Forced Draft burner blow out

Simple question: Why doesn’t a forced draft vent fan blow out the burners in a furnace? Seems like it would…


Cause there is already a draft over the burners going up the heat exchanger. The forced draft fan just sucks it up out of the heat exchanger and spits it out.

It is actually a Forced Air Induced Draft furnace.

The Blower is on the Furnace Outlet drawing exhaust gas from the Burner thru the exchanger.

You’ve both described “Induced Draft Vent Fans” which are different than “Forced Draft Vent Fans.” What I’m asking regards the latter, where the blower is at the burners… the start of the heat exchanger… and forces air through the system.


Which model Furnace are you referencing?

I have only worked with Forced Draft Furnaces / Boilers within a Heavy Industrial / Chemical Environment. I have not encountered a Residential Forced Draft Furnace.

Comfortmaker had one model forced draft. The blower was attached to the burner box forcing air into the burner compartment. Thankfully discontinued.

And now you know why it was discontinued.

I recently inspected a 2002 steam boiler with a very long horizontal run to chimney (more than ten feet). In the vent pipe about two feet from the boiler there was a daft-inducing fan, presumably because the long vent gave marginal draft. The fan was wired into the boiler safety controls, and since the fan was not turning , the boiler would not fire. So I was unable to observe the boiler operation, but I did notice a thin film of rust on the cast iron boiler sections, and a couple of small stains or drip marks on the pan below the burners. I’m wondering if the long vent length is causing the flue vapors to condense when the boiler is only working intermittently, and if the resulting condensation is running down the vent pipe and rusting the boiler. I’ve heard that this is a problem with high-efficiency boilers with induced draft, but has anyone seen it in a regular efficiency unit with induced draft in the vent pipe?

Mike O’Malley

It is always better to suck than to blow.

However, sucking causes the blower assy to come in contact with the corrosive combustion gas.

Why it doesnt blow out?
It is a confined space. All air in, goes through the heat exchanger along with the gas. The gas pressure is higher than the blower pressure so it won’t blow out.

I wonder if that would "supply " combustion air in a manner which most cat 1 Natural draft units do not have.

I am (guessing here) David ( you know this topic better than I) that this would not have that much of an effect on the burners since " pressurized jets" of N.G.) are “pulling air” (venturi??) from the open baffles (at/near) the beginning of metering value to the heat exchanger end of the burners.

Am I way off on this?

I am thinking like oxygen injection with Oxy/Acetylene torch… Burning :-k
Different concept but similar approach…maybe;)

I’m think I understand what your are getting at.
If so, you are correct.