My house has a 25 year-old dual-fuel split system heat pump. It seems to really go through contactors (the relay that turns on the compressor when the thermostat calls for heat or cooling).

The last one to fail lasted only about a year. I checked and there is solid 24vac on the coil. If I jiggle the contact with a non-conductive probe (e.g. a stick) it sparks and the compressor starts.

I had purchased what I thought was a sealed unit. But when I took off the cover (that covers the contacts-proper), there was crud in there. I also bought what is called a "one-and-a-half pole contactor), meaning it has line and load terminals for both phases (or 240vac) but one is always connected; I figure this roughly halves the probability of failure, over a two-pole one.

  • How can I prevent this from happening so often?
  • Should I get a contactor where the contacts are exposed, so I can clean them a couple of times per year?
  • Is there a brand, or grade, of contactor meant for such problematic installations?
  • Is the fact that I heat almost exclusively with wood, thereby minimizing the number of cycles of the contactor during the winter, a likely culprit?

FOLLOWUP: Based on the answers received thus far, it appears that the most likely culprit is that the contacts (in my recently-failed contactor) are not properly rated for controlling the motor load presented by the compressor. It appears that I need to be considering the IEC Utilization Category of a candidate contactor. These are described here: https://en.wikipedia.org/wiki/Utilization_categories Unfortunately, it is not clear what category is required by an HVAC compressor; AC-23 certainly would appear to fit the bill, but perhaps it is overkill and AC-3 or AC-4 would suffice. Additionally, none of the contactors I've checked online (Amazon has tons of them) actually specify their IEC utilization category. So I'm still somewhat at a loss.

  • what is the environmental temperature where the relay is located ?
    – Traveler
    Jul 12, 2022 at 1:22
  • what kind of "crud" rodent damage? Jul 12, 2022 at 11:43
  • Hard to say what kind of crud. Jul 12, 2022 at 16:22
  • Problematic compressor sits outside house. Pretty wet location, I suppose, what with summertime humidity and drip-line from gutterless roof. Jul 12, 2022 at 16:22
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    Rusty, it sounds like it would make a difference but there is no current draw until the second set of contacts close the first dosent matter, where their can be a hazard with a compressor is when the varnish starts to dissolve because of acidic oil (a leading cause of compressor failure) single pole leaves 1 side energized when the motor goes to ground it can take months prior to full failure and the frame is now hot until there is enough leakage to trip the breaker or blow the fuse. On the very few single pole thermostats or contactors I have replaced they are the cheapest and fail sooner imo
    – Ed Beal
    Jul 15, 2022 at 16:22

3 Answers 3


If you are chewing up contactors they are undersized or you starting cap is going bad causing a higher current draw and this will eat contactors and cause motors to overheat.

You are correct you can use a single pole to start a 240v device but is this wise with a hvac system , NO!

The reason you should continue to use a double pole is a common failure mode on old compressors is a little moisture gets in the loop with recharges or a low charge and the oil goes acidic and shorts to ground in some cases there is not enough current to blow the fuse or heat to open the internal safety so you have 120v on the frame and although I would not have believed it I have gotten this buzz when starting to remove covers and test. It make the job real easy to diagnose.

Make sure your cap is good and going to a slightly larger contactor is needed you need a motor listed contactor a 5hp should be enough since you have a 30 amp fuse (most likely a 3 Hp would work) up sizing the contactor will make it last longer.

If you use a smaller contactor than the motor size it will fail in 1/4 the time if it last that long.

A weak cap or undersized can cause excessive start up current but staying within 10% of the UF value will usually work.

Examine the cap if the top where the connectors are is domed it is usually on its way out they are flat when new same with the bottom, and this is a place where a higher voltage will improve the life but lower voltage will fail quickly. Almost all compressors are outside and the enclosures are iffy for moisture but usually fine unless blasting a leaf blower driving gunk into it.

So check that the contactor is properly sized, and the cap is good.

  • Ok, thanks. But see followup in my OP. Contactors don't seem to be rated for HP or IEC utilization category, just FLA, which I found a table that says that something like 12 amps fr 3HP. Jul 15, 2022 at 14:53
  • I replaced cap when I replaced this contactor a year ago. But I'll check for the signs of deterioration you mention, as well as the value. Jul 15, 2022 at 14:54
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    A 230v 3 Hp motor is 9.6a full load current by code but the starting currents will be closer to 30-50a range for a very short time. 30 years ago the contactors could handle running at FLA today I always upsize by at least 1 and they last longer, a 1 year old cap should be fine if you did get a 380v one. Many don’t realize that 240v is not the peak voltage and the cap needs to be built for peak (almost all meters measure in RMS). Some may question my use of 230v for the motor but this is the book value per the NEC table 430.250. A motor rated contactor will be larger have bigger contacts& last
    – Ed Beal
    Jul 15, 2022 at 16:14
  • I bought the failed one at an HVAC supply house. Would they even be selling a non-motor rated one ? Jul 15, 2022 at 16:29
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    Rusty, I see it all the time in an effort to save a few dollars. Most HVAC techs have very limited electrical knowledge this is not a problem if they replace with an exact same part but sometimes they get the lighter duty or not really motor rated they usually last a while.
    – Ed Beal
    Jul 16, 2022 at 0:39

It sounds like the relays you are using are not rated for the job. Resistance rated relays will not stand up to AC motor loads. When looking at the relay be sure it has a motor rating equal to or greater than your compressor motor. If the fan is also on that relay (contactor) you need to also consider its load. AS a rule of thumb, HVAC compressors are 1 ton (12000 BTU) per horsepower. If the relay has a higher rating fine, if it is the same be careful it may not last as long.

  • I guess I'm guilty of assuming that anything sold as an HVAC contactor would be rated for motor loads. But I'll check more carefully. It is a 2.5-ton unit, installed in 1997, so only 11 SEER I believe. And yes, the fan is on the same relay. Jul 12, 2022 at 16:25
  • I have added a followup to OP based on your answer. Jul 14, 2022 at 20:56

There are multiple reasons.

Undersized or defective 24 VAC transformer.

Check the 24VAC when relay is energized. Is it still 24VAC or has it dropped.

Relay electrical size.

Note the Amps rating on the relay and on the AC. Is there a match ?

Relay exposed to high environmental temperature.

What is the location of the relay? Higher temperature will limit relay life time. Using contactless temperature meter (laser thermometer) check relay temperature while operating.

  • The 24vac looks fine. Compressor is on a 30 amp breaker and it's a 40amp relay; but (as above) I did not check if it's for motor loads (merely assuming). The compressor is almost completely shaded. Jul 12, 2022 at 16:28

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