Thank you for writing to me. I am sorry that you having trouble with your Pentair heat pump. the new ones have been pretty reliable. I hate to move heaters once heat pumps are installed as funny things seems to happen when they are moved sometimes. When a compressor runs for a few seconds and then shuts down it is usually because of the low freon pressure or high freon pressure. It can also be the water flow switch, thermostat or time delay relay. Chances are it is a freon pressure switch though. If the high cut out switch is open (tripped) it can be a few reasons:
1: Too much freon
2: Water flow switch failed and compressor runs with not enough water flowing through the water pipes. (Water flow problem and bad flow switch)
3: Water valve shut on outlet of heat pump. So the water pressure is fooling the water flow switch into thinking water is flowing and heater runs until freon pressure gets too high because stagnent water can't absorb the heat.
4: Bad high pressure switch
5: Physical damage to liquid freon line obstructing the flow.
6: Freon filter drier clogged with dirt or water. (ice)
7: Capillary tubes or Expansion valve that feeds the freon to the evaporator clogged or not working.
8: Bad reversing vale on heat and cool models.
9: Trash in the heat exchanger. I have seen wood chips etc. Kinda rare though.
I am not sure why the mechanic couldn't figure it out exactly on the spot. Sounds like he is going in the right direction though. The accumulator comment threw me a bit. I think he meant the receiver. The accumulator is on the suction side of the compressor and is there to prevent liquid freon from getting into the compressor and damaging it as you can't compress a liquid. It is usually very cold and not warm or hot. The problem is probably is a restriction of some sort in the freon line but I would need to be there to tell for sure. It is simple for a HVAC guy to tell. Really shouldn't take more than an hour to get to the bottom of it. Maybe another couple to 3 hours to fix it depending on how serious it is. I would ask another mechanic to look at it. What you pay for the repair, or if you repair I guess depends on how much money you spent for the heater. Your pool will start cooling down in 2 or 3 weeks so this is a good time to get it ready if you are going to invest the money.
Please let me know how you make out. I am very interested.