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 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:
- 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.
- 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.
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
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 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 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.
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).
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. However, the fuses might be located elsewhere on the boards in the case of some 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. If you don't already, it's just a good idea to keep extra fuses on hand since this issue is so common. It's usually not required, but if you can, get some 3 amp fast-acting fuses. are perfect for furnaces. These Bussmann brand fuses
DO NOT try to substitute the fuse with one of another amp rating.
3. Make Sure the Gas Is On
Sounds pretty simple, but it's certainly worth a check. 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 performing the following:
- Remove 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.
- Make 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. For an easy guide on this (with pictures), see my article about how to light an old water heater pilot. If it doesn't light, then you probably need a new thermocouple. These are cheap and can be bought at most hardware stores. Just make sure to 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.
Calling a HVAC Professional
Trying to fix a furnace yourself is great, but experimenting can be dangerous and cause more problems than you already have.
7. Know When to Call an HVAC Professional
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.
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
Questions & Answers
After the furnace has been idle, it frequently (75%) will not provide heat when the thermostat places an order. What happens when it doesn’t work: - the thermostat places the order - the fan starts - the surface ignitor begins to glow, then goes dark - the fan continues. The remedy that works 100% of the time: - turn off the switch on the outside of the furnace - wait 30-60 seconds - turn on switch - furnace works fine until the next long idle period. Ideas?
It sounds a lot like a pressure switch. See if this helps. https://dengarden.com/appliances/Everything-You-Ne...Helpful 38
I jumped the pressure switch because the tube keeps filling up with moisture. What would cause this?
Improper venting is the most common cause of excess moisture. I can't say what may be improper, but that is usually the case.Helpful 1
My furnace will stay lit for somewhere between 30 minutes and a couple hours but then will shut off and show a lockout code. Should I clean the flame sensor?
While it doesn't hurt to try cleaning the sensor, I'm not sure that's your issue. Usually a unit fails, for whatever reason, 3 times before it locks out so I would try and see what's happening prior to that. That might give you a better idea of where in the cycle the breakdown is occurring.
Our furnace has turned on, but wouldn't ignite twice in the past week. When we reset it manually in the breaker box, it works correctly. Any ideas what could be wrong?
There are a couple of parts in a furnace that can either be going bad, or just doing what they were supposed to do due to something happening at that moment. A dirty flame sensor can be intermittent, so cleaning it is essential, or it will become more frequent. Perhaps the pressure switch is going bad, or maybe lousy timing sent a draft down or through the flue, which can trick the switch into thinking it's not getting the right pressure. If that happens more than a couple times in a week (assuming no heavy winds), you may want to replace it before it goes all together. These are common occurrences that are easy to check or fix yourself.
I have a 2003 Lennox gas fired, forced air furnace. The gas fires up but the blower only works sometimes. The HVAC company says I need a new furnace, or new motherboard and air pressure switch. My son did the diagnostic test from the manual and can’t see that I need anything more than new relay. What is your opinion, please?
I don't know what a 2003 Lennox design is but if the testing shows a fan relay is bad then I would definitely try it before making a larger purchase since they aren't that expensive.Helpful 1
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