A rooftop HVAC installation designed to protect the membrane and serve three separate zones
In Sainte-Anne-de-Bellevue, in the West Island of Montreal, we completed the installation of a HITACHI airHome Multi triple-zone multi-zone heat pump on the flat roof of a residential building. This project required a very different approach from a conventional ground-level or wall-mounted installation: the outdoor unit had to be stable, elevated, accessible for maintenance, and installed without compromising the integrity of the roof membrane.
The site photo shows a HITACHI airHome HeatForce outdoor unit installed on a freestanding metal support. The support legs rest on long pieces of wood arranged on the roof. This detail may seem simple, but it plays an important role in distributing the weight and protecting the membrane.
Several elements typical of flat roofs are visible around the heat pump: plumbing vents, mechanical outlets, ducts, other air-conditioning equipment, parapets, and roof surfaces at different levels. The location therefore could not be determined solely based on available space. We also had to consider:
- air circulation around the unit;
- technical access;
- the routing of the three refrigerant lines;
- snow accumulation;
- water produced during defrosting;
- wind exposure;
- the roof’s drainage areas;
- the ability to carry out future HVAC maintenance without moving the equipment.
The result is an orderly, elevated installation adapted to the constraints of a Montreal rooftop.
Why a triple-zone heat pump was the right solution
The building needed heating and cooling in three separate spaces. Installing three single-zone systems would have required three outdoor units on the roof, with three supports, three main power supplies, and more exposed piping.
By choosing a triple-zone multi-zone heat pump, we were able to connect three indoor units to a single outdoor unit.
Each zone can be used with:
- an independent setpoint temperature;
- an appropriate fan speed;
- different scheduling;
- operation corresponding to the room’s occupancy;
- the possibility of being turned off when the space is not in use.
In the HITACHI airHome Multi range, the triple-zone outdoor unit RAM-G24N3HAA has three indoor connections. Its rated capacity is 24,000 BTU/h in cooling and 24,000 BTU/h in heating. In a ductless configuration, its modulation can reach a range of 7,000 to 28,000 BTU/h in cooling and 9,000 to 26,200 BTU/h in heating.
This capacity is not automatically divided into three identical portions. The Inverter system modulates its output and distributes the available capacity among the active indoor units. A small enclosed room can therefore receive a unit with a different capacity from the one installed in a large living area.
Three independent controls, but one shared main mode
The term “triple zone” means that three indoor units can be controlled separately. It does not mean that the system can provide heating and cooling simultaneously.
In summer, the indoor units can operate in:
- cooling;
- dehumidification;
- ventilation.
In winter, they can be set to:
- heating;
- ventilation.
It is still possible to program different temperatures in the three spaces and turn off certain units. However, one unit cannot heat while another cools with the same outdoor unit. HITACHI documentation specifies that the units must use compatible modes depending on the season.
This explanation is part of our commissioning process. It is especially important on spring or fall days, when the sun strongly warms one room while another remains cool.
Choosing the roof: a logical solution in an urban environment
In many Montreal buildings, the available ground-level space is limited. Yards can be narrow, façades may be subject to architectural restrictions, and passageways may be used by occupants.
In Sainte-Anne-de-Bellevue, installing the unit on the roof made it possible to:
- completely free up the ground-level space;
- move the unit away from traffic areas;
- group the equipment with the other mechanical installations;
- reduce the visual impact from the street;
- simplify the route to indoor units located on upper floors.
A rooftop installation is nevertheless more demanding. The unit must never simply be placed on the membrane. Its weight must be distributed, its support must remain stable, and technicians must be able to access it safely.
The technical details that make this rooftop installation reliable
A freestanding support with no visible anchoring in the membrane
The outdoor unit rests on a four-legged metal support. Beneath this support, two long pieces of wood are placed parallel to each other on the roof.
This configuration distributes the load over a larger area than the four small feet placed directly on the membrane would provide. It also helps stabilize the support.
The photo does not show any fasteners penetrating the roof at the bases. This approach avoids unnecessarily multiplying penetrations through the membrane. Any penetration of a flat roof must be planned, sealed, and carried out using a method compatible with the roofing system.
