Beyond Spare Footage: 5 Factors to Consider When Sizing HVAC Systems

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Properly sizing your HVAC system is critical to achieving optimal performance. If the system is undersized, it will run more often and strain to maintain the desired setpoint. Oversized unit won’t cycle on long enough to fully dehumidify the space and will experience space temperature swings. In either case, the system won’t operate efficiently, wasting energy and money, and it likely won’t provide optimal comfort levels at all times, leading to unhappy occupants.

Many manufacturers have calculators to help with system design and capacity size. No matter your method, it’s critical to think beyond just the building’s square footage and geography. Here are a few other factors that should play into system capacity for heating, cooling, and ventilation.

Building functionality

Whether you’re sizing an entire building or an individual floor/tenant, it’s important to consider how the space is going to be used and what kind of equipment will be in operation. An office building floor with data servers or filled to the brim with telemarketers will generate more heat than a floor with large offices and fewer people. Everything adds up, so be sure to dial-in your calculations to include copiers, coffee makers, lighting, the number of people, type of activity, etc.


Often, calculations only consider outside temperatures and people. But if you’re using an air handler like Geniox to bring in fresh air via a heat recovery ventilator or energy recovery ventilator, that will also need to be factored in. And just like heating and cooling, the ventilation system size should take into account the building and how the space will be used, or it also won’t operate properly.

Insulation values in the walls, ceilings, and roof.

Choose your “Design Day” wisely

Calculate based on the building or space’s design day based on ASHRAE climatic data — utilizing the extreme hottest or coldest days will not result in proper unit sizing as these are extremes, exceeded a small percentage of the year.

Interior layout

Consult with the architect to determine if there are any interior features that might impact the flow of air. These might include partitions, large fixtures, or other anomalies. This also applies if the space has a large conference room or gathering space—the system will need to accommodate a larger influx of bodies from time to time.

More precise calculations for both heating/cooling and ventilation may result in a higher upfront cost, but it’s important for designers and owners to consider the total cost of ownership. Investing in the right equipment up front will improve efficiencies down the road.

To learn more ventilation system sizing, connect with Systemair at  

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