In preparing to hack my air source heat pump, I am trying to understand the tools used for getting the correct charge. I have a Fieldpiece SMAN3 unit that hooks up to the lines and can help determine the Superheat and Subcooling. I hooked it up the other day and below is what it read. The dry bulb was 59 degrees in the heated building.
I have R290a installed in the unit already. It has been working for almost a year on this charge. It was filled to be about equal to the charge of the R22 that was in there.
The plan is to install a switching valve to go between ground source and air source. But I would like charge the unit to an optimum charge with the propane refrigerant before adding the ground source part of it.
This unit was manufactured for R22. What could I do to make it more efficient with the R290a?