241 pounds on a roof membrane: installing a central system where there is no ground
In Saint-Pie, the building had no usable yard. A shared wall on one side, service access on the other, and no ground-level area capable of accommodating an outdoor unit with the required clearances. The only available surface was the flat roof.
This is a situation we spontaneously associate with the Plateau-Mont-Royal or the commercial arteries of Longueuil, not with a village core in the MRC des Maskoutains. Yet it occurs wherever a building occupies its entire lot.
The selected system is a 4-ton GREE UNIX R32 central heat pump, with outdoor unit GUD60W2/NhE-D(U) paired with the GUD48AH2/G-D(U) air handler cabinet. Here are the technical decisions this installation required, along with the indoor configuration choice that had the greatest effect on the final performance.
The installed system
- Outdoor unit: GUD60W2/NhE-D(U)—two fans
- Indoor air handler cabinet: GUD48AH2/G-D(U)
- Cooling capacity: 48,000 BTU/h, modulation from 34,000 to 50,000 BTU/h
- Heating capacity: 48,000 BTU/h, modulation from 34,000 to 52,000 BTU/h
- SEER 2: 19.0—the best value in the UNIX R32 series
- HSPF 2: 10.0 — EER 2: 12.0 — COP at 8 °C: 3.40
- AHRI code: 217120761
- Airflow: 2,040 m³/h (1,200 CFM) at a nominal external static pressure of 125 Pa (0.5 in. w.c.)
- Indoor sound level: 52 dB(A) — Outdoor: 62 dB(A)
- R32 refrigerant, charge of 4.6 kg (162.3 oz)
- Heating range: -30 °C to 24 °C — Cooling range: -15 °C to 54 °C
- 208-230 V / 60 Hz / 1 phase: outdoor MCA 39.9 A / MOP 45 A, indoor MCA 7.1 A / MOP 15 A
- Outdoor unit: 900 x 1,260 x 340 mm, 109.5 kg (241.4 lb)
- Two-stage, two-cylinder compressor, DC Inverter technology
- Refrigerant leak detector on all cabinets
- Compatible with 24 VAC thermostats and RS485 communication
Why the roof—and what it requires
A flat roof is not elevated ground. It is a watertight assembly designed to support a distributed load, not a concentrated load on four points.
Distribute the load without piercing the membrane
The unit weighs 241.4 lb. Resting on the four feet of a frame, this weight is concentrated on a few square inches of membrane. Add the fan's dynamic loads and accumulated snow, and the bearing point becomes a failure point over the medium term.
The chosen solution: the frame rests on load-distribution timbers laid flat on the membrane, perpendicular to the feet, so that the load is spread over a much larger surface and across several joists. No anchor penetrates the membrane.
This is the rule governing the entire project: a flat roof is watertight as long as it is not penetrated. Every penetration, even one perfectly sealed the same day, becomes a point requiring monitoring for the next twenty years. On a building whose roof is still under warranty, an unauthorized penetration may also void the roofing contractor's coverage.
Defrost water on a roof
In heating mode, the unit discharges water during every defrost cycle. On the ground, this water percolates away. On a flat roof, it seeks the drain.
Three checks are essential:
- The direction of the slope. The unit must not be located in a depression or block the flow of water toward the drain.
- The distance to the drain. Water traveling several meters across a cold membrane in January freezes before it arrives. We position the unit to shorten this path.
- The drain itself. A partially obstructed roof drain becomes noticeable in July during heavy rain. In February, with a unit that continuously adds water, it becomes noticeable in a different way.
Snow, wind, and the parapet
The parapet visible around the surface serves two opposing purposes. It protects against crosswinds, which is beneficial. It also creates a snow-accumulation zone, which is much less beneficial.
On a flat roof, snow does not slide off: it accumulates and compacts against obstacles. The elevated frame on timbers places the coil above the usual accumulation level, and we maintain sufficient clearance between the cabinet and the parapet so that discharged air does not flow back toward the coil.
Winds at altitude are also stronger than at ground level. The frame's stability and resistance to uplift are part of the checks before commissioning.
Vibrations in the structure
A ground-mounted unit transmits its vibrations to the earth, which absorbs them. A rooftop unit transmits them to the building's structure, and therefore to the rooms below.
Vibration-damping pads between the cabinet and the frame, a flexible loop in the refrigerant line, and a flexible electrical conduit interrupt most of this transmission. In a building where people sleep beneath the roof, this step is not optional.
Service access
A rooftop unit must remain accessible in February, not just in June. We check access — hatch, fixed ladder, or stairs — before even proposing this installation. A unit that cannot be reached in winter is a unit that will not be serviced.
Getting the unit up there
Lifting a 241 lb cabinet onto a roof is not a last-minute improvisation. It requires a lifting plan, an anchor point or suitable lifting equipment, and a sufficiently large crew. We plan this step in advance, with the equipment reserved, rather than treating it as an unexpected issue on the morning of the installation.
The project's real choice: air handler or cased coil
The UNIX R32 series offers three installation options, and that's where the project's real value lies:
- A) a complete GREE system, condenser and air handler;
- B) a retrofit with an indoor unit from another brand;
- C) a retrofit on an existing furnace with a cased coil.
Options B and C reduce labor and equipment costs. That's true, and it's often what the customer hears first. What few contractors present is the energy cost of that choice.
