Furnace blower capacitor problems are some of the most common heating challenges. A damaged or defective blower capacitor renders the furnace blower ineffective, often leading to a furnace shutdown. Alternatively, your furnace may get stuck in an endless ON-OFF cycle.
The good news is that you can easily replace a damaged or malfunctioned furnace blower capacitor. Let’s discuss how to diagnose the role of the capacitor, how to diagnose it, how to test your blower capacitor, and how to replace a damaged one.
What is a Furnace Blower Capacitor, and What Does it Do?
The furnace capacitor is an electrical component found on the furnace’s blower. It stores power to start and run the blower. Many furnaces have two capacitors – a start capacitor and a run capacitor.
Start capacitors store the electrical power necessary to start the furnace while the run blower motor stores the power to run the blower motor. However, others only have one capacitor, i.e., the start capacitor. If you’re wondering why the blower motor needs a capacitor when it can get power directly from the furnace’s main circuit board, the answer is that blowers need significant amounts of power to start and run.
Starting the furnace blower, in particular, requires a lot more power than the furnace’s control board can supply. Whereas the furnace’s circuit board can only supply 24V, blower motors require up to 200V and others up to 250V to start. Blower capacitors store between 350V and 440v to conveniently power the blower motor.
Where is the Furnace Blower Capacitor Located?
The furnace blower motor capacitor is located in the blower compartment, very close to or on the blower motor housing. Here’s how to locate your blower motor capacitor;
- Turn off your furnace at the breaker and turn off the gas supply.
- Remove the furnace’s access door located near the bottom of the furnace to access the blower assembly. You should be able to see the snail-shaped blower housing.
- Point a flashlight toward the blower housing to examine the exterior casing. Do you see a white, silver, or black can-shaped or cylindrical device on the casing? That’s likely the capacitor. It’s about 3-4-inch tall and has 2-3 connection terminals. If there’s none on the blower casing, check the blower’s immediate surroundings. The capacitor cannot be too far from the blower.
When Happens When the Blower Motor Capacitor Goes Bad?
Unfortunately, the blower motor capacitor can go bad, and when that happens, hell breaks loose! For one, the furnace may burn a little slower than usual, meaning you may not get enough warmth in your rooms.
However, that’s usually just the beginning. The slow-burning furnace is a common warning that your blower capacitor is dying.
Once the situation gets out of hand, the furnace will go off and may even enter a lockout, necessitating a manual reset to work again.
Bad Furnace Blower Capacitor Symptoms
You need to catch a dying or damaged furnace blower motor capacitor early to avert worse problems down the line. Luckily, it’s not too difficult to tell that your blower motor capacitor is bad. The following are a few telltale signs;
1. The blower doesn’t run
The first sign of a bad blower motor is a blower motor that doesn’t run. The blower motor comes on automatically once the furnace begins to produce heat. The furnace’s control panel monitors supply air temperatures and instructs the blower motor to come on as soon as internal temperatures reach the ON setting on the limit switch.
However, this will not happen if the capacitor is bad or no longer holds enough power to start the blower. Many furnaces display a warning signal when the blower motor refuses to run. So, make sure to check your system’s error codes.
2. Audible humming motor
You’re even more likely to have a defective or dead blower motor capacitor if the motor hums but doesn’t run. You may need to listen closely to hear the hum. Turn up the thermostat and wait a few minutes for the blower motor to start up. Then open the access panel and lean into the blower housing to hear the humming motor.
If the motor hums strongly but doesn’t start, the motor itself is likely good and raring to go, but the capacitor isn’t providing the necessary power. The capacitor is either dead or dying.
3. The furnace goes off
Sometimes the furnace may go off within seconds of realizing that the blower motor is not operational. You’re probably wondering why the heater must go off. The short answer is – to avert the risk of overheating and the potential consequences.
Remember that the blower motor is responsible for blowing warm air out of the furnace through the supple plenum and into the ductwork. However, this is impossible if the blower isn’t running. Thus, the accumulation of heat inside the furnace can result in internal overheating and related damages.
