Summer 2015 in Seattle has been officially declared the hottest summer here on record (check my weather station’s 2015 max temps on my Weather Underground weather history graph for geeky details). For those in the Pacific Northwest living without central A/C, it’s been miserable. And for those fortunate enough to have central air conditioning, running the A/C all day has been expensive. But things can get even more expensive if your A/C unit stops working and you call an HVAC repair man. Hopefully, this article can help you avoid that.

When your air conditioning stops working, what you’ll usually notice is that your HVAC fan runs normally, but the air coming out of your vents isn’t cold. If that’s happening to you, the #1 most likely cause is a failed capacitor in your outdoor A/C unit or heat pump. The #2 most likely cause is a failed contactor. I recently replaced both the capacitor and the contactor in one of our A/C units when it stopped blowing cold air, so I’ve documented the process to share with you here.

Disclaimer: Although these two fixes are simple enough fix for a beginning DIYer, you will be dealing with high voltage, and therefore you must take the proper precautions as explained in this article. Make sure power to the appropriate equipment is turned off, and that electricity in your capacitor is safely discharged (as explained below) before proceeding. There is a risk for serious electrical shock (enough to kill you) if you’re not careful. So be careful, and proceed at your own risk.

Recommended Tools

Before tackling any HVAC projects, I recommend having the following three tools in your arsenal:

1. A set of rubber-handled insulated screwdrivers. They are perfect for any electrical DIY work, but especially for high-voltage stuff like HVAC. You can pick up an inexpensive 7-piece set for under $20 like these Titans, but if you can stretch your budget closer to $35, this 6-piece set from Wiha is what the pros use (they’re rated to 1,000 volts).
2. A quality multi-meter (like the Fluke 117). Even though the HVAC-specific Fluke 116 is what most professional HVAC technicians use, the Fluke 117 will do most of the same functions as the 116 while also being slightly more useful for all your other electrical projects.
3. A Leatherman Wave Multi-tool. I never go anywhere without my trusty Leatherman Wave, and I strongly recommend picking up the Bit Driver Extender because the 1/4″ socket in the extender is perfect for inserting and removing those 1/4″ sheet metal screws used in most HVAC applications.

What are the A/C Contactor and Capacitor?

Your air conditioner’s contactor and capacitor work together to help power your air conditioner’s fan and compressor whenever your thermostat calls for cool (the capacitor and contactor in your heat pump work the same way).

The contactor is the “switch” that receives a low voltage (usually 24V) signal from your furnace to turn on. It’s a standard magnetic relay, meaning low voltage flows through a coil inside the contactor to creates a magnetic field, which then pulls down a piece of metal that connects both sides of the high-voltage circuit usually (120V or 240V). Closing the circuit allows high-voltage to power the fan and compressor motors. A contactor with one magnetic coil that connects one circuit is called a “single-pole,” if it has two magnetic coils that connect two circuits it’s called a “dual-pole,” and so on.

The capacitor is like a “battery” that stores some of that high voltage goodness to create what’s called a “phase shifted current,” which is what causes your fan and compressor motors to spin (thanks one of my readers, Jerry, for commenting and helping me understand that better).

Your A/C capacitor will be either a dual capacitor or a single capacitor. A dual capacitor has three sets of terminals on top: one for the compressor, one for the fan, and a shared “common” terminal for both to use. A single capacitor only has two terminals, and if your unit uses single capacitors, you probably have two of them in your system (one for the compressor and one for the fan).

Accessing Your A/C Contactor and Capacitor

Your contactor and capacitor will be protected by some sort of panel on your air conditioning unit or heat pump. The Rheem outdoor air conditioning unit at our Seattle house has a panel that removes with four screws from the top and side:

Removing panel screws from a Rheem air conditioner

The heat pump at our cabin has a curved panel on one of the unit’s corners. If you’re having trouble finding it, look for where the wires and plumbing enter the unit. That’s always where the panel will be.

Be careful when removing these panels, as some of the wires are “hot” and the capacitor may contain some charge… even if the unit is not running.

Here’s what things looked like under my A/C unit’s access panel:

Contactor and capacitor on a Rheem air conditioner.

The contactor is the rectangle switch-looking thingy on the left, and the round gooey mess on the right is the top of my capacitor. That gooey mess is a sure sign that the capacitor is leaking and needs to be replaced.

Diagnosing a Bad A/C Contactor or Capacitor

The only 100% accurate way to test for a bad capacitor is to use a decent multi-meter like the Fluke 117 (which is the one I recommend). But a quick visual inspection might be enough to indicate something’s wrong. If your capacitor is gooey (like in the above photo), it’s way to failure… if it hasn’t failed already. The other dead giveaway is a bulging top. In this image, notice the bulging top on the capacitor on the left. The one on the right looks normal:

A bulging capacitor (like the one on the left) is a sign it’s gone bad.

If your capacitor is bulged or leaking, that was probably your problem, and replace it will fix your air conditioning.

