A unit without a basement, a thirty-inch closet, and a winter spent wiping down the windows
The duplexes and triplexes from the 1960s and 1970s lining the streets of Montréal-Nord share a feature rarely mentioned in inspection reports: the upper unit has no access to mechanical space. No basement, no accessible crawl space, no exposed joists. All it has is a service closet at the end of the hallway, shared with the washer, dryer, and vacuum cleaner.
This is exactly the context of the project described here. The homeowner occupied the upper unit of her duplex, a few blocks from Henri-Bourassa Boulevard, and rented out the lower one. Since the windows had been replaced and insulation added in the attic, the air in the unit had become distinctly stuffy. In January, condensation ran down the windows to the wooden frames. The windowless bathroom took hours to dry. And the smell of cooking lingered in the hallway for the entire evening.
The building logic was clear: the house had been sealed tight without any mechanical exhaust system being added. At AirGreen, we responded by installing a FANTECH ATMO 150H air exchanger, a side-connection HRV mounted on the service-closet wall, with a network of insulated flexible ducts running beneath the ceiling.
Why choose a unit with side connections instead of one with top connections
This is the technical decision that made the project possible, and it deserves an explanation.
Most residential air exchangers sold in Quebec have all four connections on top of the cabinet. This is convenient when the unit is suspended by chains beneath basement joists: the ducts rise, run between the joists, and everything stays overhead. But this configuration requires eighteen to twenty-four inches of clear space above the unit solely for the elbows.
In this closet, the clear height below the finished ceiling was seven and a half feet. A unit with top connections would have had to extend so low that the closet door could no longer close over the ductwork.
The ATMO series by FANTECH is designed with all four connections on the sides of the cabinet:
- two collars on the left, two collars on the right, all 6-inch round;
- the unit is mounted flat against the wall, the ducts run horizontally and gently meet the ceiling;
- the required vertical clearance is reduced to just a few inches.
In the construction-site photo, the two ducts on the left, sealed with red aluminum tape, and the two loops of insulated flexible duct that go around the unit on the right before running under the ceiling are clearly visible. This was the only layout that fit in this closet.
The ATMO 150H's specifications
- Airflow: 60 to 150 cfm at 0.4 in. w.g. We aimed for approximately 85 cfm in continuous operation, which corresponds to the needs of a five-and-a-half-room dwelling occupied by two adults.
- Sensible recovery efficiency: 74% at 0 °C and 60% at -25 °C, for an adjusted efficiency of 80%. In other words, in January, about three-quarters of the heat from the exhaust air is recovered before it leaves the building.
- Power consumption: 54 W at low speed, 168 W at high speed, maximum current of 1.4 A. In continuous operation at low speed, this is a unit that costs less to run than an old-fashioned incandescent light bulb.
- Dimensions: 21 7/16 in. high × 23 7/8 in. wide × 11 11/16 in. deep. This depth of less than twelve inches matters just as much as the other two measurements in a hallway where people still need to walk through carrying a laundry basket.
- Weight: 42 lb, wall-mounted using appropriate anchors.
- Connections: four, 6-inch round, 120 V single-phase.
Fan-shutdown defrost, and why it was the right choice here
The ATMO 150H uses supply-fan shutdown defrost. When the outdoor temperature drops sufficiently, the unit briefly stops bringing in fresh air and allows the warmer indoor air to pass through the core to defrost it.
This mechanism has a bad reputation among some sellers, often because they compare situations that are not comparable. The manufacturer itself describes it as a simple, robust frost-prevention system. Its real limitation lies elsewhere: it is not suitable for integration into a central air-handling unit, because nothing mechanically prevents the central fan from drawing outdoor air through the stopped unit. For this type of connection, a recirculation-defrost model equipped with a backdraft damper is required.
Here, there was no central unit, no return duct, and no third-party fan. One independent network, one independent unit. Shutting off the fan was not only acceptable: it was the rational, less expensive choice, with no functional compromise in this configuration.
The work process in an occupied home
Dealing with the noise, because the unit is three meters from a bedroom
An HRV installed in a basement is forgiving in terms of acoustics. An HRV installed in a hallway closet forgives nothing. Three decisions were made for this reason.
First, the unit was mounted to the wall on a plate with rubber washers rather than directly against the drywall, to cut the transmission of vibrations into the adjoining wall.
Next, we deliberately kept insulated flexible duct on the first few feet of each connection rather than switching immediately to rigid galvanized steel. Flexible duct dampens fan noise; rigid duct transmits it like an organ pipe. This choice costs a few points of static pressure, which we offset by stretching the flexible duct as tight as possible, severely limiting the number of bends, and using rigid duct for straight runs as soon as we leave the cabinet.
Finally, the unit was set to run continuously at low speed, never in start-stop cycles. A fan running gently without interruption becomes background noise that the ear stops noticing. A fan that starts at full speed four times an hour wakes people up.
