Replies

1. 
   johnnyd | May 18, 2005 03:59pm | #1

   Careful...there are really strict requirements for flue size, type, and configuration...for the good of both appliances and especially the occupants as carbon monoxide poisoning is no joke and it's awfully easy to get the flue sizing etc wrong .  You'll have a hard time getting this information second-hand. 

   The only sure, safe way to do this (without the knowledge of a pro) would be to run a separate flue spec'd out per the WH manufacture.

   I you really want to DIY, at least hire a licenesed HVAC guy to do the design work..if you can find one that will do it.

2. 
   TRice | May 19, 2005 01:35am | #2

   "Can the two share an exhaust stack?" Yes, most likely. If both appliances are gravity vented, both can and in most common installations, do share the same flue. If either of the appliances is a forced draft, induced draft or power vent type, I (and most Code officials) would recommend against combining vents/flues.

   "How would I determine the size?" The easiest way is to rely on the professional that you are paying to install the vent to size it proerly based on the specifics of the installation and the appliances involved. Most manufacturers of the common "b-vent" products have tables to properly size the flue.

   A very common installation combination includes a GWH at 40,000 btuh input and a gas furnace at 80 to 100,000 btuh input, both non-condensing, vent through a 3/4/5 type of setup. 3" connection to the water heater, 4" connection to the furnace and 5" combined vent size through the roof.