Troubleshooting Common Furnace Problems With an HVAC Expert

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 22 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...

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

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.

Below, I explain each of these processes in 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.

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.

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 they aren't loose, it's unlikely.

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 of for testing a regular 24v (low voltage) circuit but not a high voltage one. In other words, don't use these instructions if you're not troubleshooting your furnace.

If you hear the furnace trying to light (ignite) or hear the blower running but aren't getting any heat, then you can skip this step, because you have power. If not, then check the following...

• Did a breaker trip?
• Is the service switch at the furnace on? (Most units have this mounted to the furnace or ceiling just above.)
• Is the little red light on the furnace's circuit board on? (Most modern furnaces have an indicator light to tell you it's getting power, and if you're lucky, tell you what might be wrong.)

Got power?

YES: Okay, then let's move on.

NO: Then you may need an electrician or HVAC professional. Before you call, though, let's check the fuse on the circuit board.

Most modern furnaces have 3 amp fuses on the circuit board (although it might be located elsewhere on the boards of certain furnaces). It looks just like the standard size fuse you find in your car and should be easy to find.

If you have an indicator light and it's not on or flashing, this may be your problem and is easy to determine and fix.

Simply pull the fuse off the board and look to see if the link inside is burned and/or broken. If so, you should be able to pick one up at the local hardware or automotive store. DO NOT try to substitute the fuse with one of another amp rating.

I've been on calls where kids playing around the furnace have turned knobs and flipped switches, costing their parents a lot of grief (and about $150).

If there's a little indicator light on your furnace's circuit board (usually visible through a tiny window on the furnace door) and it's flashing, it's trying to help.

This light blinks a sort of Morse code, a number that corresponds to a chart located somewhere on the furnace. It's usually on the doors that you remove from the front. For example, a "dot...dot...pause...dot...dot...pause" would be the number 2, which the chart may say indicates something like "ignition failure" or "pressure switch open."

Understanding the red blinking light on your furnace will help you navigate through the following possible issues that have a possibly simple solution.

Sometimes, the little red light indicates an open pressure switch, which often means a flue/chimney blockage. It's also possible the pressure switch is bad, but before we decide that, let's see if the chimney is blocked. Birds and other small animals have been known to take up residence in flues when it's particularly cold. Check for this (and/or icing) by...

• removing the chimney pipe from the top of the furnace to see if there's anything in there. If you have a high efficiency furnace, this may be difficult, but animals are less likely for you. Also check outside to make sure the ends of the pipes are clear and not iced over.
• making sure the little fan hooked to the flue (the inducer fan) is working. If not, you'll probably need a service tech.

DO NOT get up on the roof to check the chimney unless you are completely confident and comfortable in doing so. It's not worth the risk, since usually the problem isn't up there. On occasion, birds will build a nest in the chimney, but if you haven't had problems so far this winter, it's not likely your problem. This is something that becomes apparent early in the heating season.

This is easy:

• Watch and listen to your furnace as it begins to cycle. Just before you hear the gas valve click open, you should see the hot surface ignitor begin to glow or hear the click, click, click of your spark ignitor. (You might even see this spark.) If you don't see or hear these things, the furnace will stop the cycle.
• If your ignitor isn't working, the gas valve won't stay open until you replace the ignitor because the flame sensor is telling the valve that the gas isn't being lit.
• Make sure the gas is on. I have been on calls where kids playing around the furnace have turned knobs and flipped switches, costing their parents grief and about $150, so be sure the gas is turned on as well.
• If the ignitor is working but the gas valve didn't open, I am inclined to have you call a heating and cooling technician to work with a gas valve. This is not a simple repair and can cause dangerous issues if not done properly.
• If you have a pilot light, look to see that its flame is lit. If not, this is likely your problem. Try to relight it just like you would a water heater. If it won't light, then you probably need a new thermocoupler. These are cheap and can be bought at most hardware stores. (Measure how long of one you need before you go.)

If your furnace lights the burners and then shuts off just a few seconds later, this is almost surely a fault of the flame sensor. This is one of the most common furnace problems homeowners face.

Cleaning and/or replacing a flame sensor is one of the most common and simple heating and air conditioning repairs one can make. Without a doubt, you should give this a try on your own before calling an HVAC technician.

Beyond the items we've discussed, there are many other things that could be wrong with your furnace. I have identified the simple fixes above, and if you still haven't diagnosed the problem, messing around any more could compound both the problem and the expense. Although you may not want to, you should consider calling a professional HVAC technician.

Please understand that any time you work with a furnace, there is a certain amount of danger that requires your attention and care. Never perform work on a furnace if you're not sure of what you're getting into and that you've taken all the safety precautions that you can with gas, electric, and sharp edges.

I hope we were able to solve your problem but if not, at least you will have a much better understanding of your furnace which will help you with your upcoming service call. You'll be a very informed customer when they come knocking.