Combustion Air from abandoned chimney?

[jcgoldner]

Hi everyone and thanks for any assistance

I'll be installing a Navien tankless at my house and have already used up the ideal spots for direct vents for the 2 furnaces in the house. I need to get a little creative for these 2" lines. I have an idea of using my existing brick chimney (8x8 clay lined) for the combustion air only. I'd like to remove the existing single wall galv stubbed into the base of the chimney and replace it with 3 or 4" PVC, mortar it back in place, then reduce down to 2" for connection to the Navien. I will then have a few options for getting the exhaust line out horizontally from the basement.

I may be crazy but I don't see anything wrong with this. Do you see this being a problem or not passing any codes? I called Navien and they are fine with it but I've done quite a bit of online searching and haven't seen this done before. Your thoughts on this would be appreciated.
Thanks
John

 

[WorthFlorida]

All systems must be installed as recommended by the manufacturer. There are pipe type and maximum length specifications. Another problem using an old chimney is as the wind blows over it, it will lower the air pressure above the chimney and suck air from it, therefore, possible lack of combustion air.

 

[Sponsor]

 

[jcgoldner]

Thanks for the reply. I did call the manufacturer and they said it was ok but didn’t know if it would pass local code. The total length would be less than half of the allowed length for the combustion air and the unit is rated for different atmospheric locations of the intake and exhaust. Wouldn’t the same scenario of concern still be possible with a normal approved PVC vent through the roof?