I have a 23 year-old dual-fuel 2.5-ton heat pump; it is a split system, air handler and furnace in the crawlspace, compressor outside. I use it very little in the winter, mostly heating with wood; I use it a lot in the summer. Here is what has happened:

  1. Turned on A/C, after several weeks of idleness (pleasant springtime weather). Compressor did not come on (neither the compressor fan nor the compressor-proper).

  2. Next day, tried to see if compressor would work in heat mode. It did. Then switched to cool mode, and it also worked. A/C has worked fine on the few occasions I've tried it since then.

  3. Looked at compressor while running properly. Measured 24 vac signal to contactor coil and 36mv voltage drop across contacts (1-1/2 pole contactor).

What do you recommend ? Is the 36mv across contacts normal ? Sounds small, but at 20 amps, that's almost a watt dissipated in very little metal, so maybe not. (The compressor runs off a 30amp circuit). The contactor has not been replaced in many years. I am tempted to just replace it. Cheap part. DIY - I am electrical engr and have done lots of home wiring, so can do it without killing myself (turn off breaker, make sure capacitor is discharged).

But, do you think this is the most likely problem ? If not, I guess I just have to wait for it to fail again, and check to see if the coil is getting 24vac. Anything else I can check before then ?

Also, searching here, someone said capacitor is most likely cause of compressor failure. However, I wonder if the fact that my problem occurred once, and then went away, would modify this ?

  • As an interesting adjunct: I became a bit curious about the terminology "definite purpose" that one often sees in listing for HVAC contactors like the one I intend to replace. Apparently it means that they are explicitly designed for HVAC, and as such are made more inexpensively because they don't need to last for that many cycles. Which is kinda depressing. Jun 5, 2020 at 2:07

2 Answers 2


A small voltage across your contacts is normal even new. I would be looking at the start run cap as this is a normal failure , yes contactors do fail but as an intermittent failure issue the caps are usually 2:1 more likely. If you are comfortable changing a contactor a cap is even easier less connections. If your meter has a test function for capacitance turn the power off short the cap out with a screwdriver and unhook 1 side of the cap. The measured value should be within 10%, if you don’t have a cap test , use an ohm meter same process short the cap one side unhooked so it is not in circuit and with ohms function measure the cap you should see the value climb or continue to increase reverse the leads and the value should be dropping, if your meter says it is open or shorted it is bad. You may have a dual cap like 5uf and 25uf this is a start run pull the common terminal usually marked com and test each cap if either is bad the cap needs to be replaced, note if the cap looks bulged at the bottom or top it is usually close to going bad bulging is usually obvious especially if you look at a new one the bottom is rounded in extreme cases. To replace the cap the value(s) in microfareds uF is what you want to be the same or close the voltage as long as the new one is higher is ok 370v is a common value but anything higher will be fine. If you have a 5.5,25 sometimes you can only find a 5,25 that will work, you want these values to be the same as the orig but within 10% will work if you cannot find the original values.

  • Thanks (and @Machavity too). Yes, I am a EE, done lots of house wiring, have a nice Fluke meter, and this was properly installed by a moonlighting cryogenic tech, so no worries there. I guess I should just go ahead and replace both. I'm quite sure neither has been replaced in at least 10 years. Installer says that replacing contactor every so often can be thought of as a maintenance item; perhaps cap too ? Jun 3, 2020 at 16:35
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    Yes a cap can be maintenance issue probably more than a contactor. They are usually electrolytic they do change values internal shorting causes heat in the dielectric gel and the bulging , if I have a vfd on the shelf for more than a couple of years I bring the power up slow 50v per step to keep them from blowing at first the read a short but when the dielectric gell warms up it creates the separation and they work again (sometimes) this is how they can be intermittent if they have leaked or bulged best to replace them they don’t usually explode but when they do you ? How was all that in there
    – Ed Beal
    Jun 3, 2020 at 17:58
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    Yup very common Size pull the c lead with the power off and short both sides to common if you want to try and test it but anything that old this would be my first part I don’t like shotgunning but have had so many bad caps I almost go there automatically. If you notice any bulging replace it even if it measures good today as it may fail the next time, make sure to mark the leads or transfer 1 for 1
    – Ed Beal
    Jun 4, 2020 at 6:13
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    The 87 is ok it’s measured values match my z meter fairly close, with just an ohm meter you are looking for a charge and discharge not immediately jumping to OL or 0 . At that age replacing the cap may solve future problems that are many times intermittent at first.
    – Ed Beal
    Jun 8, 2020 at 1:01
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    Yes there are a lot of them out there but then they sell r134 in 2 lb cans at auto part stores. All the electrical is legal to sell to the general public even a compressor can be purchased without a license it is just the refrigerant that is truly regulated , electrical places also do this sometimes entire chains or sometimes just 1 store. I have multiple licenses and keep them active because who knows what I might want to do next or when I grow up according to my wife.
    – Ed Beal
    Jun 8, 2020 at 18:05

The capacitor adds a boost for the initial startup of the compressor and fan. It can sometimes intermittently fail before going bad entirety. The only way to know for sure is to pull it out and test with a multimeter, comparing with the rating on the side. This can be risky since it is a high voltage device and you might not have a working disconnect (make sure it is actually off, since some installers cheat by bypassing it).

Beyond that, this is very doable for a DIY. Just be careful.

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