Is your furnace not starting up, and you're not sure what to do? It is always a good idea to troubleshoot the problem and see if you can fix it on your own. Here are some areas of the furnace that should be simple to investigate as part of the startup process.
Even a gas furnace requires electricity for it to run. Verify that you have power running to the furnace, such as checking the circuit breaker to flip a circuit back on. There are also power switches that other people may have unknowingly flipped into the off position.
If your furnace has power running to it and it's not turning on, then you need to observe the startup sequence to look for what part is failing. The draft inducer is one of those parts that is easy to check, so pay attention to what it is doing. It's job is to bring air through the heat exchanger to push all fumes out through the exhaust during the combustion process. If the draft inducer is not running, then the furnace will stop working as a safety precaution. Monitor the draft inducer during startup to make sure that the motor hasn't seized up. If so, you'll need to have the part repaired or replaced.
Once you know that the draft inducer is doing its job, then the furnace's pressure switch could be the next point of failure. Low pressure is what causes the pressure switch to trigger, which prevents your furnace from turning on because it senses that those fumes cannot escape to the outside of your home.
You can rule out a problem with the pressure switch by finding other areas that would cause low pressure during the startup sequence. Look for anything that could be obstructing the flu vent or blower motor port. The pressure switch tubing can also have a blockage within it. If you find a blockage, clear it and see if that fixes the problem. If there is no blockage, then you could have a problem with the pressure switch itself and need the part repaired or replaced.
Gas Valve and Igniter
It is possible that you have a faulty gas valve that is now allowing the gas to flow through and ignite the flame. Makes sure that the gas valve is turned on and that gas can flow freely through it. If it is on and no flame is present, then it could be a problem with the gas valve or igniter. These two parts are harder to troubleshoot, and will require the help of an HVAC technician.