Venting Tankless Water Heater to Attic via existing vent

I am looking to replace my existing 50 gal water heater with an HE tankless condensing water heater. The location is in the basement and there are two 6 inch vents that go to the attic for outside air for the existing old water heater and older furnace (will replace furnace later). I have a clear path to run the HE water heater exhaust to the back of the house and could do the vent as well, but I am wondering if I can do a short run for the fresh air input vent to one of the 6 inch vents that goes to the attic and have it pull outside air from the opening or even just a 12 - 18 inch run straight up to just pull air from the room. Would this work? I'm thinking it would as air would be pulled from the attic and should be sufficient. Your advice would be appreciated.

 

It's probably WAY oversized for an air intake vent, but as long as it's a vented attic (soffit & ridge, or gable vents of reasonable size) it's fine. Galvanized vents can't be used for exhaust venting on a tankless though.

 

1: Intake and exhaust should both take the same path. This prevents problems due to pressure differentials.
2. Exhaust can not dump into attic. Needs to go outside the house.

 

I do plan to put exhaust to the back of my house. I was just hoping that the Attic vent would provide sufficient air input to the room to pull from. Is there something more to know about pressure differentials as I am not clear as to if that would be a problem.

 

It's best to follow the installation instructions.

Pulling air from the room may be an option, but there are some strict guidelines of the volume of the room verses the BTU rating of the burner. Having an opening from outside into the room that is not terminated in the device will act like a chimney and release a LOT of heat from the area up it.

When I was doing some work on my condensing boiler, with the front cover off, there was a significant cold draft from the air intake into the room. Many of these things use a vacuum switch as part of their interlock system, and expect there to be a slight drop in pressure when their forced combustion fan turns on. That may not function properly if it is not terminated properly per the instructions. The final arbiter of this is the installation instructions. Everything else is suppositions.