Homeowners guide to troubleshooting and repairing the top 3 reasons your furnace is not working...

During the coldest part of the year in Colorado, the last thing you need to deal with as a homeowner is not having heat. Getting your furnace serviced is expensive, and in the mountains, service providers usually charge a trip fee in addition to their hourly rate and a markup on materials. If you are a little bit handy, there is a good chance that you can fix some of the most common issues I have seen: your furnace fan runs, but no heat comes out.

 For the purposes of this article, we will assume you have power, and the furnace is blowing cold air for a few minutes then shuts off or lights then shuts off, and keeps repeating that cycle. You will need a basic understanding of tools like; Phillips screwdriver, flathead screwdriver, wrenches, etc.

If you are unsure at any point, please call a professional. If you smell gas turn off the gas supply and call 911.

If your furnace is supplied by propane (you have a large tank in your yard), make sure you are not empty. I have gone to several service calls, and the client did not realize they had run out of propane.
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Modern furnaces use a component called an 'ignitor' to light the fuel (propane, or natural gas) instead of a pilot flame, which then heats the air. Older pilot style furnaces use a 'thermocouple' or 'Flame sensor' to make sure the pilot light stays  lit, and they shut off the gas to the pilot flame if it goes out. Ignitors and thermocouples are prone to failure at the most inconvienant times, but are very easy to replace and inexpensive. Both styles of furnace also usually have an additional 'flame sensor' that detects the large main flame once the furnace burner has lit, and will shut down the furnace if the flame goes out due to a backdraft or other reason. These three items are the source of 80% of service calls.

If you have a furnace with a pilot light, and no heat is coming out (it may blow cold air for a minute then shut off), it is probably the thermocouple or flame sensor. try to relight the pilot light; if it will not stay lit, but you can get it to initially light, it is usually the thermocouple or flame sensor, which is a sensor that is attached next to the pilot light, and sits partially in the flame when lit. If the pilot light is lit, but the furnace lights then shuts off, you may have a second 'flame sensor' that detects the main burner flame; CLICK HERE . You can tell the difference because a  thermocouple will have a single copper tube and the flame sensor will have braided metal wire or just wire that goes to a connector on the furnace.

The following steps can help you see if one of these simple items is the cause of your furnace failure:

​FOR ALL STEPS START HERE:

• The first thing you need to do is turn off the power to the furnace. Furnaces are required by code to have a disconnect within sight of the furnace. NOTE: Turning off the thermostat does not shut down the power to your unit. They usually look like this:

• Second, TURN OFF THE GAS. There is a valve on the gas line coming into the furnace that you need to turn off. If you are unsure how to do this, the call a professional.
• ​Once you have confirmed the power is off, then you need to remove the front cover. These usually just slide up and off. They may have screws attaching them, and are often sticky or slightly resistant to removal.

​PILOT LIGHT WON'T STAY LIT:

• Once the cover is removed, if you have a pilot light that won't stay lit, then you need to remove and replace the thermocouple or sensor. To remove the thermocouple, you use a screw driver or wrench to remove it from the bracket next to the pilot flame and a wrench to remove the other end of the copper tube from the gas valve. To remove a flame sensor, you unscrew it or take off the nut and disconnect the wire. so you can go to the hardware store get a new one (McGuckins carries an assortment and they usually are $20.00-$60.00). You will need your furnace Make, Model number, and the old thermocouple or sensor when you go to the store. Some thermocouples are permanently connected to the pilot assembly, and the entire assembly needs to be replaced; This is beyond the scope of this article.

• Once you have purchased a new thermocouple or sensor, installation is the reverse. Follow any directions that came with the new thermocouple or sensor. Relight the pilot and make sure it stays lit, then replace the cover and restore power.
  
• Confirm the furnace works.

 FURNACE RUNS BUT BLOWS COLD AIR THEN SHUTS DOWN (FURNACE DOES NOT USE A PILOT LIGHT):

• If you do not have a pilot light, then you usually have an ignitor. If the furnace runs but will not light (the fan may run and you might hear a click sound several times before shutting down) then follow the initial steps above to disconnect power, turn off the gas and remove the cover.
• Once the cover is off, you need to locate the ignitor; It will be near the main burner. The ignitor operates by glowing white hot causing the burner to ignite. You can see this process if the cover is off and the furnace is running normally, or if the condition is that the furnace main burner lights then shuts off right away (see flame sensor replacement below if that is the case). Here is a typical furnace ignitor location (note the large round burner tubes)

Ignitors are identifiable by there location next to a burner. They have two wires going to a connector on the furnace and they usually have a ceramic base. Here are a few examples, with the one on the right being very common:

• Often, when you remove an ignitor it may look fine and still not work or you might see a discolored area that indicates it is not usable.  like this one:

• Ignitors are fairly straight forward, and easier to replace usually, than thermocouples. Simply unplug the old ignitor at the connector (it may have two wire nuts instead of a connector) and replace it with a matching one. You will need your furnace make, and model number. You may also be able to use a 'universal' ignitor. McGuckins and other hardware stores usually have these for $20.00-$50.00.
• If the connector on the new ignitor does not match, or the old ignitor had wire nuts twisted on, you can cut the connector off of the new ignitor strip about 1/2" of insulation off of the wires and use the old wire nuts if they are in good shape, or use the ones that come with the universal kit. 
• Reinstall the furnace cover and turn the power on.
• The new ignitor should glow brightly , then the main burner should ignite.

PILOT IS LIT OR IGNITOR GLOWS AND FURNACE MAIN BURNER IGNITES THEN SHUTS DOWN:

• The flame sensor is, as described above, a safety feature to make sure the main burner flame is lit. It is similar to a thermocouple. It will be located just above the main burners and will only have one wire going to a connector. It may be very similar in size to some ignitors but does not glow and get hot. It may also be located in the sheet metal just above the burners. Some examples:

• Remove the connector on the flame sensor
• remove the screw holding the sensor
• inspect the ceramic for any cracking or chipping. If it is damaged you need to replace it.
• you can take some sandpaper or a emory board and very lightly sand the metal rod to clean it. This sometimes works. Just be gentle. 
• Reverse the steps above to re-install the cleaned sensor or install a new one. They are usually under $30.00.
• Re-start your furnace
  

If you were successful, then CONGRATULATIONS! You are on your way to saving money, and understanding the systems in your house, as an added bonus, now your warm. 

As always, if it doesn't work, it's probably time to call a qualified service technician.