An automatic shutdown prevents such disasters.
4. Intermittent short cycling
However, sometimes the furnace may not shut down altogether but instead enter a state of intermittent shut short cycling.
This is common if the blower capacitor isn’t dead but no longer holds enough charge to start or keep the blower motor running. Such a capacitor will attempt to kickstart the blower motor but fail midway, then try again and fail.
This can go on for ages until something gives. So, you’ll hear the furnace repeatedly start and run for several seconds or a few minutes before going off. Keep in mind that short cycling consumes lots of power and accelerates wear and tear.
5. A burning smell
Finally, a burning smell from the blower compartment is another possible indication of a bad blower motor capacitor. As with any electrical appliance, the capacitor can short circuit and burn. Or, the wires connecting to it may burn.
When this happens, the electrical power supply to the capacitor is cut, meaning the capacitor can no longer work. Just be careful not to confuse furnace blower motor damages with blower capacitor damages.
That burning smell from the blower compartment can originate from either. So, you need further tests to prove that the capacitor is damaged.
What Causes Furnace Blower Capacitor to Fail?
The blower motor capacitor can fail for many reasons ranging from electrical short-circuiting to physical damage. However, the three leading causes are;
- Internal overheating
Furnace capacitors come in many types. However, many furnaces use electrolytic A/C capacitors. An electrolytic A/C capacitor contains a moist part that will dry out if the unit gets too hot inside, typically if temperatures rise above 150°F. Internal short-circuiting is inevitable when this happens.
- Defective blower motor operation
Capacitors are designed to disengage within a few seconds. Otherwise, the unit can overheat. Therefore, a defective blower motor that drags due to damage or worn bearings can cause the capacitor to burn up.
- Electrical overload/surges
Lightning and unexpected electrical line surges can also cause overload, causing the capacitor to burn up. A power surge from a lightning strike typically causes a significant current spike that can easily fry the capacitor.
- End-of-life slowdown
Finally, your furnace blower motor capacitor can also fail due to natural end-of-life slowdown. Life expectancies vary from one unit to another. However, a typical blower motor capacitor lasts six years, after which it starts to show signs of failing.
How to Test Furnace Blower Motor Capacitor
You need protective eyewear, screwdrivers, and a voltmeter to test your blower motor capacitor. Once you have the three, proceed s follows;
- Shut off the power to the furnace at the breaker and turn off the gas supply. Then wear your protective eyewear.
- Access the blower compartment and locate the capacitor. Refer to the “where is the furnace blower capacitor located” section above for details.
- Remove the wires attached to the terminals of the capacitor. Do so gently to avoid damaging the capacitor.
- Discharge the capacitor. This means releasing all the charge left in the capacitor. All you need to do to drain the capacitor is to connect it to a watt resistor.
- Check the capacitor rating on the side of the unit. You need to note down the supply voltage and capacitor capacity in microfarads.
- Set the voltmeter and connect the terminals to the two capacitor terminals. Then take the reading. If one of the readings is within the allowable range, the capacitor is still good. However, readings outside the supply voltage and microfarad setting mean you need to replace the capacitor.
How Much Does It Cost to Replace a Blower Motor Capacitor?
It costs between $150 and $300 to replace the furnace blower motor capacitor. However, you need at least $450 to replace the entire furnace blower motor. Alternatively, you can opt for DIY blower capacitor replacements, which costs $100 to $135 as you only need to purchase the capacitor.
How to Replace a Furnace Blower Capacitor
- Turn off power to the furnace at the breaker.
- Locate the blower capacitor. Then locate the motor.
- Disconnect the wiring.
- Disconnect the capacitor.
- Insert and bolt the new capacitor
- Reconnect the wiring
- Switch the power back
- Test the furnace to see if it works.
Now you know more about the furnace blower motor capacitor, what it does, and what can go wrong with the capacitor. Most importantly, you know common furnace blower capacitor symptoms and hot to test and replace a bad capacitor.