The only way to safely vent one of these gas hot water heaters is to modify the appliance by connecting the chimney to the appliance and incorporating a barometric damper
. By adjusting the damper, one can control how much combustion air enters the appliance.
A round duct with barometric damper
was discussed during construction and is still being tossed around, although the owner is not yet toasted, and 350[degrees], the Schalls would attest, is a gross exaggeration.
Causes of a low percentage of carbon dioxide include too much combustion air being supplied to the burner, a broken damper (or one that is open too wide), too much chimney draft, air leaks into the boiler or furnace, or improper setting of the barometric damper
Gravity type barometric dampers
are also an option.