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Residential · Gas & VentingFresh Air & Combustion Air Intake Installation
Fresh air and combustion air intake installation gives natural-draft gas appliances an outdoor air supply sized to their combined input, as CSA B149.1 requires when they sit in a tight house or an enclosed mechanical room. The usual build is an insulated duct from an outside wall hood that drops to near the floor and ends in a trap, so it feeds the burners without pouring cold air across the basement. If every appliance is sealed-combustion and the house simply feels stuffy, the need is ventilation, and that is an HRV, not this duct.
A draft-hood water heater or an older furnace or boiler burns air taken from the room around it, and needs more again to carry its flue gas up the vent. In a leaky house that air arrives through gaps in the building. Tighten the house with new windows and air sealing, or box the appliances into a drywalled room with a solid door, and they begin to starve: flames lift and yellow, the flue stalls, and combustion products spill at the draft hood.
The gas code sizes the fix. CSA B149.1 sets the free area of the outdoor air opening from the total input of the appliances in the space, in BTU per hour, and from the kind of appliances they are, so the duct for a water heater alone is smaller than one for a water heater and a boiler together. The duct ends close to the burner level of the largest appliance. This is gas-fitter work within our residential HVAC service, done by a TSSA-licensed technician who reads the rating plates and the code tables, not a hole sized by eye.
Comfort is why these ducts get blocked, and why the details matter. An open pipe through the rim joist is a steady cold draught in January, so homeowners stuff it with a rag and recreate the original hazard. We build it like any other ductwork installation: insulated and wrapped against condensation, run down the wall to a short distance above the floor, and finished with an upturned trap or a drop into an open bucket. Cold air sits in the trap until the burners draw it, which limits both the draught and the frost at the opening.
"Fresh air intake" gets used for three different things. Combustion air feeds burners, while a ventilation duct into the furnace return, common in 1990s and 2000s houses, brings in outdoor air for people and falls under the building code. An HRV or ERV does that second job with heat recovery and balanced flow. A house with a sealed two-pipe furnace and a power-vented water heater needs no combustion air duct at all; if its windows sweat and the air is stale, look at HRV and ERV installation.
Have the air supply checked when
- The furnace room is being enclosed during a basement renovation with a natural-draft appliance inside
- The water heater's draft hood shows scorching, or warm, damp air spills there while it fires
- An existing intake duct has been stuffed with insulation or taped over because of the cold
- New windows, doors or spray foam have gone in and the gas appliances are the original natural-draft type
- A gas technician or inspector has written up inadequate combustion air, or tagged the appliance
How it works
We total the input of every appliance in the space, note how each one vents, and judge how tight the house and the room are. A spillage check at the draft hood with the exhaust fans running, the core of our carbon monoxide testing, shows whether there is a problem today.
The opening is sized from the code tables, and the route is picked for the shortest insulated run from an outside wall. The hood goes above the snow line and clear of the dryer vent, the gas meter's regulator and other exhaust terminations.
The wall is cored, the hood sealed and screened, and the duct insulated, dropped and trapped near the appliances. Spillage is checked again with everything running before the tools are packed. Arrange a visit before the mechanical room is drywalled if a renovation is under way.
Common questions
Do I need a combustion air intake if my furnace is high-efficiency?
Not for the furnace, if it is piped as a two-pipe sealed system drawing its own air from outdoors. Look at what else is in the room. A tank water heater with a draft hood on top still takes its air from the basement and still counts. A furnace installed with a single exhaust pipe also burns room air and has its own air-supply requirements.
Can I block the fresh air duct in winter?
No. If it is a combustion air duct, blocking it removes the air the burners and flue depend on, and the result can be carbon monoxide in the house. If the draught is the complaint, the fix is insulation and a proper trap at the end of the duct. If you are not sure what the duct serves, have it identified before touching it, and keep working CO alarms near the bedrooms as Ontario law requires.
Is a combustion air duct the same as an HRV?
No. A combustion air duct is a passive pipe that lets the burners draw outdoor air; it moves air only when appliances fire and does nothing for the rest of the house. An HRV is a balanced, fan-driven ventilator for the people living there. Installing one does not remove the need for combustion air, and a badly balanced HRV can make a natural-draft appliance worse.
What does a combustion air intake cost?
It depends on the duct diameter the code calls for, the length of the run, and the wall it passes through: coring brick or stone takes longer than cutting a rim joist behind siding. There is no published band for it, so the price comes as a written quote once we have seen the room. Most installations are completed in half a day.
Would replacing the water heater solve it instead?
Sometimes that is the better spend. Swapping a natural-draft tank for a power-vented or direct-vent model removes the last appliance that needs room air, and the duct is no longer required. It is worth pricing when the tank is already past ten years. The same logic applies when a large kitchen hood is involved: see range hood make-up air.