Two systems in the same attic: why we did not connect the GREE FLEXX to the existing Unico network
At first glance, the project in Saint-Mathieu looked like an ordinary replacement. It was not. In the finished attic of this home in the Roussillon RCM, there was already a system: a Unico System Green Series, a small-duct high-velocity system installed years earlier to serve the upper floor without tearing into the ceilings. The owner contacted us for another reason: the main floor had never had anything other than electric baseboard heaters, and no central air conditioning.
The request was simple: add a central heat pump capable of heating and cooling the main part of the house without altering what was already working. This is exactly what is known as an “add-on” installation: the heat pump becomes the primary heating and cooling source, while the existing electric heating remains in place as backup for the coldest days.
We installed a GREE FLEXX R32, with the indoor unit configured as GUD24AH2/G-D(U) and the outdoor unit as GUD36W2/NhE-D(U).
The first assumption to rule out: connecting the new heat pump to the existing ducts
This is the request we receive every week, and it is also the first thing we ruled out here. The Unico system is a high-velocity system: it pushes air through small-diameter flexible ducts at a static pressure of approximately 1 to 2.5 in. w.c., which is several times the pressure for which a conventional central unit is designed. The blower in the FLEXX, like that of any residential central heat pump, operates within a much lower range.
Connecting the two would amount to asking a fan to push air through a duct network that was not designed for it. The consequences are predictable: collapsed airflow, a coil that freezes in cooling mode, repeated defrost cycles in heating mode, noise at the diffusers, and an electricity bill that rises instead of falling. A high-velocity system and a conventional system cannot be mixed—they can coexist.
So we used both systems side by side. The Unico Green Series remained in place on its concrete blocks, dedicated to the upper floor. The GREE FLEXX was installed next to it on its own support, with its own network of insulated ducts, its own drain, and its own thermostat. Two systems, two zones, two control strategies, in the same attic.
Why the GREE FLEXX R32 for this type of project
The FLEXX is a horizontal (side-discharge) airflow air handler, which matters enormously in an attic where the clearance beneath the roof slope is limited. A vertical unit would have required rebuilding the plenum and raising it beyond what the roof pitch allows.
Data for the selected unit:
- Indoor model: GUD24AH2/G-D(U) — 24,000 BTU/h rated cooling capacity, 23,000 BTU/h heating capacity
- Outdoor model: GUD36W2/NhE-D(U)
- True modulation: 12,000 to 30,000 BTU/h — the unit drops to half its rated capacity in mild weather and exceeds its rated capacity at peak demand
- Efficiency : 18 TRÉS2 (SEER2), 12.5 TRÉ2 (EER2), 10 CPSC2 (HSPF2)
- AHRI number : 217120759
- ENERGY STAR certification
- Heating range: down to -30 °C; cooling range: -15 °C to 54 °C
- R32 refrigerant, 2.5 lb factory charge, precharge covering 24.6 ft of piping
- Maximum piping length between units: 98.4 ft, maximum elevation difference 49.2 ft
- Diameters: 3/8 in liquid, 3/4 in gas
- 208–230 V power supply, 4.7 A MCA indoors, 27.7 A outdoors, 30 A MOCP
- Indoor unit dimensions: 18 1/8 x 43 1/2 x 21 1/4 in, 135.6 lb
- Conventional 24 VAC thermostat and factory-installed Wi-Fi
The most important consideration for a South Shore customer is not 18 TRÉS2. It is -30 °C. A heat pump that shuts down at -15 °C forces the auxiliary heating to operate during the most expensive weeks of winter. A heat pump that still produces heat at -25 °C or -28 °C postpones that changeover by several hundred hours per season, and that is where the real savings come from.
Sizing: why 24,000 BTU rather than 36,000
The temptation in a house this size is to go up a size “just to be sure.” This is the most common mistake with “add-on” installations.
We performed a room-by-room load calculation rather than following the rule of thumb. The main floor to be served, with its windows and partially upgraded original insulation, came in below 24,000 BTU/h at peak summer load. A 34,000 BTU/h unit would have operated in short cycles all summer: it would have reached the temperature setpoint quickly, shut off, and never run long enough to dehumidify. The classic result: a house at 22 °C that still feels clammy in July, in a municipality surrounded by farmland and drainage ditches where summer humidity is very real.
With a modulation range of 12,000 to 30,000 BTU/h, the 24,000 BTU/h FLEXX spends most of the season running at low output, in long cycles, while retaining reserve capacity for heat waves and winter nights.
The installation process, from support to startup
The support and service access
The indoor unit rests on solid, level concrete blocks, with a slight slope toward the drain connection. This tedious detail is what saves floors: equipment installed crookedly in an attic sends condensation water over the edge of the pan instead of into the drain.
We also preserved service clearance in front of the electrical panel and the access door to the coil. An attic is where technicians work least comfortably; the person who installs the equipment determines what the person maintaining it will experience over the next fifteen years.
