CAT I Horizontal Vent Limit

[BronYrAur]

I walked into a situation where I'm asked to install a condensate drain with neutralizer on a vent system. Upon investigating, I found that the installation was terrible. There are two (2) 80% upflow furnaces with draft fans in a mechanical room. Input on each is 88,000 BTUH. The vent connectors are 4" single-wall and rise up about 3'. From there they wye into a horizontal 6" B-vent. The 6" B-vent then runs 80' (yes, eighty feet) horizontally. It then elbows up through the roof with only a 5' stack. So total rise height including the connector rise is about 8'.

Shocker! They are condensing in the 80' horizontal run, and they think a drain will somehow help. I'm going to point out the problems and would like a double-check please.

According to the 2018 NFPA 54 table 13.2b, with a 3' riser and an 8' height, the single-wall connector risers should be 5". 4" is not even listed as a possibility for these conditions.

looks like the common header size of 6" B-vent would be correct, provided there were not too many elbows, etc.

Section 13.2.4 states that the length of a common vent manifold shall not exceed 18" per inch of manifold diameter. So, if I am understanding correctly, with a 6" diameter, the horizontal limit is 108" or 9'.

Does this sound right?

 

[trashcanman]

Does not sound right at all. NO DRAINS!. That is a dumb solution.
Check with the gas vent vendor. Enlarge the vent size and in addition, provide another draft fan at the termination of the vent (on roof?). Fan must be specific for gas exhaust fumes. Not cheap, but do you want products of combustion into the occupied space?