Four heads, one compressor: the calculation almost no one shows the customer
Saint-Mathieu-de-Beloeil is a 40 km² municipality with approximately 3,000 residents and some 1,225 homes. The area is predominantly agricultural, separated from the village of Beloeil since 1903, crossed by Highway 20 since 1964, and home to its own municipal airport. Its building stock is therefore distinctive: 19th-century stone houses stand alongside recent construction, and many of these buildings have never had a duct network. Without ducts, a central heat pump is out of the question without major work. This is the natural setting for a wall-mounted multi-zone system.
At this site, we installed a four-head GREE FREEMATCH R32 system. The photo shows one of them, mounted high on a wall, just below the ceiling, above a doorway, with the refrigerant line exiting the wall through an opening that was still unfinished when the photo was taken.
This case study explains the most misunderstood aspect of multi-zone systems: four heads do not make four heat pumps. They make one machine, with one compressor, one refrigerant circuit, and one shared modulation range. Everything else follows from that.
What we installed in Saint-Mathieu-de-Beloeil
The GWHD(36)ND6MO outdoor unit
- Model: GWHD(36)ND6MO, FREEMATCH R32 series
- Number of connectable heads: 2 to 4
- Rated cooling capacity: 10,550 W, or 36,000 BTU/h, modulating from 2,600 to 12,000 W (8,870 to 40,900 BTU/h)
- Rated heating capacity: 10,550 W, or 36,000 BTU/h, modulating from 2,600 to 15,000 W (8,870 to 51,180 BTU/h)
- SEER 2: 21 — HSPF 2: 10 — EER 2: 12 — COP: 3.56 W/W
- Operating range: heating from -30 °C to 24 °C, cooling from -30 °C to 48 °C
- Power supply: 208/230 V — 30 A MCA, 45 A maximum protection
- Refrigerant : R32, factory charge of 2,700 g (95.3 oz)
- Lines: liquid ø 1/4 in, gas ø 3/8 in — 40 m (131 ft) precharged length, 80 m (262 ft) maximum total length, 25 m (82 ft) maximum elevation difference, 20 g/m (0.2 oz/ft) additional charge beyond the precharge
- Dimensions: 1,020 × 826 × 427 mm (40-1/8 × 32-1/2 × 17-5/16 in) — 78.5 kg (173.1 lb)
- Sound level: 63 dB(A)
- AHRI codes: 214931589 in an all-wall-mounted configuration, 214931595 in an all-ducted configuration, 214931599 in a mixed configuration
- Technologies: G10 inverter, intelligent defrost, automatic voltage adaptation, self-diagnostics, A2L leak-detection sensor included on the indoor units
The distribution of the four heads
The room-by-room load calculation led to an asymmetrical distribution, which is almost always the right result:
- 12,000 BTU/h in the main open area, the room with the most windows and the largest volume;
- 12,000 BTU/h in the second living area;
- 9,000 BTU/h in the primary bedroom;
- 9,000 BTU/h in the second bedroom, the one whose head is visible in the photo, above the door.
Total connected capacity: 42,000 BTU/h of heads on a 36,000 BTU/h outdoor unit. This is not an error. It is the heart of the matter.
A single electrical circuit for four zones
An installation detail that matters greatly in an older home: in this product line, the indoor units are powered by the outdoor unit. There are therefore not four circuits to run to four heads, but one 45 A circuit to the outdoor unit, plus one interconnection cable per head.
Compared with four independent wall-mounted systems—which would require four dedicated circuits, four breakers, and four outdoor units—the savings at the electrical panel are considerable. In a row house supplied by a 100 A or 200 A service that is already heavily loaded, this is often the deciding argument.
The combination ratio, explained with the real numbers
Adding up the heads does not give you the system's capacity
A multi-zone system is designed so that the sum of the heads' capacities exceeds the capacity of the outdoor unit. The ratio between the two is called the combination ratio. Here, 42,000 over 36,000 gives approximately 117%.
This apparent oversizing is based on a behavioral fact: the rooms in a house do not reach their peak demand at the same time. The primary bedroom needs more power at night; the south-facing open area needs more power in the late afternoon; the office needs more power when someone is working there. A system sized for all four zones to draw their rated capacity simultaneously would be oversized 90% of the time.
The trade-off must be explained to the customer, and we explain it: when all four heads call at the same time, each receives less than its rated capacity. The system distributes the available 36,000 BTU/h. This is not a defect; it is how the system works. A contractor who promises 42,000 BTU/h simultaneously from a 36,000 BTU/h unit is promising something that does not exist.
The real risk is not a lack of capacity; it is the modulation floor.
Here is the data that specification sheets publish but almost no one comments on: the minimum capacity of this outdoor unit is 8,870 BTU/h.
In other words, when the compressor runs, it produces at least the equivalent of a 9,000 BTU/h indoor unit operating at full capacity. This has a direct and often overlooked consequence: on an October night, if only the 9,000 BTU/h bedroom is calling for heat, the system is already at its floor. It cannot go any lower. It produces heat, reaches the setpoint, stops, and starts again.
