good morning,
I am a moderately experienced amateur and have done maybe a hundred installations of small split systems in my own properties and have good equipment. what I don't have is any professional training so lack any fundamental theoretical knowledge and my question is in that area. I have a 9000 btu system that works fine in the room that it is in. the use of the room has changed so now the system needs to be a bit larger and also relocated. the 9000 uses 1/4 and 3/8 lines and those lines are " impossible" to change due to the way they were installed. the new 12000 btu system will use lines a size larger I believe ( maybe not and I haven't seen it yet but I think so ). the new inside unit will be roughly 10 feet away from the original location. the outdoor unit will be in the same position. both systems are 410a . so... the question is... can I reuse the original lines and mix the line sizes ? i.e 10 feet of the larger size to the smaller sizes and the a couple feet of the larger sizes at the other end to connect to the new outdoor unit ? my formal training is that of a mechanical engineer and theoretically all I would expect to see is an increase in flow velocity and pressure in the smaller pipes but what I don't know are any of the practical " side effects" in the refrigeration aspect of things. I really don't want to tear the wall up to do new lines if I don't have to....if you know what I mean but in the end, the thing has to work properly. thanks for your time

steve