Installation of a GREE FreeMatch R32 2-head heat pump (GWHD(42)ND6MO) on a flat roof in Rivière-des-Prairies–Pointe-aux-Trembles, Montréal

Two indoor units on a 42,000 BTU/h compressor: what the calculation says, and what the rule of thumb will never tell you

The customer called us with two quotes in hand and a specific question: why was one contractor proposing a 24,000 BTU/h dual-zone system and the other a 36,000 BTU/h dual-zone system for the exact same house in Pointe-aux-Trembles? Both had visited. Both had measured the square footage. Only one had opened the windows, looked at the orientation, counted the doors closed at night, and asked what actually heated the house in January.

We recalculated it. The result satisfied neither party: the house required two high-capacity zones and a compressor larger than either proposal, namely a GWHD(42)ND6MO from the GREE FreeMatch R32 line. This is the first time we have used this model with only two indoor units, and the reasoning deserves to be laid out in full.

The Rule of Thumb Gets It Wrong Both Ways

The most common shortcut in Quebec converts square feet into BTUs. It is quick, and it is wrong in both directions.

Square footage ignores the actual volume—a postwar flat-roof house often has low ceilings but open areas spanning its entire width—as well as the age of the building envelope, window area and orientation, air tightness, internal gains, and, above all, what is expected of the zone. A room can be easy to cool and difficult to heat. It is not the same number.

Both mistakes are costly, but not at the same time:

• Undersizing does not show in July. It shows in January, when the heat pump can no longer maintain the setpoint, the baseboard heaters take over, and the customer concludes that the unit is defective even though it is doing exactly what it was paid to do.
• Oversizing does not show in January. It shows in July: short cycles, cold but humid air, and a fan that keeps restarting. With a multi-split, it is even worse, because the minimum capacity is a property of the system, not of each indoor unit—a mechanism explained in detail in our Saint-Chrysostome article.

Third shortcut to avoid: sizing based on the unit being replaced. The capacity of an old air conditioner is not a design input; it is the legacy of a decision that no one documented. Our Anjou article tells the story of a replacement where the proper calculation led to installing fewer BTUs than before.

What We Look At Before Naming a Model

Before writing down a single product reference, we establish a room-by-room heat-loss and heat-gain calculation. For this house:

• Envelope and construction era: postwar construction, with walls reinsulated during a renovation and windows replaced at different times depending on the façade.
• Glazing and orientation: the front façade is extensively glazed, with direct afternoon sunlight across the entire open area of the main floor.
• Volume and partitioning: the main floor is open from the living room to the kitchen; the upper floor is divided around a central landing, with doors closed at night.
• Exposure: a corner house, relatively unprotected, in an area where wind moves freely between buildings.
• Existing heat sources: electric baseboards retained as backup, with no air ducts and therefore no possible ducted distribution.
• Occupancy: two adults working from home, with the main floor in use all day.

These data—not the floor area—led to two high-demand zones rather than four medium-demand zones.

The selected system: 24,000 + 18,000 BTU/h

The installed system is a GWHD(42)ND6MO with one 24,000 BTU/h wall-mounted head on the open main floor and one 18,000 BTU/h wall-mounted head on the upstairs landing, for 42,000 BTU/h installed on a nominal 42,000 BTU/h cooling capacity: a 100% ratio, and 95% of the nominal 44,300 BTU/h heating capacity.

With two zones, the diversity factor—the idea that all heads never draw their maximum at the same time—loses almost all of its statistical basis. With four or five zones, you can rely on demand not coinciding. With two zones occupied during the same hours, the worst case is also the normal case; we had demonstrated this in Vaudreuil-sur-le-Lac. So we refused to count on it and kept a 100% ratio, with no diversity margin.

That left us choosing between the GWHD(36) and the GWHD(42), which share the same cabinet, the same airflow of 3,413 ft³/min, the same sound level of 63 dB(A), the same charge of 2,700 g of R32, the same MCA of 30 A, and the same MOCP of 45 A, differing by only half a kilogram. Three differences settled the decision:

1. Instantaneous efficiency is better on the 42k, contrary to intuition: EER 3.81 and COP 3.9, compared with 3.52 and 3.56 on the GWHD(36). Here, the larger model is not the less efficient one.
2. More flexible piping limits: 50 m of precharge and 100 m of total length, compared with 40 m and 80 m. On a rooftop installation, that extra capacity is not merely decorative.
3. The ports: the GWHD(42) supports 2 to 5 indoor units. We use two.

