Problem. AC installed prior to closing but doesnt work.liability?

[Kelvin J.]

I closed on my first house with a freshly installed A/C. It hasn't worked properly since install. It is under warranty but after several visits the contractor is blaming the existing pipework (the 'line set' and drain line) and wants to charge me to replace the line set ($500). Who is liable? The rehab has been done here in Florida in August in a hot box. The rehab contractor already walked off the job once. Is there a governing body I can contact? After last nights conversation I need to pay him today to get the work done.

Details:

The house A/C was ripped out and sold in this foreclosed house. A new AC was installed in May to make the house habiltable for sale as a REO. The contractor used the existing drain line and 'line set' and put in new air handler, thermostat and condenser. AC kept tripping off or blowing warm air continuously. After several visits the existing drain was found to be blocked and backing up tripping a float switch. After I blew it out and raised it to allow condensation to drip freely the problem remained.

So far the contractor has replaced a compressor (burned out) and the condenser (with a goodman rep involved, neither of which could figure out why as they are so reliable)

New line set to go in today for $500

who is liable for this and who can I talk to about it? 

Thanks in advance!

Kelvin