A Crowded Basement, an Existing Furnace, and Five Ways to Connect the Unit
When a home already has a central forced-air system, adding an air exchanger raises an issue that most quotes dispatch in a single line: the connection method.
This is not an execution detail. The manufacturer specifies five distinct connection configurations, numbered T-1 through T-5, which must be explicitly selected in the unit during commissioning. Two of them are discouraged by the manufacturer itself. Another imposes a permanent electrical requirement that almost no one calculates before signing.
On this project in Vaudreuil-Dorion, this question took more time than the physical installation of the unit.
Vaudreuil-Dorion: Two Housing Stocks in One City
Vaudreuil-Dorion has approximately 46,400 residents and covers 73.1 km² west of Montreal, bordered by the Ottawa River and Lake of Two Mountains, at the junction of Highways 20, 30, and 40. It is Quebec's 28th-largest city by population and one of its most dynamic: a steady stream of residential projects, eight industrial parks, and a hospital expected by the end of 2026.
This growth conceals a reality we see every week in the field: the city contains two overlapping housing stocks. On one side, newly built neighborhoods constructed to be airtight, where mechanical ventilation is integrated from the design stage. On the other, the historic core and developments from the 1960s to the 1990s—the area home to the Trestler House, the Félix-Leclerc House, and the Vaudreuil-Soulanges Regional Museum—where homes were renovated in stages, insulated afterward, and where no air exchanger was included in the original design.
This project falls into the second category. And that is where the question of the connection becomes decisive, because you never start with a blank page.
The Basement as We Found It
Low-clearance utility basement with exposed joists, a steel column painted red, wiring accumulated over several decades, copper lines running diagonally, and a central forced-air system already occupying the corner of the room. Added to that was what all unfinished basements eventually come to house: storage shelves loaded with containers, pots, and boxes, built all the way to the ceiling.
The real constraint: clearance, not space
There was room for the unit. There was not, initially, enough service clearance.
The distinction escapes many installers. An HRV occupies about two cubic feet, but it requires:
- the space needed for the front door swing;
- the depth needed to remove the core in a straight line;
- side access to the collars and drain;
- and enough room to stand in front of it without a stepladder.
We repositioned the unit a few feet from the initially planned location, specifically to preserve this clear space in front of the door. And before leaving, we explained to the homeowners that the shelves must not be moved into this space. It is the most useful conversation of the day, and the one that takes thirty seconds.
The suspension: chain and spring
The unit is suspended from the joists by chains, with a damper spring inserted in the suspension line.
This spring is not a decorative accessory. In an older home where the joists are more flexible and the floor less massive than in a recent construction, residual motor vibration is easily transmitted to the room above. The chain alone already partially decouples the unit from the structure; the spring absorbs the fraction of vibration that the chain still transmits through direct tension.
In new homes, we often do without it. In 1970s homes in Vaudreuil-Dorion, Laval, or the North Shore, it makes an audible difference.
The mechanical connection
The connection between the unit's collar and the existing ductwork was made using a sheet-metal transition fabricated on site, then sealed with aluminum tape around its entire perimeter. The insulated flexible duct with a black jacket is secured to it with a clamp, the insulation brought over the transition and sealed in turn, so that no section of cold metal remains exposed to the humid basement air.
It is this continuity of the insulation, more than the connection itself, that determines whether condensation will reappear in the spring.
The control
A wall-mounted display control was installed in the immediate vicinity of the unit. It provides access to the three commissioning screens printed on the manufacturer's label:
- the mode selection — standby (STB), minimum, maximum;
- the airflow setting in CFM, entered in increments using the buttons;
- the installation configuration, from T-1 to T-5.
The label also bears a QR code providing access to up-to-date installation and maintenance documentation. We systematically take the opportunity to scan it in front of the customer and save the link on their phone—the paper manual always ends up in a bin at the back of the garage.
T-1 to T-5: the five ways to connect an air exchanger, and the ones to avoid
Here is the content we wish we had found online when we started. These five configurations are not cosmetic variations: they change the unit's operation, its energy consumption, and the risk associated with the installation.
T-1 — The fully independent system
The air exchanger has its own complete network: exhaust ducts to the bathrooms, kitchen, and basement, and supply ducts to the bedrooms and living areas. It does not connect to the heating system.
- Recirculation mode is available.
- No interlock with the furnace fan is required.
- This configuration offers the best control and the lowest operating cost.
- It is also the most expensive to install in a renovation because the entire network must be created.
T-2 — Source exhaust, independent distribution
Stale air is extracted through dedicated ducts at the locations where pollutants and humidity are produced. Fresh air is distributed through an independent network.
- Recirculation mode is not available.
- No interlock with the furnace fan.
