[I] have a Heat pump with A/c. We are changing the indoor air handler to a gas unit. Can the outdoor condensing unit be used with a new coil in this system?

Yes it should be able too. Why are you changing coils?

The coil on the original indoor piece is 25 years old, gigantic,and does not match the new furnace.

Ah, OK. I would recommend you get a txv valve for your new coil. And get the exact match do not get a half size bigger. What size is your heat pump?

Josh, Thanks for the feedback. We got the system charged (no txv) and are getting a very high pressure on the liquid line. Would the lack of a txv cause this? I also read that the heat pump condensing units have back check or one way direction valves. What are we missing?

What size coil did you install? Tonnage? And what size heatpump do you have? Did you change the piston in the coil or use the one that came with the coil?

I am told the serial number states the size of the unit. To the best of my knowledge it is a 3.5 ton.

OK, is the new coil a 3.5 or a 4 ton? Was this unit charged in the heating mode or the ac mode? What was the suction and head presures you are getting?

We did not change anything in the coil.

4 ton coil. Charged in idle. Head pressure 75# and suction pressure pegged.

Outdoor fan is running normal? Did you try to turn it to heat and see what liquid pressure you were getting? 75 on the suction sound high also may be over charged how hot was it where you are today?