The ducts
The new duct run branches laterally from the unit into a large-diameter insulated flexible trunk, then divides toward the main-floor drops. Three rules we systematically follow in an attic:
- Flexible duct is pulled taut, never compressed. A sagging flexible duct loses a significant portion of its airflow and creates low points where moisture accumulates.
- Bend radii are maintained from the very start at the collar. A tight elbow placed directly against the unit costs more in pressure drop than three metres of straight duct.
- The duct insulation must remain on the warm side of the enclosure. A poorly insulated cold duct in a ventilated attic causes condensation in the insulation, followed by a ceiling stain in August.
Condensate
The unit’s drain connection slopes down through flexible corrugated hose to a PVC system equipped with an accessible U-shaped trap and cleanout plugs. A float safety switch was added: if the drain becomes clogged, the system shuts down instead of overflowing into the ceiling.
A poorly made trap is the most common cause of water damage with an attic air handler. Without a trap, the blower’s negative pressure holds water in the pan. With a trap that is too short, it loses its prime. We prime it with water at startup and show the customer where the plugs are for annual maintenance.
The R32 refrigeration circuit
The 3/8 in. and 3/4 in. piping was brazed under nitrogen, then evacuated to a deep vacuum, with a pressure-rise test performed before opening the valves. The run remained within the factory charge length, so no additional refrigerant was needed—but we systematically document the actual length in the file because that is the information the manufacturer requests in the event of a claim.
The A2L leak detection sensor: the detail many people still overlook
R32 is a refrigerant classified as A2L, meaning it is mildly flammable. The FLEXX comes with an integrated leak detection sensor, and the label affixed to the unit’s panel is explicit: the sensor’s position depends on the air discharge direction. On a horizontal-flow unit that can be configured for left- or right-hand operation, an on-site conversion without repositioning the sensor renders the protection ineffective.
We check this point during every commissioning and test the associated shutdown logic. This is a new aspect for many installers in Quebec, and it is precisely the kind of detail that an inspector or insurer will examine in the coming years.
Power supply and controls
A dedicated circuit compliant with the outdoor-side 30 A MOCP, a disconnect within sight, 24 V wiring to a conventional thermostat, and a Wi-Fi connection. The upper floor retains its own Unico control. The two systems do not conflict because each regulates its own zone.
What this project teaches homeowners in Saint-Mathieu, La Prairie, Candiac, and Delson
The most common mistakes with a central “add-on” heat pump
- Reusing an incompatible duct network simply because it is already there. A high-velocity, undersized, or leaky duct network negates the efficiency stated on the technical data sheet.
- Oversizing. An oversized heat pump costs more to purchase, dehumidifies less effectively, and wears out faster.
- Forgetting the changeover to auxiliary heat. On an “add-on” system, the transition logic between the heat pump and the existing electric heating must be set up and verified. If configured incorrectly, it runs the baseboards at the same time as the heat pump, causing the bill to skyrocket.
- Neglecting the outdoor unit in winter. A unit installed too low disappears under the snow and becomes an ice-covered box again. We elevate it on a support and keep the defrost drainage clear.
- Failing to document commissioning. Without records of the pressures, temperatures, airflow, and AHRI number, a subsidy application becomes a weak case.
Subsidies: the state of things in 2026
Two important changes for homeowners on the South Shore:
- The Chauffez vert program ended on March 31, 2026. In any case, it only covered the conversion of oil or propane systems.
- Hydro-Québec's LogisVert program remains the main incentive for a central heat pump. The key conditions are: being a Hydro-Québec customer, having the work carried out by a contractor holding a RBQ licence and registered with the program, installing an ENERGY STAR certified unit with a valid AHRI number, and submitting the application within the months following the invoice. The incentive is paid as a rebate after the fact.
The number one reason for rejection that we see is not the model: it is a missing AHRI number or one that does not match the indoor/outdoor combination actually installed. The FLEXX in the GUD24AH2/G-D(U) + GUD36W2/NhE-D(U) configuration bears the 217120759 number, which is included in the file provided to the client. Amounts vary depending on the unit and configuration; we validate them before signing rather than afterward.
The Rénoclimat program may also apply when other energy-efficiency work is carried out at the same time.
Why homeowners in the Roussillon RCM call us
We install central heat pumps, wall-mounted heat pumps, mini-split systems, air conditioners, and heat-recovery ventilators throughout Montreal, Laval, Longueuil, the North Shore, and the South Shore, including smaller municipalities in Montérégie that many contractors avoid. Saint-Mathieu is one of them.
What this project illustrates: the value does not come from the model selected; it comes from assessing the building before the purchase. A low attic, an existing high-velocity system, a ground floor with baseboard heating, and a clearance constraint—each of these factors could have turned a good machine into a disappointing installation.
If you are considering a central add-on heat pump, replacing a central unit, or adding air conditioning to a home that has never had it, write to us or call us. We travel to your location, take measurements, calculate the load, and provide you with a clear proposal including the AHRI number, actual efficiency ratings, and eligibility for available rebates, already verified.
AirGreen — Air Conditioning & Heating. RBQ 5645-9605-01.