❄ Total Cooling Tonnage
🔥 Heating Capacity
Sizing Breakdown
Full Calculation Table
| Component | Basis | Value | Result |
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How to Use the RTU Sizing Calculator
Choose single-zone constant volume for a directly-served single zone, or multi-zone VAV if the RTU feeds several downstream VAV boxes.
Enter the cooling load and heating load the RTU needs to serve, and select a heating source appropriate for your climate.
Enter minimum outdoor airflow from your ventilation calculation to confirm whether ventilation or cooling load governs total supply airflow.
Click Calculate RTU Sizing to see required cooling tonnage, heating capacity, supply airflow, and per-unit sizing if multiple RTUs serve the load.
Understanding RTU Sizing
Packaged rooftop units combine cooling, heating, filtration, and often ventilation air handling into a single factory-built package mounted on the roof, connecting to the building only through supply and return ductwork. Their self-contained design makes them a popular choice for single-story or low-rise commercial buildings where a central chilled water plant is not economical, and their capacity and efficiency ratings are governed by the binational AHRI Standard 340/360.
Single-Zone Constant Volume vs. Multi-Zone VAV
A single-zone constant volume RTU serves one thermal zone directly, cycling capacity to match that zone's load with relatively simple controls, common in small retail units or single-tenant spaces without significant internal zoning needs. A multi-zone VAV RTU feeds several downstream VAV boxes, each independently controlling airflow to its own zone, while the RTU modulates total supply airflow and capacity to match combined system demand across all zones, similar in concept to a smaller-scale version of a central AHU serving VAV boxes.
Supply Airflow: Cooling Load or Ventilation, Whichever Governs
Just like a central AHU, RTU supply airflow follows whichever requirement is larger: the airflow needed to remove sensible cooling load at the design supply air temperature difference, or the minimum outdoor air requirement for the zones served. In a densely occupied space, ventilation can exceed the cooling-driven airflow, meaning the unit ends up sized larger than cooling alone would suggest. Confirm your outdoor air requirement with the commercial ventilation calculator before finalizing RTU size.
How Many RTUs a Building Needs
Building size, roof structural capacity, and zoning requirements all factor into how many RTUs a project uses. A single-story retail building might use one RTU per few thousand square feet of sales floor, while larger buildings often split across multiple smaller units rather than one large unit, both to limit how much conditioned space is lost if a single unit needs service, and to better match zone-specific load patterns across different building orientations or occupancy types, a tradeoff worth running through the lifecycle cost calculator since more, smaller units cost differently over time than fewer, larger ones.
Canadian Climate Considerations for RTU Selection
RTU compressors and outdoor coils sit fully exposed to outdoor conditions, unlike indoor mechanical room equipment. Canadian winters bring extended periods of extreme cold that affect refrigerant pressures and can cause outdoor coil frost formation, so units specified for Canadian climates need low-ambient cooling controls that permit shoulder-season cooling operation at low outdoor temperatures, along with reliable defrost cycling for heat pump configurations, sized against your city's actual winter extremes from the design temperature lookup. Gas-fired heating remains common where natural gas rates are favorable relative to electricity, while heat pump RTUs are gaining adoption alongside electrification incentives, typically paired with backup electric resistance heat for the coldest design conditions when heat pump capacity derates significantly.
Comparing RTU to Central AHU and Chiller Plant Approaches
For buildings large enough or tall enough that roof-mounted equipment and ductwork runs become impractical, a central AHU connected to a chiller plant, covered by the AHU sizing calculator and chiller plant calculator, generally becomes the more economical and practical choice, a crossover point the ASHRAE Handbook, HVAC Systems and Equipment volume discusses across its separate unitary equipment and central plant chapters.
Frequently Asked Questions
A single-zone constant volume RTU serves one thermal zone directly, cycling on and off or modulating capacity to match that single zone's load, common in smaller retail units or single-tenant spaces. A multi-zone VAV RTU serves several zones through separate VAV boxes downstream, varying airflow to each zone independently while the RTU itself modulates total supply airflow and capacity to match combined system demand.
Building size, roof structural capacity, and zoning requirements all factor into RTU count. A single-story retail building might use one RTU per few thousand square feet of sales floor, while larger buildings often split across multiple units to limit how much space loses conditioning if one unit needs service, and to better match zone-specific load patterns across different building orientations.
RTU compressors and outdoor coils sit directly exposed to outdoor conditions, and Canadian winters bring extended periods of extreme cold that can affect refrigerant pressures and coil frost formation. Units specified for Canadian climates need low-ambient cooling controls that allow operation at low outdoor temperatures for shoulder-season cooling calls, plus a reliable defrost cycle for the heating mode outdoor coil in heat pump configurations.
Gas-fired RTU heating remains common in many Canadian commercial applications due to lower operating cost per unit of heat in regions with low natural gas rates relative to electricity. Heat pump RTUs are gaining adoption where electrification incentives or lower Canadian electricity rates make them competitive, though capacity typically derates at very cold outdoor temperatures and may need backup electric resistance heat for extreme cold snaps.
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