A/C Compressor wiring

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The contactor on my old 3 ton A/C has been making an usual noise and I'm getting ready to purchase and install a new one. I was surprised to see that the original contactor is only a 1-pole unit and that only one hot leg is interrupted. Turns out this is common, one reason being to allow a crankcase heater to be active when no cooling is energized. (I always thought heaters were for heat pumps or maybe operation in cool climates although why need air conditioning?)

Any one know if and/or why the compressor uses a heater in summer operation? Just wondering if it would be OK to disconnect the heater wires and use a 2-pole contactor for added safety?

[BTW: The unit stopped working several weeks ago and it turned out one of the crimped wire terminals at the compressor had deteriorated and created an "open". I fixed it but will replace all 3 with new ones when I get the contactor.]
 
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You could easily leave it disconnected as long as you don't run the unit when ambient temperature is below about 40º. Crankcase heaters are primarily used on compressors in heat pump systems because those compressors run in all seasons. The heater is only needed in low temps to prevent liquid refrigerant from entering the compressor and causing premature failure. Looks like you know this already. The contactor on my 4-ton unit is a single pole (30A) with shunt, like you're describing. I replaced it a few months ago along with the condenser fan motor and dual run capacitor.
 
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The crackcase heater is to prevent refrigerant migration and condensation in the compressor. It helps all ac and heat pump units but is actually rarely installed on residential ac units with scroll compressors because they typically aren't operated in those conditions often enough to cause failure and the manufacturer can save 5 bucks.

If you have a heater then either the manufacturer or your installer deemed it necessary and you should leave it active. Modern ones typically have thermostats on the the heater to run it only when needed.

It isn't any safer but yes 2 pole contactors can typically be used instead.
 

Touring5

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What do you mean " For added safety " ?
If both legs are disconnected (by the two poles on the contactor) nothing beyond that will be "hot". As it is, one leg is connected (by the shunted leg) to wiring inside the unit, specifically the fan motor. The two wires for the crankcase heater are fed from the line side of the contactor (i.e. aren't switched).
The crackcase heater is to prevent refrigerant migration and condensation in the compressor. It helps all ac and heat pump units but is actually rarely installed on residential ac units with scroll compressors because they typically aren't operated in those conditions often enough to cause failure and the manufacturer can save 5 bucks.

If you have a heater then either the manufacturer or your installer deemed it necessary and you should leave it active. Modern ones typically have thermostats on the the heater to run it only when needed.

It isn't any safer but yes 2 pole contactors can typically be used instead.
I was surprised to see that the unit does appear to have a crankcase heater connected (two black wires coming from the bottom part of the compressor, connected to L1 & L2. This is a 1989 Heil unit with a piston compressor I believe.

It just got me wondering. I don't know what temperature the heater thermostat is set for but it would seem the unit would be drawing (wasting) power all Winter as long as the circuit breaker and disconnect are On. (After Summer I usually turn off the circuit breaker until the next hot weather).

I looked on a hvac forum and some guys said they only carry 2-pole contactors on the truck (simplify inventory).
 
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The contactor has nothing to do with any crankcase heater. Heater wires would be connected to always-live wiring on the input side of the contactor.
 
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I was surprised to see that the unit does appear to have a crankcase heater connected (two black wires coming from the bottom part of the compressor, connected to L1 & L2. This is a 1989 Heil unit with a piston compressor I believe.

It just got me wondering. I don't know what temperature the heater thermostat is set for but it would seem the unit would be drawing (wasting) power all Winter as long as the circuit breaker and disconnect are On. (After Summer I usually turn off the circuit breaker until the next hot weather).

I looked on a hvac forum and some guys said they only carry 2-pole contactors on the truck (simplify inventory).

Ahh that would be why then. Piston compressors do not tolerate liquid very well at all.

Being that old you are probably right that it has no thermostat and simply is on all the time. You wouldn't have any issues flipping the breaker off over the winter but I'd recommend turning it on 24 hours before using it for the first time in the summer. That will give it time to boil any refrigerant out of the oil.
 

Touring5

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Ahh that would be why then. Piston compressors do not tolerate liquid very well at all.

Being that old you are probably right that it has no thermostat and simply is on all the time. You wouldn't have any issues flipping the breaker off over the winter but I'd recommend turning it on 24 hours before using it for the first time in the summer. That will give it time to boil any refrigerant out of the oil.
Thanks for that info. In the past I recall hearing a bubbling or boiling sound that seemed to come from the evaporator coil (in heating season). I wondered if there was some refrigerant flow occurring due to the cycling of the furnace (an upflow with the coil box directly above the furnace heat exchanger). There have been Winters when I neglected to turn off the breaker - perhaps some convective like refrigerant circulation (?).

It's been a few decades but I didn't remember simple condensing units using (or having) crankcase heaters.
 
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I've never owned a unit with a crankcase heater . The contactors have always been double pole with one side strapped through . Not much need for a crankcase heater down here apparently .
 
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If your unit has a crankcase heater it will usually look like a big hose clamp around the compressor with two wires coming off of it.

Single-pole contactors are used for one reason only, it saves the manufacturer money.

It is perfectly OK to replace it with a double-pole contactor.

Unless your unit is a heat pump or otherwise operated in conditions when the outdoor temp is lower than the indoor temp (like a computer room AC unit), it does not need a crankcase heater.

And even newer heat pumps with scroll compressors often don't have a crankcase heater. Mine doesn't. Been that way for 5 years. Trane says it's not needed.
 
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Thanks for that info. In the past I recall hearing a bubbling or boiling sound that seemed to come from the evaporator coil (in heating season). I wondered if there was some refrigerant flow occurring due to the cycling of the furnace (an upflow with the coil box directly above the furnace heat exchanger). There have been Winters when I neglected to turn off the breaker - perhaps some convective like refrigerant circulation (?).

It's been a few decades but I didn't remember simple condensing units using (or having) crankcase heaters.

Sounds like it. Refrigerant moves to the coldest point so changing the temp on one component will make it move around to balance out.

Normally they don't since scrolls can actually pump some liquid. Pistons will not pump liquid.
 
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Before u get excited about the heater need to put a amp meter or ohm meters on the leads most over a few years are bad and should be replaced ( pain in the butt). Single pole contactor like others wrote save the manufacturer a few pennies. Wire to the line side. Heater does one thing keep oil warm to drive refrigerant from the oil in the off cycle so when comp starts oil doesn't turn to foam from refrigerant boiling off. A 5 minutes time delay is very important most electronic thermostats have this in their setting or there i a cube time delay relay under $20.
 

Touring5

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Many units do not have a heater. Mine does not.
Yes, I'm surprised this one does. In the 80's when I worked at a Carrier assembly plant I only remember heat pumps using heaters. I wondered if maybe this manufacturer just used compressors with heaters and optionallly connected them based on the type of unit they were going in (?).

I'll be putting the new contactor in today and replacing the crimp terminals that connect to the compressor and I'll take a better look (just for s&g).
 
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On a scroll compressor, the shell (and the oil sump) is part of the high pressure side. So there's no concern about liquid condensed there.
 
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