My Plumber says the condensate line to my third floor HVAC unit cannot go outside as it will freeze. Is this correct? I see many other houses with lines outside. I want to drain it under my porch and am not concerned about acidity there nor any issues leaking back into the foundation as that area drains well.

it will freeze..

There is no condensate unless it is humid AND you are running the air conditioner, Unless you are referring the to combustion condensate on a high efficiency unit

i assumed because he said HVAC that it did include heat??
we yankees call them AC units and we don't normally run them when it is 4 below zero outside.

Heating Ventilating Air Conditioning

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

Unless it is a condensing unit, there is NO "condensate" when it is heating.

Most of the heating systems in my area are condensing units and will freeze up if a 1" pipe is run thru an exterior wall. The vast majority of them are piped to a floor drain, the others are piped underground to a small gravel pit.

Thanks for the warning Packy. I won't go outside. Next question. If it goes into my drain piping must it be an open pipe to a sink? Can it empty into a sealed trap like a dishwasher does?

yes it can..

If it's the winter and the air conditioner is not used couldn't the trap dry up from evaporation unless he uses a trap primer?

I asked about condensate from a dehumidifier and was told it needed to drain into a standpipe with a trap like a washing machine.

My AC (no heat) unit drains condensate via 1" PVC pipe protruding from the fascia into the gutter outside. Makes me wonder now why they couldn't run the condensate into the plumbing stack in the attic?