- It is an excellent compromise for renovations: it captures pollutants at the source, where it matters most, without imposing a permanent electrical load on the house.
T-4 — The simplified installation
The fresh-air intake and exhaust are connected to the central system's return duct. This is the quickest and least expensive configuration to install, which is why many contractors offer it as the default option.
It comes with one strict requirement: the furnace fan must be interlocked with the operation of the air exchanger. Without this interlock, the fresh air introduced into the return is immediately drawn back into the unit instead of being distributed throughout the house. The unit runs, but the house is not ventilated.
The cost no one calculates
This is where the true cost of the simplified configuration lies, and this calculation guided the decision on this project.
An air exchanger runs continuously. In a T-4 configuration, this means that the furnace fan must also run continuously. However:
- A furnace fan with a conventional (PSC) motor commonly consumes 400 to 600 watts. Running continuously, that amounts to approximately 3,500 to 5,200 kWh per year.
- An ECM motor fan running at low speed typically consumes 80 to 150 watts, or approximately 700 to 1,300 kWh per year.
In a Vaudreuil-Dorion house whose furnace predates the arrival of ECM motors, choosing the simplified configuration to save a few hours of installation can cost, in fan electricity, several times the price of installing a complete dedicated ductwork system over the appliance's service life.
This is a legitimate trade-off when the furnace is recent and has an ECM motor. It is a poor choice when it does not. We therefore check the type of motor in the central system before recommending a configuration — a two-minute check that most bids omit.
T-3 and T-5 — The configurations the manufacturer discourages
These two configurations also connect both airstreams to the central system's return, using different routing variations. The manufacturer explicitly identifies them as not recommended.
The reasons are technical and serious:
- T-3 requires interlocking with the central fan to prevent condensation — and, eventually, mold — when the air conditioning is running. The humid fresh air introduced into a cooled ductwork system condenses on the duct walls.
- T-5 adds a warning concerning high-velocity central systems, whose high static pressure can cause the air exchanger to operate erratically.
When a contractor proposes one of these configurations, demand justification for the choice. There are situations where T-3 or T-5 are the only physically possible connection. They are rare, and the bid should explain them in writing.
The ten-foot rule
It applies as soon as the appliance is connected to a central system's ductwork, and is specific to climates where the temperature drops below freezing — meaning all of Quebec.
The fresh-air distribution outlet of the air exchanger must be connected to the central system's return duct at least ten feet from the appliance's return point, with the distance measured along the duct.
This separation prevents the appliance from immediately drawing back in the air it has just introduced, and gives the fresh air time to mix with the return air before reaching the heating system's heat exchanger. An insufficient distance causes thermal short-circuiting and an actual airflow rate far below the displayed rate.
We measure this distance along the actual duct, not by eye on a plan.
What we selected for this house
The existing furnace did not justify forcing its fan to run continuously. We therefore rejected the simplified configuration in favor of a connection with source extraction and independent distribution, which captures humidity and pollutants where they are produced, without parasitic electrical load.
The installation cost was higher. The annual electricity bill, however, contains no hidden line items. We presented the two costed scenarios to the homeowners and let them decide—which is the only honest way to approach this trade-off.
Five connection errors we regularly correct
- Leaving the factory configuration in the unit instead of selecting T-1 to T-5 according to the connection actually installed.
- Connecting to the return without interlocking the furnace fan. The unit runs without effect.
- Failing to maintain the ten-foot separation on the return duct.
- Interrupting the insulation at the junction between the flexible duct and the sheet-metal transition, creating a permanent condensation point.
- Installing the unit in a space that will later be enclosed—shelves, storage, or future partitioning—without reserving service clearance.
About financial assistance
The answer has not changed, and we state it as is: no amount is currently paid in Quebec for adding an air exchanger to an existing home. The Rénoclimat program remains the only entry point, and it funds the building envelope, not ventilation.
However, the configuration trade-off described above has an immediate and recurring financial effect, starting in the first year. That is where the real savings lie, and they do not depend on any program.
AirGreen in Vaudreuil-Dorion and the West Island
We serve Vaudreuil-Dorion, L'Île-Perrot, Pincourt, Saint-Lazare, Hudson, Rigaud, Les Coteaux, Saint-Polycarpe, and the entire Vaudreuil-Soulanges RCM, as well as Montreal, Laval, Longueuil, the North Shore, and the South Shore.
For any HVAC ventilation installation in a home that already has a central system, we document three things before even preparing a quote: the motor type of the existing fan, the available return-duct length, and the actual service clearance around the proposed location. These three pieces of data determine the configuration, the configuration determines the price, and the displayed price then corresponds to what the customer will really pay—with installation and operation included.
It takes longer to prepare than a per-square-foot quote. That is also why we do not come back to correct our own installations.