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Hi-I haven't worked on too many heat pumps so bear with me, but just from my A/C experience I know this one has a problem. It is a Carrier heat pump R-22 unit with flow check pistons and solenoid operated reversing valve, defrost board and a suction line accumulator(I think they all have these) Got a call that it was icing up on a 20 F day, but didn't get there till monday when it was around 36 F outside. Seemed to be operating normally with a bit of frost on the coils, I checked the defrost and reversing valve operation, and all seemed normal....fan stopped, it thawed out, and then when it began operating again there was heat inside. I put my thermometers and gauges on and took the following readings: Compressor discharge temp 136F Liquid line temp 76F Indoor temp 65F outdoor temp 36F. On the high (discharge) side, I was reading 168 PSI which gives me 88-90 condensing temp so I believe this gives me 12F of subcooling....a bit off the mark according to the chart on the unit. The readings that threw me were these: From the liquid line service valve coming back to the outdoor coil, before the flow check piston, I was getting 55 PSI / 30F and it was frosting up right at the service valve, then up towards the piston and on. Shouldn't the pressure be the same as the discharge here since it is before the metering device? Does this indicate that the piston on the indoor coil is stuck/ blocked? It seems that the pressure is not high enough to get the refrigerant through the outside metering device. I wasn't able to measure the superheat because it looked as though someone had removed the third schrader valve and crimped/brazed it shut, so I'm kinda in the dark here, but it seems as though there is either a restriction at the service valve, or at the indoor piston. Put it on emergency heat and am planning on heading back when it stops raining/snowing. Any thoughts/suggestions are appreciated.