Multiple flues

I inspected a side by side duplex today and was checking the code check reference for the following.

There are 2 water heaters and 2 furnaces venting in one tile flue.

Suprisingly there was no exterior deterioration to the chimney.

Is this acceptable?

Also I am disapointed with the Preston guide.

Is Ducane listed under another manufacture for air conditioners? They are listed in the furnace section.

Also for example. I am referencing a Goodman furnace M# GMP075-3
S# 7255300333

Preston has this listed from 1992 to 2000 for the same BTU. So how do you know if it is 1992 or 95 or 2000?

Why cant manufactures just list the date?




Multiple vents going into the same flue are fine so long as they’re burning the same fuel. I can tell you that I see many chimney flues in multi-families taking on 2 water heaters and two furnaces. That’s fine. Any more than that, I’d be suspicious.

As for your dating code, Prestons has a huge window in that particular model #. As you stated, it has the same model # from 1992 to 2000. Prestons always confuses me.

According to my HVAC records, I have a different formula, but your numbers don’t make sense.

Goodman: First two digits of the serial number are the year. Second two are the month.
Btu rating is model number…ie; GMP075-3 (75,000 bonnet input)
Compressor is same :serial # 9709088872 (yr1997 wk 09) Tonnage is model # CK361C The first two numbers after the CK (letters) divided by 12 is tons. ie: 36 div by 12= 3 tons.

Are you sure the first two numbers in the Serial # are 72? According to this code it’s got to be a 9. That’ll make it a 1992.
But then the week would be 55? We only have 52!!!

Multiple appliances into a single flue can be correct, but if you have a doubt always recommend a professional inspections, combining appliances requires a good knowledge of flue venting practices to properly size the piping especially if you have a combinations of natural draft and powered draft units. Look at the venting for signs of corrosion, water leakage (flue gases cooling and running back down the piping) etc