On Jan 3, 3:40*pm, "Ed Pawlowski" wrote:
On Jan 3, 1:06?pm, Newton wrote:

I'm looking at replacing my 17-year-old 75-gal Hot Water Heater with
two 50 gallon units.

The current one vents into the chimney by way of galvanized ductwork.

What is the recommended way to vent the dual heater setup?
I am concerned that if one heater goes off, exhaust gas from the other
one might find its way back into the house by traveling backwards thru
the ductwork of the heater that is off.

AFAIK, two appliances are not allowed on one chimney. *That would include
two of the same type. *.

What kind of heat do you have? *If you have a hot water boiler, consider
adding an indirect fired water heater. *My 40 gallon has a first hour rating
of over 230 gallons. *You can go with a direct vent model, such as this onehttp://www.ho****er.com/lit/spec/res_gas/AOSRG45101.pdf
It will give you 164 gallons first hour. *Two of them *if you really need
that 320 gallons.

This one is larger but has a lower first hour ratinghttp://www.ho****er.com/lit/spec/res_gas/AOSRG45700.pdf

Before you make a final decision, look over the offerings available and you
may find you can get more output from a good single unithttp://www.ho****er.com/lit/spec/r-gas.html

Note that on the web page (left side) there is a link to dsireusa.org *Check
it out as there may be some rebates or state money to help you buy a more
efficient unit.

Do you mean a furnace-boiler and water heater cannot share a chimney,
Its code here.