Installation of a GREE FLEXX R32 “add-on” central heat pump in Saint-Philippe, Montérégie

A 1.9-metre crawl space, a furnace to retain, and an A2L sensor that determines its position: the Saint-Philippe project

The homeowner called us in mid-June, after two summers of running two portable air conditioners in a bungalow in the residential area of Saint-Philippe, a few minutes from montée Monette. The house was already heated with electricity, with a complete ductwork system and a furnace in good condition. The request was clear: central air conditioning, more economical heating during the shoulder seasons, and above all, no throwing out a heating unit that was still working.

This is exactly the situation in which a central “add-on” heat pump is the right answer. And that is the configuration we installed: a GREE FLEXX R32 connected to the existing ductwork, with the furnace retained as a backup.

Why choose an “add-on” configuration instead of a complete replacement

The term “add-on” describes a simple reality: a heat pump is added to an existing heating system instead of tearing everything out. The existing unit remains, but its role changes. It stops being the primary heat source and becomes the backup source, taking over when the outdoor temperature drops below the heat pump’s balance point.

The benefits are tangible for a homeowner on the South Shore:

• We reuse the ductwork, which often represents 30 to 40% of the cost of a new central system.
• We maintain true redundancy. If the heat pump is shut down for maintenance in January, the house does not lose heat.
• We add central air conditioning, which a furnace alone will never provide.
• We make the project eligible for financial incentives for high-efficiency heat pumps, which a simple furnace replacement would not allow.

The “add-on” trap is always the same: two units competing for the same network and thermostat. We’ll come back to this, because that’s where the difference lies between an installation that works and one that drives up your energy bill.

The chosen unit: GREE FLEXX R32, 34,000 BTU/h

After calculating the load room by room and measuring the existing ductwork, we selected the following matched system:

• Outdoor unit: GUD36W2/NhE-D(U)
• Indoor unit (horizontal air handler): GUD36AH2/G-D(U)

The figures that matter:

Three features of the FLEXX weighed heavily in the selection, and they are directly related to the add-on installation.

Conventional 24 VAC control. The FLEXX does not require a proprietary thermostat or a proprietary communication gateway. It can be controlled with a conventional 24 V thermostat, making integration with an existing heating appliance much cleaner and much easier to troubleshoot five years later.

Horizontal (side-discharge) airflow design. The indoor unit is designed to be laid on its side and discharge air laterally. In a Saint-Philippe crawl space where the clear height does not exceed 1.9 m, this is not a catalog detail: it is what makes the installation possible.

Dual Fuel and Ultra Heat compatibility. Even though this home is fully electric, the platform supports dual-energy scenarios. For our customers in Saint-Constant, Delson, or Candiac who still use natural gas, it is the same appliance that can be used.

Add to that the factory-installed Wi-Fi—not a module to order separately three weeks after commissioning—and a heating range down to -30 °C, covering nearly all winter nights in Montérégie.

The A2L leak sensor: the constraint that dictated the installation

This is the most misunderstood component of systems at R32, and the one that distinguishes an installation done properly from one done in a hurry.

R32 is classified as an A2L refrigerant: mildly flammable. The standard therefore requires leak detection integrated into the appliance, with a mitigation strategy. The FLEXX comes with the A2L detection sensor included, installed at the factory. But—and this is stated in black and white on the warning label affixed to the blower-section panel—the position of this sensor depends on the unit’s discharge direction.

With a unit lying horizontally, there are two possible orientations. If the appliance discharges to the right, the sensor must be located in the position shown in the figure. If the airflow direction is reversed without repositioning the sensor, the sensor ends up on the wrong side of the coil. R32 is heavier than air: it descends and accumulates at the lowest point of the section. A misplaced sensor may therefore monitor an area where the refrigerant will never go.

At Saint-Philippe, the discharge direction was dictated by the basement's geometry. We therefore:

1. Confirmed the airflow direction before even lowering the unit, based on the layout of the connection plenum.
2. Physically verified the sensor's position in the heat-exchange section, with the panel open, before pressurizing the system.
3. Documented the position in the file, with a photo, so the next technician who opens this panel knows what they are looking at.
4. Tested the mitigation logic during commissioning: upon detection, the compressor stops and the indoor blower is activated to dilute and disperse the refrigerant throughout the duct network.

That is three quarters of an hour's work. It is also the difference between a functional safety device and a decorative one.

The actual site constraints

Insufficient overhead clearance for suspended installation

The first instinct with a horizontal appliance is to suspend it from the joists on threaded rods. That was impossible here: the exhaust duct passing through the ceiling slab ran exactly along the usable axis, and there was not enough clearance above the cabinet.

We therefore mounted the unit on a base of solid concrete blocks, leveled with a straightedge and shimmed with composite shims. Three requirements guided the installation:

• A very slight slope toward the drain connection, so the condensate pan drains by gravity and does not retain standing water.
• Sufficient base height to create the condensate trap beneath the pan. A trap crushed flat against the slab is the number-one cause of the water backups we see in service on horizontal units on the South Shore.
• Access clearance maintained on the service-panel side, along the entire length of the 48 in. cabinet.

Connection to the existing network

The FLEXX indoor unit discharges laterally into a custom-built transition plenum fabricated in the workshop, then connected to the existing heating appliance retained as backup, which is itself connected to the main distribution trunk.

This plenum was sealed with aluminum tape at all its joints, including the inside corners—no duct tape, ever, on a pressurized plenum. The insulated flexible ducts leading to the secondary branches were stretched out, never compressed: a collapsed flexible duct adds invisible bends that no one accounts for in the static pressure calculation.

