On Aug 27, 5:45 pm, Speedy Jim wrote:
bill allemann wrote:
On common natural draft NG water heaters, I usually see 4" vent pipe, but I
was looking at specs on a 40gal unit on the Ruun web site, and the chart
shows 3" for residential sizes up to 60 gallons. Somewhere else I was
reading that it is not a good idea to have oversize flue vents on gas
appliances. I can see where oversize vents would also be an energy waster.
Are a lot of people installing vents that are too large?
Anyone know of a good reference for sizing vents.

Often, it comes down to what the local
inspection requirements are. Some cities
won't permit 3" vent.

The draft hood bypasses most of the air
around the heater in standby mode so I
doubt that losses are significantly
different for 3" or 4".

Jim

Locally, we can't use a 3" vent if the water heater is the only
appliance on the chimney.

JK