What this choice costs, frankly: 79 kg to lift, dimensions of 1,020 × 826 × 427 mm, 63 dB(A), an MOCP of 45 A that could have forced a panel upgrade in a less well-served house—as in our articles on Napierville and Saint-Édouard—and a modulation floor of 8,870 BTU/h shared by only two zones. In the shoulder seasons, a single head calling for heat places the system near its minimum. The instruction given to the client is therefore to keep both zones active at moderate setpoints rather than heavily relying on just one, following the same logic as in Montréal-Est.

Finally, SEER2 21 and HSPF2 10 are identical across all five models in the FreeMatch range. The seasonal ratings do not distinguish them. The real differences lie in the EER, COP, and the “min. – max.” line, whose interpretation is developed in our articles on Dorval and Baie-D'Urfé.

Selected technical data sheet

Total line lengths: 21 m and 17 m, for 38 m total, within the 50 m factory charge. No R32 was therefore added or removed, and the A2L minimum room-size calculation remained based on the factory charge. Approximately 4 m of elevation difference, well below the 25 m limit.

Only two heads, one of them 24,000 BTU/h: what this requires from the building

The large wall-mounted head and the wall that must support it

A 24,000 BTU/h head is not a 9,000 BTU/h head turned up higher. It is a different object, and the HVAC literature rarely discusses it.

• The wall section. A unit in this class is considerably longer. The geometry that accommodates a small head above a door does not necessarily accommodate it: we measure the clear section between the nearest obstruction on the left and the one on the right, not the width of the wall.
• The anchoring. More mass, more leverage, and a unit that vibrates slightly every time a fan starts. The mounting plate must be secured to the actual structure at several points, not to drywall with toggle anchors. In this house, the stud locations shifted the head a few centimeters from the initial drawing; the mounting comes first, and the aesthetics adjust.
• Reach. A large unit projects air a long way. In a wide but shallow room, the air jet hits the opposite wall and drops too soon; in an open-plan ground floor extending lengthwise, that is exactly what we want. The axis was therefore chosen along the longest dimension of the open area, including the kitchen.
• Air noise. At high speed, a head of this size can be heard. In a living room, airflow—not the compressor—determines acoustic comfort; commissioning set a lower cruising speed, with maximum speed still available for recovery.
• The drain. The condensate outlet side is determined by the line-set outlet, and on a large cabinet the mistake costs more: correcting the slope on a unit of this size means reworking the wall.

When a wall cannot meet these five points, the right answer is not to force it: it is two smaller heads, or a console-type unit, and a recalculated load.

The roof: what we will not rewrite here

The outdoor unit is installed on the building's flat roof. We will not paraphrase our own pages on the subject.

Our article on Dollard-des-Ormeaux covers the assembly: we never pierce a membrane to anchor equipment; the assembly is ballasted on sleepers with an abrasion barrier, the load is distributed well beyond the equipment footprint, the parapet is crossed from above with a drip loop, exposed insulation receives a continuous UV-protective sleeve, and the age of the membrane must be requested when the quote is prepared, not afterward. The one on Plateau-Mont-Royal reminds us that written authorization must name the roof and the support method. The one on Pierrefonds-Roxboro discusses support aging: galvanic corrosion between the aluminum frame and steel fasteners, equipment that must remain level so the base pan drains, and vibration pads that harden. The one on Pointe-Claire covers the roof's mechanical surroundings—vents, hoods, chimneys—and the disconnect mounted to the support when there is no wall. For exterior wiring in general, our article on Côte Saint-Luc covers the subject comprehensively.

For this project, let us remember only this: the indoor units are powered by the outdoor unit, so there is only one circuit to run to the roof for the entire system. The panel load calculation belongs to the master electrician; our contribution is the manufacturer's data, MCA 30 A / MOCP 45 A.

