Installation of a GREE FLEXX R32 central “add-on” heat pump in Sainte-Catherine, Roussillon RCM

Keep the existing unit, add the heat pump: the project that reshaped the heating bill for a Sainte-Catherine home

There are homes where completely replacing the system makes no sense. This was one of them. A low basement, a still-functional furnace, an existing duct network, and homeowners who mainly wanted to stop paying for heating twice: once in energy costs and once in repairs. Our response was a GREE FLEXX R32 central heat pump installed in an “add-on” configuration, connected to the existing distribution system rather than replacing it.

Here is how this de Sainte-Catherine project, in Montérégie, on the South Shore of Montréal, unfolded—with the actual figures, the basement's physical constraints, and the technical decisions we would make again.

The context: a house a few streets from the Seaway

Sainte-Catherine occupies a strip of land wedged between the St. Lawrence Seaway and Route 132, in the Roussillon RCM. The residential housing stock there consists largely of bungalows and split-level homes built between the late 1970s and mid-1990s, with basements finished later and limited headroom beneath the joists.

The house in question checks all these boxes: a partially finished basement, an open mechanical section, spray foam on the ceiling and along the rim joist, and a galvanized sheet-metal duct network that, unlike much of what we see in the area, had been properly sized from the outset.

The homeowners called us for a simple reason: they heated with electricity and a furnace, had no central air conditioning, and the basement could not accommodate a full-height vertical unit. Three constraints, one coherent solution.

Why choose an “add-on” rather than a complete replacement

The term “add-on” refers to a central heat pump added to an existing heating system, rather than a system that replaces everything. The original heating unit remains in place and becomes the backup; the heat pump handles the main load, for both heating and cooling.

Three reasons motivated this choice here:

• The existing distribution system was sound. Reusing a functional duct network costs a fraction of rebuilding it. We checked the duct sizes, joint tightness, and return-air path before committing.
• Redundancy has real value in Montérégie. During extreme cold snaps, having a second independent heat source is not a luxury.
• The budget became manageable. The client could spread out the investment: install the heat pump now and replace the supplemental unit later without losing the investment.

We often tell the same thing to our clients in Longueuil, Laval, and the North Shore: the best HVAC installation is not always the most complete one. It is the one that solves the real problem without creating three new ones.

The GREE FLEXX R32: What the Technical Data Sheet Really Says

We selected the three-ton configuration: GUD36W2/NhE-D(U) outdoor unit paired with the GUD36AH2/G-D(U) indoor unit.

The Numbers That Matter

The FLEXX lineup goes up to 19 TES2 and 10.5 HSPF2, but those peak ratings belong to the 4- and 5-ton models. We want to make this clear because a brochure summary and the table row corresponding to your unit do not always tell the same story. The model installed here is ENERGY STAR certified and has a verifiable AHRI number—that number, not the brochure, is the authoritative reference when applying for a rebate.

-30 °C: What It Means—and What It Doesn’t

The FLEXX’s heating range extends down to -30 °C, and its cooling range covers -15 °C to 54 °C. One nuance is important: an operating range is not a guarantee of capacity. At -30 °C, the unit still operates; that does not mean it delivers 34,000 BTU/h. A heat pump’s capacity decreases as the outdoor temperature drops—that’s physics, not a manufacturing defect.

This is precisely where the “add-on” approach becomes elegant. Rather than oversizing a unit to cover five nights per winter—and condemning it to short cycling the rest of the year—we size it for the actual load and let the supplemental system handle the peak. In Sainte-Catherine, the calculated balance point was around -12 °C: above that, the heat pump is sufficient on its own; below that, the supplemental system takes over.

R32 and the A2L detection sensor: what changes in a basement

The R32 is classified A2L, meaning slightly flammable. Its GWP (global warming potential) of approximately 675 is significantly lower than that of R-410A, which explains the industry’s transition. But this classification imposes requirements that many installers discover on the job.

The FLEXX comes with a factory-installed A2L leak detection sensor—clearly visible in the job-site photo, mounted directly in the coil section. Its role: if an abnormal concentration is detected, the module shuts down operation and forces the indoor fan to disperse the refrigerant through the duct network, where it cannot accumulate.

What this means in practical terms:

1. The sensor is not an optional accessory. It must be left in place and powered, and its wiring must never be bypassed to “silence” a system.
2. The room volume matters. A 2.9-lb charge in a small, enclosed mechanical section does not have the same profile as in an open space. We verify the served volume before determining the piping length and additional charge.
3. Brazing and evacuation procedures change. We purge with nitrogen during brazing and pull the vacuum below 500 microns, followed by a rise test. In an R32 system, residual moisture is no more forgiving than elsewhere, but the consequences arrive sooner.

