Heating and Cooling your home is one of the most basic necessities and most of the time things work without issue but there are times when things go wrong. In this How To we will look at some of the most common problems with a Gas Furnace. Some of them you can easily fix yourself and others you might want to call a professional. If you are in doubt then always contact a professional to do the work and you should not take this How To as professional advise it is just a review and overview of what things might go wrong and how they are fixed.
The Number 1 Gas Furnace Problem Dirty Flame Sensor
The most common problem for Gas Furnaces is the flame sensor. The flame sensor is a small metal rod mounted near the main flame of your furnace. When your furnace is ignited the flame sensor senses if the burners are running and then tells the computer to allow the furnace to continue running. Diagnosis: If the flame sensor is dirty or not operating correctly you will watch your furnace begin to run, your igniter will start to glow and then gas will be ignited. Within 2 seconds the furnace will shut down and this process will happen again. Eventually the computer will lockout and stop attempting to ignite the furnace. Fix: In most situations the flame sensor is just dirty. It is possible to clean the sensor by using a fresh dollar bill that is rough and rubbing off the carbon. This would be equal to about 2000 grit sand paper. You do not want to scrub the sensor with anything more aggressive or you can damage it. In other cases the flame sensor might have completely failed and will need to be replaced. The cost for a new sensor can be as cheap as $5 or maybe as high as $20 so if you are someone that likes to work on their equipment and it only costs $7 you might want to pick one up and keep it in storage. To replace the sensor you need to first find it. The sensor has a single wire to it and it is located near the burners. It should be held in place by a small bolt. Remove the wire from the sensor and then remove the sensor. You can then either try cleaning it which works 80% of the time or replace it.
The Number 2 Gas Furnace Problem Bad Igniter
We just mentioned the igniter and what it does when we were diagnosing the flame sensor but lets go over it again. When your furnace starts your small inducer fan will send air to your burners it can be seen in your furnace as a small black wheel that spins. It provides air only to let the gas burn. After the inducer fan starts the igniter should begin to glow very brightly and then after that the gas valve will open to provide gas to the burner. If the Igniter is not glowing then the gas will be released but the flame sensor will turn the furnace off. Diagnosis is very similar to the flame sensor but what you are looking for is the igniter not glowing.The furnace will attempt to light a few times and then lock its self out. The Fix: unfortunately you can’t just rub the igniter with money and make it work. That sounds so weir but hey it works for the flame sensor. To verify that your igniter is getting power you can use a Volt meter and when the Igniter Circuit is active it should provide 115v power to the connector. If it has power then you can suspect the igniter needs to be replaced. Igniters normally run about $20 at the time we are writing this and they are easy to replace just like the Flame Sensor there should be 2 wire cable going to it and you remove the wire at the connector and then undo the bolt holding it in place. These igniters can be fragile especially the older ones that are shaped like a W vs the newer ones that are just a straight rod. You can replace old ones with a newer style but if you are doing the work yourself as long as you are careful you can use your direct replacement part to make the fix. Since this is a replacement only item then its not a bad idea to pickup an extra if yours is cheap.
The Number 3 Gas Furnace Problem Dirty Filters
Yes, your Furnace Filter or the Duct Vent Filters or even too many closed ducts in your house can stop your furnace from running. What happens is as we went over before the small inducer fan supplies air to your burners starts to run and your furnace lights normally but because you have restriction of the main fan that is supplying air that flows through the house the furnace shuts down. If the furnace continued to run the burners would get too hot and it might damage the furnace. So even though it might look like everything is running and the furnace is just shutting off it could be because of a restriction of air due to a dirty filter. Diagnosis: Remove and inspect the furnace filter and replace it if you have a new one. Sometimes even filters that look clean can be restricted especially if you have a humidifier running in the winter. Humid air can mix with dust and form a thin film of packed dust on the filter restricting it. Then visit the vents in the home and make sure they are open and if they have vent filters you should remove them. Fix: If the furnace runs after removing the restrictions then you know what the problem was.
The Number 4 Gas Furnace Problem Main Blower Capacitor
There are two capacitors on most HVAC Systems. The first is on your outside condenser unit for your air conditioning and you can expect to replace those capacitors every 5-7 years so you should always have an extra on hand. There is also a capacitor on your inside furnace main blower fan. This capacitor does not normally blow out but it can. If it does you will have a good blower motor that just won’t start because the capacitor is failing. Diagnosis: Watch your furnace attempt to run and it should fail to start the large blower motor fan and shut off relatively quickly to prevent the burners from overheating. Gain access to the blower motor by removing the filter or by removing an access panel and after the furnace ignites spin the blower motor fan blade with a screw driver or wood dowel not your fingers. If the blower motor runs when you bump start it by hand then you know the capacitor is blown. When Capacitors go bad they normally puff up but you can also test them with a volt meter if your meter can do capacitance tests. Remove the capacitor and the wires and run a capacitor test. Fix: Capacitors are inexpensive about $25-35 so you should get the numbers off of the one in your system and keep a backup on hand. Replace the capacitor and your furnace should run correctly.
The Number 5 Gas Furnace Problem Is Your Thermostat
Today most thermostats are powered by low voltage that they get from the furnace. If there is a problem with this connection or the circuit providing power to your thermostat your furnace will not function. For LED thermostats you can see that the panel is not lighting up. Another problem that is very common is that the batteries in unpowered thermostats go bad. Replace the batteries and everything will function correctly. Thats why it is often best to have a very basic thermostat that does not require batteries. Finally you could just have a malfunction of your thermostat and it needs to be replaced. However the worst feeling is when you look at your thermostat and it is set to off. Yes this does happen so turn it back on and everything will be ok. Diagnostics can be performed at the furnace to see if the thermostat is passing power to call for heat. This test is a little beyond the basics but your furnace manual will probably have a diagram and instructions. If you have a fancy thermostat then you need to read its manual. Newer home automation systems can get pretty complex so read the manual correctly and make any tests and repairs necessary.
Final Note
These are the most common reasons your furnace might fail to operate however there are a few more that are also common. Failure of your Gas Valve does happen and they can cost from $50 to $300 just for the valve. A Computer Control Board can also go out and it is very difficult to predict how much you will pay for one especially on older models. They might not even be available and unlike oil furnaces you can’t always find one that will work other than the original. That could lead to furnace replacement.
As we said some of these parts are cheap and you can check prices online and order backups to have on hand when you might have problems.
When in doubt always call a professional.