Tankless Venting Question

[Steve Soldo]

Replacing an Bosch Aquastar with a Navien condensing unit, indoors. Previous system had a 5 inch exhaust galvanized ducting.

I can run the 2 inch exhaust line PVC through the existing 5 inch duct. But I am not sure how to secure the PVC as it travels through there, especially at the roof.

Any ideas how to do this ?

 

[Fitter30]

What's the full navian model most need fresh air also. Two 5" sheet metal caps, in plumbing a 2" Pipe hanger and a tube of polyurethane caulk. After the bottom cap to flue by offsetting the hole for pvc to the out side edge with the 2" pipe hanger think you could catch the pvc with hanger attached to the flue pipe after ell (90°). On the roof could center the pvc center of the other flue cap. Use the poly caulk to seal everything. Caulk is little messy but I've had good luck using it. If pipe at the flue isn't plumb can use a hair drier to straighten it just watch the heat. Could also put some insulation between the pipes.
www.homedepot.com/p/The-Plumber-s-Choice-2-in-DWV-Pipe-Hanger-in-Galvanized-Steel-02HDWP/309604114

 

[Sponsor]

 

[Steve Soldo]

The unit is NPE-150S2

Thank you for your input.

 

[Fitter30]

Navian recommends two pipe venting but single would work. Look at pages 45 & 47.

https://www.google.com/url?q=https://whci.requested-info.com/brands/NNA/documents/installSheets/NPE-2_Installation_Manual_EN_210216.pdf&sa=U&ved=2ahUKEwjerdr-qoGFAxXVrokEHdAxBk4QFnoECAQQAg&usg=AOvVaw3nm7x8DyNSToHucBgrlYCI

 

[Steve Soldo]

Well, this unit is very expensive and I probably should vent it by the book. I am not so much concerned about the air inlet, but rather the exhaust. I am going to pass the 2" PVC through the existing vent pipe. But I have to figure out a way to secure the pipe and also seal around the pipe at the roof.

 

[Bannerman]

I am not so much concerned about the air inlet, but rather the exhaust.

A gas burning appliance requires sufficient air for combustion, and as most residential tankless WHs consume as much as 199,000 BTU/hr, the combustion air requirement can be substantial.

If the tankless unit is located within the home's conditioned space, unless provision is made to supply combustion air from an external source, it will likely be consuming conditioned air from within the home, thereby resulting in increased discomfort due to drafts, while also needlessly adding to the monthly heating or cooling expense.