A postwar bungalow that breathed through its cracks — until they were sealed
Côte Saint-Luc is among the areas on the island where the housing stock has been extensively modernized over the past fifteen years: windows replaced, doors weatherstripped, attics blown to R-50, basements finished. Each improvement reduces the heating bill. Together, they also eliminate the “accidental” ventilation that kept the air breathable. The homeowner called us in February with three classic symptoms: condensation on the lower third of the ground-floor windows, cooking odors that lingered until the next morning, and a finished basement that smelled musty despite a dehumidifier running nonstop.
The diagnosis did not require exotic instrumentation. A hygrometer indicated 58% relative humidity at an outdoor temperature of -12°C — well above the 30% to 40% target for winter. The house was not lacking heat; it was lacking air exchange.
Why an air exchanger instead of simply a bathroom fan
An exhaust fan expels heated air and replaces it with… whatever happens to enter. In a tight house, it creates negative pressure that draws air through the only available paths: the crawl space, the floor drain, the water heater flue. This is the mechanism behind combustion-gas backdrafting, and it is precisely what a balanced system prevents.
An air exchanger — a heat recovery ventilator, or HRV — circulates two equal airflows in opposite directions through a core: outgoing stale air transfers its heat to incoming fresh air without the two airstreams mixing. The air in the house is replaced without throwing heat out the window and without depressurizing the building.
Choosing the Fantech FLEX 100H for this house
Space was the main constraint. The finished basement in Côte Saint-Luc had only one available mechanical area, near the water heater, with 2 x 8 joists and limited ceiling clearance. A unit with side ports would have required more clearance width than was possible without demolishing drywall.
The FLEX 100H from Fantech has top-mounted ports, which makes all the difference in a tight space: the four connections run directly up into the joist cavity instead of extending sideways. Its actual specifications:
- Airflow range: 49 to 104 CFM at 0.4 in. water column
- Connections: 5 in., oval type (adapted to round with transition collars)
- Dimensions: 17 7/8 in. (H) x 21 1/2 in. (W) x 14 1/2 in. (D)
- Weight: 39 lb — light enough to suspend on chains between two joists
- Power supply: 120 V, 1.6 A max
- Defrost: by recirculation, without stopping the supply fan
- Sensible recovery efficiency: 60% at 0°C, 56% at -25°C (low speed)
The defrost mode deserves an explanation because it is the point homeowners understand least. On units with supply-fan shutdown, the fresh-air supply stops during the defrost cycle — during a cold snap at -25°C, this can amount to a significant portion of an hour without fresh air. The FLEX 100H’s recirculation defrost closes off the outdoor intake and circulates indoor air through the core to thaw it. Indoor ventilation never stops, and stagnant air that contributes to winter condensation does not build up.
For a larger house or a higher performance requirement, we generally specify the HERO series instead — the HERO 150H-EC achieves 84% sensible recovery and a fan efficiency of 3.0 CFM per watt thanks to its EC motors. It is a unit we regularly install in Ville Mont-Royal and Hampstead, in homes of 3,000 sq. ft. and larger. Here, with seven main rooms and a calculated requirement below 100 CFM, the FLEX 100H met the demand with a comfortable reserve, in a cabinet that actually fit the available space.
The installation process, step by step
Day 1 — Suspension and electrical connection. The unit is suspended by four chains fitted with isolation springs, never screwed directly to the joists. This detail visible in the photo is not cosmetic: a rigidly bolted HRV transmits its vibrations into the wood structure, and the bedroom above becomes a drum. The springs decouple the unit from the framing. A dedicated 120 V outlet powers the unit — no extension cord and no sharing a circuit with the sump pump.
Day 1 — Exterior openings. Two wall hoods, one for fresh-air intake and the other for stale-air exhaust, are spaced more than 6 feet apart to prevent air short-circuiting. We also position them away from the dryer vent, water-heater vent and garage entrance. In Côte Saint-Luc, the height above the ground must also account for snow accumulation: an intake hood buried in January chokes the airflow of a unit that appeared perfectly balanced in October.
Day 2 — Ductwork. The exterior sections are made with insulated flexible duct featuring a continuous vapor barrier and joints sealed with aluminum tape. An uninsulated fresh-air duct running through a heated basement condenses along its entire length and drips onto the finished ceiling — this is one of the repairs we most often perform on installations completed by others. Bend radii are kept generous: flexible duct crushed at 90 degrees can add as much pressure loss as 20 feet of straight duct.
Day 2 — Condensate drain. The core produces water. The drain is connected with a trap to the floor drain. Without a trap, the unit draws air through the drain line, making it impossible to maintain proper balance.
Day 2 — Balancing. The label on the unit door shows the relationship between airflow in CFM and the pressure measured at the supply and stale-air ports. Balancing is performed at Normal (high) speed with a differential pressure gauge, adjusting the dampers until the two airflows are equal within a 10% tolerance. An unbalanced HRV is a unit that continuously pressurizes or depressurizes the house — exactly the problem we were trying to correct.
The mistakes we most often see with air exchangers
- No balancing. The unit is connected, it blows air, and the customer is satisfied for the first month. Three winters later, the house is under negative pressure and the basement smells like a drain.
- Exhaust only in the bathrooms, supply only in the basement. Fresh air must reach the bedrooms and living room, where occupants spend their time.
- Hoods too close together. Stale air is immediately drawn back in; the unit recycles its own odor.
- Failure to maintain the filters. Filters should be rinsed or replaced every three months, and the core should be washed once a year. A dirty core loses efficiency and makes the motors work harder.
- Wrong control selected. A simple switch leaves the unit running continuously or not at all. A multi-mode wall control, or a VOC-sensing control such as the ECO-Touch IAQ, modulates airflow according to the actual air quality.
The measured result
Three weeks after commissioning, relative humidity stabilized between 34% and 38% during the day at an outdoor temperature of -15°C. Condensation on the windows had stopped, except on the oldest pane on the north façade. The homeowner reported the detail we tend to stop thinking about after twenty years in the trade: cooking odors now disappear within an hour instead of lingering until the next day.
Our approach to air exchanger installation in Greater Montreal
At AirGreen, we install residential ventilation systems in Montreal, Laval, Longueuil, the North Shore and the South Shore, in settings ranging from a Plateau triplex to a Chomedey bungalow, as well as homes in Brossard and Terrebonne. Each area brings its own constraints: municipal regulations governing façade openings, shared walls in Côte-des-Neiges duplexes, and 6 ft. 4 in. ceiling heights in Saint-Léonard basements.
We calculate the required airflow based on the number of rooms before proposing a model, never the other way around. We systematically balance the system during commissioning and record the values. And we explain to customers how to maintain their unit, because an HRV neglected for five years becomes nothing more than a metal box hanging from the ceiling.
For an assessment of HVAC installation or replacement of an air exchanger in Côte Saint-Luc or elsewhere in Greater Montreal, our team comes to your home, takes measurements and proposes a system sized for the actual house rather than for a catalogue.