Boiler Vent Design

[rahim16]

Hi Guys,
I am installing one boiler with 2400MBH output capacity with two(2) 18" vent size (Boiler actually consist of two small units each has 18" stack). As per regulation we need Type B vent. Boiler is natural draft & has hood on it & installed in bldg basement.
Building has one existing 24" OD stack hole in the first concrete floor & in roof.Height of basement & first floor is total 28' (boiler height will be 7')

My questions are;
Can I combine the two 18" individual stacks to one common & use exisiting stack?
How I calculate the combined stack size?
By using thumb rule of 18+9=27"+2 = 29" or 30" (2" for type B vent)BUT I wanna know is there any calculation for finding exact combine stack size for my case?
Should we need to combine the two 18" stacks at different levels (we follow CSA B149.1 code in Canada).

Thanks in advance
Rahim

 

[lownox]

Boiler IOM should contain venting information that covers commom venting practices, vent height, etc. If it doesn't you should contact the boiler manufacturer about their recommendations. What is the make of the boiler?

 

[cry22]

A rule I used and verified with manufcaturers who sized the combined stack for me.
square root of (18 square + 18 square) = square root of 648 = 25.4" common stack.
This rule has proven to be oversized somewhat, so your existing 24" appears to do the job.

Don't forget the barometric dampers for each 18" stack.

You should run your sizing thru a boiler MFTR such as cleaver brooks.

Voila Rahim.

 

[DrRTU]

The boiler O&M should have vent sizing tables. I have used the Hart & Cooley sizing guide in the past.

http://www.hartandcooley.com/vent/all_vent.htm

You may want to look the Field line of draft controls; they have examples of multiple boilers in the catalog.

http://www.fieldcontrols.com/draftcontrol.php