I have a natural gas furnace and most times, when it lights, it lets off an
increasingly loud 'whoomp'...
I checked the pilot light and it is up pretty high...
Knowing some basic physics, it is obvious that the gas is filling the
chamber and then when the fuel-air mixture hits the right spot, it lights...
I wouldn't really care, but it didn't used to do this...
How old is this furnace. No matter.
My guess is that the burners are dirty. Call a repairman to diagnose the
problem and probably remove and clean or replace the burners. If you
leave it like this you could crack the heat exchanger, or worse.
Good thing you know physics. Do you know the one where you hear a loud
noise and look down finding yourself 20 ft in the air?
Have you thought of having a "tune-em-up, clean-em-up" done to your
As some of guys told you, you need proper cleaning in and around the
furnace but also check that your location of furnace is not on vacuum
in other words if you have clothes dryer in same are make sure your dryer
is not creating vacuum in the area or you are having some type of blockage
in smoke stack, the items above can spell disaster.
If you have a typical 4-burner furnace, the pilot light will be
located between the middle 2 burners. The pilot should have 2 jets
pointed in opposite directions directly towards the adjacent burners
(burner #2 and #3).
When the main gas valve is turned on, gas flows into all burners, but
ignition starts from the pilot and ignites burners 2 and 3, then 1 and
4. What could be happening is that one side (the left or right) is
not being ignited by the pilot, or the ignition is not jumping from 3
to 4 or from 2 to 1 until enough gas has filled the burner area to
ignite from an un-intended place - and by then there is enough of a
gas buildup to create a small explosion.
You'd have to watch the entire ignition sequence (from a safe
distance) in order to see what's going on. As has been mentioned, the
burners may need to be cleaned (usually - of rust that has fallen from
the walls of the heat exchanger onto the burners). The burners should
just lift out. Don't do that until you turn off the gas supply to the
Another explanation is that there is a hole in one of the chambers and
the return air is flooding through the hole into that chamber and
disrupting the proper distribution of gas or leaning it out. This
would only happen if the fan is already running when the thermostat
calls for heat.
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.