I would not have installed it... however here is a thought:
Next time tell the builder to sign off you warned him, you are NOT responsible, and sign off on a load calc. If the builder will not sign off... then ask him 'Why not?'... and just look at him...
Builder will find a Craig's list guy... you need to do your best or leave your tools in your van.
GA-HVAC-Tech
Your comfort, Your way, Everyday!
GA's basic rules of home heating and AC upgrades:
*Installation is more important than the brand of equipment
*The duct system keeps the house comfortable; the equipment only heats and cools (and dehumidifies)
*The value of comfort, over the long term; leave economic choices behind!
Choose your contractor wisely!