A flat roof, two zones to cool, and an R32 system that changes the gameેણ
The project we received in à Sainte-Victoire-de-Sorel initially looked like a standard wall-mounted heat pump installation: two rooms to condition, a set budget, and a homeowner tired of running her electric baseboards at full capacity from November through April. What made this project unusual was where the outdoor unit had to go: on a flat roof with an elastomeric membrane, more than six meters above the ground, with ladder-only access.
At AirGreen, we regularly carry out this type of rooftop installation—the triplexes in the Plateau, Rosemont, and Villeray have given us plenty of experience—but every roof has its own requirements for load distribution, drainage, and membrane protection. Here is how we approached this one, and why the GREE Freematch R32 was the right technical choice rather than simply a catalog choice.
The context: fully electric heating and uneven comfort
The home was built in the 1980s and properly insulated, but heated exclusively with electric baseboards. Two zones were problematic:
- The open-concept living room, facing due south, overheated in summer and difficult to keep at the right temperature in winter because of the large windows.
- The upstairs primary bedroom, consistently 3 to 4°C warmer than the rest of the home in July.
The homeowner had already consulted two contractors. One proposed two separate single-zone wall-mounted air conditioners, while the other suggested a complete central system. Neither truly met the need: the first would have multiplied the outdoor units on a roof that could not support two, while the second would have required ductwork that was impossible without demolition.
Why the GREE Freematch R32 GWHD(24)ND6MO
We selected the GWHD(24)ND6MO model, a multizone outdoor unit that supports 2 to 3 indoor units, configured here with 2 wall-mounted heads.
The technical data that influenced the decision
| Feature | Value |
|---|---|
| Nominal cooling capacity | 24,000 BTU/h (7,030 W), modulation from 7,500 to 31,400 BTU/h |
| Nominal heating capacity | 24,000 BTU/h, modulation from 7,500 to 31,400 BTU/h |
| SEER2 | 21 |
| HSPF2 | 10 |
| EER2 / COP | 12 / 3,9 |
| Heating range | up to -30 °C |
| Refrigerant | R32, factory charge of 1,700 g |
| Sound level | 58 dB(A) |
| Power supply | 208/230 V, MCA 19.5 A, MOCP 25 A |
| Total piping length | up to 60 m, maximum elevation difference: 15 m |
| Warranty | 10 years + 2 years compressor and parts (with registration) |
Three factors truly made the difference in this case.
First, the heating operating range down to -30 °C. Sainte-Victoire-de-Sorel is not Montreal: January nights there regularly fall below -25 °C, and the St. Lawrence Valley adds a wind factor that appears nowhere on a technical data sheet. An ENERGY STAR Cold Climate-certified heat pump is not a marketing luxury in this area; it is what ensures the unit still produces heat on the nights when baseboard heaters cost the most.
Second, the modulation range. A G10 inverter compressor capable of dropping to 7,500 BTU/h avoids the short cycling that wears components and creates discomfort. In the shoulder seasons—April and October—the system runs continuously at low speed instead of starting and stopping every eight minutes.
Third, the R32 refrigerant. Its global warming potential is approximately three times lower than that of R410A, the required charge is lower at the same capacity, and heat transfer is superior. GREE includes an A2L leak detection sensor as standard on this range, directly meeting the requirements of recent codes governing mildly flammable refrigerants.
Rooftop installation: where craftsmanship shows
Distribute the load without puncturing the membrane
The outdoor unit weighs 51 kg empty. Add the structure, accumulated snow, and late-winter ice, and you get a point load that no elastomeric membrane should have to bear directly.
Our installation, visible in the jobsite photos:
- Load-distribution slabs placed on the membrane, without mechanical fastening — no roof penetration, and therefore no possibility of infiltration.
- Treated cedar beams placed on the slabs, serving as a continuous base and thermal break between the metal and the roof surface.
- Elevated metal support bolted to the beams, positioning the bottom of the unit approximately 45 cm above the roof.
- Recycled rubber mats arranged around the concrete base to protect the membrane from foot traffic during future maintenance.
This clearance height is not arbitrary. It serves three purposes: defrost water falls freely without forming an ice base beneath the unit, accumulated snow does not obstruct the air outlet, and the technician can access the underside of the unit during annual HVAC maintenance.
