What causes a natural gas boiler to backdraft?

Larry Savino

Please define back drafting?

Please describe your home also?

What is the tallest part of your home? Your chimney to the heater that you say is back drafting? Or is it some other chimney or the attic

What type of chimney is it,is it a B vent or is it an old masonry unlined and over sized and on an outside wall.

What has changed in the home over the last year or so that could have caused this.Sealed up the house? New solid fuel burning appliance added to another area of the house,removed the doors to a masonry or ZC fireplace,added high hats to the top floor,added new exhaust fan to kitchen, added attic ventilator to attic area with a moisture stat on it,Is the attic scuttle closed off, Do you have a whole house fan louvers that are not closed 100%

As stated before this sounds like a problem with pressures and it sounds like the boiler room is in a negative pressure,unless the chimney is clogged,

This is a dangerous situation and should not be allowed to go another 5 months for another cleaning.

If you cannot get this figured out with your Heating contractor I would suggest you hire a heating expert from this organization BPI
http://www.bpi.org/bpi/www/pages/about_bpi/about_bpi.asp

Just about anybody certified by this organization has the knowledge and the needed tool to diagnose this problem quickly,

With more and more renovations, installing high CFM exchaust fans, Older homes being sealed up to keep the cold out, Adding wood or pellet products to a home for heat we all are going to have to get a better understanding of pressure in a home and how to identify when there is a problem with them, Because the heating unit is most effected by it

Dwayne

Boiler backdrafting

I have a natural gas boiler that is backdrafting.I had the boiler cleaned 5 months ago by a company and it worked good for those 5 months.It is now backdrafting again.

-It has the proper combustion air I was told.

ed wallace

backdrafting

is the chimney causing the problem?

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Brad White_38

Backdrafting

To your particular case: You said you had it cleaned 5 months ago and it worked for 5 months. Are we to assume that it had been backdrafting prior to the last cleaning? In that case I would look for local causes such as debris within the chimney coming to roost near the breeching connection to the chimney or atop the draft hood/regulator.


To understand it in it's simplest form, backdrafting is essentially a condition of lower pressure in your basement/boiler room and a higher pressure at the top of the chimney. (Pressure always moves from high to low, we all know).

In short, the pressure may be artificially lower within the boiler room/enclosure or artificially high outside the chimney exit, or a combination of both.

Armed with that, the first things I look at are sources of negative pressure within the boiler room or affecting the boiler enclosure. Laundry clothes dryers, exhaust fans for example. You would be surprised how often boiler rooms are exhausted for cooling/ventilation purposes, to "draw in" combustion air. I have seen boiler rooms which had subsequent woodworking shops adjacent. The shop dust collector affected draft and that was solved by super-filtering and returning the air to the shop to make it "pressure neutral". The source of negative pressure could be more remote than that of course.

How constant is the condition? Has anything happened so recently that negated 5 months of successful operation?

The other thing I would look at is, as Ed suggested, the chimney. That should always be your dominant source of negative pressure. If the height was initially marginal (draft sufficient but not a lot of excess under ideal conditions), an animal or bird nest, wind damage or other reduction could be the culprit. A tipping point of sorts.

Another factor may be what is known as temperature inversion, common in mountain areas but can happen anywhere as in a microclimate. If the backdraft condition is sporadic rather than constant, I would look at this. Not a lot can be done about that but to put in a draft inducer for "positive negativity".

Locally high pressure, the construction of a higher building next to yours, especially "downwind". Think of a big scoop forcing down prevailing winds. Another draft inducer solution.

Absent an identifiable physical cause such as debris blockage which must be addressed as it's own issue, I would strongly recommend a stack-top draft inducer such as Tjernlund or Exhausto, interlocked to assure pressure-based draft at all times of operation.

Wayco Wayne_2

I had a

problem with a furnace backdrafting and found the attic fan was stuck on and someone closed the gable vents with plywood. D'oh. WW

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S Ebels

Back drafting/spilling is pretty straightforward

It happens when the atmospheric pressure is lower in your house than it is outdoors. It can also occur when your chimney flue or breeching pipe is blocked. One other factor, often overlooked is a chimney that is oversized for the btu's going into it.

Pull your breeching pipe and hold a lighter up in front of the hole in the chimney. See if the flame is drawn toward the hole. Now turn on all the exhaust equipment in the house, bath fans, dryer, range hood, anything that sucks air from your house and blows it outside. Let it all run for 5 minutes and then do the lighter thing again. See if there is any change in the flame.

You can also look up your chimney to check for blockage in the flue passageway with a mirror. There was a thread here last week about all the stuff and critters people have found in chimneys.

That'll give you a start.

frankfick

backdraft

i would agree it is caused by backdraft. i have an indoor garden with a very strong exhaust system that runs 24/7. i fired up my furnace for the first time this fall and had a huge backdraft. i read your post and turned off my exhaust. no more backdraft. going to have to have some kind of intake air for my setup.