I am trying to determine the rules on where a direct vent exterior termination for a water heater may be in relation to the furnace’s direct vent termination. Can someone please advise or refer me somewhere for these specs?
The manufacturers specifications of the water heater and furnaces, some are different then others.
In looking at those specs they referred to mechanical and non-mechanical air intakes which I take to be for room air and not other appliances.
In general, direct vent intakes and exhausts should be separated by at least 12 inches to prevent exhaust getting sucked into intake. (Concentric types don’t necessarily need 12" separation as their design ensures that exhaust can not easily flow back into the intake.)
The intake and exhaust vents for the same appliance should be in the same air pressure zone eg, the same side of the house, not too far apart, and be the same length, number of elbows etc…
Intake and exhausts should be free of any potential obstructions, such as being below the maximum expected level of snow height. They should be not be under decks or the exhaust pointed toward anything that can be damaged by water vapor. They should always grade evenly back to the appliance, there should be no dips that can trap water vapor.
Most direct vented appliance have safety controls that will turn off the gas valve if they detect pressure variations or obstructions between intake and exhaust, so nobody dies from CO, but maybe they freeze to death because the furnace won’t work.