the b vent or liner size for Hwt

[hmmbm]

Hi?I have a question about replacement a furnace. now the furnace and water heater using a common vent 6" or 5", I need to change the 96% furnace so do I have to change the vent to 4" for the water heater. Can I keep the 6" or 5" vent for the water heater?
I checked the code book it only shows the max input btu for vent size.

[samihabib]

You have to switch to a 4" because code says maximum for proper stack temp , if you oversize stack temp will drop to a point of condensation and back draft can occur

[Admin]

You can vent the water heater using the existing 6" vent if you can satisfy TSSA Advisory FS-065-05.

Download it here,

https://www.hvactechgroup.com/article.php/20070429181100239