Furnace and water heater.... Venting

The smaller vent is for the water heater and is lower than the furnace vent, Had a HVAC tech tell me today that this was fine. I tried to tell him that the lower BTU appliance needed to vent above the larger BTU appliance. Working on the report just want to be sure. THANKS in advance for your thoughts.

Yes you are correct unless the plenum type ducting somehow negates the issue in a way I am not aware of.

Did he light a match and check draft or just blurt.:slight_smile:


He was there only to start up the system it was a forcloure. Just blurt ya da ya da LOL

I am looking into this right now and think there may be exemptions which brings us into the foggy area to be educated on further.
There is what is referred to as an increaser in place and let me upload a diagram in a minute.

Just thinking that maybe the larger diameter acts like a plenum and changes need for water heater position to be above somehow.
This is a guess on my part.

Notice the water heater is at bottom and connected to an increaser .
If more than one appliance is to be connected
to one venting system, the common vent must
be correctly sized. It is a good idea to make
a sketch of the proposed installation, labelling
the components you will need. Adjustable Pipe
Lengths are available to make up odd lengths.
Minimize the number of turns and lateral
runs, as the National Fuel Gas Code places
limitations on these. A 45° turn is preferable to
a 90° turn. The appliance reference material
should be consulted at this time, as well as any
Local Authority having jurisdiction. In most
localities, building permits are required for any
new appliances, or modifications to existing
venting systems.
3. Figures 1, 2, & 3 show examples of some
typical residential installations.

The smaller vent pipe connector and lower BTU should be at top. If the HVAC tech says it’s OK, have him put it in writing.

If someone can correct me I’m listening.


Wow, That diagram is really close to what I have here, The two vents meet and go vertical to the exterior. So its never as clear as we think, Thanks Bob !!

From what is pictured.
I disagree…
we can only defer to Contractors (allegedly more knowledgeable)
If an HVAC Contractor says OK…
it is…

You know the answer…
There are limits to the recommendations given in a report
Due Diligence prevails based upon recommendations…


It won’t be accepted here in Canada. CSA B149 approval or the highway. 8.16.1 When two or more vent connectors enter either a common vent or common chimney, they shall not enter at the same horizontal plane. The smallest vent connector should be highest or in other words be the lowest BTU rating.

Bob, do you know if that increaser has any mechanism in it or is it just a plain reducing coupling?

It is just a coupling.Google “b vent increasers” and find tons of them.
Not an expert on the subject but if you think about it the fact you have two smaller diameter vents mixing in to a larger diameter vent decreases likely hood of back draft.After all that would reduce the pressure.

I may be totally wrong but hope Charlie chimes in.http://publicecodes.citation.com/icod/irc/2006f2/icod_irc_2006f2_appb_par001.htm

Seems the common vent may act as a manifold and stop increased air pressure from causing backdraft in this case.
Below is excerpted from a Hart and Cooley PDF.
When manifolding a fan-assisted appliance with a drafthood-
equipped appliance, the fan-assisted appliance
should be positioned closer to the common vertical

Last note or thought here.
How could a common vent be used in a multistory building if smaller appliance must always be above the larger ?

Not right … I would call it, after that may the best salesman win


You cannot determine based on a picture. What was the btu output of the water heater and the btu output of the furnace. I have seen older water heaters with much higher btu output than a a newer, more efficient furnace. Size of the vents can be, but are not necessarily the only clue. Pictures don’t give us the needed information.

That change would require a review and redesign of the existing flue (Level 2 at minimum).
Should not be a problem for the Homeowner and/or HVAC Contractor of the Installation to produce when questioned.

My point again is that you cannot make that determination by a photo alone. I would venture to say that the vast majority of installs in this area use 6" b-vent for a combination furnace / water heater. Lowering the btu rating of the furnace by even half would not require a change in the size of venting.

Chris and others are dead on. All the discussion and research is interesting and beneficial but it won’t occur out in the field during an inspection. There are always going to be exceptions but the general rules rule:

Larger vent below smaller
Larger Btu below smaller
Forced draft below open draft hood

Outside of that, unless you have the time to research it and be certain your specific setup passes, the safest bet is to call it out.

They are the experts for sure.

Joe, I see a stat on the wall.

Is that unit for the garage or the whole house?

An inspector friend of mine sent me that. He wrote it up, so I’m assuming is zoned with the house. Regardless, it voids the fire barrier also.