The job-site mechanics

Connecting a horizontal-airflow unit to an existing plenum

The indoor FLEXX is designed for horizontal (side-discharge) airflow, which was decisive here: at 21 1/4 in. wide and 21 1/4 in. deep, the unit can lie on its side where a 48-in.-tall vertical model simply would not fit beneath the joists.

The connection between the new unit and the original plenum was fabricated on site from sheet metal: gradual transition, cleated joints, and complete sealing with aluminum tape around the entire perimeter. We never reuse an original transition that “looks like it will work.” A two-inch misalignment in a transition means permanent turbulence, noise, and a pressure drop that the fan will pay for over fifteen years.

The indoor unit weighs 163 lb empty. It was seated on a rigid base rather than simply resting on the plenum sheet metal—an error we regularly see when taking over jobs on the South Shore.

The filter: the part we neglect

We integrated a hinged-door filter cabinet upstream of the unit, with a pleated filter accessible without tools. The media choice deserves a sentence: an overly fine filter on a central system with an undersized return-air system reduces airflow, freezes the coil in cooling mode, and makes the compressor work in a vacuum.

Our rule: choose filter efficiency only after measuring the available static pressure, never before. On this job, the measured external static pressure left us enough margin to increase filtration efficiency without reducing airflow.

Condensate: a slope, a trap, and two cleanout plugs

In air conditioning, a central three-ton coil can produce several liters of water per day in the July humidity of the Montérégie region. The drain was therefore installed in rigid PVC with:

• a trap suited to the cabinet pressure (on the suction side, a trap that is too short will empty and let air in, blocking drainage);
• two accessible cleanout plugs, so the drain can be flushed without taking anything apart;
• a continuous slope, checked with a level, all the way to the discharge point.

We emphasize this because we have already been called in for water damage caused by an improperly routed condensate drain. A pipe that rises three-quarters of an inch over twelve feet guarantees a puddle in August.

The refrigerant connection and the 30-meter question

The FLEXX refrigerant piping uses 3/8 in. liquid and 3/4 in. gas lines. The system is precharged for 24.6 ft (7.5 m), with a maximum length of 98.4 ft (30 m) between the indoor and outdoor units and a maximum elevation difference of 49.2 ft (15 m).

Beyond the factory charge, you need to add 0.32 oz per foot (30 g/m). On this job, the actual run measured approximately 12 m, so we weighed and added the corresponding supplementary charge using a scale, then documented the value on the commissioning sheet. A system charged “by gauge and experience” with R32 is a system whose actual charge no one will ever know when it is time to diagnose it.

The 24 V thermostat and dual-fuel logic

FLEXX operates on a 24 VAC power supply and accepts a conventional thermostat, which greatly simplifies integration in an “add-on” setup. No proprietary protocol, no gateway: a heat-pump-compatible thermostat with auxiliary heat, properly configured, is all that is needed.

Configuration is the part that too many installations get wrong. Here is what was set up:

• the balance point below which auxiliary heat is allowed;
• auxiliary heat lockout above this temperature, to prevent the house from being heated by auxiliary heat while the heat pump is running in vain;
• the restart delay after a defrost cycle, so that the system does not switch to supplemental heat every time defrosting briefly cools the supply air.

A poorly configured dual-fuel system costs more than a simple system. Properly configured, it becomes the most cost-effective installation one could put in a 1980s bungalow.

What this Sainte-Catherine project reminds us

The mistakes we see most often

• Sizing by eye. Three tons because the house is 1,400 sq. ft., without a load calculation—that is the costliest shortcut in the trade.
• Reusing an existing refrigerant line without validating it. An old R-410A line is not automatically compatible in terms of diameter, cleanliness, or residual oil.
• Ignoring the return air. A central system that is properly installed on the supply side but restricted on the return side will never deliver its rated performance.
• Neglecting service access. We systematically leave the clearance required to remove the panels and coil. Equipment that cannot be serviced is serviced by no one.
• Treating the A2L sensor as a nuisance. It is a safety device, not a design flaw.

Rebates and eligibility

The FLEXX is ENERGY STAR certified and has an AHRI number—the two basic requirements for most programs. Depending on the heating source being replaced or supplemented, various government programs and local rebates may apply. Criteria, amounts, and dates change from year to year: we confirm eligibility with the client before signing, using the exact AHRI number of the installed matched system rather than the product line’s commercial name. This is the only way to avoid an unpleasant surprise afterward.

Our coverage

We carry out this type of HVAC installation throughout Montreal, Laval, Longueuil, the North Shore, and the South Shore, including Sainte-Catherine, Delson, Saint-Constant, Candiac, Châteauguay, and the entire Roussillon RCM. Every project begins with a technical visit: measuring available static pressure, inspecting the ductwork, performing a load calculation, and assessing the physical constraints of the mechanical room.

If your home has a furnace that is still in good condition and no central air conditioning, the “add-on” configuration deserves serious consideration before contemplating a complete replacement. Contact our team for an on-site assessment.