defective. Usually, when it’s not working, it’s because it’s not getting the 24 volts from the circuit control board. So, here is the thing, if you can hear the gas valve open when there is a call for heating, then there is probably no problem with the gas valve.
And if the utility company supplies your gas, you want to ensure that you open the manual gas valve fully. Some homeowners think it is OK to partially open the gas valve, but that is not true. When you need the gas valve open, it should be fully opened.
However, another problem is you could be running low on gas. Make sure there is enough gas in the tank. If you are not on natural gas from a utility company but on a propane tank, ensure there is enough propane in the tank otherwise, you could be running low on fuel.
This is especially the case if you are experiencing this problem mainly at night, and this is what could be happening.
During the day, the sun would heat the propane tank enough to increase its temperature and pressure. This forces whatever little fuel is left in the tank to fire up your furnace. But at the night, the tank will cool and be left with little pressure not enough to force the fuel to fire up your furnace.
4) Thermostat issues
Now, this is the most overlooked issue. Remember to put your thermostat to heating. If not, no matter how much you raise the temperature point, your furnace will not kick on. Go to your thermostat and check what is currently there; if it shows a fan, change it to heat.
5) Dirty flame sensor
The flame sensor is a device that detects if ignition has been successful. Meaning if there is no flame, the flame sensor will send a signal to the control board, which will, in turn, close the gas valve. This is to prevent leakage of the gas into your home.
A dirty or defective flame sensor could be why your furnace is not kicking on when the temperature drops.
Disconnect the flame sensor and use sandpaper to clean it. Consider getting a new flame sensor if this doesn’t fix the problem. If you are buying from a local store, don’t forget to go with the sample to ensure that you buy a compatible one.
6) Problems with the drain
There is a condensation pan that collects excess water. When blockages are in the drain, the reservoir will become full, preventing the furnace from starting.
While draining the water will temporarily fix the issue. A long-term solution is to fix the blockage.
In a nutshell
In summary, these are the common issues that can prevent your furnace from kicking on when the temperature drops:
- Defective circuit control board
- Bad ignitor
- Problem with the gas supply
- Dirty flame sensor
- Thermostat issues
- Problems with the drain
We hope you have fixed the problem with the tips in this guide. If not, it’s a good idea to call professionals for help.
HVAC tech with over 30 years of experience. Retired and doing repair work on the side around Madison County, AL.