Two units, one roof: the question no one asks before ordering the equipment
In the photo taken at the end of the project de Saint-Hyacinthe, two GREE outdoor units can be seen aligned on a gravel roof, each on its own aluminum support. They do not belong to the same unit. One serves the Freematch R32 multi-zone system we had just installed for the owner of the ground-floor unit; the other was already serving the upstairs apartment.
This is a common situation in the plexes of Montérégie and the Island of Montreal, and it is also one of the main sources of poorly designed installations. Because a unit that works perfectly on its own can see its performance drop by 15 to 20% simply because a neighbor installed theirs three meters away, on the wrong axis.
The assignment: air-condition two zones without affecting the neighboring unit
The building is a two-unit stacked duplex, a few streets from downtown Saint-Hyacinthe. Flat roof, gravel-covered membrane, access through an interior hatch.
The owner occupied the downstairs unit and wanted to:
- to air-condition the double living room facing the street;
- to cool the rear bedroom, which had poor ventilation and where the summer temperature regularly exceeded 28°C;
- to keep the electric baseboards as supplemental heating rather than replacing them;
- to be able to add a third zone in two or three years, when finishing the basement.
This last point directly determined the choice of equipment.
Why the GREE Freematch R32 GWHD(24)ND6MO instead of the smaller model
We installed the GWHD(24)ND6MO, an outdoor unit that accepts 2 to 3 indoor units, connected here to 2 wall-mounted heads.
An 18,000 BTU system would have been sufficient for the immediate needs. It would also have made adding a third head impossible without completely replacing the outdoor unit — meaning redoing the roof support, refrigerant lines, and electrical circuit. The additional cost of the larger model at the time of installation represented a fraction of the cost of a complete reinstallation three years later.
Outdoor unit data
| Feature | GWHD(24)ND6MO |
|---|---|
| Connectable indoor units | 2 to 3 |
| Rated cooling capacity | 24,000 BTU/h (7,030 W) — range 7,500 to 31,400 BTU/h |
| Rated heating capacity | 24,000 BTU/h — range 7,500 to 31,400 BTU/h |
| SEER2 / HSPF2 | 21 / 10 |
| EER2 | 12 — COP 3.9 |
| Heating down to | -30 °C |
| Refrigerant | R32 (A2L), factory charge 1,700 g |
| Airflow | 2,236 ft³/min — 58 dB(A) |
| Power supply | 208/230 V — MCA 19.5 A — maximum protection 25 A |
| Dimensions / weight | 964 × 660 × 402 mm — 51 kg |
| Piping | 30 m precharge — 60 m maximum — 15 m elevation difference |
| Warranty | 10 years + 2 years compressor and parts |
The unit is ENERGY STAR certified, approved for cold climates, and incorporates the A2L leak detection sensor required for mildly flammable refrigerants—the orange label visible on the front of the unit.
Two outdoor units sharing the same roof
This is where the Saint-Hyacinthe project becomes instructive for anyone who owns a plex or is considering a HVAC installation on a shared roof.
The recirculation phenomenon
An outdoor unit in cooling mode discharges warm air through its front. In heating mode, it discharges cold air. If a second unit draws in this discharged air, it operates with a distorted ambient air temperature—warmer in summer, colder in winter—and its compressor compensates by consuming more energy.
On an unobstructed roof, nothing is visible. The customer only notices that their bill does not decrease as much as promised.
The clearances we systematically apply:
- 60 cm minimum behind the unit (coil intake side);
- 1.5 m minimum in front of the discharge outlet, clear of all obstructions;
- never place two units facing each other, nor one unit blowing toward another unit's intake;
- 30 cm minimum above the roof surface, or higher depending on the area's historical snow accumulation.
In Saint-Hyacinthe, we positioned our unit laterally offset from the existing unit, with both blowing in the same general direction, toward the unobstructed area of the roof. Neither unit draws in air discharged by the other.
The support: aluminum on timbers, without penetration
The assembly visible in the photo:
- Treated wood timbers laid flat on the gravel, distributing the load across the membrane.
- Aluminum support bolted to the timbers; aluminum has the advantage of not rusting—an inexpensive galvanized steel support stains the membrane within five to seven years and becomes a weak point.
- Unit mechanically secured to the support using its original feet, with anti-vibration pads.
No fastener penetrates the membrane. This is an exception-free rule for us: as soon as a screw punctures a roof, the roofer's warranty becomes questionable and infiltration is only a matter of time.
The mistake to avoid: placing the unit directly on gravel. Gravel shifts, the unit tilts, the compressor operates in an improper position, and defrost condensate accumulates beneath the unit instead of draining away.
The electrical aspect, specific to the plex
In a multi-unit building, a heat pump must be powered from the panel serving the dwelling it serves, on a dedicated circuit. Connecting the lower unit to the upper panel because the cable route was shorter is a mistake—and a source of disputes between co-owners or between tenant and landlord.
We therefore ran a dedicated circuit from the ground-floor panel, with a disconnect within sight of the unit, in accordance with the Quebec Electrical Code. 25 A protection, with conductors sized according to the unit's 19.5 A MCA.
Running the lines and commissioning
The refrigerant lines—1/4-inch liquid, 3/8-inch gas—run along the parapet in a UV-protected insulated conduit, then descend through an existing chase to the two wall-mounted heads. Total length: approximately 19 metres, well below the 30 metres factory-precharged for this model. No additional R32 was required, which we confirmed by weighing rather than estimating.
The commissioning procedure applied:
- Pressurization with dry nitrogen and verified hold on both branches.
- Evacuation to 500 microns, with an insulation test using an electronic vacuum gauge.
- Opening the valves and checking superheat and subcooling.
- Measurement of the compressor's actual amperage under load.
- Supply-air temperature readings at each indoor unit.
- Registration of the 10+2-year warranty in the owner's name.
Duration: one and a half days, including assembly of the support and running the electrical circuit.
Grant eligibility
The Freematch R32 is ENERGY STAR certified and has distinct AHRI numbers depending on the installed configuration. For an electrically heated dwelling in Montérégie, the efficient heat pump component of the LogisVert program from Hydro-Québec is generally the applicable option.
A plex has a specific requirement: eligibility is assessed per dwelling, and the electricity account in the applicant's name must correspond to the dwelling being served. A landlord installing a system for a tenant must verify this before submitting an application — this is the most common reason for rejection we see in this type of case.
Minimum performance criteria and grant amounts change from year to year. We verify eligibility when preparing the quotation, never based on documentation from the previous year.
What we have learned from shared roofs
Every year, we install dozens of wall-mounted heat pumps on plex roofs in Montreal, Laval, Longueuil, on the North Shore, the South Shore, and in Montérégie. The lessons are consistent:
- Survey what is already there on the roof before choosing the location, including neighbouring units and plumbing vents.
- Document the selected location with photos and measurements for the next contractor who climbs onto that roof.
- Leave a clear access pathway: a technician who has to step over refrigerant lines to reach a unit will eventually crush one.
- Plan for future changes — a third zone, a finished basement, or a change in the use of a room.
For a multi-zone heat pump, wall-mounted air conditioner, or HVAC maintenance project in a multi-unit residential building, our team surveys the roof before recommending a model, not the other way around.