Installation of a GREE FreeMatch R32 two-head heat pump in McMasterville (La Vallée-du-Richelieu Regional County Municipality)

Two outdoor units on the same wall: what the second machine does to the first

McMasterville covers 3.1 km². It is a town born from a factory: the Hamilton Powder Company purchased the land in 1874, began producing nitroglycerin and dynamite there in 1879, and the territory occupied by the company was separated from Belœil on July 31, 1917 to form a village named after William McMaster (1851–1930), the first president of Canadian Explosives. The factory—later CIL—produced TNT at a rate of one million eight hundred thousand pounds per month at the end of the First World War, followed by fertilizers, plastics, and pharmaceutical-grade nitroglycerin. It closed in February 1999, and its 62-meter tower came down in December 2000. A memorial park, a TNT-crystal-shaped monument, and a dense town where houses stand close together are all that remain.

This density explains the photo of this project. In a paved side corridor, a few feet wide, two outdoor units are mounted on wall brackets, one behind the other, on the same facade. The closer one is the new multizone heat pump—the four white-sheathed refrigerant lines can be seen curving toward the wall. The farther one is an older system that is still in place. Two electrical disconnects. And opposite them, about one and a half meters away, a row of mature cedar trees that closes off the corridor along its entire length.

This is an extremely common configuration in Montérégie, yet it is the one about which the least technical literature exists. Let's fill that gap.

The project at a glance

The second machine is not an accessory to the first

Here is the principle to remember, and it fits in one sentence: two outdoor units aligned in a narrow corridor are not two independent installations; they are two stages of the same airflow circuit.

On this GREE family, the unit blows horizontally from the front and draws air in from the rear and sides. Place two cabinets in a row in a passage closed off by a wall on one side and a hedge on the other, and the air discharged by one becomes part of the air available to the other. No one designs this intentionally. It happens because the second unit arrived years after the first, and the available space was there.

The consequences are measurable but invisible:

• In cooling mode, the downstream unit receives air that has already been heated. Its condensing pressure rises, its efficiency drops, and its consumption increases. No error code appears: the unit does exactly what it is asked to do, simply less effectively.
• In heating mode, the error reverses and worsens. Both units draw their heat from the same volume of air and discharge it colder than when it entered. A poorly ventilated corridor becomes a cold sink. The downstream coil operates with artificially cooled air, which brings the freezing threshold forward and multiplies defrost cycles. This is the mechanism we had demonstrated in Ahuntsic-Cartierville for a single unit; with two, it compounds.
• The asymmetry is permanent. It is not one unit out of two that suffers: it is always the same one.

What we check before adding a unit to an already occupied façade

1. Which unit is upstream in the airflow, and whether the order can be reversed. The new unit is not necessarily the one that is easiest to place: sometimes it is the one that should be placed first in the airflow.
2. Whether the discharge axes can be offset rather than aligned—staggering the mounting heights or orienting the faces makes a greater difference than moving them a few inches closer together or farther apart.
3. The clear space behind and beside each cabinet, separately. The intake side is what matters, not the front.
4. Renewal of the corridor itself: two open ends, with nothing that closes them afterward.
5. The spacing between the two units, as specified in the manufacturer's installation manual—and nowhere else. We do not publish a distance: it belongs in the manual for the specific model, and any contractor who quotes one from memory is improvising.

Two units, two disconnects, two circuits

The photo shows two disconnect enclosures, and that is the correct answer. Each outdoor unit has its own means of disconnect, visible and accessible from its position, and its own circuit sized according to its own ratings. For this GWHD(24), that is MCA 19.5 A and MOCP 25 A—figures that belong in the master electrician’s calculations, not in an estimate.

Two systems on the same house also mean two loads added together at the panel. The question of available capacity arises before ordering the unit, not on installation morning. Saint-Jérôme publishes our method on this point, and Montréal-Est explains why MCA and MOCP are not the same as the rated amperage.

One practical point we document systematically: which of the two disconnect switches controls which of the two units. Labeled, dated, and recorded in the file. Ten years from now, a technician who shuts off the wrong circuit loses an hour—or cuts power to a running machine.

