2. I understand that commercial gas WH's are using sched 40 PVC for
the
exhaust stacks because the exhaust is cool. How can this be?

2) Residential too. They are called "Power Vernted" water heaters.
Basemement air is drawn into the pvc through the inducer motor
diluting the warm gases.
Bubba

What's the advantage of a power vented wh?
I see a lot of disadvantages over "standard" ones, like higher cost,
more components to fail, less heating via center exhaust, motor and air
noise, etc. Also, I thought PVC deteriorated in sunlight so what's the
vent above the roof going to do?
Bob S.