HVAC compressor contactor premature failure

Question

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.

Answer 1

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.

Answer 2

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.

Answer 3

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.