The condensate drain in a home without a floor drain
This is the detail that complicates half of all upper-floor air exchanger installations. The unit produces condensate, which must be drained by gravity.
The drain connection is located beneath the housing. We installed a loop trap immediately at the outlet — visible in the site photo — then routed the tube on a continuous slope, secured with clamps, to the washing machine's drainpipe. Three non-negotiable rules apply to this section:
- No high points along the route. A single rise can trap the water, fill the pan, and cause it to overflow.
- A trap is mandatory. Without one, the unit draws air through the drain and brings the drain's odor back into the home.
- A connection to the drain line, never into empty space. We have seen HRV drains end above a hardwood floor, in a bucket that no one emptied.
The system was tested with water, in the homeowner's presence, before the cabinet was closed.
The duct network and the exterior vents
The system was installed with four branches: two leading outdoors—fresh-air intake and stale-air exhaust—and two leading into the home. The exhaust draws air at the source, in the bathroom and near the kitchen; the supply delivers fresh air to the bedrooms and living room.
The two exterior ducts were insulated and fitted with a vapor barrier along their entire interior length, without exception. An uninsulated fresh-air duct passing through a heated home becomes a cold surface, and a cold surface in a humid home will always eventually dampen the drywall around it.
The two wall hoods were installed through the side wall, spaced apart to prevent the appliance from drawing back in what it had just expelled, and positioned high enough to remain clear after a heavy snowfall. In a dense area such as Montréal-Nord, we also check what is near the intake: a neighbor’s dryer vent, a plumbing vent, or a parking area. A fresh-air intake placed next to an exhaust vent does not provide fresh air.
Balancing, measured and recorded
The appliance door has a table correlating measured pressure with airflow. We connect a manometer to the two pressure taps, take the reading, cross-reference it with the table, and then adjust until the supply and exhaust flows are within ten percent of each other.
This step takes an hour. It determines everything that follows. An appliance that extracts more air than it supplies puts the home under negative pressure and draws air in through electrical outlets, around windows, and from the shared stairwell—in a duplex, that also means drawing air from the neighboring unit. An appliance that supplies more air than it extracts pushes humid air into the walls. Both errors can be corrected with a manometer and a screwdriver, but they are costly when no one corrects them.
One day of work, two technicians, and the home restored to order that same evening.
The six points that distinguish a properly installed air exchanger from an appliance gathering dust on the ceiling
- Is the control accessible and understood? An appliance no one knows how to use invariably ends up in “standby” mode. For this home, we chose a simple control set to run continuously, supplemented by a bathroom timer that switches the appliance to full speed for twenty, forty, or sixty minutes. In a rental building, a preprogrammed “set it and forget it” control avoids unnecessary service calls.
- Is the system independent or connected to the central system? These two configurations require different equipment and different types of defrosting.
- Are the outdoor ducts insulated all the way to the unit? This is the leading cause of water damage incorrectly attributed to the air exchanger itself.
- Does the drain have a trap and a continuous slope? Check it visually—in ten seconds.
- Has the unit been balanced with a manometer? Ask for the final airflow readings.
- Is there enough clearance to remove the core? A unit that cannot be opened will never be serviced.
Eligibility for grants
The Québec government's Rénoclimat program provides assistance of $500 for the installation or replacement of an HRV or an ERV, subject to two conditions: the unit must be listed in the Home Ventilating Institute (HVI) directory of certified products, and an energy assessment must be completed before and after the work. The initial assessment must take place before the project begins, with no exceptions or possibility of completing it afterward. Provincial program terms change from year to year, and we recommend verifying the current conditions before setting a work date.
Maintenance explained to the owner before we left
- Every three months: remove the filters, rinse them with lukewarm water, and let them dry completely before putting them back.
- Once a year: remove the core, vacuum it, wash it with warm soapy water, and check the drain trap.
- Twice a year: clear the grilles of the outdoor hoods, where lint, leaves and seeds accumulate.
HVAC maintenance performed at this frequency keeps the airflow at the level measured during commissioning. Without it, a unit can lose one-third of its capacity in three winters without any warning light coming on.
AirGreen in Montréal-Nord and Greater Montréal
We serve Montréal-Nord, Ahuntsic-Cartierville, Saint-Léonard, Rivière-des-Prairies and the entire eastern part of the island, as well as Montréal, Laval, Longueuil, the North Shore and the South Shore. Duplexes, triplexes, apartment buildings, single-family homes: we adapt the ductwork configuration to the building rather than the other way around.
If the air in your home feels heavy, your windows run with condensation in winter, or your rental building has never had mechanical ventilation, call us. We assess the available space, tell you what type of unit will actually fit there, and provide a complete quote, including the ductwork.