My house is 3400 Sq Ft. 8 years old, 2 story. Each unit is 2 1/2 ton, Rheem Heat Pump units. They have been working very well. I can get about 22 degree difference downstairs and about 19 upstairs. I have added extra blown insulation in the attic. Roughly about 36", with a power fan and whirly birds.
My problem started 3 weeks ago. My upstairs compressor went out. My Brother in Law has his own Commercial HVAC Company. He and I replaced the compressor with a 2 1/2 ton size. After that we noticed it would not hold a vacuum. We discovered the upstairs coil was bad. We replaced it with a 3 ton size. I cannot get the air as cold up there as before. He did tell me that with different size componets it was tricky to get a typical charge reading like you normally would.

My Brother In Law says I need to wrap my plenum with duct board 1st off. I can't hardly hold my hand on the plenum because its so hot being in the attic. I feel its low on freon. The big line, return I believe, is cold and sweating but not as much as my downstairs unit.

This is my 1st post here and thank you for taking the time for any suggestions.

If the new coil has a thermostatic expansion valve on it then the system should be charged by the "sub cooling method" if it does not have a txv then it should be charged by the "super heat method" these two ways of charging a system are the correct ways verses feeling the suction line. If it does not have a txv then you might want to have this added to your coil, Good luck. Mike

Thanks Mike, I don't believe it has a TEV. I know we didn't install one. Could you elabarate a little on what this does please? When we charged it we simply did it the "old fashion" way of using the vacumm and then charging by gauges. Is this what you call the sub cooling method? Just trying to get my questions ready for my Brother In Law.
Thanks again Mike.

A TXV is a valve that automatically controls the refrigerant that flows into the evaporator coil, it has a sensor that senses the temperature of the refrigerant line leaving the coil and adjusts accordingly to temperature and pressure.
Sub cooling is done by measuring the temperature of the liquid line leaving the condenser and checking the head pressure, converting this pressure to a temperature reading and subtracting the line temp. from this will give you the subcooling.
By the way, insulating the plenum can not hurt any thing. Mike

Mike, Thanks for the explanation. I will run this by him.

I just got through doing some duct work repair in the attic. Man, I could kick that sub. Some of the duct work had a lot to be desired. I took them off and used mastic and screwed the collars back on real good, then used some straps. With the condition it was in I should getsome help right there. I think I have about 1 return and 1 supply left to do. My digital temp gauge on top of my plenum, which is about 6 ft from the roof was 106.

I have been wondering about something for awhile. When the subs install the duct work why do they strap it up against the roof instead of laying it down on the ceiling where the insulation will be? It seems like you are working backwards.

Funny thing about strapping the duct, some inspectors think the duct will collapse the insulation or some contractors want to be able to crawl under it so they strap it up. I have had some inspectors insist on strapping it down so that it does not move around during an earth quake. So you do what they want and move on.

I can surely tell a difference since I worked on some of the duct work upstairs. Last night I was getting about 20 degree difference between supplyand return. I am going to wrap the plenum unit today so that might help as well.