The figures side by side for the same 4-ton capacity
| Air handler | Cased coil | |
|---|---|---|
| Indoor unit | GUD48AH2/G-D(U) | GCAC48H/NhA |
| AHRI code | 217120761 | 216768110 |
| SEER 2 | 19,0 | 15,2 |
| HSPF 2 | 10,0 | 8,5 |
| EER 2 | 12,0 | 10,0 |
| COP at 8 °C | 3,40 | 3,20 |
| Airflow | 1,200 CFM | 1,100 CFM |
The difference is clear: 19.0 versus 15.2 in SEER 2, and 10.0 versus 8.5 in HSPF 2. In other words, the same outdoor unit, the same refrigerant, the same rated capacity — yet seasonal heating efficiency is approximately 15% lower with the retrofit solution.
Our decision rule
A retrofit with a cased coil remains the right choice in many situations:
- A newer, well-maintained furnace that the owner wants to keep as supplemental heat.
- A configuration in which replacing the cabinet would require major sheet-metal work.
- The budget does not allow for the complete system, and the real alternative is to do nothing.
A complete air handler is the right choice when:
- The existing unit is nearing the end of its service life anyway.
- Heating dominates the annual energy bill, as it does for the vast majority of Quebec homes.
- The project is designed to maximize eligibility for performance-based incentive programs.
In Saint-Pie, the existing indoor unit was nearing the end of its service life. The calculation was simple: pay once for the complete cabinet rather than accept fifteen years of lower efficiency.
Ultra Heat: what the specifications really say
The UNIX R32 series claims 100% heating capacity at -20 °C and stable operation down to -30 °C.
This wording deserves careful attention because it is more precise than the usual slogans. It does not say that the unit produces 48,000 BTU/h at -30 °C. It says that it maintains its rated capacity down to -20 °C, then continues operating steadily down to -30 °C at reduced capacity.
For Montérégie, this means in practical terms that electric backup heat is needed only during the winter's most severe cold snaps, not as soon as temperatures reach -12 °C, as with an earlier generation of central units.
The two-stage compressor and communication
The two-stage, twin-cylinder compressor combined with DC Inverter technology provides a useful modulation range of 34,000 to 52,000 BTU/h in heating mode. The unit adjusts to actual demand rather than cycling on and off.
On the controls side, the series accepts a standard 24 VAC wired thermostat—which considerably simplifies a retrofit—and also supports RS485 communication for installations requiring more advanced integration.
The refrigerant line on a rooftop run
The manufacturer's limits: 3/4-in. suction line, 3/8-in. liquid line, standard length 7.5 m (24.6 ft), maximum length 30 m (98.4 ft), and maximum elevation 15 m (49.2 ft).
This is the value that matters in a high-rise building. The elevation difference between the indoor unit and the rooftop unit must be calculated and documented, not estimated by eye from the sidewalk.
The horizontal run across the membrane is protected by a rigid, elevated conduit so it does not obstruct rainwater drainage or remain in standing water. Both lines are insulated along their entire length, and the passage through the parapet or wall is sealed according to building-envelope waterproofing requirements.
Mistakes to avoid when installing on a roof
- Puncturing the membrane to anchor a frame without coordinating with the roofer.
- Installing the frame directly on the membrane without load distribution.
- Blocking the path of water to the roof drain.
- Neglecting vibration isolators on an occupied structure.
- Placing the unit in a parapet corner, where snow accumulates and air recirculates.
- Running the line flat through standing water.
- Confirming the installation without having verified winter access.
Rebates
The GUD60W2/NhE-D(U) + GUD48AH2/G-D(U) combination, under AHRI code 217120761, delivers the series' best efficiency ratings: SEER 2 of 19.0 and HSPF 2 of 10.0.
This point deserves emphasis in the context of Quebec assistance programs, which award their amounts based on certified performance and heating capacity. A retrofit configuration using a cased coil, with its HSPF 2 of 8.5, may not qualify in the same way—or at all, depending on the program and the year.
This is an additional reason to settle the question of the air handler before preparing a budget. We verify the eligibility of the exact pairing and its inclusion on the equipment lists before ordering, and we compile the file using the AHRI code for the pair actually installed.
What Saint-Pie adds to our practice
Two lessons stand out from this project, and they apply far beyond a flat roof.
The first: the absence of land does not rule out a central system. It requires a different approach—load distribution, water management, and service access—but it does not close any doors.
The second: the choice of indoor unit has a greater impact on the bill than the choice of brand. The same UNIX R32 condenser, depending on whether it is paired with an air handler or a cased coil, delivers an HSPF 2 of 10.0 or 8.5. This difference is not visible in any installation photo. It shows up over fifteen years of Hydro-Québec bills.
Our coverage in Montérégie and Greater Montreal
At AirGreen, we design, install, and service central heat pumps, wall-mounted heat pumps, air conditioners, VRF systems, and air exchangers in Montreal, Laval, Longueuil, on the North Shore, on the South Shore, and throughout Montérégie, including Saint-Pie, Saint-Damase, Sainte-Cécile-de-Milton, Saint-Dominique, and Saint-Hyacinthe.
Rooftop installations are part of our everyday work on urban buildings in Greater Montreal. The same methods apply to a village building whose lot is completely built up.
Have your building assessed, whether or not you have land
If you ruled out a central heat pump because there is no room on the ground, the question deserves to be revisited. A roof, structured balcony, load-bearing wall, or service yard: there are more possible installation locations than it may seem, provided they are assessed by someone who understands building-envelope constraints as well as HVAC requirements.
Contact AirGreen for a building assessment, load calculation, and detailed quote for a GREE UNIX R32 system. We will present the possible configurations with their performance figures, not just their prices.