A good functioning furnace is essential, especially in the colder months when you want to warm up your home to keep cozy. However, it can be frustrating if your furnace won’t cooperate. It’s quite common for furnace blower motors to have issues, but what if the blower motor hums but won’t start?
The number one cause of your furnace blower humming without starting is the capacitor going out, which is very easy to replace. However, the less common but more expensive cause would be a faulty blower motor.
In this article, we will talk about the signs of a bad capacitor and how you can diagnose the problem without contacting a professional. Moreover, we’ll provide instructions on how you can test your capacitor to ensure that this is the problem.
Table of Contents
Why Is My Furnace Humming But Not Won’t Start?
Humming but refusing to start is not an uncommon problem with furnace blower motors. Most likely the capacitor is the culprit. This is a small and inexpensive electrical component that’s not difficult to find and replace. However, a faulty blower motor is another possibility. That would involve more expense, but as it’s less common, we’ll start with the capacitator.
There are two kinds of capacitors: Start capacitors and run capacitors. Most newer furnaces only have a run capacitor. Dual-run capacitors, found in AC condensing units and packaged heat units run two fans at the same time – for example, the blower fan and the AC fan.
Can I Replace A Capacitor On My Own?
The most cost-effective way to replace a capacitor is to use a quick DIY fix. If you don’t feel confident to carry out the work, a contractor will of course involve more expense. The fix is relatively easy and friendly for DIY beginners. If we had to rate this on a scale from 1 to 5, with five being the most difficult, we say it’s about a 2.
However, the difficulty depends on what needs to be removed to allow you to access the capacitor bracket. In certain models, it’s immediately accessible. In others, you’d need to remove a panel or control board.
Diagnosing The Issue
First, let’s identify the issue
There could be several signs that something is wrong:
- There’s no air being pushed through the grates, although the blower is humming
- The light is buzzing, but you cant hear the blower fan moving
- The thermostat is set higher than the temperature it’s showing
- No air is flowing
Furnaces typically make a soft humming noise whenever the power is on – however, what you need to look out for is a louder, buzzing noise.
Troubleshooting A Bad Capacitor
The heavy motor of your furnace blower requires a lot of power to get started. The furnace voltage of 110-120 is insufficient for this task. The capacitor can provide an extra 400+volts. When this stored power is released, the furnace can start drawing in cold air to provide heat.
You could try to get the blower started manually. Here we stress that this is not what we recommend, as it involves serious safety risks. However, as some people choose this option we’ll explain what they do.
- Gather the equipment you will need, such as a wooden spoon or long screwdriver. On no account, use your fingers – it’s just not safe.
- Figure out the way the blower is facing. Work out which way the blower rotates in operation. You may see a directional arrow on the blower housing which will help you.
- Turn off the furnace. Next, reduce the temperature on the thermostat, and switch off the furnace using the main switch located on or near the furnace.
- Try rotating the blower fan. Using your selected tool, try and rotate the blower fan in the direction it should turn. If it moves freely, the motor hasn’t seized up. This is a clue that it’s the capacitor creating the malfunction.
- Turn the fan on. With the furnace still switched off, turn the thermostat to Fan On mode (rather than Auto mode which is probably the default).
- Power the furnace. Does the hum start again while the fan blades don’t begin rotating?
- Manually spin the fan. Use the dowel or long screwdriver (never your fingers) to try to start the blades spinning in the direction they should move. You’ll just be providing a little extra energy to get the blower moving.
Does the Blower re-start? If the answer is yes, you’ve identified that the capacitor is almost definitely the issue. If it doesn’t fire up, it is probable that the motor is burned out. That will require a different repair which should be carried out by a qualified furnace repair technician.
How To Test A Capacitor
If you already have a multimeter and want to be certain that your run capacitor is at fault, it’s quite simple. Just follow the steps below. However, before you go any further, just a word of caution. As we previously mentioned, the run capacitor stores up to 400 volts of additional energy.
Please pay close attention to Step 1 below, to avoid an electric shock. It’s also understandable if you prefer to leave this work to a professional – it will ensure there are no accidents.
- Discharge the capacitor. Take an insulated screwdriver, ie one which has a rubber, synthetic or plastic handle, and make sure the handle is 100% crack-free. Next, place the screwdriver shaft horizontally across the two terminals. This must be carried out with the furnace switched off and the capacitor still in place.
- Take out the capacitor. Remove the screws which hold the bracket in place. Draw out the capacitor and detach the connectors. This process is also known as ‘shorting.
- Understand the reading you‘re looking for. The capacitor’s rating will be clearly written on it – this is known as the micro-farad (MFD) number. Typically the label will say something like “+/- 10%. This means that a “good” reading on a 10 MFD capacitor could be 9, 10, or 11 on the multimeter when it’s turned to capacitance (see next step).
- Test the furnace run capacitor. Switch your multimeter dial to the ‘capacitance’ setting. Attach a meter lead to each of the capacitor terminals. Hold them in place and observe the meter. Wait 30-60 seconds. If nothing registers it means the capacitor is dead. But, if the number does go up, but if after 60 seconds, it hasn’t reached the microfarad rating (plus/minus 10%), this means it is deteriorating and needs to be replaced.
If the meter reading shows the capacitator is working at the correct capacity, we recommend that you call a technician to service your furnace. He or she will also test each component and give you an estimate for a replacement if required.
Where To Buy A Capacitor
Furnace run capacitors can be found online at HVAC sites such as RepairClinic.com and SearsPartsDirect.com, and sometimes on Amazon.com. However, they aren’t always easy to find locally. Home improvement stores and hardware stores don’t often stock them.
Local HVAC parts stores often only sell on a wholesale basis to HVAC companies, so you’d have to be prepared to do some searching to locate the part you want at the right price.
Furnace Capacitor Cost
The price will typically be anything from $5 to $20. It’s worth bearing in mind that your capacitor may fail at some point. It makes sense to invest in a spare (and also an extra igniter) so that in case of need you can replace it immediately. While this isn’t necessary, most people find it convenient so that they aren’t scrambling to locate the part they need.
To have a furnace capacitor installed by a qualified HVAC technician can cost anything from $80 to $200. Expect a minimum fee of $75, even though the part is inexpensive and the repair can be completed in between 5 -20 minutes. If you need the repair carried out at night, or the weekend, expect to pay more. If there’s no danger of additional damage due to frozen pipes, it will save you money if the repair is carried out during normal working hours.
Where To Buy A Furnace Blower Motor
Unlike capacitors, you can usually find furnace blower motors at your local Home Depot or Lowes. Just get the model number of your blower motor and call the store so they can order it for you.
Furnace Blower Motor Cost
On average, a furnace blower motor will cost you around $450. However, this price can fluctuate from $200 all the way up to $800. The price depends on the model of your furnace, as well as the size. For instance, the blower motor, an industrial type furnace, will be around $800, if not more.
To have an HVAC technician install it can cost you between $500 to $1,200, depending on the size of the job and who you hire. This is why it’s a good idea to call around for prices before hiring someone.
When your furnace refuses to start but hums anyway, this can be frustrating. However, it’s most likely your capacitor, which is an easy fix. By walking through to test the capacitor as well as the blower motor, you will be able to tell what the problem is so that you can fix it and have your furnace working like it’s brand new.