[marty69]

Two 4" flue tied into one... what should the diameter be

[smoothy]

I think that's a code violation.

[massplumber2008]

Hi Marty

The answer isn't really going to be an easy answer, per say, as each state has its own requirements and mathematical formulas for figuring this stuff out...;)

As an example, in my state, we have to know the chimney flue size to start, distances of each appliance from the chimney, BTU count on each appliance, location of chimney (outside wall or runs up through the inside of the home), etc., etc., etc.!

With that being said, in most cases where you have 2 appliances on an outside chimney that connect together, we would calculate that the two 4" flues must connect together to a 5" x 4" x 4" WYE fitting and then 5" must connect into the chimney. Like I said, however, there are many factors that can mess this simple case up and make it so that the chimney is not properly sized (undersized), or is even OVERSIZED and will not allow a proper draft to pull on the appliance flues which would then require that a chimney LINER of the correct size be installed....UGH!!

Anyway, I would suggest talking to a local plumber and have him do a proper calculation according to your state requirements....chimney flue work is something that can kill whole families if done wrong!!

Good luck!

Mark

[speedball1]

It's better to be certain. Contact your local building department. Good luck, Tom

[Milo Dolezal]

To compute the size of your vent for two water heaters, you have to know what is the combined value of BTU's for both water heaters. Based on that number, you will install proper size vent. See the chart.

http://www.hotwater.com/lit/misc/aostd63110.pdf

Hope that helps

Milo