The wooden pieces serve several purposes:
- distribute the weight of the heat pump and the support;
- limit concentrated pressure on the membrane;
- create a straight base;
- improve the stability of the assembly;
- keep the metal feet away from direct contact with the roof;
- facilitate future work beneath the support.
The exterior wood should nevertheless be monitored over time. It must remain stable and not warp to the point of altering the unit's level.
Vibration-dampening pads beneath the heat pump
Between the unit's feet and the support's upper crossbar, we can see vibration-dampening pads.
Even though an Inverter heat pump often operates at reduced speed, its compressor and fan produce mechanical movement. On a roof, these vibrations can be transmitted to the structure and become noticeable in the units beneath the equipment.
The supports help to:
- limit transmission to the framing;
- reduce indoor humming;
- protect the connections from repeated vibrations;
- improve acoustic comfort;
- avoid metal-to-metal contact.
Their effectiveness also depends on the support being level. If a single foot bears too much of the weight, the system may vibrate more and place uneven stress on the chassis.
Sufficient clearance for snow and defrosting
The heat pump was installed well above the roof surface. This elevation is essential in the climate of Sainte-Anne-de-Bellevue.
On a flat roof, snow does not always accumulate evenly. Wind can create substantial drifts near parapets, mechanical outlets, and equipment. A heat pump installed too low may become surrounded by snow even if the average depth on the roof seems reasonable.
The support height helps maintain:
- the rear air intake unobstructed;
- the front fan above the snow;
- clearance beneath the lower tray;
- a drainage area for defrost water;
- access for removing ice.
In heating mode, the outdoor coil may accumulate frost. The system then initiates a defrost cycle, during which a sometimes significant amount of water is released. This water may freeze on the roof.
The space beneath the unit allows this ice to form without immediately reaching the chassis. Nevertheless, the area must be checked in winter and accumulations that could impair operation must be removed.
A ventilation face oriented toward an open area
The fan is directed toward an unobstructed part of the roof. No tall parapet or other equipment is directly in front of the grille.
This orientation reduces the risk of recirculation. In cooling mode, the discharged warm air must be able to move away from the unit. In heating mode, the discharged cold air must not immediately return toward the heat exchanger.
Excessive recirculation can cause:
- reduced efficiency;
- unfavorable operating pressures;
- longer cycles;
- more frequent defrosting;
- premature wear;
- increased electricity consumption.
On a roof with multiple pieces of equipment, we must also prevent the discharge from one unit from being drawn into another system located nearby.
The horizontal route of the refrigerant lines
An insulated line is visible at the front of the support. It follows the roof toward the penetrations or the other sections of the route.
A triple-zone installation includes three sets of refrigerant lines. For the RAM-G24N3HAA model, HITACHI specifies:
- three liquid connections of 1/4 po;
- three gas connections of 3/8 po;
- a maximum total piping length of 200 feet;
- a maximum length of 82 feet between units;
- a maximum total height of 66 feet;
- a minimum total length of 16 feet;
- a maximum elevation difference of 16 feet between the indoor units.
On a roof, the line route must be protected against:
- ultraviolet rays;
- standing water;
- movement caused by wind;
- future work on the roof;
- roofers’ tools;
- snow and ice;
- deterioration of the insulation.
The black insulation visible around the lines limits heat gain or loss and prevents condensation on the gas line. It must remain continuous and in good condition.
We also make sure not to install the lines along the main pathways used by technicians or roofers.
Roof penetrations require precise coordination
The lines may eventually need to pass through the building envelope. On a flat roof, this penetration must be watertight and arranged so that it does not create a weak point.
A poorly executed penetration can cause leaks long after the heat pump is installed. We must therefore coordinate:
- the location of the penetration;
- the sleeve type;
- the height above the roof;
- watertightness;
- protection against snow;
- the possibility of replacing the membrane;
- the bend radius of the lines.
We avoid routing the lines directly through an area where rainwater accumulates. The lines must also be supported so that their weight does not pull on the entry point.
A wind-exposed installation
The roof is generally more exposed than ground level. Gusts can be significant near parapets and between buildings.