Two units, but only one noise threshold

This is where McMasterville stands out, and it must be read carefully.

The regulation 346-00-98 on noise nuisances specifically names, in its section 3, “a heat pump, an air-conditioning unit, a ventilation unit, a refrigeration unit”, with two thresholds: 60 dB(A) between 7 a.m. and 11 p.m., and 50 dB(A) between 11 p.m. and 7 a.m. Fines range from $100 to $4,000 depending on the offender and whether it is a repeat offense.

Fifty decibels at night is the strictest threshold we have encountered in the municipalities covered by this series—five decibels below Saint-Jean-sur-Richelieu’s, and it applies from 11 p.m. to 7 a.m., precisely the hours when a heat pump operates for heating.

Two clarifications, and they point in opposite directions.

The first protects the owner. The GWHD(24) technical data sheet lists 58 dB(A), and it would be wrong to draw any conclusion about compliance from that: these are two different quantities. The manufacturer’s value is sound pressure measured at a short distance under test conditions, with the fan at maximum. The regulatory threshold concerns noise perceived in the neighborhood, in the real world, with the distance and obstacles that exist. We developed this distinction on our Saint-Jean-sur-Richelieu page; it applies in full here.

The second goes the other way, and it is the one no one mentions: the regulation does not grant you a separate threshold per unit. Two machines running at the same time, at night, in the same corridor, constitute a single situation to be assessed. The fact that each one, considered separately, would be beyond reproach is not a defence. That is why, on a façade that already has a unit, the nighttime silent mode of both systems is configured together at commissioning—not just that of the new one.

We will not elaborate further on acoustic physics: Huntingdon covers outdoor noise, and Saint-Chrysostome covers noise perceived indoors.

The unit that was already there has an age

This is a matter for the quotation, not a job-site discovery: how old is the existing system, and who maintains it?

Three practical reasons:

• An older unit will be replaced before the new one. If removing it requires dismantling the support or crossing the new unit's refrigerant piping route, that must be planned now, not six years from now.
• It probably does not use the same refrigerant. An older system is likely using R-410A, while the new one uses R-32 (A2L). Two fluids, two sets of cylinders, two procedures, and a built-in leak detector on the new unit that the old one does not have. Our Saint-Césaire page explains what switching to R-32 actually changes.
• Two units do not necessarily mean a single maintenance contract. The customer must know, in writing, what our mandate covers and what it does not. An orphaned unit on a façade is a unit that no one cleans.

A hedge is not a fence

The other wall of this corridor is alive. That changes everything, and it is regularly treated as a landscaping detail.

A cedar hedge advances

A fence stays where it was installed. A mature hedge gains a few centimetres each year, in both dimensions, and it does so on the side where there is light and space—that is, toward the corridor. The clearance measured with a tape measure on the day of installation is not the clearance the unit will have in five years.

So our rule is simple: rear clearance is measured by allowing for growth, not the current state. We established it in Côte-des-Neiges–Notre-Dame-de-Grâce for a solid fence, and Saint-Valérien-de-Milton extended it to plantings. A hedge that gradually closes off the intake side produces exactly the same loss of performance as an enclosed cabinet—only more slowly, and therefore more difficult to attribute.

What a hedge does that a fence does not

• It sheds needles. A thuja releases them all year round, and they stick to the coil that draws in air. This is gradual fouling that cannot be seen from the house.
• It produces pollen and seeds in spring, which are drawn into the fins.
• It reacts to discharged air. A plant placed in front of the discharge side is exposed to a continuous, dry airflow; browning on only one side of a hedge, at the height of the unit, is not a disease.
• It has roots. The root system of a mature hedge occupies the first few centimetres of soil across a width that extends well beyond its foliage—this is the constraint we documented in Rosemère. This is less of a concern for a wall-mounted installation, but much more so for a paved corridor, which can heave.
• Its density changes in winter. A conifer remains opaque all year round: it therefore blocks the wind in January as well as in July, and retains snow.

The paved corridor, water, and access

The paving visible in the photo is a surface that does not absorb water. Defrost water dripping from a suspended unit spreads over it, freezes, and ends up on a walkway that someone will use in winter. The water's path must be determined before drilling, and nothing—neither a drain pan, tray, nor container—should be placed under a cabinet to “collect” it: that merely builds a column of ice.

