Venting - Common Vents

[Zzz]

As stated in the gas code you must use 2 base tee's for a induced draft furnace and a water heater with common venting.
My question is why do most furnces downdraft the water heater before firing and getting the b-vent hot and working? I know back a few years they went through some dumb ideas, individual vents, y-branches etc. I have seen so many hot water tanks with the plastic rings burnt off (inlet and outlet) it makes me wonder why the geniuses at the saftey branch can't come up with a better solution. The way I see it they are using the old theory that the appliance can downdraft for 7 minutes. I bielive the solution is vertical b-vents and at least one foot between bace tee's which is not practical in either case.
This is not a problem if the water heater is not firing when the furnace comes on since, there are no  products of combustion untill the furnace (there's that F word again lol) fires. Howeve I have seen it cause pilot outages on the water heater.
Your input will be appreciated.

[Admin]

The two causes of your problem are insufficient chimney capacity and lack of combustion air.

It's important to calculate the capacity of the common vent, and to use Annex C of the gas code to apply reductions.  You can't replace the water heater that's onsite, assuming that everything is fine and to code.

I interpolated the data found in the gas code vent tables to the foot, and included the deductions for a liner, tee, extra fittings ect...  It makes calculating the common vent capacity fast and easy.  If you see a chimney liner with elbows between the liner and tee, then chances are the common vent is undersized.  Most people end up going with a power vented or instantanous water heater if their chimney won't support a second appliance.  If your chimney calculations are close look at installing a low input water heater which has a lower BTU input.

You can download the vent tables - Here

You have a good eye to notice the melted rings around the water piping.  That's a good sign there's a venting issue.

[Zzz]

If while your out there in the field and you check the draft hoods on water heaters you will find the h/w tank downdrafts when combined with a power vented furnace 50% of the time even when the venting tables are followed to the rule, but only untill the furnace fires up.
Hehe try it ya might like it.
29 years out there in the field and still learning every dayI love it!!!