Make-Up Air Unit Ontario 2026: When Your Home Needs an MAU, What It Costs, and How It Works With Your HVAC

What a Make-Up Air Unit Actually Does

A make-up air unit is a small mechanical appliance that brings outdoor air into the home through a dedicated duct, usually opening up near the kitchen so the air is introduced close to where it is being exhausted. The unit includes a fan sized to the volume of exhaust it is compensating for, a motorized damper that stays closed when the unit is off, a filter, and (in nearly every Ontario installation) a heater that tempers the incoming air before it reaches the living space.

The purpose is pressure balance. When a range hood pulls 600 cubic feet per minute of air out of a house, 600 cubic feet per minute has to come back in. In a leaky older home, that air finds its way in around windows and under exterior doors, and nobody notices. In a modern airtight home built to current SB-12 requirements, there is no easy path, so the house goes into noticeable negative pressure. Doors stick, exhaust fans stall, and in the worst case the combustion flues on a natural-draft water heater or fireplace reverse and pull flue gases back into the house.[3]

Why This Became a Residential Problem

Twenty years ago, residential make-up air was a commercial problem. Houses were leaky enough that the envelope did the job automatically, and kitchen hoods were modest. The shift happened on two fronts. First, Ontario's energy efficiency requirements under Supplementary Standard SB-12 have pushed new homes toward much tighter envelopes, with blower-door performance that would have been considered exceptional a generation ago.[1]Second, kitchen design trends brought professional-style range hoods into ordinary homes, with airflow ratings four to six times higher than the builder-grade hoods they replaced.

The result is that a house designed to leak almost nothing now has to accommodate a peak exhaust event an order of magnitude larger than the baseline ventilation rate. Residential MAUs exist to bridge that gap. Natural Resources Canada and CSA F326, the residential mechanical ventilation standard referenced throughout Ontario practice, both address the interaction between principal ventilation and large exhaust, and the practical result is that any hood meaningfully above principal ventilation capacity needs engineered make-up air.[2]

The 400 CFM Line

Ontario contractors and building inspectors generally treat 400 cubic feet per minute as the working threshold for when a kitchen range hood requires engineered make-up air. The number is a rule of thumb, not a hard code citation, but it lines up with the point at which a normal single-family home can no longer supply replacement air through envelope leakage and principal ventilation alone. Professional-style hoods rated 600, 900, or 1200 CFM are always well past the line, and inspectors expect an engineered make-up air solution on any such installation.

The envelope tightness of the specific home matters as much as the hood rating. A certified Net Zero or Net Zero Ready home under the Canadian Home Builders' Association program is tighter than a code-minimum SB-12 home, and the grey zone at 300 to 400 CFM collapses: essentially any meaningful exhaust needs engineered make-up air.[7]

Powered versus Passive Make-Up Air

The simplest form of make-up air is passive: a motorized damper opens when the hood turns on, and outdoor air is pulled in through a dedicated duct by the hood's own exhaust. Passive designs work at low CFM and in homes that are not extremely tight, but they have practical problems. The air is untempered and dumps at outdoor temperature. The damper has to open reliably and seal when closed or it becomes an uncontrolled leak. And the pressure balance depends on the hood actually pulling enough suction to draw air through the intake duct, which is not always the case on longer or more complex duct runs.[4]

Powered make-up air units include their own fan matched to the expected exhaust rate, their own motorized damper, a filter, and tempering. The fan runs on demand (usually via a hood interlock or a pressure sensor) and delivers a known volume of conditioned air regardless of hood suction. For anything above roughly 400 CFM, powered units are the practical standard in Ontario, and the code intent is met much more reliably.

Tempering: Electric versus Gas

Residential MAUs in Ontario are almost always tempered. The two common options are electric resistance and gas-fired (indirect or sealed-combustion) heating. The choice comes down to capacity, operating cost, and available gas service.

Electric tempering is simple, quiet, compact, and the default choice on small to mid-size residential units. Installation is cheaper because no gas rough-in or venting is needed, and the unit is easier to place. The tradeoff is operating cost: raising 500 CFM of outdoor air from minus 15 Celsius to 16 Celsius consumes several kilowatts, and on an Ontario time-of-use rate the per-hour cost is noticeable. For a home where the hood runs thirty minutes a day on average, the operating cost is easily absorbed; for a home where the hood runs hours at a time, gas-tempered units often pencil out better over time.[5]

Gas-fired tempering is significantly cheaper to operate in Ontario natural-gas territories and scales better to larger airflows. The installation is more involved because a gas supply and a combustion venting arrangement are required, and the installer must coordinate with the home's other gas-fired appliances. TSSA-certified installation and gas safety work is required, and the commissioning includes confirming that the MAU itself does not create backdraft risk on other appliances.[3]Tempering setpoints on residential units typically land between 15 and 18 Celsius, which is warm enough to avoid a cold draft across the kitchen and cool enough to let the kitchen's own heat gain finish the conditioning.

Interaction With the HRV or ERV

A common source of confusion is the relationship between the make-up air unit and the home's HRV or ERV. These are different jobs that serve different purposes, and in a modern Ontario home both are typically required.

