Is it ok...

Is it ok to have a larger duct on a water heater? Here’s one that has the same size duct as the furnace. (You can’t tell from the picture but it enters the pipe “behind” the furnace duct.)

Mark, best I can tell the water heater is 4" and the furnace is 5"

Is this an accurate comment?

  1.   The water heater duct is entering the flue behind the location where the furnace flue enters. The larger (furnace) BTU appliance should be closer to the stack than the smaller one (water heater). In this case they are reversed. This could cause “back drafting” of the furnace exhaust gasses down the water heater flue and into the home. Major Concern. Recommend repair.

Mark, are you sure about the diameters of the two flues?

A side shot by any chance?

No it’s not accurate. WH does not have a duct, it has a vent. ;-):mrgreen:

As a side note; How many levels of concern do you use? You have “Major concern” here, curious as to what other verbiage you use there. TIA

See this document.

I think I had it bassackwards in my original post so I removed that comment.

I can’t tell from the photo but they need to be offset(not entering the flue at the same place).

Read this document and note the examples:

Louvers are awful high on the furnace door. Does this furnace has a inducer draft/power vent?

According to your pdf, what I have pictured is fine. I’m still not sure about the “vent ;-)” sizing. I’ll be back at the house tomorrow, but just at quick glance I thought they were the same size, and had always seen smaller vent pipes for WH’s.

It looks like a 5 inch to me. Being a little big should not matter.

The flue on the water heater must be the same size as the collar supplied with the appliance.

As James hinted at things may change if the furnace is a fan assisted draft.

My concern is that when the furnace runs the fan assisted exhaust will blow fumes out the water heater collar because of the lack of a wye fitting.

A better pic and more info would certainly help.

looks like 6" ducting

Remove the T fitting add a Y fitting connect the water heater to the strait end and the furnace to the Y and your good no chance of back draft

Ok, heres some more pictures from today…

The first picture is looking at the duct as it enters the chimney on the right.

The second is “caddy corner” to it so you get perspective.

The next three are the HW flue, the Furnace flue, and the combined double walled flue entering the chimney.

Here the picture on the left is a close up of the single flue entering the chimney. (The washed out white area of the picture is bad photograpy. That is the cinder block chimney.) Note there is no fitting here, which was “tagged” by our local gas company. If you look closely though, the flue connects to a flexible metal flue inside the masonry chimney. This is the exact same material I found in a closet that was being used as ductwork (of course with duck tape.)

Comments? :mrgreen::smiley:


Pics show the situation much better.

This is my opinion.

I think I would consider it OK as long as the water heater does not back draft when the furnace is running.

There probably should have been a wye instead of a tee for the water heater connection and 12" vertical above the water heater draft hood would be better as well.
Some manufactures specify the minimum vertical section above the draft hood before a change in direction.

This one specifies 6" of straight before the next fitting. See Figure 11


Is the flexible “flue” and duct work ok? I know the duck tape isn’t.

See this

Cool! I love this job. Learn something every day. Love this message board as well!


I’ve learned much here as well and like to pass it along.:D:D