The support must therefore provide sufficient stability to withstand movement. The unit itself has a large vertical surface area, particularly when elevated.
We consider:
- the unit’s weight;
- the width of the support;
- the length of the bases;
- load distribution;
- the prevailing wind direction;
- the height of the parapets;
- the building’s specific requirements.
The RAM-G24N3HAA triple-zone model has a net weight of approximately 121 lb and dimensions of 33.5 in. wide, 31.5 in. high, and 11.7 in. deep.
These data influence the choice of support and the stabilization method. A structure designed for a small single-zone unit is not automatically suitable for multizone equipment.
A roof already occupied by several systems
The photo shows other units and several mechanical outlets around the installation. This density requires detailed planning.
We must maintain:
- access to vents;
- the routes used by technicians;
- the areas required by other equipment;
- roof drains;
- the possibility of replacing the membrane;
- an appropriate distance from exhaust sources.
A heat pump should not be placed in front of an outlet that could project contaminated, greasy, or very humid air toward its heat exchanger.
The selected location provides a clear façade and a reasonable work area around the technical side.
Performance and comfort of the HITACHI airHome Multi triple-zone system
Seasonal efficiency suited to a ductless system
For the RAM-G24N3HAA triple-zone ductless configuration, HITACHI documentation indicates:
- a TRÉ2 of 12.5;
- a TRÉS2 of 22.0;
- a CPSC2 of 9.5 in Region 4;
- a CPSC2 of 7.8 in Region 5;
- a COP of 3.5 at 8 °C.
TRÉS2 represents seasonal cooling efficiency, while CPSC2 allows comparison of seasonal heating performance.
These values are useful for comparing equipment, but actual consumption depends on many factors:
- the building’s insulation;
- the requested temperature in each zone;
- the number of units used simultaneously;
- exposure to sunlight;
- the tightness of the windows;
- how often the filters are cleaned;
- the condition of the outdoor heat exchanger;
- winter conditions on the roof.
Heating operation down to -20 °C
The airHome Multi range can operate in heating mode down to -20 °C. For the triple-zone model, the specified maximum capacity is 18,500 BTU/h at -15 °C and 13,000 BTU/h at -20 °C.
These values show why a heat pump should not be sized solely according to its nominal capacity of 24,000 BTU/h. As the outdoor temperature drops, the available capacity decreases.
On a roof, exposure to wind can also increase the building’s heat loss. We must therefore consider the actual needs of the three zones and the presence of supplemental heating during the coldest periods.
Several indoor unit formats
The airHome Multi range allows different indoor unit styles to be combined:
- airHome 400 wall-mounted unit;
- airHome 650 wall-mounted unit;
- airHome Floor console;
- airHome Mini Cassette;
- airHome Ducted unit.
Available capacities generally range from 7,000 to 24,000 BTU/h, depending on the format.
This flexibility is useful for a three-zone building. It is not necessary to install three identical indoor units. A large common area can receive more capacity, while an office or bedroom can be served by a smaller unit.
Sizing must take into account:
- floor area;
- fenestration;
- orientation;
- the building level;
- ceiling height;
- solar gains;
- the use of the room.
Sound levels suitable for residential use
The triple-zone model has a specified maximum outdoor sound level of 54 dB(A).
However, how sound is perceived on a roof depends on the structure, supports, and position relative to occupied rooms. A poorly isolated unit can transmit vibrations through the framing, even when outdoor noise seems low.
Anti-vibration mounts and distributed bases therefore contribute to comfort just as much as the unit’s acoustic characteristics.
FrostWash and Mold Guard for compatible indoor units
Depending on the selected indoor units, the range may offer FrostWash technology. It uses a freeze-and-thaw cycle to remove some of the contaminants present on the indoor coil.
The Mold Guard function circulates air for approximately 60 minutes after certain air-conditioning or dehumidification operations to dry the internal components.
These functions can help maintain the unit, but they do not replace:
- filter cleaning;
- drain inspection;
- fan cleaning;
- coil inspection;
- periodic professional HVAC maintenance.
