air supply inlet jargon

[Glevin]

What is the difference between nonmechanical and mechanical air *

I am investigating purchasing a tankless water heater. When deciding location, horizontal vent location appears to be a big issue.

The distance between the HVAC air supply inlet and the water heaters concentric, combustion air inlet differs between mechanical and nonmechanical systems air supply inlet systems.

My HVAC unit is in the basement. I have a duct (* *) going from outside to the return plenum before the filter. Would this be considered a nonmechanical air supply *

Thank you.

[4ftBanger]

That would be considered a mechanical air supply inlet as air is being drawn into this inlet by a fan. Pay close attention to these clearance guidelines to prevent exhaust gasses from the water heater from being drawn back into the house.