The HRV or ERV provides continuous balanced ventilation for the whole home. It exchanges stale indoor air for fresh outdoor air at a relatively low rate (often 60 to 120 CFM in a single-family home) and recovers a large fraction of the heat or, in the case of an ERV, a fraction of the latent moisture as well. It runs continuously or on a timer, and it is the workhorse of indoor air quality in a tight home.[2]

A make-up air unit, by contrast, runs only when a large exhaust event is active. It moves much more air at a time (typically matched to the range hood, so 400 to 1000 CFM), does not recover heat (the air is passing through once and is tempered directly), and is not a whole-home ventilation solution. Using an HRV to try to keep up with a 900 CFM hood is not a viable substitute; the HRV is not sized for it and would be destroyed trying. Likewise, running an MAU continuously as a whole-home ventilation solution wastes energy because there is no heat recovery. Good design uses each for the job it is built for.[4]

Combustion Safety and the Natural-Draft Problem

The single biggest safety reason the Ontario Building Code and TSSA care about make-up air is the interaction with natural-draft combustion appliances: standard-vent water heaters, mid-efficiency furnaces, and wood-burning fireplaces or stoves. These appliances rely on the buoyancy of hot flue gases to move exhaust up the chimney. When the house goes into strong negative pressure, the chimney can reverse, and combustion byproducts (carbon monoxide in particular) flow back into the home instead of up and out.[3]

Every Ontario home with a natural-draft appliance and a large exhaust fan needs either sealed-combustion replacement equipment, power-vented equipment, or engineered make-up air. On a kitchen renovation where the hood is being upgraded, this is the single most important check the mechanical contractor and inspector make. If the home still has a natural-draft water heater or an older mid-efficiency furnace, the renovation may need to include equipment upgrades alongside the make-up air work. Skipping this step has caused carbon monoxide incidents; it is not a theoretical concern.

Commissioning and Balancing

A make-up air unit that is installed but not commissioned is only half a system. Commissioning is the technical step of measuring actual airflow, confirming that the MAU and the exhaust it is matched to are producing the designed volumes, balancing the two so the house stays near neutral pressure, and verifying that the interlock between the hood and the MAU works reliably.[2]

A proper commissioning visit includes: airflow measurements at the hood and the MAU under load; pressure measurements in the kitchen, the adjacent living space, and relative to outside; a combustion safety check on every natural-draft appliance in the home with the hood running at maximum; a damper and interlock functional test; and a written report. In Ontario, an HRAI-trained technician or a TSSA-certified gas contractor (for gas-tempered units) is the right person for the work. Commissioning labour typically runs a few hundred dollars on top of equipment and installation and should never be skipped to save cost.[4]

Typical Installed Costs in 2026

Installed pricing varies with home layout, tempering choice, retrofit versus new construction, and whether the MAU is being paired with an ordinary mid-tier hood or a professional-style setup. The ranges below are typical Ontario installer pricing in 2026.

Homeowners planning a kitchen renovation should ask for the make-up air scope to be broken out as a line item on the quote, with the equipment make and model, tempering type, and commissioning work separately priced.[6]

Why Inspectors Flag Kitchen Renovations

Ontario municipal building inspectors see kitchen renovations as a common flashpoint for make-up air issues for three reasons. First, the renovation is visible: cabinets come out, a hood goes up, and the upgraded appliance is obvious from the street or on the permit drawings. Second, the trade-off is predictable: homeowners almost always upgrade to a larger and more powerful hood as part of a kitchen remodel. Third, the risk is concrete: a new 900 CFM hood dropped into a tight home with a natural-draft water heater is exactly the recipe for a carbon monoxide incident.

The right sequence on a kitchen renovation is to confirm the planned hood rating before the cabinets are built, consult the mechanical contractor and the municipal building department, and scope the make-up air work into the permit application. Homeowners who discover the requirement after the drywall is up face the worst-case retrofit pricing and sometimes have to rework finished cabinetry to accommodate the MAU intake duct.

Common Mistakes

• Treating the HRV as a substitute for make-up air on a 900 CFM hood.
• Installing an untempered passive damper and assuming it will handle a 600+ CFM hood in January.
• Skipping commissioning because the unit appears to be running.
• Leaving a natural-draft water heater in place after adding a large hood without combustion safety testing.
• Placing the MAU intake too close to an exhaust vent, driveway, or gas meter.
• Not interlocking the damper with the hood, so the damper stays closed or stays open at the wrong times.
• Oversizing the MAU and creating positive pressure, which drives humid indoor air into wall assemblies in winter.

Where This Fits in the Renovation Process

Make-up air is a mechanical scope item, not a cosmetic one, and it belongs in the quote-reading and contractor-selection conversation early. See our how to read an HVAC quote Ontario 2026 guide for what to check on any mechanical quote, our HVAC contractor insurance check Ontario 2026 guide for verifying the contractor handling the work, and our HVAC repair vs replace decision Ontario 2026 guide for the equipment-age math that often comes up alongside a kitchen renovation.

Frequently Asked Questions

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