Thermostats get replaced, filters get changed, condensers get washed down. The flue gets looked at when something has already gone wrong. This page is about why we check it on every gas appliance we touch — in a house and in a mechanical room.
A gas furnace has two jobs beyond making heat. It has to get its combustion products completely out of the building, and it has to get enough fresh air in for the burner to work properly. Both of those depend on parts that are out of sight, do not have a warning light, and are frequently disturbed by work that has nothing to do with heating.
That last point is the one that catches buildings out. A re-roof, a new water heater, an attic conversion, a tenant improvement that moves a partition, a bathroom extractor that someone fitted at the weekend — none of those is HVAC work, and every one of them can change how a flue behaves.
Not a theoretical list. These are the things that turn up in older housing stock and in multi-tenant commercial buildings around Pomona, where equipment has been changed piecemeal over thirty or forty years.
The single-wall pipe between the appliance and the vent runs hot and wet at every start-up. Where it passes through a damp attic or an unconditioned crawl space it corrodes from the inside, and the first visible sign is usually rust staining on the outside — by which point there is a hole.
Double-wall vent sections are held together by twist-lock and by the fasteners at each joint. Anyone working in an attic — an electrician pulling cable, an insulation crew, someone storing boxes — can knock a section loose. It stays sitting roughly where it was, and exhaust goes into the attic instead of the sky.
Older houses often vented a furnace and a water heater into one flue, sized for both. Then the furnace gets replaced with a modern one that vents differently or produces far less exhaust volume, and the water heater is left venting alone into a flue now far too large for it. It will not draw properly, and that is the point where products of combustion start coming back into the room.
A furnace or boiler in a closet or a small mechanical room needs a deliberate air supply. We find those openings drywalled over during a remodel, blocked by shelving, filled with stored stock in a retail back room, or louvred into a space that was itself later sealed. The appliance then has to take its air from wherever it can get it.
Kitchen exhaust, a bathroom fan, a dryer, a commercial kitchen hood, a workshop extractor — all of them pull air out of a building. If nothing is bringing an equivalent amount back in, the easiest replacement path can be down the flue. A natural-draft appliance can end up venting backwards while running perfectly well otherwise.
Vents that end too close to a window, an air intake, a walkway or a neighbouring tenancy's fresh-air opening. Usually created by later changes — a new intake added to the roof, an extension built out under an existing termination, a patio roof enclosed.
A visual inspection catches the obvious: a disconnected joint, visible corrosion, a blocked opening. It does not catch a flue that draws correctly on a mild morning and fails when the kitchen hood is running, or an appliance that is combusting badly while looking perfectly normal.
So the check has an instrument side. On gas appliances we work on, that means reading the combustion, confirming the vent actually establishes draft under the conditions the building really operates in, and testing with the exhaust appliances running rather than with the building sitting quiet.
What we will not do is publish the readings we consider acceptable, or a table of numbers for you to check yourself. Those limits belong to the appliance manufacturer and to the code in force where you are, they are not the same for every appliance, and a number lifted off a website is exactly how someone talks themselves into leaving a real fault alone.
Most of the housing stock in this part of the valley puts the furnace in a hallway closet, in the attic, or in the garage, and runs the vent up through the attic to the roof. All three arrangements are fine when they are intact. All three are easy to disturb.
When we service or replace a residential system we follow the vent from the appliance to its termination — the connector, its rise and support, every joint, the clearances to combustible material, the cap — and we check that the room or closet still has the air supply it was built with.
If your house has a furnace and a water heater sharing one flue and only the furnace has been replaced, tell us when you call. It is common, it is worth checking, and it is not something you can see from inside the house.
Stop reading web pages. Get everyone out, leave the door open, and call the gas utility or the fire department from outside. That is the correct order, and it is not us — an emergency response is faster than a contractor. Call us afterwards and we will find out why it happened.
In a commercial building the same physics has more ways to go wrong, because more people change the building. A suite gets re-partitioned and the louvre that fed the mechanical room now opens into a storeroom. A restaurant upgrades its hood and nobody adds make-up air. A landlord encloses a service corridor. Each change is reasonable on its own; the combination is what causes the problem.
Shared mechanical rooms are the most common source of confusion, because the appliances in them often belong to different tenants and no single occupier feels responsible for the room itself. We document what is in there, what condition the venting is in, and who it serves — which is frequently the first time anyone has written it down.
For through-wall and sidewall venting, the questions are clearances, terminations relative to windows, doors, intakes and walkways, and whether anything has been built since installation that changes those distances. On a building that has had two or three tenant fit-outs, something usually has.
It is part of the service or replacement price, not an extra. If we find a fault in the vent system that needs correcting, that is quoted separately and you decide — but the checking itself is included, because an appliance we have worked on is an appliance we are responsible for.
Yes, and new systems are not exempt. Two of the faults on this page — a shared flue left oversized, and a vent disturbed by later work in an attic — are more likely on a recently-replaced appliance than on one that has sat untouched for twenty years.
No. We can tell you whether what you are describing is worth looking at quickly, and we will be honest if it sounds urgent. But a vent system is assessed on site, with instruments, with the building running the way it normally runs.
House or building. We will tell you what the inspection involves and what it costs before anyone comes out.