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Old 07-10-2007, 11:44 PM   #1
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Default Heat Pump compressor theory

So it looks like my heat pump took a bath during my basement flood and decided to pack it in. I've gotten estimates from $2000 to replace the compressor to over $8000 to replace the unit. I want to be sure that it's really blown before I fork over all those dollars so I'm hoping that there's a homebrewer with some expertise that can help me out. Please assume I'm highly technical.

The bottom of the compressor got wet to about 2 inches which covered the entrance of the power cables into the compressor. There is some white sealant around the wires but it's possible some water got in past the barrier. In addition to the compressor, the positive temperature coefficient resistor (PCTR) was submerged. I took it apart to drain it, but I have to confess that I'm unsure of its operation and whether or not it was damaged in any way. It seems to be simply two ceramic plates with a metal disc between them. I reassembled the resistor and one time it got very hot, so I took it apart again and this time there was no heat.

I checked the voltage to the compressor and there is 220 volts between the run and common terminals, and between the PCTR the common. The compressor doesn't seem to want to start at all.

I was alerted to the problem when the breaker to the furnace was thrown. I reset the breaker but did not hear any compressor operation. The breaker did not throw again immediately and I ran it for about 30 seconds per test without it tripping. I didn't want to push it any further than that if there is indeed a fault.

I haven't checked continuity to ground yet, or terminal to terminal because I've only just found a very basic schematic for a compressor online. Maybe some HVAC person can direct me to more information or tests to perform. I have access to a Fluke DMM and can borrow a scope from work if need be. In fact, I should be able to access any equipment that can be found in a well supplied electronics maintenance shop, but no specialty HVAC tools.

So if anyone can help me out, I'd appreciate it. I'm not expecting that much can be done, but I'd be wasting a great resource if I didn't ask. We can go to PMs if we start to bore anyone.

Thanks in advance, HVAC guys(gals).

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Old 07-11-2007, 04:27 AM   #2
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More than likely, if there was any damage done, it would be the electronics and not the compressor. The seal where the wires pass into the compressor crankcase is designed to hold back refrigerant pressure. I can guarantee that no water got into the refrigerant circuit.

Even if the compressor was bad you would see voltage reading across the common terminal to the other terminals. This is due to the fact that most residential units only use a single pole contactor to start/stop the compressor. One leg of the 220vac goes straight to the common terminal, so it's live even when not running

If you want to check the compressor windings for opens, shorts or grounds, turn off the power and remove the wires from the three terminals on the compressor. Make sure you mark them so that you can return them to their proper positions.

To get accurate readings your meter must be able to check for resistance in different ranges (400 ohm, 4000 ohm, 40,000 ohm etc). If it can, set it to it's lowest ohm setting (400 ohm) and check for opens and shorts between the windings. Check terminals 1-2, 1-3 and 2-3. You should get some measurable reading. If you see no continuity, you have open windings. If you see zero resistance, you have a short between windings.

To check for grounded windings, set your meter to its highest range (40 meg ohms). Scrape some of the paint off the compressor body so that you have good contact with one of your meter's probes. Place the other probe on each of the three terminals, one at a time. You should not see ANY continuity from ANY of the windings to ground. If you do, you have grounded windings.


If the compressor was running when the flood reached the terminals, or if it started after the terminals were covered with water it would have created a direct short and tripped the breaker and most likely would not damage the compressor. Don't let some hacks start cutting out the compressor until you are certain it is shot. Let me know how you make out.

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Old 07-11-2007, 01:02 PM   #3
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The compressor was not running when the water came up. I shut off the breaker myself to ensure that it would not come on when the power was restored. I inspected the unit when the water receeded and the only component under water was the PTCR. I pulled it apart and drained it. Other than that and the compressor body, no other components touched the water. Of course I suspected the PTCR as the culprit but it appears to be assembled properly and still no luck.

I'm at work right now so I can't check for shorts and continuity until I get home, but doing that is no problem. I understand what you mean about the single pole contactor, but when the furnace starts up, you can hear the bang of the contactor and the voltage on the windings does go to line voltage. That tells me the compressor is getting its proper juice.

What can you tell me about the PTCR? That's the one variable I'm not familiar with. I tried taking it out of the circuit but leaving the capacitor in. Can the system run without it? As I said, it was the only thing that got wet and I did take it apart.

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Old 07-11-2007, 04:02 PM   #4
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I know that a PTCR has an increase in resisitance in response to an increase in temperature. However, I'm not sure what function it has in your particular unit. What kind of heat pump do you have that has the compressor located inside in the basement? It is a water source heat pump, or geo-thermal? Let me know who the manufacturer is and maybe I can find some more info.

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Old 07-12-2007, 02:17 AM   #5
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To me, water source and geo-thermal are one and the same. I draw from one well and discharge into another. The heat pump either cools the house or warms it, depending on the direction of the reversing valve. The weather here is too cold to have an external compressor and an air to air unit.

I checked the the compressor windings with an expense auto-ranging Fluke multimeter and it looks like I have open windings. I couldn't get any impedance readings at all between any of the terminals. They weren't shorted to ground but they appear open.

By the way, my unit is a Florida Heat Pump (FHP) and it's an older 5 ton model (SL055). According to FHP, the replacement for this unit is the GT054. You can see the propaganda at www.fhp-mfg.com

I don't know if I'll buy another FHP to replace this one. I think I may go with a locally produced machine instead. I tend to have more confidence in machines that were designed in Canada for a Canadian climate.

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