I know 3/4" is spec, but existing in wall is 5/8 can't replace so will 5/8 damage 3 t unit or will it even work. Another option is I have a redundant 5/8 line is it possible to combine these or would that be to much capacity?
I would not want to comment if the 5/8 would work or not. It may function, it may not but it would not have near the proper capacity I don't like to give advice to people when I'm not positive what the outcome would be.sorry I would just open the wall and change the lines and make it right
25' to 30'. So all I will loose is a small % of eff? A 5/8-1/4" line set won't harm unit?
Assuming a few LR 90º fittings, your looking at loosing in the neighborhood of 500 btuh of the systems capacity, vs around 150 btuh loss with a 3/4 inch suction line, so around 350 btuh difference.
If the line is in good shape, its not worth chasing that 1% difference in total capacity, as you would likely never see a payback in the life span of the system.
The higher refrigerant velocity in the 5/8" suction line will result in more reliable oil return, so could potentially actually extend the life of the compressor.
You didn't mention the 1/4" liquid line in the OP. That is a no go for a 3 ton system, even with a short line set, due to the pressure drop.
5/16 would be the minimum.
If more government is the answer, then it's a really stupid question.
I would say more then lickly if you have 5/8 suction line then your liquied line will be 5/16 which might appear to be 3/8 really hard to tell then apart sometimes. Will it work yes but expect btu loss.
I would confern that you do have 5/8 and 3/8 and get installer to see what the loss will be and if you have 5/8 and 5/16 same applys you are pushing the limits of the line set. If its 1/4 on the liquid line then it will not work propely and all kind of problems can occur.
Installer needs to run this through pipeing software and see what the loss is or will be. Only real way to know for sure.