Loads & Energy Modeling for a 13-Storey Hospital Tower

Building Loads & HVAC Optimization

Energy model geometry of a 13-storey hospital tower on a three-storey clinical podium, with surrounding buildings modeled for shading. Representative model shown; project details have been changed to protect client confidentiality.

Case Study Summary

Service	Building Loads & HVAC Optimization, with a parallel NECB 2020 Energy Modeling compliance workstream
Building Type	Hospital, 13 storeys: patient rooms, operating rooms, imaging suites, labs, and support space
Location	Canada (Climate Zone 7A)
Floor Area	≈24,000 m² (≈258,000 ft²)
Software	IES-VE
Rooms Modeled Individually	1,114
Central Plant	Condensing gas boilers; magnetic-bearing chillers plus a heat-recovery chiller; four main AHUs with enthalpy-wheel heat recovery
Compliance Result	20.8% better than the NECB 2020 reference building; ≈440 tonnes CO₂e saved annually

The Challenge

A 13-storey, 24,000 m² hospital tower packs an unusually dense mix of clinical space types into one building: MRI and CT suites, operating rooms, clinical labs, a laundry, patient care areas, and standard office and support space, each with its own occupancy pattern, equipment load, and code-mandated airflow requirement. Two different disciplines needed two different, defensible numbers from essentially the same building at the same time. The mechanical designer needed airflow requirements for every room, reconciled against CSA’s healthcare ventilation standards, to size the AHUs correctly, and the project needed a defensible NECB 2020 energy compliance result for that same design. In a hospital, getting either wrong isn’t a minor miss: undersized ventilation in a clinical space is a life-safety and code issue, not just an efficiency one.

Our Approach

Room-by-room load modeling

All 1,114 individual rooms in the tower were built out as a full internal-gains model in IES-VE. Instead of typical space-type averages, each room has its own entries for lighting power density, occupancy, plug loads, and named specialty equipment (CT scanners, MRI equipment, laundry machines, lab refrigeration, and more), each captured as its own distinct internal gain. Every room’s simulated heating and cooling load reflects what’s actually inside that specific room, not a category-level assumption applied building-wide.

CSA airflow reconciliation

For every one of those 1,114 rooms, two independently-derived airflow numbers were compared side by side: the CFM required to meet CSA’s minimum air-change-rate requirement for that room’s healthcare occupancy classification, and the CFM required to offset the room’s simulated cooling load — frequently driven higher by solar exposure or dense equipment loads in spaces like labs and imaging suites. The higher of the two, room by room, set the final design airflow, and that room-by-room maximum rolled up into the sizing basis for the building’s AHUs.

This wasn’t a one-size-fits-all check. CSA’s air-change minimums assume a fairly generic space; a CT or MRI suite with heavy equipment heat gain, or a south-facing lab with significant solar exposure, often needs meaningfully more airflow than the code minimum just to stay within its cooling setpoint — in which case the IES-VE load result, not the CSA minimum, governed. Reconciling both numbers room by room, rather than applying a blanket CSA multiplier across the building, is what let the mechanical designer size every AHU for the room that would actually be built, not an averaged approximation of it.

Boiler curve customization

Rather than use IES-VE’s generic default boiler performance curve, the actual condensing boilers’ manufacturer performance table (efficiency as a function of firing rate and return-water temperature) was fitted to a custom bi-cubic efficiency curve in IES-VE, reproducing the manufacturer’s published data to within about 1 percentage point across its full operating range. That distinction matters for a hospital heating plant in a cold Canadian climate: condensing efficiency rises as return-water temperature drops, so a hot-water-supply-reset strategy during shoulder seasons produces a real, quantifiable efficiency gain that a generic default curve would have missed entirely.

Heat-recovery chiller & winter cooling

Imaging equipment and other dense internal loads mean some spaces in the building need active cooling even in the middle of a Canadian winter, a condition a typical seasonal heating/cooling model doesn’t handle well. The design’s modular water-to-water heat pump system was represented as two heat-recovery modules connected to both the chilled- and heating-water loops (recovering condenser heat to offset boiler load during simultaneous heating and cooling) plus a third module modeled as a dedicated heating-only unit, sequenced ahead of the boilers as a preheat stage. All of it was built from manufacturer-published capacities, efficiencies, and water temperatures rather than simplified assumptions.

Results

1,114

Individual rooms modeled with their own internal gains

20.8%

Better annual energy performance than the NECB 2020 reference building

≈440t

CO₂e saved annually versus the reference building

The proposed design’s energy use intensity came in at 390 kWh/m²/yr against 492 kWh/m²/yr for the NECB 2020 reference building, a 20.8% improvement. The biggest drivers were enthalpy-wheel heat recovery (71.3% sensible effectiveness versus a 50% baseline), a reduced window-to-wall ratio, and the heat-recovery chiller’s contribution to the heating plant. Unmet heating and cooling hours stayed within the NECB’s required tolerance in both the proposed and reference models, confirming the HVAC systems as modeled could actually maintain their setpoints, not just produce a favorable energy number.

Proposed vs. NECB 2020 reference building energy use by end use. The MRI/CT equipment load is identical in both models because it is fixed by the equipment itself, not something the design can improve, which is exactly why it needed to be modeled explicitly rather than folded into a generic plug-load assumption. Fan energy is higher in the proposed design, but the much larger drop in space-heating energy more than offsets it.

Why It Mattered

For a hospital, the room-by-room load model became the shared foundation for both disciplines. The mechanical designer used it to size airflow defensibly, room by room, against both CSA requirements and each space’s actual equipment and solar loads. The same underlying model then rolled forward into the NECB 2020 compliance submission that showed the design beating code by 20.8%. Building both from one coordinated model, rather than as two disconnected exercises, meant the HVAC sizing and the energy compliance story were never at odds with each other: every AHU was sized for the building that was actually going to be built, and modeled to match it.