Lifting, itemized in the quote rather than diluted into the total

This is the price difference that customers understand the least and entrepreneurs explain the least.

On the ground, you unload a unit from a truck and set it down. On a roof, you have to lift 79 kg of cabinet, plus the support, the sleepers, the tools, and the piping materials, and bring down the old equipment if there was one. Three scenarios are possible, and they don't cost the same:

1. Roof hatch or interior staircase: the least expensive when the layout allows it, but the most demanding for protecting the premises—the unit passes through the finished house.
2. Ladder and two-person handling: possible for small equipment, never for the cabinet.
3. Boom truck: the fastest and safest option, but it requires clear street access, sometimes a temporary occupation of public property, and a time window.

We price this item separately and explain it. A roof-installation quote that shows the same labor price as a ground-level quote hasn't looked at the building. And the question arises again later: when the membrane is replaced, the unit will have to come down, which means recovering the refrigerant, removing the unit, storing it, reinstalling it, and recommissioning it. That's also why the roof's age is one of our quoting questions.

What the jobsite photo shows

The ivory outdoor unit rests on an aluminum frame with adjustable feet, which is itself set on wood sleepers distributed across the roof, with a gravel area visible in the background. The insulated, jacketed refrigerant line can be seen running flat along the sleepers before reaching its drop point, along with the gray disconnect switch downstream and the blue-tinted finned coil behind its protective grille. The manifold gauge is connected to the service valves: the photo was taken during commissioning, not afterward. The plastic tray placed on top of the cabinet is a jobsite container, removed at the end of the day—we mention it because nothing should be left on an appliance, whether a tool, tarp, or container, for the reason explained in our Côte Saint-Luc article about bent fins.

Nearby are neighboring flat roofs and a garage: that's precisely why the roof was chosen instead of the yard. On these lots in the eastern part of the island, usable side-yard space is narrow and is needed for snow removal.

The mistakes we see most often on this type of project

• Sizing by floor area, then blaming the unit in January.
• Using the removed unit's capacity as the starting point.
• Mounting a large wall unit in drywall because the stud wasn't in the right place.
• Penetrating a membrane to anchor a support and jeopardizing the roofer's warranty.
• Install the unit above a bedroom, then discover the vibration in February.
• Leave a “lifting to be confirmed” or “electrical work to be confirmed” line in a rooftop quote, when those two items are precisely what determine the price difference.
• Cover the outdoor unit for the winter: it has never protected anything and prevents the unit from breathing.
• Assume that a multi-split system can heat one area and cool another simultaneously: there is a single refrigerant circuit and a single reversing valve, so only one mode can operate at a time.

Warranty, financial assistance, and documents provided

The GREE warranty is 10 years, extended to 12 years upon registration with proof of installation submitted to the distributor. We complete this registration with the customer at the end of the project: the difference costs nothing, and an unregistered warranty remains a ten-year warranty, which is still good but unnecessarily short.

An eligible system may qualify for current residential programs, including Hydro-Québec's LogisVert and Rénoclimat. The Chauffez vert program does not apply here because the existing heating is already electric. Conditions and amounts vary by program and program year: we verify eligibility when preparing the quote rather than announcing a figure. The AHRI codes for the combination actually installed (214931590 / 214931596 / 214931600, depending on the configuration) are included in the file because it is the certified combination, not the outdoor unit alone, that is recognized.

The end-of-project file provided to the customer includes the models and serial numbers by room, developed line lengths, proof of commissioning, the circuit number at the electrical panel, the RBQ licence, and the invoice.

Our service areas

At AirGreen, we design and carry out installations of wall-mounted heat pumps, wall-mounted air conditioners, and multi-split systems throughout Montreal, including the borough of Rivière-des-Prairies–Pointe-aux-Trembles — 51.26 km², home to more than 117,000 residents, bordered by the Rivière des Prairies and the St. Lawrence River, with 135 parks and green spaces, two nature parks, and a heritage mill dating back to 1719 — as well as in Laval, Longueuil, on the North Shore, and on the South Shore. Every project begins with a calculation, a technical visit, and a written quote, and ends with documented commissioning and scheduled HVAC maintenance.