Tim C. wrote:
Greg,

Regarding venting, the outside of the house is brick. The current vent
is on the roof. I realize that a hole could be knocked in the brick
for side vent, but I prefer to use the existing path out the roof, just
a bigger vent.

If we put in a 4" vent, and later some year down the road get anothe
water heater that only need a 3" vent, can we use the 4" venting system
with that heater? I assume that would be OK, just not the other way
around.

Thanks.

In most cases yes. You just should not keep a 3" when a 4" is called for.

ChrisGW