Rectangular flue pipe

[Jon mackay]

Is it acceptable to use a rectangular pipe for a flue?

I would imagine that this would be rated for heat ducting but not flue venting.

[Markus Keller]

Good question, don't know but I would say probably not. Can't cite a section though.
My bigger concerns with this install are two things
- The left is probably a boiler, the right probably the water tank. If the boiler and HWT are running at the same time the boiler flue volume could hinder evacuation of HWT flue gas. The way the boiler flue comes in straight across the HWT flue path could block flue flow and cause back spillage at the HWT. Conceivably also at the boiler but I'm picking on the HWT because of the smaller volume.
- The large volume of the rectangular duct could also negate the flue effect of the chimney.
Of course either one of these issues could be minimal if the primary chimney has a kickass flue draw.
On installs like that I like to do the match test with units on and off. Stops any future questions when the client sees the match blowing backwards.
I would write it up. Maybe Bob will post with some NFPA section to back you up.

[Bob Harper]

NFPA 211-2010 Chapter 9 table 9.2.2.3 gives you the minimum gauge metal for connectors based upon diameter. Draw a circle inside this square duct to determine your "effective flue area" and apply. Also, this cannot violate the 7x rule, which this one might. Just because it is rectilinear does not make it illegal. Poor choice? Yes. Illegal--does not have to be round. Can also be oval up to an aspect/ ratio of 2:1.

Lots of other questions here. Main visible issue is inadequate clearance to combustible stud and cable. That scrap of sheetmetal gives you zero clearance reduction. Use B-vent connector at min. Probably needs a liner, too. Conduct Level II inspection. If no liner, still needs a cleanout for starters.

Target rich environment....