It's easy enough to figure out and close enough to get you by. It varies from seer to seer. Let's look at a 10 seer. The condensing temp on the inside scale of the liquid guage is approximately = to the discharge air temp of the condenser. We know a 10 seer matched unit has a 20 degree delta t across the condenser coil. Wrap the condenser coil and maybe even block the condenser fan to drive the liquid pressure up until you reach 95 on the inside scale. Take your 95 and subtract your condenser delta t and now you know you can charge the unit like it is a 75 degree day. Take your wet bulb and everything else as usual. Just make sure your liquid pressure is holding steady. You must have the home up to 74 degrees inside and a little humidity from the humidifier doesn't hurt. If there is no indoor load there are no tricks in the world to help with that.
Originally posted by BaldLoonie
30 degree drop? You ain't movin' much air! Do the customer a favor, go back on a warm day. Wrapping the coil simulates a warmer day but most charging charts ask outdoor temp. What is "outdoor temp" on a 55 degree day with plastic around the coil? If charging a piston system properly, you need indoor wet bulb. In cool weather, so low it is off the chart.