heat pump refrigerant line sizing

[gmansr]

I am considering updating my exist Rudd 9 seer unit with a new Rheem 15 seer unit. The new units are 15JPL30A01 heat pump with a RHKLHM3617A air handler. The existing refrigerant line size is composed of 1/4 " and 3/8 " copper piping. The vertical rise using the 1/4 " is approx. 16 ' followed by approx. 20 ' of horizonal, it then switches over to 3/8 " for the remaing 18 ' run to the air handler located in the attic. Total run is approx 54 feet. The problem is that the vertical run is inside an insulated wall, the contactor said that he is confident the system will work with the existing runs, but if it doesn't he will run a new line but go the 16' vertical run on the outside of the house which I don't want to do. Rheem specs state the complete run should be 3/8 ", any suggestions? Is there a different system I could use?

[monkey spanners]

In general its best to follow the manufacturers recomendations about pipe sizing etc if you want the unit to work as the manufacturers intended. You may not get the benefit of the 15 seer rating if the pipework is restrictive.
I am not familiar with Rudd or Rheem equipment as i'm in the UK, mostly we fit mini splits here for residential air con.
Personally i'd want to run a new line set out, especially if the old ac was on R22 as i expect the new will be on R410A.

Jon