This short cycling is the leading cause of dissatisfaction with a poorly designed multizone system. It causes:
- noticeable temperature fluctuations in the room;
- poor dehumidification in summer, because the coil does not have enough time to condense moisture;
- repeated start-up and shut-down noise, particularly noticeable at night in a bedroom;
- greater compressor wear.
That is why we systematically refuse to install a single indoor unit on a large outdoor unit “to leave room for expansion later.” That extra capacity comes at the cost of comfort every day.
The two opposite mistakes
- Too many indoor units, oversized. The instinct to install 12,000 BTU/h units everywhere “just in case.” The result: a 10 × 12-foot bedroom receives twice the capacity it needs, the indoor unit runs at minimum output, intermittently blows cool air, and the customer sleeps with the door open because the room is uncomfortable.
- Undersized indoor units on an oversized outdoor unit. The ratio drops below 100%, the compressor never has enough load to operate in its efficient range, and the 21 SEER 2 ratings shown on the specification sheet never materialize in the home.
Proper sizing falls somewhere in between. It is calculated room by room, taking into account each room’s orientation, windows, insulation, and actual use. We perform this calculation before proposing a configuration, never afterward.
What a multizone system cannot do
Three limitations you should know before signing—ones a rushed salesperson will not mention.
- Simultaneous heating and cooling are impossible. All indoor units share a single refrigerant circuit. If one zone calls for heating while another calls for cooling, the system cannot satisfy both. In a home where a south-facing office overheats while a north-facing bedroom stays cool during the shoulder seasons, that is a real limitation.
- An outdoor-unit failure takes all four zones offline. That is the trade-off of a single circuit and a smaller electrical panel. Four independent systems cost more and occupy four exterior-wall locations, but they do not all fail at once.
- Piping length is a fixed budget. This unit allows 80 m of total length, across all indoor units, and 25 m of elevation difference. Four indoor units distributed throughout a house consume this budget much faster than one might imagine, and every meter beyond the 40 m factory charge requires 20 g of additional R32, weighed and recorded.
An Indoor Unit Above a Door: The Constraint the Photo Reveals
The indoor unit visible in the photo is installed high on a wall, a few centimeters below the ceiling, directly above a door's framing. This is not a default location. It is a deliberate choice.
Why This Wall, and Why So High
In a bedroom, there are rarely many possible locations for a wall-mounted heat pump. The exterior wall is occupied by the window. The side walls hold the bed and wardrobe. That leaves the interior wall, the one with the door, and the only free surface there is the strip above the door framing.
This choice comes with rules:
- The ceiling clearance must be respected to the letter. An indoor unit mounted too close to the ceiling draws in its own discharged air, which throws off the room-temperature sensor and causes the unit to shut off before the room has been conditioned.
- The airflow must sweep across the room, not toward the door. An indoor unit installed above an opening naturally blows toward the opposite wall. The louver direction and sweep programming are adjusted during commissioning, not left to chance.
- The bed must not be in the airflow. In a room with a door wall, the bed is often opposite it. We discuss this with the client before drilling, because moving an indoor unit afterward leaves a scar in the wall and in the relationship.
The Lintel You Don't Drill
Above every door opening is a lintel, the structural element that carries the load of the wall above the opening. In the photo, the refrigerant line penetration is deliberately offset to the side of the framing, rather than above its center.
This is deliberate. A lintel must not be drilled, notched, or bypassed through improvisation: it carries a load, and weakening it compromises the wall's structure. On a wall-mounted heat pump installation, this means deciding where the line will exit by reading the framing, not by looking for the shortest route. When the only available passage crosses a structural area, the right answer is to move the indoor unit, not drill anyway.
The condensate drain when the head is at the highest point
A head mounted at the very top of a wall has an advantage and a pitfall.
The advantage: gravity. Condensate drains naturally, without a pump, provided the slope is continuous along the entire route to the outside. One fewer condensate pump means one fewer moving part, one less source of noise, and one fewer potential failure.
The pitfall: on an interior wall, the drain must cross the entire house to reach a discharge point. The slope is easily lost, and a counter-slope of just a few millimeters is enough to cause water to collect, followed by an odor and then an overflow into the head's drain pan. We check the slope with a level along the entire length, and when the route does not allow a continuous slope, we install a pump rather than hope for the best.
The penetration, sealing, and finishing
In the photo, the penetration is still open: the framing, insulation, and bundle consisting of the two insulated refrigerant lines and the interconnection cable are visible. This is the condition of the worksite at the time of connection, not the finished condition.
A completed penetration, in our work, includes:
- an outward slope for the penetration, so that water running along the bundle exits rather than entering;
- restoring air-barrier and vapor-barrier continuity around the bundle; otherwise, the penetration becomes a path for air leakage and a condensation point in the wall;
- durable mechanical sealing rather than a simple bead of expanding foam, which deteriorates under ultraviolet light outdoors and does not provide long-term airtightness;
- protecting the bundle against vermin on the exterior side;
- and repairing the drywall, joint compound, and paint on the interior side, because an installation that leaves a hole in a finished wall is not complete.
