The York units are R-22 not 410a. High side pressures should be below 200 psig, assuming there is adequate airflow on the indoor unit. I initially had problems with airflow (the contractor who did the original install undersized the returns), but once that was sorted out the pressures were still way too high (it had to be > 300 psig to trip the limit switch). I finally determined that the subcooling was very high which is consistent with having excess refrigerant flooding the condensor. In effect too much of the condensor is being used to hold liquid refrigerant and not enough is available for heat transfer. The HP compensates by raising the pressure and hence the temperature to get sufficient heat transfer across the reduced condensor surface. I've read that the newer HPs are sensitive to charge level and my experience bore this out. I only had to remove a small amount of charge (into an EPA certified recovery tank) and everything worked right. I could also see the effect on the very next electric bill.
I went w/ the older R22 unit primarily because I was replacing an old R22 AC-only unit and I wanted to reuse the old linesets. R410a is not compatible w/ the mineral oil used in R22 systems, and so most installers recommend you replace the linesets if you upgrade to R410. I didnt' want to have to do that (my linesets were fine and changing can be a messy operation in an existing structure).
Also for the York units I was considering, the R22 model put out slightly more heat than the R410a unit, so I stuck w/ R22. I picked up a 30 lb bottle of R22 from ebay (thats one thing the EPA cert allows you to do) and so I should be good on refrigerant for a long time.
When I was installing the system, I spent some time looking at the type of thermostat I wanted. Most brands have "dual-fuel" capable models that will switch on the backup heat (propane in my case) if the outdoor temp goes below some preset temperature. The York has a builtin demand-defrost controller that has this ability, but what I wanted was some way to run the heat pumps as long as the heat they produced was cheaper than propane -- even if the outdoor temp was below the point where the heat pump could supply sufficient heat to maintain the set temp. The solution I found was to get a fairly simple dual-fuel thermostat that had a 2 stage differential -- the heat pump turns on when the temp is .5 degree below the set point, and the propane turns on when the temp is 2.5 degrees below the set point. This allows the heat pump to run even when it can't keep up with the heat loss from the home. In this case the ASHP will continue to put out heat while the indoor temp slowly drops -- when it gets to 2.5 degrees below the set point, then the propane furnace comes on and cranks the temp back up to the set point and the whole cycle starts over again. That said it has to be very cold out (<20 degrees) before this will occur. At higher temps the heat pump handles the whole load except during defrost cycles.
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