R32 refrigerant filtration and monitoring
The system uses refrigerant R32. Depending on the type of indoor unit, the range may include a leak sensor accompanied by a visual or audible alarm.
Compatible units may also offer:
- a PM2.5 activated carbon filter;
- a ViroSense S filter;
- an integrated leak alarm;
- an optional control interface.
Installing an R32 system requires precise flared connections, a rigorous leak test, and evacuation performed with the appropriate equipment.
Optional control with airCloud Go
An optional Wi-Fi interface allows use of the airCloud Go app. This feature is convenient in a three-zone building, especially when the units are distributed across multiple floors.
It allows you to:
- adjust temperatures;
- schedule operating hours;
- turn off an unused zone;
- prepare the building before occupants arrive;
- monitor certain operating parameters.
The units remain independently controllable while operating in the same heating or cooling mode.
Electrical data for the triple-zone system
The RAM-G24N3HAA model uses a 208 to 230 volt, single-phase, 60 Hz power supply. The documentation indicates:
- minimum circuit ampacity of 19 A;
- maximum overcurrent protection of 35 A.
On a roof, the electrical run must be protected from the weather and installed so that it does not create an obstruction. The disconnecting means must also remain accessible near the unit.
We check the following before commissioning:
- voltage;
- conductor gauge;
- circuit protection;
- grounding;
- communication wiring;
- terminal tightening;
- the operation of each zone.
Eligibility for rebates: the complete combination must be validated
The HITACHI brochure states that the airHome Multi range may qualify for most provincial heat pump incentives. This does not mean that every combination automatically receives the same financial assistance.
Eligibility depends in particular on:
- the outdoor model;
- the three indoor models;
- the certified combination;
- the recognized capacity;
- the building type;
- the program requirements;
- the installation date.
We therefore verify the exact combination before confirming a price.
Mistakes to avoid when installing on a flat roof
Placing the unit directly on the membrane
The weight would be concentrated beneath the feet, and the vibrations would be transmitted directly to the roof.
Using a support that is too low
Snow and ice could quickly reach the chassis and reduce airflow.
Placing the unit near a drain
The structure and bases must not prevent rainwater from flowing toward the drainage points.
Blocking access to other equipment
Roofers and technicians must be able to access the vents and nearby equipment.
Leaving the lines unsupported
Wind and work on the roof can move them or damage their insulation.
Installing the unit facing a nearby parapet
The discharged air could flow back toward the heat exchanger and impair performance.
Neglecting defrost water
A significant layer of ice can form beneath the heat pump during the winter.
Forgetting about future roofing work
The installation must allow the membrane to be replaced or repaired without requiring the complete removal of all the lines.
Roof maintenance differs from ground-level maintenance
A heat pump installed on a roof must be inspected with particular care.
We recommend checking periodically:
- the condition of the supports;
- the level of the support;
- the condition of the wooden bases;
- the membrane beneath the installation;
- the stability of the lines;
- snow and ice accumulation;
- the cleanliness of the heat exchanger;
- the clearances around the fan.
Roof access must always be safe. The owner should not approach an unprotected edge or work on the unit without the necessary equipment.
The final result in Sainte-Anne-de-Bellevue
This multizone heat pump installation in Sainte-Anne-de-Bellevue now makes it possible to heat and cool three indoor zones with a single HITACHI outdoor unit.
The project includes:
- a HITACHI airHome Multi triple-zone heat pump;
- a 24,000 BTU/h outdoor unit adapted to three connections;
- a freestanding metal support;
- anti-vibration supports;
- long bases distributing the load across the roof;
- elevation adapted to snow and defrosting;
- a fan oriented toward a clear area;
- protected, horizontally routed lines;
- access maintained for future maintenance;
- a cohesive integration among the other rooftop equipment.
This project demonstrates that a rooftop installation should never be treated as a simple replacement solution when ground space is limited. It requires a complete assessment of the membrane, wind, snow, drainage, weight, and accessibility.
At AirGreen, we adapt every HVAC installation to its environment, whether it is carried out on a flat roof in Montreal, on wall brackets in Laval, on a balcony in Longueuil, or on the ground on the North Shore and South Shore.