Ok backdafting

Inoticed evidence of backdrafting on a recent inspection and called for an HVAC Technician review of this gas fired furnace.

His response was “we tested the furnace and found it operational with the exception that it was backdrafting on startup. After the flue warmed up, it was drafting OK. We always recommend carbon monoxide monitors in every home”

I’ve been asked to comment - anyone?


I don’t believe the manufacturer designed it to operate like that.


I went to a seminar on gas heating, if you are checking a unit that has been

off, you can give the flue up to 10 minutes before it is required to draft

properly. After that it there should be no back drafting.


"If one or more of your combustion appliances is backdrafting for longer than about 30 seconds at start-up, you should fix the problem. But poor air quality may be already causing health problems - sore throats, irritation to the nostrils and eyes and asthmatic symptoms - and may be making the indoor environment overly humid. If you ignore the problem, the performance of the burner will deteriorate, and the situation could well become deadly. Experts take two approaches to solving backdrafting problems. One method goes after the mechanism of backdrafting, the other eliminates sources. The potential for backdrafting can be controlled by balancing indoor and outdoor air pressure with a fan that pushes air into the house. "

Quote from: http://www.homesafe.com/coalert/bkdraft.htm

Here are some sobering CO/backdrafting stories:


Can back draft up to 10 minutes? :shock:

And if the combustion is burning really dirty that is scenario for a disaster.

That seminar must not have been taught by Rudy Leatherman or Bob Dwyer from Bachrach; Jim Davis from National Comfort Institute; Tim McElwain or Dr. Thomas Greiner. This goes against everything that they cover in their training.

Great links guys!!

Here are a couple of things that contribute to the problem.

What did you guys see in that picture?

One is A vent in the return air duct within 10 FEET of the FAU… A MAJOR no no period .

Before any flue can adequately warm up the effective “draw” is weak but 10 minutes… :shock:

All these play a part in the equation in a natural draft , gas burning appliance:

Length of flue? Slope of flue to chimney?
Size of flue, Greater then 7 times rule? Also calculated for multiple appliances.
Size of chimney itself? Too large, Brick, Unlined?
Does the flue share a connection with any other appliance (gas or otherwise)? Bad if mixed fuels!
If so how is it connected? Larger on the bottom of breach or “tee” fitting into flue before breach into chimney?
Does the appliance fit into the orphan category?

Combustion air / dilution air requirements. Is the room sized to small or sealed off ?
Is that a closet where the return air duct “pulling” “gas” down the chimney for combustion instead of moving products of combustion up and out of the house?

Take a look at :

Go to “Venting Done Right”

If you have about ten minutes , it’s a great video!

My gas company training / certification on Natural gas appliances tells me this is NOT a safe operating appliance period… :wink:


It came from Joe Zaabadick,

The Program Instructor is Mr. Joe Zaabadick, an experienced 40-year veteran of the gas industry. He spent 35 years as an employee of the Southern California Gas Company. He has held the position of Senior Instructor of Customer Services Training at the Company’s Pico Rivera Training Facility

“with the burners burning for 10 minutes, there should be no spillage after 10 minutes max, it should draw withing 10 minutes, it should be hot enought by then, if it doesnt draft in 10 minutes refer it out”

You can catch him Sept. 25-27 at the Inspection Expo in Las Vegas, he is one of the of the featured speakers.


Don’t like the return air duct where its located, very close to burners.
Also the house could be to air tight and the furnace is starved for air, creating downdraft situation.