Common mistake to avoid: placing an outdoor unit directly on concrete blocks on the roof. It's quick, it's inexpensive, and it's the number-one cause of service calls in February — the unit ends up encased in a block of ice formed by its own defrost cycles.
The vibration-absorbing loop in the refrigerant lines
The photos show that the insulated refrigerant lines form a wide loop before entering the unit. This is not excess tubing that was poorly managed. On a rooftop, the building structure transmits every compressor vibration to the living spaces directly below. This loop absorbs micromovements and prevents the connection from undergoing mechanical fatigue.
We added:
- Closed-cell insulation with increased thickness, protected against UV rays — the main cause of performance loss in rooftop installations is line insulation degraded by the sun after three summers.
- A flexible electrical conduit with a disconnect switch in the immediate vicinity of the unit, compliant with the Quebec Construction Code.
- A condensate drain routed to an existing drain point, without a trap where water could freeze.
The Vacuum, the Step No One Sees
The project's total piping was nearly 22 meters, well within the system's 60 m limit, so the 30 m factory charge covered the entire installation without adding refrigerant.
The system was evacuated down to 500 microns, followed by a 20-minute standing test under an electronic vacuum gauge before the valves were opened. With R32, this step is non-negotiable: residual moisture in an R32 circuit reacts with POE oil and produces acids that destroy the compressor windings within a few seasons. A contractor who “pulls a vacuum for about fifteen minutes by guesswork” is selling you a 10-year warranty they know they cannot honor.
The Indoor Units and the Measured Results
Two wall-mounted indoor units were installed:
- 12,000 BTU in the living room, positioned so the airflow sweeps across the south-facing glass area rather than blowing directly at the occupants.
- 9,000 BTU in the primary bedroom, set to night mode with reduced fan speed.
The nominal total capacity of the indoor units (21,000 BTU) is intentionally lower than that of the outdoor unit. This approach leaves reserve capacity that can be directed to a single zone during peak demand—the bedroom during a heat wave, the living room at -25 °C—without ever pushing the compressor to its limit.
Total project duration: one and a half days, including installing the roof support, drilling and sealing the wall penetrations, running the dedicated electrical circuit, and commissioning.
After the first winter, the homeowner reported a marked reduction in baseboard use, with the baseboards only taking over during the coldest spells. This is exactly the expected behavior of a system with an HSPF2 of 10 in the Montérégie climate.
Grants: What to Check Before Signing
The GREE Freematch R32 is certified by ENERGY STAR and AHRI, making it eligible for financial assistance programs for high-efficiency systems. For a home already heated with electricity, the efficient heat pump component of Hydro-Québec's LogisVert program generally applies.
Three checks to perform systematically:
- The AHRI number for the exact combination installed in your home must appear on the eligibility list—not just the outdoor unit model. For the GWHD(24)ND6MO, reference numbers are provided by configuration (ductless, ducted, or mixed).
- Minimum performance criteria change from year to year. What was eligible last year is not necessarily eligible today.
- The contractor must hold the appropriate RBQ licence, and the form must be submitted within the prescribed timeframe after commissioning.
At AirGreen, we prepare this documentation for our clients and verify eligibility before installation, not afterward.
Our expertise in rooftop installations
This Sainte-Victoire-de-Sorel project is part of a series of rooftop installations we carry out each year in Montreal, Laval, Longueuil, on the North Shore, and on the South Shore: Rosemont plexes where the unit must fit between two skylights, Griffintown condos with condominium restrictions, and Laval commercial buildings where the load must be calculated based on the existing structure.
What these projects have taught us, and what we systematically apply:
- Never use mechanical fasteners in a membrane without the involvement of a roofer.
- Always raise it above the area's historical snow depth.
- Always plan for maintenance access from the support-design stage—a unit that cannot be cleaned is a unit that will lose 20% of its efficiency within five years.
- Always document line lengths, the final charge, and commissioning readings so the 10+2-year warranty can be defended.
If you are considering a rooftop HVAC installation or a multi-zone heat pump for your home, our team assesses structural feasibility, zone configuration, and eligibility for subsidies before even recommending a model. We serve the entire Greater Montreal, North Shore, and South Shore.