The minimum installation height and the A2L label
The label affixed beneath the head shows two values derived from the requirements for R32, a refrigerant classified as A2L, meaning mildly flammable: a minimum installation height and a minimum room area, the latter being around 7.1 m² (approximately 76.5 sq. ft.) for a unit of this size.
These two values go together. R32 is denser than air, so it sinks in the event of a leak. The higher the unit is mounted, the better the refrigerant disperses before reaching the floor, and the smaller the room served can be. A head mounted high on a wall naturally meets the height requirement. A console unit installed at the bottom of a wall, on the other hand, requires a larger floor area for the same charge.
The practical consequence is simple: a small bedroom, a cramped office, or an enclosed room smaller than 7 m² cannot take just any head, and this must be verified before the equipment is ordered. Each indoor unit in this range is also equipped with a leak-detection sensor that must remain powered continuously to perform its function.
The most common mistakes with a four-head multi-zone system
- Sizing the heads by eye instead of using a room-by-room load calculation.
- Ignoring the outdoor unit's modulation floor and ending up with short cycling in the smallest zone.
- Installing only one head on a large unit in anticipation of a future addition.
- Promising the customer simultaneous heating and cooling on a single circuit.
- Drilling through a lintel or beam to shorten the line route.
- Exceeding the piping-length allowance without adding, weighing, and recording the additional 20 g/m charge.
- Neglecting the condensate drain slope over a long indoor run.
- Sealing the wall penetration with expanding foam alone and considering the work complete.
- Relying on the wrong AHRI code when submitting a rebate application.
Rebates and eligibility: three AHRI codes for the same outdoor unit
This point deserves further explanation because it is specific to multi-zone systems and can be costly when mistakes are made.
The same GWHD(36)ND6MO outdoor unit carries three distinct AHRI codes depending on the type of indoor units connected to it:
- 214931589 when all heads are wall-mounted or non-ducted;
- 214931595 when all indoor units are ducted;
- 214931599 in a mixed configuration, that is, as soon as a ducted unit is paired with wall-mounted units.
Adding a single ducted unit to a set of three wall-mounted units therefore changes the certified combination, its performance ratings, and the code to enter on the rebate application. We regularly see applications denied or delayed because the declared code does not match the combination actually installed. At AirGreen, we enter the exact code on the quote and help our customers assemble their applications. Since the amounts and eligibility criteria are revised periodically, we always recommend validating them before signing.
Two technical data figures not to repeat without verification
We read technical documents line by line, and we prefer to flag anything that raises questions.
Maximum piping length. The range’s presentation page states 100 m. The table, however, lists 80 m for the 36,000 BTU/h model; the 100 m applies only to the 42,000 model. A quote based on the 100 m stated on the cover page may describe an impossible routing.
The performance ratings displayed at the top of the document. It states “up to 21 SEER 2 / 12.5 EER 2 / 10 HSPF 2.” The SEER 2 rating of 21 and the HSPF 2 rating of 10 do indeed apply to the entire range, but the EER 2 rating of 12.5 belongs to the 18,000 and 28,400 BTU/h models. The unit installed here is rated at 12. The difference is slight, but it illustrates a general rule: the table is reliable, while the cover page is a promotional summary.
We are adding a third observation, which is not an error but a deficiency: the supplied documentation contains no information about the indoor units. No capacities, dimensions, sound levels, or models. Yet in a four-head system, this is precisely half of what matters for everyday comfort. This information is available in the indoor-unit product specifications, and we provide it to the customer with the quote rather than leaving them to choose heads blindly.
Our installations in the Vallée-du-Richelieu and on the South Shore
Saint-Mathieu-de-Beloeil joins a series of projects carried out in the MRC of La Vallée-du-Richelieu and throughout the rest of Montérégie: Saint-Charles-sur-Richelieu, McMasterville, Saint-Mathias-sur-Richelieu, Saint-Roch-de-Richelieu, Saint-Sébastien, Venise-en-Québec, Saint-Liboire, Sainte-Madeleine, and La Présentation. The problems are never exactly the same: a 42-inch crawl space, a slab that cannot be drilled, an overloaded electrical panel, a finished wall that must be opened, and here, four zones to serve in a building without ductwork.
At AirGreen, we design, install, and maintain wall-mounted heat pumps, multi-zone systems with two, three, four, and five heads, central heat pumps, wall-mounted air conditioners, and commercial solutions in Montreal, Laval, Longueuil, on the North Shore, and on the South Shore. We hold RBQ licence 5645-9605-01, including the ventilation, refrigeration, and electrical subcategories.
If your home has no ductwork and you are considering a multi-head system, the first step is neither choosing the brand nor deciding on the number of zones. It is a room-by-room load calculation, followed by verification of the combination ratio and modulation minimum. We do both and show you the figures.