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 backdraft on hot water heater
Author: Anonymous User

I just replaced the gas hot water heater. It is 6 inches shorter than the old one so I added 6" of vertical vent pipe (goes to an elbow and then runs to the chimney). It backdrafts (the old one might have too, but I never checked). The furnace right next to it has drafts fine. When I disconnect the 6" vertical vent pipe and the elbow, and check the draft at the mouth of the "horizontal run" (which is about 1/4" pitch) where the elbow attaches the drafting is A-OK telling me that the "horizontal run" is fine? The elbow and 6" piece are not clogged, I can see
all the way through-poke my hand in there, etc.. Any ideas out there? Me and the plumbing inspector are stumped!
Thinking hard in the Twin Cities

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 Re: backdraft on hot water heater
Author: Edward429451 (CO)

How much initial rise do you have (from the draft diverter) to the first elbow? (I like to see at least a foot)

How many BTU's is the waterheater?
What size is the vent connector?
How long is the run? (meaning linear feet from the wh to the vent stack where it goes up.)

Even some properly vented wh's will backdraft (a little) upon startup until the vent is warmed throughout. But it should stop fairly quickly and begin positively drafting.

Can tell more with additional info.

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 Re: backdraft on hot water heater
Author: Edward429451 (CO)

In a lot of cases, wh's are coming through with bigger burners than before and the new code out here (CO) is that if it's 40K BTU's or more it has to have a 4" vent. Any wh will draft better with a 4". Upsizing in a masonry chimney is easy with a hammer & morter patch.

Forgot to ask how many elbows also. And what's its source of combustion air? Closet? open basement? Small mech room turned towards the wall? Solid doors / louvered?

Many things to consider.

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 Re: backdraft on hot water heater
Author: Dunbar (KY)

Flue draw cannot be gauged by visual inspection alone.


A draft gauge is used to determine F.P.M.'s (Feet Per Minute).


Minimum allowable draw off of any water heating device is 300 fpm's. A good operating water heater pulls anywhere from 500 to 850 fpm's.



When initially checking the draft on a water heater, you must wait 5 minutes before checking with the water heater running.


Sometimes the water heater will not draw until the flue pipe or brick chase warms up to allow proper draw.



If the water heater is in a closed and confined area with no fresh air intake for the water heater and furnace, this could be another reason for the problem you are in.

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 More info: backdraft on hot water heater
Author: Anonymous User

Here is more info. 36K btu unit in an open basement next to the furnace. 4" vent pipe, 6" initial vertical rise (after the vent hood) to a 90 degree elbow, then 3 feet horizontal to an "S" bend (combo 90 & 60 bends) where it feeds into furnace vent (5 or 6";) which proceeds 1 foot to chimney. Furnace vents fine in all scenario's (running or not, or when hw running or not).
The tech support said needs to run 12" vertical or more and then recommends a softer bend than 90 (45 to straight to 45).
Any other thoughts?

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 Re: More info: backdraft on hot water heater
Author: Dunbar (KY)

You need to call a licensed plumber. You seem not to grasp the seriousness of your situation, and a expert is needed to correct your problem.


There are many possibilities involved, and a trained eye is needed.

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 Re: backdraft on hot water heater
Author: Anonymous User

It's time to call a Licensed Plumber mpls-diyer. I had a call a few months ago, 4th plumber called for the problem, 3 year old home (very tight home), owner had a commercial grade vent install for his viking stove and wondered why the pilot kept blowing out on his water heater.It was the first thing I saw walking into the house, problem solved even before I set foot in the mechanical room.

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 Re: backdraft on hot water heater
Author: Edward429451 (CO)

<Any other thoughts?>

What other thoughts? It needs revented. Get rid of the "S". Have a pro come in and upgrade the vent to what the manufacturer told you.

Carbon monoxide is deadly. Don't put your family's safety in jeapordy tryin to save a few bucks.

Maybe this is a good time to have the furnace and water heater completely revented from draft hoods to chimney top with a reliner kit. It's not that big of a job. Let us know what you wind up doing.



Post Edited

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 Re: Thanks again!
Author: Anonymous User

Thanks everyone. 1914 home, quite drafty. FYI it vents fine with out the draft hood (such that vertical rise is stabilized 1-2" directly over the exhaust port (I have been checking it hourly). Seems like needs reventing and am much happier having a plumber do this than ending up dead. Just didn't know if it was a something simple that was being overlooked. Thanks again.

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