A rooftop installation designed to protect the building and ensure stable operation in every season
Installing a dual-zone heat pump on a flat roof requires a much more rigorous method than a conventional ground-level installation. In Sainte-Anne-de-Bellevue, we installed a Fujitsu AIRSTAGE Aquila XLTH system designed to heat and cool two distinct zones of the building from a single outdoor unit.
The project photo immediately reveals its main distinctive feature: the outdoor unit was installed on a membrane roof, amid several ventilation outlets and near the parapet. The space was open, but every decision had to account for the roof structure, water drainage, future maintenance, and increased exposure to wind and snow.
We therefore designed a raised base consisting of a metal structure, long wooden load-distribution members, and protective surfaces placed on the roof. Anti-vibration mounts were also installed directly beneath the heat pump. This configuration distributes the load, limits vibrations, and keeps the unit above the immediate level of the membrane.
The photo also shows a level placed on top of the unit. This detail corresponds to an essential step in the project: before permanently connecting a heat pump installed on a roof, we check its alignment and stability in several directions. An improperly leveled unit may vibrate more, drain defrost water less efficiently, and place an uneven load on its base.
Why the roof was the best location
In some buildings in Sainte-Anne-de-Bellevue, it is difficult to install a ground-level outdoor unit without obstructing access points, windows, common areas, or the appearance of the façades. The roof can then become the most logical location, especially when the two indoor units are on the upper levels.
This solution notably makes it possible to:
- clear the pathways around the building;
- reduce the equipment’s presence on the façades;
- shorten certain refrigerant lines;
- group the technical components in an area already designated for mechanical equipment;
- protect the unit from impacts, vandalism, and ground-level maintenance work;
- serve two zones with a single outdoor unit.
However, choosing a rooftop location should not be viewed simply as a way to hide the equipment. A roof is a more exposed environment. Wind, freezing temperatures, solar radiation, snow accumulation, and temperature variations are often more significant there than at ground level.
We must also maintain an appropriate distance from air outlets, plumbing vents, and other equipment visible on the roof. The heat pump fan must be able to expel air without obstruction or recirculation toward the heat exchanger.
A base that distributes the load without compromising the membrane
The roof visible in the photo is covered with a granulated membrane. It was essential to prevent the narrow feet of a metal support from exerting concentrated pressure on this surface.
We therefore used long load-distribution bases. These increase the contact area and reduce the load exerted on a single point. They are placed on protective surfaces to limit friction and the risk of damaging the membrane.
This method serves several purposes:
- keep the outdoor unit stable;
- distribute the weight of the unit and the structure;
- protect the roof from sharp metal edges;
- maintain sufficient clearance beneath the heat pump;
- limit movement caused by vibration;
- make the entire assembly accessible for HVAC maintenance.
A common mistake is to place a metal support directly on the roof without adequate protection or load distribution. Over time, vibration and weight can mark the membrane or create a wear point. A roof leak may then appear long after installation, sometimes far from where the water becomes visible indoors.
Why we avoid penetrating the roof when it is not necessary
Any penetration of a roof represents a potential risk. When mechanical attachment to the structure is not required, a properly sized base can support the equipment without creating multiple penetrations.
This does not mean that all rooftop installations can be carried out this way. The equipment weight, support height, wind exposure, structural requirements, and building recommendations must be assessed.
In this project, the selected configuration made it possible to create a stable base while protecting the existing surface. We also took care not to obstruct the natural flow of water across the roof.
A height suited to snow and defrost cycles
The metal structure clearly raises the Fujitsu unit. This height is not chosen solely to make the technician’s work easier. It plays an essential role during the winter.
When a heat pump operates in heating mode, its outdoor heat exchanger may accumulate frost. The unit periodically initiates a defrost cycle that melts this buildup. The water must then be able to drain beneath the unit.
On a flat roof, a heat pump placed too low can encounter several problems:
- defrost water freezes beneath the chassis;
- a block of ice gradually reaches the base of the unit;
- snow reduces airflow through the heat exchanger;
- the lower openings become partially obstructed;
- winter maintenance becomes difficult;
- defrost cycles become more frequent.
