This is a common furnace problem. It’s probably the capacitor, a small and relatively inexpensive electrical part that is easy to locate in your furnace and replace. It could be a bad blower motor, which is a moderate-to-expensive repair, but let’s hope for the best at this point. Again, it’s usually the capacitor.
Capacitors come in two types: Start capacitors and run capacitors. Most furnaces currently being made have only one, and it is typically referred to as a run capacitor. Dual run capacitors are used in AC condensing units and packaged heat units – they run two fans at once such as the blower fan and AC fan.
The good news is that replacing a capacitor is a quick fix whether you DIY (cheap) or call a pro (moderately expensive).
The DIY rating on this repair is, in our opinion, a 2 or 3 out of 5. The rating depends on how much you have to remove to get to the capacitor
Let’s Be Sure We’re On the Same Page
Is your furnace blower humming but no air is blowing through your grates? You’re not hearing the blower fan rotating – just the light buzzing?
Thermostat setting is higher than the temperature reading on it: Check
No air is flowing: Check
Most furnaces emit a soft hum at all times when the power is on.
However, we’re talking about a louder hum that could also be described as a buzzing noise.
Troubleshooting a Bad Capacitor in your Furnace
Furnace blower motors take a lot of juice to get moving, since they are heavy. Your furnace voltage is 110-120, and that’s not enough to do the job.
The capacitor stores up to 400+ volts of energy. The stored energy is released to kick start the blower when it’s time to start dispersing furnace heat and pulling in cold air to be heated.
Here’s how to troubleshoot a bad furnace capacitor:
Approach 1: If you’re not planning to do the repair, call a furnace company. A humming blower that won’t rotate when the thermostat is calling for heat is enough to point a finger at the run capacitor and shout, “It’s your fault!”
Approach 2: Try to manually start the blower. OK, we can’t recommend this for liability reasons. But here’s what some people do.
1. Tool: Find a wooden dowel, paper towel tube or long screwdriver. Fingers aren’t recommended, but it’s your call, of course.
2. Direction: Determine which way the blower is supposed to be rotating. There is often a directional arrow on the blower housing.
3. Turn the thermostat down, and shut off the furnace using the switch on or near the furnace.
The switch, like a common light switch, is easy to see on this Carrier.
4. Try to rotate the blower fan in the direction it should be going. If it rotates freely, then the motor isn’t seized up. There’s another clue that it’s the capacitor putting a chill on your home.
5. Fan On: With the furnace switch still Off, go back to the thermostat. Turn the Fan to On mode (it is probably in Auto mode).
6. Furnace On: Turn the furnace switch On. Does the hum begin again but the fan doesn’t start rotating?
7. Spin the Fan: OK. This is where it gets dicey. Use the dowel or fingers you can afford to lose, and with a quick flick or nudge, attempt to spin the blower in the direction it should go. This is you supplying the extra energy needed to overcome the blower inertia.
8. Does the Blower Fire Up? If it does, you’ve verified that the capacitor is almost-certainly the problem.
9. No? If the blower motor doesn’t fire up, it is likely burned out. That’s another repair, and one we recommend leaving to a furnace repair technician.
How to Test a Capacitor to Be Sure
If you have a multimeter and want to know for sure if the run capacitor is bad, it’s not hard.
Remember we said the run capacitor stores energy?
The amount can be more than 400 volts!
Please pay attention to step 1 below, or it might release it on you. That’s a pretty good poke, but is easy to avoid.
If you aren’t interested in the job, that’s understandable.
Call your favorite furnace repair company. If you don’t have one, use our Free Local Quotes Tool or phone number, and you’ll get no-obligation quotes from at least 3 pre-screened, license and certified furnace repair technicians in your area.
Here are the steps.
1. Discharge the capacitor. Get an insulated screwdriver (rubber/synthetic/plastic handle), and make sure there are no cracks in the handle. Then, lay the screwdriver shaft horizontally across the two terminals like this.
Note: This should be done with the furnace switch Off and the capacitor still in its place.
2. Remove the capacitor. Remove the screws holding the
3. Know what reading you should get. The capacitor rating is printed on it. You’re looking for the micro-farad number. Look for the Greek “m” followed by an F. A Greek “m” looks like this:
Microfarads are also referred to as MFD.
Often the printed label will say “+/- 10%. So, a “good” reading on a 10 MFD capacitor would be anywhere from 9 to 11 on the multimeter with it turned to Capacitance (see next step).
4. Test the furnace run capacitor: Turn your multimeter dial to capacitance. It is this symbol:
Put one of the meter leads on each of the capacitor terminals. Hold them and watch the meter.
- Dead: If nothing registers after 30-60 seconds, the capacitor is dead.
- Wearing out: If the number rises, but after a minute doesn’t get to the microfarad rating (plus/minus 10%), then it is wearing and should be replaced.
- OK: If the meter reading shows the capacitor is still good, then we recommend calling for furnace service. A technician will conduct a similar test on the blower motor and give you a price to replace it, if the motor is bad.
Here’s a quick summary video to show you what has been described in the steps above.
How to Buy a Capacitor – And Where
How: There are two ratings to match.
- The microfarad / MFD rating, as described above.
- The voltage rating.
The new capacitor should be rated the same on both numbers to ensure you’ve got the right part.
Where: You can buy a furnace run capacitor online at HVAC parts sites like RepairClinic.com and SearsPartsDirect.com.
Locally, they are harder to find. Home improvement stores and hardware stores usually do not have them. Many local HVAC parts stores only sell wholesale to HVAC companies.
You might have to make a few calls to find a local HVAC parts dealer that sells to homeowners.
Furnace Capacitor Cost
The cost ranges from about $5 to $20 for the part. Even if the capacitor isn’t bad this time, it might go bad at some point. Savvy DIY homeowners buy one, along with an extra igniter (another common, but easy to fix problem), to have the parts ready for use if needed, so they don’t have a day or week to get a part.
The cost of a furnace capacitor installed by an HVAC technician will be $80 to $200. The part is cheap, and it only takes 5-20 minutes for the repair. However, most HVAC repair technicians charge a minimum fee of at least $75.
Repairs on nights and weekends bring higher costs. If you can wait in the cold without risk of the pipes freezing, you can save money by scheduling the repair during normal hours.