Quick rundown.. 10 year old Lennox heat pump with elec aux. Since I purchased the house 5 years ago, I have practically rebuilt the system.

- Replaced 24v transformer - old one failed. Replaced with HD unit for industrial control
- Replaced 3A fuse with "Lil Popper" external breaker - great for when system is fucking up
- Replaced defrost control board on compressor - original board had cracked PCB
- Replaced compressor contactor - old contactor had eroded points
- Replaced condenser fan and air handler fan run capacitors - why not? They were 10 years old.
- Replaced reversing valve coil
- Had small leak in reversing valve assy. brazed, system evacuated, recharged, and pressure tested good

Before I replaced the reversing valve coil, the system would blow the 24v fuse about once a month. Usually would happen during the cold months when the system was cycling into defrost. When it did, it seemed like I was chasing a ghost to find the problem, which is why I installed the 3A 24v breaker. After replacing the coil, the issues seemed to go away. Later on during the summer, I discovered a small leak in the reversing valve assy, and had it eventually repaired after several HVAC outfits claimed that it could not be done - I just identified the leak location at a failed factory braze, and had a tech glob a bunch of silver solder onto it. He then evacuated the system, nitrogen purged it, recharged and tested it. It ran great through the summer and through the winter.

Just this morning I woke up and found that the 24v breaker was blown. Resetting it had no effect. Went outside and checked everything on the compressor, blew out some insect debris, and restarted everything. Breaker does not reset. Wait for compressor to kick in. It kicks in, and then almost immediately blows the 24v breaker again.

I tried disconnecting the reversing valve coil at the defrost control board, and reset the 24v breaker. It now stays on.

I did a resistance test on the reversing valve coil and got 15.8 ohms which seems to be within spec. I also set the system into cooling mode with the compressor shut down, and verified that the coil had good magnetic pull, which it did. I then reinstalled the coil, put the compressor back together, and energized the whole system. It started up and has been running normally for several hours now.

The only part of the defrost system that I have not replaced, is the temperature sensor on the compressor. The system has both demand and timed defrost. The timed defrost is set to 90min accumulated runtime since it never gets that cold here. The demand defrost sensor seems to work as the unit will go into defrost on its own if it begins to frost.

I'm just about to purchase a new reversing valve coil just in case, but I grow tired of throwing parts at the system. Anyone have an idea or stuff to check?

Heat pumps

Price point.

Cheap now and enough to get past the warranty period.

Check start/run capacitors, should be +-6% of rating.

Oil from the reversing valve leak can break down the insulation on the coil.

You do have a 10 year old system shit breaks, if you had a 10yr old car what would you do?

A new AC system is less than a new car

Reversing valve should be energized on cool, not heat on a Lennox.  

That is correct that is how it tested out. Coil is not energized on heat, but it does energize when system cycles into defrost (cooling) mode.

The 24v breaker reset again lastnight while the heat pump was running. It was down into the 30s so it would've been cycling into defrost. It seems that the 24v breaker pops when it gets into situations where defrost is called.

The most frustrating thing is that it worked great for over a year, then back to this again.

Even though the reversing valve coil tested OK on the ohmeter, I think I'm going to replace it and the pigtail - the combo goes for $20 on eBay. I wonder if it is shorting out internally when the coil itself gets too cold. Maybe I'll take the coil and throw it in the freezer then see what the resistance readings are.

If I had paid a tech to do all this work I might have just replaced the whole system by now. The only time I've had to call a tech was for the leak.

Disconnected 24V+ to defrost board and the 24V breaker no longer trips. Guess I will go from there. Going to guess that a relay on the board is shorting out internally

Replace the whole defrost board.

These things do happen, and honestly, as a professional, I have had to parts push to solve and issue. Low voltage and electronics as well as circuit boards are a pain to diagnose. I've had ambient temps making shorts because of expansion and contraction. I hate to say to part push but sometimes you have to with these types of controls

Yesterday I had a closer look and found that the 220V+ was intermittently grounding out to the frame of the heat pump. There was a stamped hole from the wiring area of the unit, where the harness passes through to the pump, and the 220V+ had chafed its insulation on the poorly stamped chamfered hole. It had enough room to hang free most of the time but I could see burn marks from where it had occasionally arced to the frame, and half of the wire strands were gone from the exposed area from arc damage. This has always been obscured by the defrost board so I never noticed it until now. Repaired the damaged section of cable, wrapped the harness in protective sheathing through the hole, and everything is working fine now. That certainly explains why when the 24V breaker popped, it usually happened in combination with the heat pump 220V breaker, as it appears that when the 220V+ grounded to the frame it probably sent 220V into the 24V circuit through some of the 24V components that are attached directly to metal within the unit e.g. the reversing valve coil. Also explains why the unit seemed to work fine sometimes after I pulled the cover and jostled the wiring around.

I also notice that since repairing the damaged section of cable that the pump is no longer drawing as heavily (e.g. lights temporarily dimming) as it used to when it cycles on.

I really can't believe that I just now noticed that since I feel like I've been over the unit and its wiring many many times.

Normally the common side of the 24VAC is grounded to the metal.

So putting 110 to get metal back feeds 110 to the transformer for a sec till the breaker trips

Years ago when we were having one of those spring days where it is cool in the mornings and evenings, but 80 during the day I turned on the A/C when I came home at 5 pm.  Wife got home late from work. Around 8-9pm.  It was cool outside by then. She came in the house, felt the AC running and like a pissed off bitch, she flipped the AC into heat instead of the off position.  

Cost her $600 for a new reversing valve and compressor swap.   She never threw another switch hit temper tantrum again.  

Don't have a pissed off wife running the thermostat do you??