The raised base allows water to drain more freely and provides additional clearance when snow accumulates on the roof. However, the area must still be inspected after major storms, and the air inlets and outlets must be kept clear.
Vibration control on a roof
The visible vibration-damping blocks beneath the four corners of the outdoor unit are a major part of the installation. On a roof, vibrations can be transmitted to the building structure and become audible in the rooms directly below.
Even when the heat pump itself operates quietly, a poorly designed base can amplify certain frequencies. The roof, joists, and ceilings can then act as a resonance chamber.
We therefore seek to interrupt vibration transmission at several levels:
- between the heat pump and the metal support;
- between the support and the long distribution bases;
- between the bases and the protected roof surface;
- along the refrigerant line routing;
- at the points where the pipes enter the building.
The lines must not be pulled too tightly. They need some flexibility so they do not transmit compressor vibrations directly to the structure.
A Fujitsu dual-zone heat pump suited to the needs of two spaces
The operating principle of the Aquila XLTH multizone system
The installed system allows two indoor units to be connected to a single outdoor unit. Each zone has its own control and can operate at a different setpoint temperature.
In practice, this means that a room exposed to the sun may require more cooling than a room on the shaded side. In winter, a zone with more exterior walls may require more heating than the other.
Fujitsu documentation for AIRSTAGE H Series multizone systems lists 18,000, 24,000, 36,000, and 45,000 BTU/h outdoor units that can power up to two, three, four, or five indoor units, depending on the model and approved combinations. For a dual-zone configuration, the table on page 2 includes several indoor-unit combinations, such as 7,000 + 7,000 BTU/h, 9,000 + 12,000 BTU/h, or 12,000 + 12,000 BTU/h.
The exact capacity of the photographed installation cannot be confirmed from the image alone. We therefore avoid assigning a specific BTU rating without consulting the nameplate, quotation, or project technical file.
Separately controlled indoor units
Each indoor unit can be controlled independently. Occupants can adjust:
- temperature;
- fan speed;
- airflow direction;
- the schedule;
- turning the zone on or off.
However, the system remains subject to an important rule: both zones must operate in the same general mode. They can both heat or both cool, but one zone cannot provide cooling while the other calls for heating.
Fujitsu documentation describes this operation and indicates that a multizone outdoor unit can power multiple indoor units simultaneously, with individual control of each zone. It also mentions a connection capacity of up to 130%, without increasing the outdoor unit’s total production capacity.
Sizing a dual-zone system
Selecting a multizone heat pump does not involve arbitrarily adding together the rated capacity of the two indoor units.
We assess in particular:
- the area of each zone;
- solar exposure;
- ceiling height;
- insulation quality;
- window dimensions;
- air infiltration;
- pipe length;
- height differences;
- low-temperature heating capacity;
- the level of simultaneous use of the two zones.
An improperly sized system can provide disappointing comfort even if it is a high-quality unit. An oversized outdoor unit may operate in short cycles. Insufficient capacity can result in continuous operation during cold periods.
The manufacturer’s combination tables are used to confirm that the indoor units can be connected together, but Fujitsu specifies that they must not be used alone to select the system capacity. The manufacturer recommends its Design Simulator software to validate the configuration.
A system designed for cold conditions
The Fujitsu Aquila range includes models designed for cold climates. The documentation provided indicates that CTEB units can operate in heating mode down to -26 °C and in cooling mode at an outdoor temperature of up to 50 °C.
This data must be interpreted correctly. The minimum operating temperature indicates that the unit can continue heating under these conditions, but the available capacity varies according to the outdoor temperature.
To determine whether the heat pump can meet the building’s needs on its own, we must compare:
- the building’s heat losses;
- the unit’s actual low-temperature capacity;
- the desired comfort level;
- whether backup heating is present;
- the intended operating strategy.
In many projects, the heat pump becomes the primary heating source, while electric baseboard heaters or another system takes over during certain periods. In other cases, the installation is designed to cover a very large part of the heating season.
The R32 refrigerant visible on the unit
The marking R32 is clearly visible on the front of the outdoor unit. This refrigerant is used in many recent systems. It has a lower global warming potential than R410A, but it requires an appropriate installation procedure and tools.
