When roof access determines the model we install
Before discussing SEER2 or the modulation range, there is a question any serious contractor asks when climbing onto the roof for the first time: how will the equipment get up here, and how will it get back down twelve years from now?
In Saint-Pie, in the Maskoutains RCM, the answer to this question directly changed the model we proposed. Roof access was through a modest-sized interior hatch, with no exterior ladder and no possibility of parking a lift on the alley side. This seemingly purely logistical constraint has very real technical consequences.
The building context
A gravel-covered flat-roof building in the village core, with the typical characteristics of the building stock in this part of Montérégie:
- aging membrane but in good condition, last replaced about ten years ago;
- plumbing vents and ventilation outlets distributed across the surface with no apparent logic;
- wood access hatch in the center of the roof;
- heating by electric baseboards, no forced-air ventilation;
- no ground-level space available for an outdoor unit — tiny backyard, street-facing façade.
The owner wanted to air-condition two zones and reduce heating consumption during the shoulder seasons without undertaking major interior work.
The choice: GREE Freematch R32 GWHD(18)ND6MO
We selected the GWHD(18)ND6MO, the outdoor unit of the Freematch R32 system designed for two indoor units.
Weight as a selection criterion
It is a criterion that technical specifications list last and that online comparison tools completely ignore. Yet it should have been at the top of the list in this case.
| Model | Net weight | Dimensions |
|---|---|---|
| GWHD(18)ND6MO | 35.5 kg | 822 × 550 × 352 mm |
| GWHD(24)ND6MO | 51 kg | 964 × 660 × 402 mm |
Fifteen kilograms and an extra fourteen centimeters of width make no difference on a ground-level patio. They make all the difference when the unit has to pass through a hatch and be carried by two people up a ship's ladder. The 24,000 BTU model would have fit only after partially dismantling the hatch frame. The 18,000 model fit without any modifications.
Since the load calculation showed a peak demand compatible with 18,000 BTU/h for both zones, there was no reason to complicate the job.
Performance data
| Feature | Value |
|---|---|
| Connectable indoor units | 2 |
| Rated cooling capacity | 18,000 BTU/h — range 7,300 to 19,800 BTU/h |
| Rated heating capacity | 18,000 BTU/h — range 7,300 to 18,100 BTU/h |
| SEER2 | 21 |
| HSPF2 | 10 |
| EER2 / COP | 12,5 / 3,9 |
| Guaranteed heating down to | -30 °C |
| Refrigerant | R32 (A2L), 900 g factory charge |
| Sound pressure | 54 dB(A) |
| Airflow | 1,354 ft³/min |
| Electrical | 208/230 V — MCA 14.5 A — maximum protection 20 A |
| Piping | precharge 10 m — maximum 40 m |
| Certifications | ENERGY STAR, AHRI, cold-climate certification |
| Warranty | 10 years + 2 years compressor and parts |
A SEER2 of 21 and an HSPF2 of 10 place this unit in the upper tier of the residential market, and its cold-climate certification guarantees useful heat output down to -30 °C — a non-negotiable requirement for an installation in Montérégie, where January nights in open agricultural areas regularly drop below -25 °C.
Jobsite logistics: what happens before the first connection
Hoisting the equipment
We made three trips: the aluminum support and lumber first, the outdoor unit next, and the tools and lines last. The equipment was strapped and hoisted by two technicians, with a third on the ground stabilizing the ladder.
What a homeowner should require from their contractor before the work:
- A preliminary roof inspection, not just a property inspection. A contractor who bids on a rooftop installation without having been on the roof is bidding blind.
- Written confirmation of the access method—hatch, exterior ladder, or lift. If a lift is required, it is a budget line item, not a surprise on the morning of the work.
- An inspection of the membrane's condition before work begins, documented with photos. This protects both parties.
Position it without blocking what is already there
The roof already had several constraints. Our placement follows three simple rules that we apply everywhere, in Montréal and on the South Shore alike:
- Never block the access hatch. A unit placed in front of the hatch turns every future maintenance job—its own and the roof's—into an acrobatic manoeuvre.
- Never place it in the path of water. On a flat roof, water flows toward one or two drains. A support that blocks this path creates a permanent puddle, and a permanent puddle wears out the membrane.
- Maintain the required intake and discharge clearances from vents, parapets, and other equipment.
The unit rests on an aluminum bracket bolted to treated-wood beams, which are themselves laid flat on the gravel. No screws penetrate the membrane. This is the only method we accept on an existing roof.
The electrical connection visible in the photo
The grey box attached to the support post is the disconnect switch, required within sight of the unit. It is connected by a liquid-tight flexible conduit, the only acceptable type for outdoor use: standard rigid conduit would fatigue under compressor vibrations and eventually crack at the connection.
The circuit is dedicated, protected at 20 A, and sized according to the unit's 14.5 A MCA.
Common mistake: powering a heat pump from an existing circuit shared with another appliance. The breaker will trip on the first startup under load, in the middle of a heat wave, on a Sunday.
The critical point: piping length
The GWHD(18)ND6MO is precharged for only 10 metres of piping. This is the lowest value in the Freematch range, and it is also the one most often overlooked.
On a rooftop installation, the total line length for two heads can easily reach 15 to 20 meters, simply because of the vertical drop. Each meter beyond 10 requires an additional 20 g of R32, weighed on an electronic scale and recorded.
An undercharged system behaves as follows:
- heating efficiency reduced by 10 to 20% in extreme cold, precisely when it is needed;
- excessive compressor superheat, resulting in accelerated wear;
- no error code displayed—the unit works, simply poorly.
This is the most common silent failure we encounter during HVAC maintenance work on installations completed by others.
Commissioning and results
Protocol applied:
- Pressurization with dry nitrogen, with controlled pressure holding.
- Evacuation to 500 microns, with isolation verified using a vacuum gauge.
- Weighing and adding the additional R32 based on the actual measured length.
- Checking superheat and subcooling on each branch.
- Measuring the compressor amperage under load.
- Providing the client with the commissioning sheet and registering the warranty.
Project duration: one and a half days, including transporting the equipment onto the roof.
Our work in Montérégie and Greater Montreal
At AirGreen, we install wall-mounted heat pumps and multi-zone systems in configurations where access affects the outcome just as much as the thermal calculations: plex rooftops in Montreal, buildings without elevators in Longueuil, North Shore residences with a unit in a narrow backyard, and buildings in agricultural villages such as Saint-Pie where the truck has to back down a dirt alley.
The constant: we climb onto the roof before preparing the quote, and we choose the model based on what we saw there—not the other way around.
For an assessment of your heat pump, wall-mounted air conditioner, or rooftop HVAC installation project, our team travels throughout Montérégie, Greater Montreal, the North Shore, and the South Shore.