If your in a no A/C needed area. Then sizing for the heat load is ok. Duct work if its large enough for the 3 tons, or even large enough or 4 tons has little bearing on temp delta across the coil as long as the blower is set up for the right CFM for the heat pump size.
Originally Posted by LibertyTree
A 2 stage/multi stage heat pump just slows the indoor blower when the heat pump is running in first stage. So with a 4 ton 2 stage heat pump. When its in first stage you basically have a 3 ton running on a 4 ton duct system. And still have a good temp rise.
Get the air flow correct, and then find out why it has a poor temp delta. Could easily be that it was over charged if they had the blower speed/CFM set too high.
Assuming R22 system 216psi head=>107F- 72F LLT= 35F subcooling. Overcharged?
The ductwork should not be a problem. As long as it's not too small, it will not adversely affect performance of the equipment.
You said you weighed in the charge, so I don't think it could be overcharged enough to perform that poorly. A 15 deg temp rise assuming nominal 1200 cfm is less than 20,000 btu. I would look for a possible restriction or problem with the TXV in the outdoor unit. The suction pressure would help. I'm guessing it's low.
I'm interested in what you find.