Troubleshooting Common Furnace Problems With an HVAC Expert
Figuring Out Furnace Problems
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.
Furnace Troubleshooting Checklist
- Check the thermostat settings.
- Check for power via the breaker, switch, indicator light, and fuse.
- Check that the inline gas valve is open.
- Check filter.
- Check ignitor for ignition.
- Check flame sensor for ignition confirmation.
- Check for flue/chimney blockage: pressure switch.
- 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.
#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
for a jumbled digital display
need replacing (we can test this to be sure)
for a flashing battery indicator
need new 'AA' batteries
not be turned up high enough
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
Mercury-Controlled Thermostats Gotta 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.
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.
- Locate the circuit board where the thermostat wires connect to the furnace.
- 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.
- Connect your jumper wire between the "R" and "W" terminals.
- 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.
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 only. In other words, don't use these instructions if you're troubleshooting a line voltage circuit. (120v or higher)
#2: Check for Power to the 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.)
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.
Did the Furnace Blow a Fuse?
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.
#3: Make Sure the Gas Is On
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).
Did You Check the Filter? (I Had to Ask.)
Though it doesn't cause as many problems for heat as it does A/C, did you check the furnace filter?
#4: Check the Flashing Red Light
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.
#5: Check for Blockages
Furnace Pressure Switch Problems
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.
#6: Check Furnace Ignition
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.
NOTE: The igniter part can be tricky because it could be the pressure switch that won't let it light so you may have to run some checks on that switch to be sure which is causing the issue.
- If the igniter glows or sparks then 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 light an old water heater pilot. If it won't light, then you probably need a new thermocouple. These are cheap and can be bought at most hardware stores. (Measure how long of one you need before you go.)
A Furnace Flame That Keeps Going Out
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.
#7: Know When to Call an HVAC Professional
Trying to fix a furnace yourself is great, but know when you need an HVAC professional.
Experimenting can be dangerous and cause more problems than you already have.
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.
~ We're all in this together ~
Questions & Answers
Our furnace will run, and after a few minutes the thermostat shuts off, comes back on, and a few minutes the furnace will shut off and then back on. My thermostat is connected to the C on the mainland, so I know the circuit board is restarting. Top of core looks fine, but I can't figure out if my secondary is cracked? It was installed in 2001.
I would need to run some tests to figure out why your unit is kicking off and on or witness what part of the cycle it's stopping. As for the heat exchanger, you'd probably need to have someone run a smoke or CO detection test and you're likely still to be under a parts warranty. Get a CO detector to be safe.
My unit turns on, but the blower fan will not start unless I start it by hand then it does fine until the furnace cuts off then starts up again. Then I have to give the fan a hand again, could it be that the fan motor is dirty and just needs cleaning?
I hope you're using something besides your hand to do that, but it sounds like the capacitor if that's what's happening. Most often mounted to the front of the fan housing.
I have an Ultra SX 80. It seems like everything is working except the main blower fan. It makes a humming noise and does not turn on even when the thermostat is set to on. Any thoughts? Board or motor maybe?
If the furnace is going through its processes and then after lit, the fan doesn't come on, and the unit shuts down again it would likely be the blower which if changed should change the capacitor on the blower too. That's really the best I can offer you from here.
How do you know when your circuit board on a forced air HVAC unit is defective?
Typically you'll see bad power or no power going from point A to point B. However, there are a lot of A's and B's, so you really need to know what you're testing. There are some "flags" that can give you the idea it's bad but electrical testing is the only true way.
When I turn the furnace on, it starts the blower fan and the inducer fan and slowly the inducer fan slows to a stop by the blower fan keeps running. While all this is going on the hot surface ignition switch never stars to glow and it never fires up what could be causing this?
I believe it's the pressure switch. Check that the chimney isn't blocked or the inducer seeming weak or grinding but if those seem ok I believe that's the problem based on what you've told me.
© 2014 Dan Robbins