flushing old lines

[osagebow]

Hello, John from Western VA here. First post, thankls for the oppurtunity to pick y'alls brains.

I am getting an 30 year old 2.5 ton lennox electric heat pump replaced by a local trane guy. He is putting a XL 15i compressor and XL cabinet in but he is keeping the existing lines and flushing them. My old lennox was serviced by a technician several years ago that supposedly cleaned it and replaced the refrigerant, but it quickly stopped cooling, (but continues to heat.)

-what are some reasons the old unit still heats but cannot cool? I am concerned the lines may be part of the problem.

-what would be left in the lines that could contaminate the refrigerant in the new unit, and what can I look for during the install that would be a "red flag" for possible contamination.

- what is "green slime" in HVAC lines? I saw that somewhere either here or elsewhere researching hvac replacement. Can flushing the lines be effectively done if it is present?

Thanks
John, AKA Osagebow

[2old2rock]

More importantly, are the lines the right size?
If not, you'll have problems, especially with a heat pump.
Most reputable companies would replace the lines, unless absolutely impossible to access.