Three water heaters on one vent pipe.

Is it ok to connect 3 water heaters to one vent to the exterior?

I think it’s fine. But we’ll see what the code guys say when they chime in.

Sorry thought I had answered this last night.
It is most likely fine.
Common vents are allowed if sized properly.
The thing I see wrong in the picture is the burner port plate missing on the far left unit.

Email me if you want a copy of the pdf.

Oooops you must have posted this twice as the bottom half of the picture from last night is not there anymore.

To my understanding a flue if sized properly you can have however many appliances using it. There is a math formula using btu’s and such of each appliance to figure out the proper flue size. I just ran into a furnace and 3 water heaters all gas using one flue and when I spoke to a professional chimney guy he started going thru the whole formula to figure it out and is sending me some book to use for future ref… In my situation the chimney flue was undersized based on BTU’s.

My general rule of thumb is… the flue pipe diameter should increase (step-up) as every additional appliance if connected.

In the OP, the flue does increase when #2 is added, but doesn’t appear to when #3 is added. That would cause me to call it out for further evaluation of proper sizing and repair.

Approved by AHJ on a 4 unit condo building on the Plaza in KCMO. Built in 1938, rehabbed in 2005.

4 furnaces, two hot water heaters. One chimney. No liner, no cap.

Furnaces are 2005 all with inducer motors.

I highly recommended carbon monoxide detectors. About all I could do in that situation.

Jeffrey is correct and the formula is in every one of the B149 Gas codes.