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Jeremy Martin
14-05-2013, 02:32 AM
Recently I came across a Bryant heat pump with a 1.5-pole contactor. They had the crankcase heater wired up across the side of the contactor that breaks the circuit, i.e. one side of the heater connected to L1 and the other side connected to T1. This was consistent with the wiring diagram.

When the contactor is pulled in, there will be no potential difference between L1 and T1, so the heater won't do anything.

Here's where it seems funky to me. When the contactor is not pulled in, the crankcase heater is essentially wired in series with the compressor windings. Apparently the voltage drop through the heater is high enough that the compressor won't spin; instead, the windings will heat up (and help keep the crankcase warm).

Am I understanding this correctly? It seems like that would be hard on the windings. And what happens if the crankcase heater partially shorts to ground?

Tesla
14-05-2013, 05:32 AM
Hi Jeremy
It would seam probable that you have miss interpreted - all good the contact energises and compressor comes in and cch drops out, on de - energise the comp drops out and the cch comes in but not in series with comp. It should be comp on NO contact and cch on NC contact. If not lease post some pics.

Brian_UK
14-05-2013, 07:34 PM
Some manufacturers do indeed use the compressor windings for oil heating in the off-mode.

monkey spanners
14-05-2013, 11:32 PM
I have read about similar set ups on American style ac systems.

Assuming the CCH is about 70W that works out at .3A at 230V so i don't think it will harm the compressor windings. Likewise the CCH would be about 755 ohms so another 3 or 4 ohms in the compressor winding won't make much difference.

If you are concerned about the CCH shorting out and causing the compressor to run you could fit a small amperage fuse in line with the heater.

Jeremy Martin
15-05-2013, 02:05 AM
@Tesla it was a standard A/C compressor contactor (at least standard here in the US of A), no NC contact.

@monkey spanners Those numbers make sense. I never had to mess with a crankcase heater so I didn't know how many watts they're supposed to draw. When you look at the ohm numbers though, that's definitely not a problem.

Thanks for the info.

Tesla
15-05-2013, 07:00 AM
Well I learnt something out of this post. Now I wonder why they all don't wire up like Jeremy said, could we just get away with a big 170ohm resistor monkey spanners and let the windings do the heating?

Brian_UK
15-05-2013, 11:11 PM
Copeland have a document with wiring diagrams....

Www.hvacrinfo.com/ae_index.htm
Scroll down to section 22 and select document 2-1230

Sorry, can't master cut and paste on my new tablet yet.