Tankless Water Heater Air Intake Question.

I have a question on inside tankless water heaters and makeup (combustion) air that feeds them. I have had problems with plumbers saying it is o.k. to have the makeup air come from inside the home, and not be vented into the intake of the unit from outside (Mostly A.O. Smith brand). The installation directions say it can be installed both ways, but does not specify if it is in a Garage ( where more air is available through the garage doors, or inside a home). Especially with newer homes and these units drawing lots of inside air.

Opinions and facts please….

Does this not answer your question?

I get 2 different answers from 2 different factory reps.

I would go with whatever is documented in the installation instructions.

If those instructions permit it and the room is provided with a source of external combustion make up air I see no problem.

The room it is in is 10 x 24 and the end of a basement, and the door will be closed most of the time as this is the unfinished protion of the basement, and there is no other source of air into the room. While the A.O.Smith says it will through a code if not enough air is available, I want to make sure we do not have a problem after the new home owner moves into the new house. Then we are at issue with cutting into walls or bringing in vent pipe.

In my area a combustion air duct to the outside is required at the time of the build if there are gas consuming appliances and it has been that way for decades.

What about your area?