To test your contactor, a multi-meter is also helpful to confirm that you have high voltage running to the contactor (and that your problem isn’t caused by something as simple as a blown fuse or breaker) and continuity on both sides of the circuit when the contactor is “closed.”  But if you’re confident that your contactor is receiving power, you can do this quick-and-dirty test with a rubber-insulated screwdriver (the rubber is important so you don’t shock yourself). With the power to your air conditioner turned on and your thermostat not calling for cooling, hold the rubber-insulated end of the screwdriver and use the other end to hold in the “button” (circled in red in the image below) on the contactor for 5-10 seconds to manually close the circuit. The fan and compressor on your air conditioner or heat pump should run normally until you release the button.

Press this button to close the circuit on your contactor

If pressing the contactor button turns everything on, the capacitor is probably OK, but keep reading to test the contactor’s coil.

If you press the button and hear a hum, and the fan and/or the compressor doesn’t turn on, you’ve probably got a good contactor and a bad capacitor. The hum you hear is the fan and/or compressor motor getting some power… but without that “extra oomph” from the capacitor, it’s not enough to get it moving.

If absolutely nothing happens (and you’re certain you’ve got power to the contactor), you probably have a bad contactor.

If everything works manually, you still need to verify that the magnetic coil is automatically closing the circuit on the contactor when the thermostat calls for cool. Stay outside with your air conditioning unit and have a helper inside the house turn the thermostat down so the system calls for cooling. You stay outside and watch the contactor (you can do this from your phone by yourself if you have a smart thermostat like an ecobee). If the contactor’s button doesn’t get pulled down by the magnet to close the circuit when the system calls for cooling, the “coil” portion of the contactor is likely your problem and the contactor needs to be replaced.

In my case, neither the contactor nor the capacitor had failed completely, but the capacitor was leaking (as seen from the first photo) and the contactor was making a loud buzzing sound when the system was running — both of which are signs that failure is imminent. Because the parts are cheap and easy to replace, and because a healthy contactor and capacitor can extend the life of expensive compressor and fan motors, I decided to replace both the capacitor and the contactor at the same time, and it might not be a bad idea for you to do the same.

Buying the Correct Replacement A/C Contactor or Capacitor

The best way to find the right replacement for your contactor or capacitor is to go to Amazon, type in the model number of your old one, and purchase the same kind. Both items are destined to fail eventually, so if you need to replace them now, I recommend buying two so you have a spare on hand when this project is done. Or, if you’re reading this but haven’t had either of them fail on you yet (trust me, it will happen eventually… and probably when it’s least convenient), you should go through these steps to find the correct replacements and order spares now so it’s a quick and easy process to repair your A/C or heat pump when the time comes. The parts are cheap, and you’ll be the family hero when you can fix it before the house gets above 76F. 🙂

If you need to disconnect or remove anything to get a better look at the part numbers on your capacitor or contactor, do the following three things first:

1. Make sure power to the air conditioner or heat pump is shut off. This is different than the power to your furnace. There should be a large shut-off lever or a fuse panel outside and near the unit.
2. Don’t touch the terminals of your capacitor until it’s been discharged. Use a rubber-handled screw driver or rubber-handled need-nose pliers (the rubber insulates you from electric shock) to “short” the terminals and dissipate any residual charge in the capacitor. On a single capacitor, just use the tip of the tool to touch both sets of terminals with the metal part of the tool the same time. On a dual capacitor, dissipate both sides by using the tool touching the C and the FAN together and then the C and the HERM together. It’s worth noting that this is not exactly the “recommended” way of dissipating a capacitor (as one of my buddies mentions in the comments below). The “recommended” way to drain a capacitor is to use a 1000 Ohm resistor that’s rated for the voltage and hold it between the terminals for 5 seconds. With that said, I’ve seen lots of HVAC professionals use the screwdriver trick. As always, proceed with caution (and eye protection probably isn’t a bad idea).
3. Take close-up photos of the connections with your mobile phone (get a few different angles) so you know how everything goes back together.

When shopping for a replacement capacitor, you’ll need to make sure:

1. You’re buying the right type of capacitor.
2. The capacitor is able to deliver enough voltage to help power the compressor and fan motors.
3. The capacitor has enough capacitance to store that voltage until it’s needed.

Concerning capacitor type, there are two general categories: “start capacitors” (used only when the motor starts) and “run capacitors” (used the entire time the motor is running). Most A/C capacitors are run capacitors, but there are some units that will require a start capacitors instead, so make sure you get the right kind for your unit (again, thanks to my reader Jerry for his education in the comments about capacitors).

As for voltage, most air conditioners and heat pumps have compressor and/or fan motors that require either 370VAC or 440VAC to start. If your unit requires 440VAC, you cannot use a 370VAC capacitor. But if your unit only requires 370VAC, it’s totally fine (and not a bad idea) to use a larger 440VAC capacitor. With voltage, you can always go bigger than required, but you can never go smaller.

For the capacitance, make sure the replacement capacitor has the same micro-farad rating (shown as MFD or μF) as your old one. For dual capacitors, the capacitance is shown as “45/5” or “45+5” MFD, which means the compressor side is rated at 45 μF and the fan side is rated at 5 μF. For single capacitors, you’ll just have one value. Unlike with voltage ratings (where it’s OK to go bigger), you should not get a capacitor that’s rated higher in capacitance than the original. Keep the same capacitance in your replacement.

