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Where does your furnace get the air it burns?
Every fuel-burning appliance needs air to burn and a way out for its exhaust. Both are designed in when it is installed, and both can be undone afterwards — by insulation, by a new range hood, by a basement remodel, or by a door somebody started closing.
If a carbon monoxide alarm is sounding, or anybody in the house feels unwell, leave the building and call 911 from outside. Then call your gas utility’s emergency line. Do not open windows first, do not look for the cause, and do not ring a contractor. They arrive faster than we do, they do not charge you for a false alarm, and making the building safe is their job. Ring us afterwards — working out why it happened is ours.
Natural draft, sometimes called atmospheric
The oldest and simplest arrangement, and still very common in Cheektowaga basements. There is no fan moving the exhaust: it rises up the flue because it is hot, and it does that only as long as nothing else in the house is pulling harder. The air it burns comes straight out of the room it stands in.
This is the type most affected by everything else on this page, because it has no way to force its exhaust out. If the room goes into depression, the flue can reverse, and the exhaust comes back into the building instead of leaving it.
Induced draft
A small fan on the appliance pushes the exhaust out through the flue, so the draft does not depend on heat alone. That makes it considerably more tolerant than a natural-draft appliance — the fan will win most arguments a bathroom extractor can start.
But the air it burns still comes out of the room. The fan solves the exhaust half of the problem and not the supply half, and an induced-draft furnace in a sealed cupboard can still be short of air. It is also very often installed alongside an older water heater that is natural draft, sharing the same chimney — and that appliance has none of the protection.
Sealed combustion, also called direct vent
Two plastic pipes through the wall: one brings outside air directly to the burner, the other takes the exhaust out. The combustion is sealed off from the room entirely, which means the house cannot starve it and the house cannot pull its exhaust back in.
This is the arrangement that makes the rest of this page mostly irrelevant, and it is why it is worth knowing which one you have. It is not a reason to skip a service — the pipes themselves need checking, and the condensate that a high-efficiency appliance produces is its own subject — but the depressurisation problem is largely designed out.
An open basement or garage
The most forgiving place for it to be, because the volume of air around it is large. In a house built before about 1975 that has not been heavily air-sealed, an open basement usually supplies what an appliance needs without anything being added.
What changes it: finishing the basement. A wall across the space and a door on the furnace end turns an open basement into a closet, and it is done for perfectly good reasons by people who are not thinking about the flue.
A closet with a louvred door
The louvres are there on purpose. The appliance was installed on the assumption that air comes through them, and the free area of those openings was — or should have been — calculated for the input of the appliance.
What to look at: whether the louvres are still open. They get papered over, blocked by shelving, taped up for noise, or replaced with a solid door when a hallway is redecorated. A louvred door that has been swapped for a plain one is one of the commonest changes we find, and nobody remembers making it.
A closet or small room with a solid door
A small sealed volume around a fuel-burning appliance. If the appliance takes its air from the room, then unless there are ducts or grilles bringing outside air in, the only air it has is what leaks past the door frame.
What to look for: a duct or a grille high up and another low down, either to outside or to a large open space. If there are two, the arrangement was designed. If there are none, and the appliance is not sealed combustion, something has been changed or something was never right.
A utility room with the laundry in it
The hardest place, for one reason: a clothes dryer throws a large amount of air out of the building every time it runs, and it runs for an hour at a time. In a small room with a fuel-burning appliance, that is a machine actively competing with the flue.
Lint is the second problem. It settles on burners, blocks intakes and coats the inside of a sealed-combustion air pipe, and it does it invisibly over years.
The room cannot starve it
A sealed-combustion appliance brings its own air in from outside, so where it sits makes very little difference to the combustion. A closed door, a tight house or a running extractor fan do not affect it the way they affect an older appliance.
Two things still worth checking. First, that the intake and exhaust terminations outside are clear — in this climate they can be blocked by drifted snow, and a blocked intake shuts the appliance down at the worst moment. Second, whether anything else in that space is not sealed combustion. A high-efficiency furnace beside an older natural-draft water heater is a very common pairing, and the water heater has none of the protection the furnace has.
