Why would a mechanic bypass the heat?

sounds like your heater core in your environmental controls ruptured. He would a bypassed it to stop the leaking but unfortunately does not fix the issue. In order to fix a heater core the entire dash has to be removed from the car to get at it. This would in turn make it a costly repair.

The dash doesn't have to be removed at all on these Audi's. Actually one of the easier Heater Cores to remove. However, once the heater box is pulled out from the firewall bay, the whole box has to be disassembled to get to removing the actual heater core radiator part. You'd want to hire someone who understands these Audi system, and pay them. Its about a 5 or 6 hour job for someone who knows what they're doing. And the heater core itself is only like $30 if you order it online.