If your capacitor’s exact replacement is too expensive or hard to find, you can replace it with any unit that has the proper ratings. For example, my Rheem RAKA-037JAZ air conditioner came with a dual capacitor marked “Aerovox TT 10000 AFC SH1238.” I couldn’t find an exact replacement anywhere except direct from Rheem, and they wanted way too much money. But with some searching I found out that particular Aerovox is a 370VAC 45+5 MFD capacitor, so I purchased this TRCFD455 unit on Amazon for less than $11 (including shipping), which is rated up to 440VAC and 45+5 MFD. Again, it’s fine to go bigger on the voltage, but you can’t go smaller.

My existing contactor was a 24V Honeywell R8242A-1032, which isn’t available any more. The replacement is listed a Honeywell DP1025A 5006, which I found for $20 shipped on Amazon (note: the wiring for the new one was a bit different than my old one — read more on that below if you’re using this same contactor).

If you’re coming up empty, try contacting a local HVAC supply house. They might be able to tell you what you need based on your equipment’s model number. And some HVAC repair companies will sell parts direct to consumers if your house is too hot, you’re in a rush, and don’t want to wait a day or two for Amazon shipping. Remember when I said you should order spares before they break? Seriously. It’s cheap insurance!

Installing the Replacement A/C Contactor or Capacitor

Make sure you’ve followed the three steps mentioned above (power to the A/C unit or heat pump off, capacitor discharged, and photos taken) before you proceed.

My A/C unit had a large shut-off switch like this:

Shut off the power to your A/C unit or heat pump before you proceed, and don’t forget to discharge the capacitor.

Remove the access panel (if you haven’t already) and locate your contactor and capacitor. Here’s a reminder of what my old ones looked like:

Contactor and capacitor on a Rheem air conditioner.

Triple-check to make sure there’s no power to the unit (I did mention that 240V is nothing to scoff at, right?), then disconnect all the spade connectors from the part(s) you’re replacing. If they’ve been on there for a while, using your Leatherman’s pliers will make things easier. Remove any screws holding the part(s) in place. Check out how rusted my old contactor was on the bottom:

This rusty old contactor was on its way out.

Remove the old parts (in my case, I replaced both the capacitor and the contactor). If you’re installing a capacitor, double-check it to make sure it has the correct rating. If it’s too low, it won’t start the compressor and fan motors:

Replacement dual capacitor ready to install.

Drop the new part(s) into place and secure them (you can wait to the end to secure them if you prefer). The replacement capacitor should drop into the old hole, but it’s OK if you need to figure out a different way to mount it:

Securing the new capacitor.

Once the new capacitor and/or contactor are installed, re-connect all the wires using your pre-disassembly photos as a guide. If the markings aren’t clear, the C (common) terminal on your dual capacitor will have the most spade connectors (mine had four), the HERM (compressor) terminal will probably have one fewer than the C (mine had three), and the FAN will probably have only one. Here’s how my new contactor and capacitor looked when re-connected:

Reconnect your capacitor and/or contactor just like the old ones.

If you’re replacing an old Honeywell R8242A-1032 contactor with the newer Honeywell DP1025A 5006 contactor, read this next part carefully. The terminals on the older R8242A-1032 are marked T1, T3, L1, L3, C, and C. On the newer DP1025A 5006, the terminals are marked T1, T2, L1, L2, A1, A2. Logic might suggest that you simply hook up the T1 and L1 connections just like the old one, then connect the wires from the old T3 and L3 terminals to the new T2 and L2 terminals, right?

Wrong!

The internal wiring on the two relays is different, so when replacing a Honeywell R8242A-1032 with a Honeywell DP1025A 5006, here’s how the old contactor’s terminals map to the newer contactor:

• 240V to fan: T1 (old) -> T2 (new)
• 240V to compressor + capacitor: T3 (old) -> T1 (new)
• 240V from panel: L1 (old) -> L1 (new)
• 240V from panel: L3 (old) -> L2 (new)
• 24V from Thermostat: C (old) -> A1 (new)
• 24V from Thermostat: C (old) -> A2 (new)

It really doesn’t matter which old C wire goes to which new A1 or A2, and it actually doesn’t even matter if you swap the L1 and L2 incoming 240V power wires on the newer relay. But the old T1 wires must connect to the new T2, and the old T3 wires must connect to the new T1, or things won’t work.

When everything’s re-connected, turn your A/C unit’s main power back on:

Main power to the A/C unit turned on.

Then tell your thermostat to call for cooling again.

If you did everything correctly (and as long as you were careful, there’s no reason you believe you didn’t), your A/C unit or heat pump compressor and fan should both be running (instead of just humming) and chilled air will once again be flowing from your vents. Congratulations on a great DIY fix!

I won’t blame you if you walk around the house demanding high-fives from the spouse and kids.

Go ahead. You’ve earned it.

As always, I welcome your questions, comments, and feedback below!