Refrigerant piping

The FLEXX 36 uses a 3/8 in. liquid line and a 3/4 in. gas line. The unit is precharged for 24.6 ft (7.5 m) of piping. Beyond that, add 0.32 oz per foot (30 g/m).

The actual run to the outdoor slab was 41 ft. We therefore added the supplementary charge using an electronic scale, weighed it, and recorded it in the commissioning report—not estimated from the gauges. On a modulating unit, overcharging by a few ounces costs performance and causes cycling, and it is not apparent during the first summer.

The limits to observe for this model: 98.4 ft (30 m) maximum length between the units and 49.2 ft (15 m) maximum elevation difference. We always design our runs according to the model-specific table, never according to the generic value on the cover page of a brochure.

Flared connections tightened with a torque wrench, evacuation below 500 microns with a 15-minute standing vacuum test with the valve closed, then the valves are opened. On a system R32, rigorous evacuation is non-negotiable: residual moisture and R32 do not mix well with POE oil.

Condensate

The pan drains through a corrugated hose into a PVC line equipped with an accessible cleanout plug, then to the condensate pump. We systematically add a high-water safety switch wired in series with the control: if the water level rises, the heat pump shuts down before the water spills out. We have seen too many basement floors damaged by a $6 hose to cut corners on this precaution.

Control integration: where most “add-ons” miss the mark

Two units, one system, one thermostat. If the wiring is hastily done, we get one of the following two scenarios:

• The furnace starts at the same time as the heat pump. The backup’s warm air passes through the coil, discharge pressure rises, efficiency collapses, and we pay for electricity twice.
• The heat pump is locked out at too high a temperature. The balance point is set to -5 °C “as a precaution,” and the backup runs all winter. The customer paid for a heat pump to heat the home, not the baseboard heaters.

In Saint-Philippe, we wired a two-stage conventional 24 V thermostat with auxiliary heat lockout and set the balance point based on the home’s calculated load, not a default value. The sequence is simple: the heat pump operates on its own as long as it can meet the load; auxiliary heat is called only at the second stage, when a temperature difference persists, and it is locked out until the compressor has had time to do its job.

Airflow was measured and adjusted to the actual duct network, with external static pressure recorded on both sides of the cabinet. We never rely on a theoretical airflow from a technical data sheet—it is a laboratory value, not the value in your basement.

How this project changes your bill, and what you are entitled to

Financial assistance: the calculation rule few people know

Hydro-Québec’s LogisVert program pays $50 per 1,000 BTU/h for an efficient heat pump, and $120 per 1,000 BTU/h for a unit rated for a cold climate. The unit must be ENERGY STAR certified—this is mandatory for any installation completed since November 26, 2025—and provide a minimum of 12,000 BTU/h in cooling.

The detail that makes all the difference: the amount is calculated based on capacity at -8 °C, not nominal capacity. The calculation uses not the 34,000 BTU/h listed on the technical data sheet, but the certified value at -8 °C associated with the AHRI number for the specific indoor-outdoor combination you are installing. Changing the indoor unit without changing the outdoor unit changes the AHRI number, and therefore the amount.

At AirGreen, we verify the match in Hydro-Québec’s search tool before ordering the equipment, and we assemble the application with the customer. It is the only way to avoid the unpleasant surprise of a rejection after installation.

The Chauffez vert program, for its part, has not accepted applications since March 31, 2026. Homeowners still using oil or propane on the South Shore must therefore work with the programs that are still active, and we review the options with them when preparing the quote.

The mistakes we see most often with “add-ons”

1. Oversize “to be safe.” An oversized unit short-cycles, dehumidifies poorly, and ages quickly. Load calculations are not an administrative formality.
2. Keep an undersized duct network. A furnace tolerates high static pressure. A variable-capacity heat pump, much less so.
3. Ignore the A2L sensor position on a horizontal unit. See above.
4. Placing the outdoor unit directly on the ground. In Montérégie, snow accumulates and the defrost cycle produces water. We systematically install units on raised bases.
5. Failing to record the added charge. Without the exact amount documented in the file, the next diagnosis starts blindly.
6. Neglecting indoor filters. A clogged filter on a modulating system results in a gradual loss of capacity that the customer attributes to the unit.

Our experience on the South Shore and elsewhere

The Roussillon RCM area—Saint-Philippe, Saint-Constant, Delson, La Prairie, Candiac, Saint-Mathieu, Châteauguay—keeps us busy because the housing stock there includes both new homes with complete ductwork and homes from the 1970s to the 1990s whose basements and crawl spaces require horizontal installations like this one.

We install the same configurations in Longueuil, Brossard, and Saint-Hubert, in Montreal neighborhoods with low basements, as well as in Laval and on the North Shore, from Terrebonne to Blainville. Our teams also know the municipal regulations applicable to outdoor units—property-line setbacks, permitted noise levels, and screening—and our office answers these questions by phone even before providing a quote.

The homeowner in Saint-Philippe wrote to us three weeks after commissioning: “The basement is finally usable in July, and I can't hear the unit from the living room.” The FLEXX 36 outdoor unit is rated at 61 dB(A) at high speed—and in modulating mode, it rarely runs at that speed.

Let's move forward with your project

If you already have a ductwork system, a heating unit that is still serviceable, and want to add central air conditioning without redoing everything, a GREE FLEXX R32 add-on central heat pump deserves serious consideration.

At AirGreen, every quote begins with a load calculation, an assessment of the existing ductwork, and a verification of eligibility for financial assistance. Contact us for an evaluation of your HVAC system in Saint-Philippe, on the South Shore, in Montreal, Laval, Longueuil, or on the North Shore.