The room cannot starve it — the dryer can still coat it
The combustion is sealed off from the room, so the dryer competing for air is not the problem it would be with an older appliance. The depressurisation question is largely designed out.
Lint is the live issue here. It settles on everything in a laundry room over years, including the outside intake termination and the appliance cabinet. It is worth having the terminations looked at on a service, and worth keeping the dryer’s own vent clear for its own reasons.
And the same check as anywhere else: is anything in that room not sealed combustion? An older water heater beside it is the usual answer, and that one is affected by the dryer.
It takes air from the room, and the room is large
This is the arrangement most of this housing stock was built with and in most houses it works. The volume of an open basement is normally enough.
What would change that: a basement that has been finished or partitioned since; substantial air-sealing or new windows, which most houses here have had; and a powerful range hood, which in a modern kitchen can move more air than everything else in the house combined.
The check is not complicated. With the appliance running and the house closed up, a technician looks at whether the flue is actually drawing — and then does it again with the dryer, the bathroom fan and the range hood all running, which is the condition that matters and the one nobody tests.
It takes air from the room, through those openings
The appliance was installed on the assumption that the louvres supply its air, so the whole arrangement rests on them still being open and still being large enough.
Worth checking yourself: that nothing is stacked against the door on either side, that the louvres have not been papered, taped or painted shut, and that the door itself is still the louvred one. A solid replacement door looks better in a hallway and it is a genuine change to how the appliance works.
Whether the free area is correct for the input of that appliance is a calculation, not a glance — it depends on the appliance’s rating and on whether the openings go to outside or to another room inside the house. That is worth asking for on a service visit.
A room-air appliance behind a closed door
This is the combination we most often find has been changed since installation. Either the space was always ducted for combustion air and you simply have not noticed the grilles, or somebody put a solid door on a closet that used to have a louvred one.
Look for two openings — one near the ceiling and one near the floor — leading either outside or into a much larger space. Two openings means it was designed. No openings, with a fuel-burning appliance that takes its air from the room, means it should be looked at.
On a natural-draft appliance this matters more, because there is no fan to force the exhaust out against a depressurised room. On an induced-draft one the fan handles the exhaust, but the supply of air to burn is still the room’s job.
None of this is a reason to panic and it is not a reason to open anything yourself. It is a reason to have it measured, and measuring it takes about twenty minutes.
A room-air appliance sharing a room with a dryer
A clothes dryer is an exhaust fan that runs for an hour at a time. In a confined utility room with an appliance that takes its combustion air from that room, the two are in direct competition, and the dryer usually wins.
This is a genuine mechanism, not a theoretical one. It is behind a pilot that will not stay in on laundry day, a burner that sounds different when the dryer is running, and rust or soot staining around a flue connection. Add a bathroom fan upstairs and a range hood and the effect compounds.
On a natural-draft appliance it is the highest-priority version of this whole page, because nothing forces the exhaust out. On an induced-draft one the fan protects the exhaust side, but the air to burn still has to come from somewhere.
Lint on top of it: it settles on burners and blocks intakes over years, invisibly. Both problems are checked on the same visit and both are ordinarily fixable — a combustion air duct, a grille, or in some cases simply moving what the room is used for.
Four things that change a house without anybody noticing
New windows and air sealing. Worth doing, and the single largest change to how a house behaves. The appliance in the basement was installed on the assumption that the house leaked.
A powerful range hood. A modern one can move more air than every other fan in the house put together. It is the most common recent addition that changes the answer.
Finishing the basement. A wall and a door turn an open basement into a closet, and the decision is made by somebody thinking about flooring.
Replacing a louvred door. Done for looks, in a hallway, years after anybody remembers why the louvres were there.
Every one is a reasonable thing to do to a house. The point is only that the appliance was set up before they happened.