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windswept4 01-22-2011 11:33 PM

heat pump Trane Executive II BWD724A

This heat pump is vintage 1986 and for 25 years has been heating and cooling the downstairs (1200 sq ft) until sometime this winter, the compressor failed to activate. Its "twin" heat pump heats and cools the upstairs (900 sq ft) and still performs fine. The wall thermostat to the downstairs unit still controls an operating air handler fan and the "blue light" back-up auxilliary electric heat (resistance) coils within the air handler (the place under the house and within the ductwork where the "A" shaped evaporation coils and the squirrel cage blower fan are hidden). The compressor outside will not activate with the flick of the thermostat control on the wall. No worries about having no heat, because I have natural gas ventless logs in the fireplace (the flue well packed and sealed with insulation to eliminate updrafts and eliminate a tremendous loss of heat right up the chimney); and that one "pitiful" gas log, alone, keeps the house toasty.

I'm going to begin this thread with the hope that those of you who have no knowlege or experience with this sort of thing can travel the road of enlightenment along with me and we can learn together and experience by trial and error and by the good graces of those who have travelled this road before and are kind enough & willing to share their suggestions, photos, charts or sketches that will make us better informed.

My first efforts will be to focus on the electrical circuit from the line voltage source through the fuses, contactors, relays, and safety switches (if any) all the way to the start & run capacitors and the compressor itself.

If I can get the compressor to activate (and there is no assurance of this at this time), I'll move on to the refrigerant pressures (high and low pressure sides of the compressor).

I'm going to start by posting some pictures and wiring diagram for use by anyone having similar unit and as I learn and become aware of the step by step process of trouble shooting this unit, I will share my findings with you. Any guidance or help, of course, will be greatly appreciated.

As the compressor simply will not activate upon fingering the wall thermostat, my first suspicion is that a start or run capacitor or a contactor has failed. I'm also going to guess that the problem will be fairly easy to isolate and cheap to fix, notwithstanding the age of the unit. I'm also banking on another 5 years of service from each of the 25 year old Trane units. More to follow when I get time to get outside again and begin testing the line and thermostat voltages and circuits.

The usual disclaimers: i'm fumbling my way through this...not knowing much other than this: capacitors do hold a charge, until safely discharged; and i'm working with 220 Volt & 24 Volt AC circuits, which means please do not insert self, or fingers, into the circuit. Also do not provide a convenient path to ground through oneself for any electrical component. Always assume the hardware has not been wired correctly...and with that in mind proceed with deliberate safety practices in mind.:thumbup:

windswept4 09-03-2011 10:35 PM

heat pump contactor discussion Trane Executive II BWD724A
Today the cooling function failed. Replacing the contactor fixed it. I thought it would be useful to post a few key observations that helped pinpoint the problem. I ran through some quick general tests to see what worked and what didn't.
1. The air handler inside was activated by the thermostat. This indicated that the fan inside the ductwork of the house was fine, the thermostat seemed to control the air handler which was essentially quick proof that the power to the fan was working, the thermostat and its 24 V power supply seemed to be working fine.
2. With the thermostat cool setting set low enough to activate the inside air handler as well as the outside components (the condensing coil, the compressor and the condensing fan), I then moved outside to see what was happening out there. The condensing fan was blowing ambient (air temperature) air fine, but the compressor would not come on. By waving my hand over the fan discharge air stream one could tell that it was just ambient air blowing without any added heat that would have been picked up from the condensing coils. Ordinarily in a working unit, the discharged fan air would feel somewhat warm if the compressor was actually compressing freon and the freon was condensing within the condensing coils. The absence of feeling that warmth and the absence of hearing the compressor motor itself, were 2 indications that the problem was compressor or compressor activation related. That the condensing fan was operating was proof enough that power through the cutoff panel was intact. In addition the condensing fan could be controlled by the thermostat (yet more proof that the thermostat circuits were ok).
3. So now my focus was on why the compressor was not activating and there could be a zillion reasons within the outside unit to explain that. For now though I removed the cover and inspected the visible wiring...there were no obvious burns or shorts. I then activated the unit thermostatically and noticed at the contactor some obvious sparking and arcing around the contact points. Knowing that more often than not the contactor is a problem I stopped at that point and removed the contactor and replaced it as best I could on a Labor Day weekend.
This particular contactor was a 2 pole contactor (230 Volts rated at 30 Amps). Since 2 pole contactors were simply not available this weekend because the parts stores were closed, the only replacement contactor I could find was from a trailer supply store. They happened to have a single pole contactor (230 volts rated at 30 amps). Cost: $15. I bought it and then the fun began.
I had to learn a bit about contactors and here's what I learned about the old contactor: L1 is connected to one side of the source power supply (do not connect this to the ground lead and do not connect L1 to the return line). The other side of the power supply source I will call the "return" (this is not the ground) and the "return" is not connected at all to the contactor. L1 is essentially the source power and it is partnered with T1. T1 is the "pick off" terminal which leads to the loads. There are a number of loads within the condensing unit, but the principal loads are the compressor motor and the condensing fan motor. My contactor also had a second pole. This was labeled L2 and it was partnered with T2 in the same manner as L1 was partnered with T1. L1-T1, when the circuit was complete powered the compressor; L2-T2, when the circuit was complete powered the condensor fan motor. As noted earlier, the condensor fan motor seemed to work fine, but not the compressor. This is a strong clue that there was a problem in the L1-T1 connection (contact) point within the contactor points. Added proof of this (though I did not make the measurement) could come by simply measuring the voltage at L1 with respect to the return line, which would have been 220 Volts. A quick test of T1 with respect to the return line also should have revealed 220 volts with the contactor activated. Again, I did not test this, but the same tests with respect to ground (as opposed to the return line) would have revealed 110 volts at L1 and 110 volts at T1. The reality however was that the voltage at T1, when the contactor was activated was 0 volts with respect to the return, indicating no power from L1 was making it through to T1. This in fact was my problem. The contact point was so burned and pitted that no electrical connection could result and therefore the compressor had no power to run. The fix? Replace the contactor so that the L1 power supply can flow through to the T1 point (and from there power could flow to the compressor).
The purpose of the contactor is that a small current & small voltage activated by the thermostat (24 volts) pulls shut a pair of contact points connecting the huge power source (L1) of 220V and up to 30 amps to the wires (T1) that supply the loads (the compressor and the condensing fan among other loads).
In my case simply replacing the two pole contactor with another 2 pole contactor would have solved the problem. As the only replacement contactor I could find was a single pole, this meant the following "modifications".
On the two pole contactor, there is an L1-T1 pair and an L2-T2 pair. On the single pole contactor, there is only an L1-T1 pair and there is no L2-T2 pair (this pair was present, but it was hard wired closed, which means that anything connected to L2 would automatically be hard wired/connected to T2). What I did was to move all the old L1's to the new L1 and all the old T1 wires to the new T1. I also moved the old L2 wires to the new L1. I also moved the old T2 wires to the new T1. Essentially all the old L1 and L2 sources were combined into the new L1 and all the old T1 and T2 loads were combined onto the new T1. The new hardwired linked L2 to T2 were not used at all and they were left not connected to anything. As this "fix" is to a 25 year old unit, i'm not concerned about balancing the loads between a two pole contactor as the original specs called for, because even though the useful life of the single pole contactor may be shortened by doubling up the loads this way, it is still within specs and, if my capacitors are acting properly, the arcing between the L1 and T1 contact points should be minimized. The points should last a few more years, anyway, as my unit approaches obsolescence…remember its already a 25 year old heat pump.

windswept4 09-03-2011 10:42 PM

contactor: heat pump Trane Executive II BWD724A
1 Attachment(s)
Here is a pic of the old contactor with the related wires dangling nearby

Master of Cold 09-03-2011 10:56 PM

That unit was born with a single pole contactor.
The black (wire)side should be on the switching side, and red compressor lead on the non switched side. The purpose of this arrangement was to feed a very small voltage to the windings and act like a crankcase heater.

windswept4 09-04-2011 10:18 AM

Single pole two pole contactor elaboration Trane Executive II heat pump
Hi Master of Cold:

Thanks so much for the comment. I will double check the contactor and the lead configuration to make sure the all year cold weather "heater" lead to the compressor oil etc is truly "hot" year round. After reading your comments, I'm thinking that the contactor that went bad on me may not have been the original contactor (instead a replacement installed by a technician along the way). I've had the units for about 15 years and there is no telling what may have happened service wise during the first 10 years of its operation. Your feedback will lead to my re-examination of the old contactor to make sure it really was a 2 pole and to check the way I moved the wires from that contactor to the new single pole contactor. Thanks so much for your thoughts.

Master of Cold 09-04-2011 11:27 AM

I wouldn't worry about it too much. That single pole setup didn't really work too well for a crankcase heater anyway. If it runs, let it be. I don't fix what's not broken.

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