During commissioning, we perform, among other things:
- a check of the flared connections;
- a pressure test;
- a leak check;
- a measured evacuation;
- a verification of the line-set lengths;
- a charge check according to the specifications;
- a complete heating and cooling test.
A simple evacuation performed for an approximate duration is not enough. We must confirm that air and moisture have been removed from the system before opening the service valves.
Routing the lines across the roof
The photo shows several insulated lines following the roof surface before reaching the outdoor unit. On a roof, their protection is essential.
The lines are exposed to:
- sunlight;
- freezing;
- rain;
- movement caused by wind;
- future roof work;
- technicians’ movements;
- birds and small animals.
We secure the route to prevent it from shifting while maintaining some flexibility. The insulation must remain continuous, and the connections must be protected from moisture.
The route must also not create an unnecessary obstacle for roofers or access to mechanical equipment. An orderly installation reduces the risk of a line being crushed or damaged during future maintenance.
Challenges specific to installing a heat pump on a flat roof
Transporting the equipment
Transporting an outdoor unit to the roof requires advance planning. The unit’s weight and dimensions, access to the building, and technician safety must be assessed.
Depending on the worksite, we may use:
- a freight elevator;
- a crane;
- a lifting device;
- an interior staircase wide enough;
- a roof hatch;
- specialized material-handling equipment.
The unit must be moved without damaging its chassis, heat exchanger, or internal components.
Worksite safety
The presence of a parapet does not always replace fall-protection measures. The location of technicians, tools, and materials must be organized to avoid unnecessary movement near the roof edge.
The photo shows that the work is still in progress: packaging, tools, and fastening materials are grouped near the unit. Before we leave, we remove waste, screws, pieces of insulation, and packaging that could obstruct a roof drain.
Coordination with the roofing
A heat pump should never prevent the membrane from being inspected or repaired. We therefore leave enough space to check the surface beneath and around the base.
When a roof needs to be replaced a few years later, a well-designed support also makes the roofer’s work easier. An installation that is too low or too bulky may require the owner to have the system temporarily disconnected.
Mistakes to avoid on this type of project
Place the unit directly on the membrane
This method increases the risk of wear, ice buildup, and vibration.
Using a base that is too narrow
An unevenly distributed load can deform the membrane or the underlying support.
Blocking roof drainage
The bases, pipes, and cables must not prevent water from reaching the drains.
Neglecting wind exposure
A unit installed on a roof is generally more exposed than a ground-level unit. Its stability must be assessed with this in mind.
Placing the unit near a problematic vent
Air discharged by certain vents must not be drawn in by the heat pump. The distance and orientation must be checked.
Forgetting to leave service clearance
The technician must be able to remove the panels, access the valves, and clean the heat exchanger without moving the unit.
Commissioning performed by AirGreen
Once the structure and outdoor unit are installed, we carry out a complete inspection:
- level check;
- base stability check;
- inspection of the protections beneath the supports;
- clearance checks;
- refrigerant line routing verification;
- leak test;
- vacuum evacuation;
- electrical inspection;
- proper identification of the two zones;
- testing of each indoor unit;
- heating and cooling checks;
- drain inspection;
- final roof inspection;
- site cleanup.
We then explain to the customer how the system works, the limitations of multi-zone operation, and routine maintenance procedures.
An installation adapted to Sainte-Anne-de-Bellevue
This installation in Sainte-Anne-de-Bellevue demonstrates that a dual-zone heat pump can be effectively integrated into a flat roof when every detail is planned from the outset.
The project’s success depended on several elements:
- a raised base;
- proper weight distribution;
- membrane protection;
- anti-vibration supports;
- a perfectly level unit;
- sufficient clearance;
- a clean piping route;
- convenient access for maintenance;
- a multi-zone configuration adapted to the building’s needs.
The Fujitsu AIRSTAGE Aquila XLTH system can now heat and cool two spaces with a single outdoor unit, while freeing up façades and ground-level walkways.
At AirGreen, we install multi-zone heat pumps in Montreal, Laval, Longueuil, the North Shore, and the South Shore. Whether the installation is at ground level, on wall brackets, or on a roof, we adapt the method to the building, Quebec’s climate, and the manufacturer’s technical requirements.