Thread: Induced Draft Furnaces

Inspected a rental property today and came across a situation I have not encountered before. 3 induced draft furnaces and a water heater all hooked up to a masonry chimney. All located in the basement - water heater was 26,000 BTU input, 2 furnaces at 45,000 BTU and 1 furnace at 75,000 BTU. total of 191,000 BTU all going into an 8" metal flue and then into a masonry chimney.
1. Is this too much for the size of flue pipe?
2. Should the furnaces be hooked to a chimney or PVC out the side wall?
3. Won't the draft from 1 furnace be forced out the water heater flue or other furnace's and into the basement?
Kind of hard to tell from the pictures, not an easy spot to get to and it was a real pipe nightmare.
Thanks, Lee

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Lee, that looks pretty instucting though we don't run across many basements here in Hou. maybe these things can help.
1) here it is not evan allowed to run exaust from equip. to masonary chimneys.
2) unless those furnaces are high efficency;90% or better the flues cannot be pvc type;and depending on location of doors and windows above that basement the basic rules apply for termination points.

Lee, there are condensate stains all over those flue pipes indicating a drafting issue. Refer for evaluation and all repairs as needed by an HVAC professional.

Like Randy said, only furnaces with efficiency levels of 90%+ can vent via PVC pipe. That's a pretty standard piece of knowledge for an HI. If you are unsure of this, you may want to get some additional HVAC training.

Hi,
Thanks for the input. Yes I knew about the 90% going into PVC. I thought these were 90+ furnaces and doing research this morning I found them to be 80+. I have already noted the the condensate stains on my report and am going to recommend evaluation, but I guess more for my knowledge is there a table for the size of a flue vs. BTU's going into that flue?

Thanks, Lee

Common vent capacity sections of Tables G2428.3(1) or (2) and 504.3(1) or (2).

First are IRC numbers and the second from Int Fuel Gas Codes but they are identical tables. The (1) or (2) depends on whether the connectors are single or double wall.

If I'm reading those tables right for your situation (fan+nat) then the allowable combined BTUs for an 8" common vent start at 310,000 and go up from there as the vent height increase. I believe that is assuming the common vent goes all the way up to the termination. I'm sure it get's "muddier" if you are connected to a masonry chimney, which would of course need to be lined, but the 8" seems plenty big enough for what you have there.

Having said all that, your photos gave me a headache and I'd defer to an HVAC expert, which I'm certainly not! One concern I'd have is the small, natural draft water heater operating by itself during the summer and whether that will achieve sufficient draft in such a large common vent. Someone else will have to answer that one.

Thanks Richard.