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Troubleshooting Common Furnace Problems With an HVAC Expert

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Dan has been in the HVAC industry for 23 years with experience ranging from installation and service to sales and distribution.

Diagram of an HVAC system, which also includes furnaces.

Diagram of an HVAC system, which also includes furnaces.

Before You Start

Before you call out the pros, why not take a shot at fixing that furnace yourself?

I have been in the HVAC industry for over 23 years. There are a lot of things that can go wrong with a furnace, but many issues are easy for anyone to figure out and fix.

Even if you need a part, don't be discouraged; many of the most commonly replaced parts are easy to install and can usually be bought from a local supplier. Your odds for getting the heat back on sooner rather than later are actually not too bad. Below, you'll find the following in this article:

  • A furnace troubleshooting checklist
  • Tips on common furnace repairs
  • A video guide to help you fix it yourself

Furnace Troubleshooting Checklist

Below, I explain each of these processes in more detail. We start with the simplest problems and then test for more difficult issues. It's important to be methodical and systematic in your diagnosis.

  1. Check the thermostat settings.
  2. Check for power via the breaker, switch, indicator light, and fuse.
  3. Check that the inline gas valve is open.
  4. Check filter.
  5. Check ignitor for ignition.
  6. Check flame sensor for ignition confirmation.
  7. Check for flue/chimney blockage: pressure switch.
  8. Know when to call a service technician.

1. Check the Thermostat

I know it may sound silly, but before we get carried away, let's check the thermostat for a couple of things. You might be surprised how much money per year is spent having guys like me come and turn up peoples' thermostats or replace their batteries.

Thermostat Problem Checklist

Sometimes big problems have easy fixes.

Check...It Might...

for a jumbled digital display

need replacing (we can test this to be sure)

for a flashing battery indicator

need new 'AA' batteries

temperature setting

not be turned up high enough

function selection

not be turned to "heat" (switch off and on again to be sure)

inside for loose or touching wires

just need tightened or tidied up

How to Test a Thermostat

Testing your thermostat is really easy. All you need is a small or medium sized screwdriver and a short (6") piece of 18 gauge wire (what we'll call a "jumper wire"). If you go to the hardware store, ask for "18/2."

Simply go to the furnace, remove the doors, and follow these steps:

  1. Locate the circuit board where the thermostat wires connect to the furnace.
  2. Disconnect the wires connected to the "R" and "W" terminals. (The wires should be red and white, but aren't always.) Push them safely to the side.
  3. Connect your jumper wire between the "R" and "W" terminals.
  4. Re-secure the door and see if the furnace works.

Did it work?

NO: Then your thermostat isn't the problem.

YES: Then it's likely your thermostat is bad and needs replacing. It could be the wires connecting the thermostat to the furnace, but if they aren't loose, brittle, or broken, it's unlikely.

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NOTE: Do not leave the jumper wire on the furnace to run it. It will make the furnace run continually without any temperature control, which can be extremely dangerous and cause other problems. Only use the jumper wire to see if the thermostat is faulty.

ALSO: These instructions are for testing a regular 24v (low voltage) circuit only. In other words, don't use these instructions if you're troubleshooting a line voltage circuit. (120v or higher.)

Mercury-Controlled Thermostats Must Go

If you have an old thermostat that senses the temperature with a mercury bulb, get it replaced as soon as possible. It's not nearly as accurate as newer digital models, and since it contains mercury, it's not safe.

Replacing Your Thermostat

If you decided you need to replace your own thermostat after performing the steps above, or you have an old mercury controlled version, you'll need to select a thermostat to use.

Most reputable thermostat makers will provide very clear directions with their units on how to install them, and it really isn't too difficult for the homeowner to do themselves if you take your time and follow the supplied directions.

You can get all fancy and buy a Nest thermostat that connects with your Alexa and all the other modern home-automation devices. Or, if you're like me, you just want something simple, easy to install, and easy to read. I prefer something like this Orbit Clear Comfort Thermostat (below).