Finally, a question to ask before installing: will the unit be able to be removed? Between a wall and a hedge, the usable width is that of the narrowest point, not that of the average corridor. A unit that can only be removed by severely cutting back a thirty-year-old hedge becomes expensive by the twelfth winter. That is the Villeray rule, and a vegetated corridor makes it more restrictive than a fence barrier because a hedge cannot be dismantled.

Permits and regulations in McMasterville: what is checked, and what is not

Two established facts, cited from the city's regulations.

An authorization certificate is required. Regulation 387 on permits and certificates lists, in section 2.11, the work requiring an authorization certificate issued by the designated official, and this list expressly includes “installing a heat pump”. This is neither a formality nor an option.

The front yard is prohibited, without exception. Zoning By-law 382, in section 4.19, states:

“The following are specifically prohibited: meters, satellite dishes, radio, television, communication or other antennas, air-conditioning and heat-exchange equipment, heating-oil tanks, clotheslines, gas cylinders, and panels and installations that collect all or part of solar energy.”

This is a clear prohibition, and it explains much of the side-yard placement seen in this town: when the front yard is excluded by by-law and the lot is only a few dozen metres deep, the side is what remains.

However, we will not publish any setback in metres, maximum height, or screening requirement for a heat pump in McMasterville. Chapter 5 of the zoning by-law, which deals with accessory structures and equipment, is truncated in the available consultation; a specific rule may be found there. Pay particular attention to a figure that is circulating: the 1.5 metres in section 4.21 applies to pool filters and pool heating equipment, not heat pumps. Confusing the two means publishing a rule that does not exist.

The reliable route: Urban Planning and Sustainable Development Services, 255 Constable Blvd., 450 467-3580.

The selected unit

Two 15,000 BTU/h wall-mounted heads, totaling 30,000 BTU/h on a 24,000 BTU/h rated system — 125%. This oversizing is the very principle of a multi-zone system: the two zones never demand their maximum at the same time, and the compressor distributes the load. What matters at the other end of the range is the 7,500 BTU/h modulation floor, which allows a single head to operate alone without short cycling.

The GWHD(24) accommodates two to three heads. In a house where a system already exists, this range deserves careful consideration when preparing the quote: adding a third zone later is not a checkbox—it means reopening the wall, checking the load, and recommissioning the system. Our Sainte-Hélène-de-Bagot page explains why.

Mistakes to avoid

• Aligning two cabinets in a corridor without asking which one is drawing in the other's air.
• Treating the new unit as a project independent of the one already mounted on the wall.
• Believing that the noise threshold applies to each machine individually.
• Comparing the technical sheet's 58 dB(A) with the regulation's 50 dB(A).
• Measuring clearance from a hedge without allowing for its growth.
• Confusing the 1.5 m clearance for pool equipment with a heat pump setback.
• Considering the front yard: it is prohibited by section 4.19.
• Installing without verifying the corridor's usable width for the unit's future removal.

Rebates and service area

These units are eligible for local rebates, and the AHRI codes above are required by the programs. We support every application for LogisVert, Rénoclimat and Chauffez vert, recording the codes in the project-completion file without promising amounts whose eligibility schedules are beyond our control.

In the La Vallée-du-Richelieu RCM, we have already documented an installation in Belœil, from which McMasterville separated in 1917. Mont-Saint-Hilaire, Otterburn Park, Saint-Basile-le-Grand, Chambly, Carignan and Saint-Mathieu-de-Belœil complete the service area on this side of the Richelieu. Our teams cover Montréal, Laval, Longueuil, the North Shore and the South Shore.

Why AirGreen

A façade that already houses a machine is not a blank wall. At AirGreen, we look at what is already there before proposing what will be added: airflow direction, the age of the existing unit, panel capacity, the actual width of the passage, and what the hedge opposite will look like in five years. We quote regulations without interpreting them, call the planning department when the wording is unclear, and refuse to publish a figure we cannot substantiate.

For an assessment in McMasterville, write to us: we measure before we propose.