Rooftop unit selection for commercial buildings

Packaged rooftop units (RTUs) are the most common HVAC system in low-rise commercial buildings across Canada. Retail stores, strip malls, restaurants, schools, warehouses and single-storey offices all rely on them. An RTU contains the fan, filters, cooling coil, compressors, condenser and heating section in one weatherproof cabinet, connected to ductwork through a roof curb.

Because RTUs are a mature, mass-produced product, it is tempting to pick one from a catalogue based on tonnage alone. That shortcut causes many of the comfort complaints, high energy bills and early failures seen in RTU-served buildings. Good selection balances capacity, airflow, heating, ventilation, efficiency and climate.

Start with an accurate load

RTU capacity should come from a proper load calculation, not a rule of thumb or the size of the unit being replaced. Use a commercial cooling load calculation and a heating load calculation based on outdoor design conditions for the location, as listed in the National Building Code of Canada climatic data or the applicable provincial code.

Key outputs for selection:

• Total and sensible cooling load, because RTUs have a fixed sensible heat ratio at a given airflow
• Heating load, including ventilation air heating at the winter design temperature
• Required outdoor air, from ASHRAE 62.1 or the code referenced ventilation standard
• Design supply airflow, based on sensible load and supply air temperature

Oversizing is common and costly. An oversized RTU short cycles, removes little moisture, wears compressors and costs more to buy and run.

Cooling capacity and airflow

RTU cooling capacity is rated at standard conditions under CSA C746 in Canada and AHRI 340/360 in the US. Actual capacity depends on outdoor temperature, entering air conditions and airflow. When selecting:

1. Check capacity at your local summer design dry-bulb and entering wet-bulb temperatures, not just the nominal rating.
2. Confirm that the sensible capacity meets the sensible load. High-latent spaces such as restaurants and assembly areas may need lower airflow or a unit with enhanced dehumidification.
3. Typical airflow is about 50 to 60 L/s per kW of cooling (350 to 450 cfm per ton). Lower airflow improves dehumidification; higher airflow improves sensible capacity.
4. Check that the fan can deliver the design airflow against the actual external static pressure of the duct system. Many units are selected at a nominal static that is lower than the real ductwork imposes.

Heating options

Heating is where Canadian RTU selection differs most from warmer markets.

For gas heat, select the heat exchanger and burner for the full heating load including outdoor air at the winter design temperature. Two-stage or modulating gas valves give much better temperature control than single-stage burners, especially for units with high outdoor air fractions. Gas installation must comply with CSA B149.1.

For heat pump RTUs, check capacity and COP at your local design temperature and decide on supplementary heat. Cold climate heat pump RTUs are improving quickly; see our guide on cold climate heat pumps.

Ventilation and economizers

Outdoor air

Every RTU serving occupied space must deliver the required outdoor air. Options include:

• A fixed minimum damper position, simple but often inaccurate
• An outdoor airflow measuring station, which allows the controls to hold a measured airflow setpoint
• Demand-controlled ventilation using CO2 sensors in variable-occupancy spaces

Heating large amounts of outdoor air at -30°C (-22°F) or colder is a big load in Prairie and northern climates. An energy recovery wheel or plate exchanger can reduce it, but frost control must be addressed.

Economizers

An air-side economizer uses cool outdoor air for free cooling. In Canadian climates, economizers can provide a large share of cooling hours. NECB and ASHRAE 90.1 require economizers on units above certain capacities, with exceptions. However, economizers are also one of the most common sources of RTU failures: stuck dampers, failed actuators and wrong changeover settings. Specify quality dampers, a proper controller and fault detection. Our guide on economizers covers the details.

Efficiency ratings

Minimum efficiencies are set federally under Canada's Energy Efficiency Regulations and by NECB or provincial energy codes. Exceeding the minimum is often justified, particularly IEER, because RTUs spend most of their hours at part load. Variable-speed compressors and fans offer the biggest part-load gains.

Fans and controls

• Supply fan control: many codes now require multi-speed or variable-speed fan control on larger units, reducing fan speed when cooling demand is low. Constant-speed fans running 24/7 waste a great deal of energy.
• Controls: specify a unit controller that can communicate with the building automation system, often via BACnet, and that supports scheduling, setback, economizer control and alarm reporting.
• Fault detection: economizer fault detection and diagnostics are required by some energy codes and are valuable for any unit.
• Smoke detection: duct smoke detectors and fan shutdown may be required by the building code depending on airflow and building type.

Physical and site considerations

1. Curb and structure: check the roof structure for unit weight, and whether a new unit fits an existing curb or needs an adapter.
2. Clearances: maintain manufacturer clearances for condenser airflow and service, and keep intakes away from exhaust fans, plumbing vents and flues.
3. Snow: in heavy snow regions, raise curbs and protect intakes with hoods or snow guards.
4. Noise: consider unit sound levels near residential neighbours and above noise-sensitive rooms.
5. Wind and seismic: restraint requirements vary by location and building importance.
6. Service access: provide safe roof access and guardrails or fall protection as required by occupational health and safety rules.

Useful options

• Hinged access doors and filter racks for easier maintenance
• Low ambient cooling control for spaces that need cooling in winter
• Modulating gas heat or hot gas reheat for humidity control
• Factory-mounted disconnect and convenience outlet
• Corrosion-resistant coil coatings near coasts or industrial exhaust
• Phase and voltage monitors to protect compressors

Frequently asked questions

Should I match the tonnage of the old unit?

Not without checking. Loads change as buildings are renovated, lighting is upgraded and uses change. A load calculation often shows that a smaller unit is adequate. See replacing rooftop units.

Is a heat pump RTU practical in Canada?

In milder regions such as coastal British Columbia, yes. In colder regions, heat pump RTUs usually need supplementary heat for the coldest hours, but they can still cover a large share of the annual heating load.

How long do RTUs last?

Typical service life is often quoted at around 15 to 20 years, depending on maintenance, climate and duty. Coastal exposure and poor maintenance shorten it.

Key takeaways

• Size RTUs from a load calculation, not from the old unit or a rule of thumb.
• Check capacity at local design conditions and actual external static pressure.
• Select heating for the full load including outdoor air at the winter design temperature, and prefer staged or modulating heat.
• Specify reliable economizers, outdoor air measurement and variable-speed fans where practical.
• Commission and balance the airflow after installation to confirm that the unit delivers what was selected.