Crankcase Heaters and Single-Pole Contactors

carrier

We keep two-pole, 40-amp 24v coil contactors on all of our vans. They are versatile and reliable, and you can replace most residential A/C contactors with them.

There are a few things to watch for, though, especially when you have a crankcase heater. Many brands power the crankcase heater constantly and shut it on and off with a thermostat, often mounted on the discharge line (looking at you, Trane). When you replace a single pole with a two-pole contactor in this type, you need to make sure you connect BOTH sides of the crankcase circuit across the L1 and L2 line side of the contactor to ensure the heater can function when the compressor is off.

Want to know something even more confusing than that? Look at the diagram at the top. Focus on the top left part of the diagram where the crankcase heater is located…

How do you think that works? I will wait while you think it through. Don't cheat. Look at it.

This is a common Carrier heat pump crankcase heater configuration.

You notice that one side of the heater goes to L1 line side Terminal 1, and the other side is going to L1 load side terminal 2.

So, the crankcase heater ONLY functions when the compressor contactor is OPEN. Even then, it does so by back-feeding through the compressor common and back through the run winding of the compressor to the constant powered L2 side of the contactor.

This means if you replace this contactor wire for wire with a two-pole contactor, the crankcase heater will never work. You must put the compressor run wire (yellow) to the bottom of the contactor (L2 line side) instead of the top, as it should be if you want the crankcase heater to function in this situation…

All of this is to remind you: DON'T BE A PARTS CHANGER! Know what you are replacing, why you are replacing it, and what each wire and component actually does.

—Bryan

Related Tech Tips

Underlying Problems That Get Missed
Today, I ran a service call with another tech where the previous tech had diagnosed an intermittent piston restriction. I read the history beforehand. For the past several years, there were many assorted comfort complaints and a lot of minor charge adjustments in both the summer and winter. It is worth noting here that the […]
Read more
Don't Squeeze a Radiant Barrier
You are probably all familiar with radiant barriers. Sometimes, it is thin foil draped under the roof deck. Other times, it's used on the inside of stud walls or over furring strips before the drywall goes up, and there is even plywood with a radiant barrier attached to one side that is used for roof […]
Read more
Newton's Laws of Motion
“Isaac Newton” by paukrus is licensed under CC BY-SA 2.0 Sir Isaac Newton was an English physicist and mathematician born in a small Lincolnshire village called Woolsthorpe in 1642. His father died before he was born, and his mother remarried when Newton was three years old. He stayed with his maternal grandparents thereafter, and his […]
Read more

5 responses to “Crankcase Heaters and Single-Pole Contactors”

  1. I’ve bee telling people this for years. They look at me like I’m an idiot and keep on putting 2 pole contactors in place of single pole contactors with out making the change

  2. What is the advantage for keeping the compressor coil in the active circuit? Only to energise the crankcase heater?
    In case of short circuit between cch and the unit earth, the comp winding, being thinnest in the circuit, would it not blow like a fuse?

    • That’s why you need to have the right size fuses, so that they blow up first.
      The problem with putting a 2 pole contactor comes because a lot of techs when they see a 2 pole contactor assume there is nothing energized, when you have the Run leg of the compressor energized. I prefer to put a relay that energizes the crankcase heater when the contactor is not energized.

  3. You see those little asterisk by the crankcase heater that means it is optional. I cases where you don’t have the crankcase heater it is using the compressor winding and cap to act like a crankcase heater.

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.

loading

To continue you need to agree to our terms.

The HVAC School site, podcast and daily tech tips
Made possible by Generous support from