Office building HVAC

Office buildings make up a large share of Canada's commercial floor space, from three-storey suburban buildings to downtown towers in Toronto, Calgary and Vancouver. Their HVAC systems look straightforward on paper: keep people comfortable during business hours and supply enough outdoor air. In practice, offices are where most comfort complaints, tenant disputes and avoidable energy waste happen.

This guide covers the main design drivers, the systems commonly used, and the operational issues that matter most to owners and property managers.

What drives office HVAC loads

Office loads are dominated by the building envelope at the perimeter and by internal gains in the core.

• Perimeter zones, typically the outer 4 to 5 m (13 to 16 ft) from the exterior wall, see solar gain, conduction through glazing and winter heat loss. A south or west perimeter office can need cooling on a sunny January afternoon while a north office on the same floor needs heat.
• Interior zones see only internal gains from people, lighting and equipment. They need cooling almost year-round, even in a Canadian winter.
• Internal gains have dropped significantly over the past two decades as LED lighting and laptops replaced fluorescent fixtures and desktop towers. Older design assumptions of 15 to 20 W/m2 (1.4 to 1.9 W/ft2) for plug loads are often higher than actual measured loads in modern offices.

Because of these differences, good zoning separates perimeter from core and separates each exposure. A single thermostat serving both a glass corner office and an interior meeting room will satisfy neither.

Ventilation

Office ventilation is designed to ASHRAE 62.1 through Part 6 of the NBC or the provincial building code. For office space the standard specifies 2.5 L/s per person (5 cfm) plus 0.3 L/s per m2 (0.06 cfm/ft2), with a default density of 5 people per 100 m2. Conference rooms are much denser and need roughly three to four times the outdoor air per unit area of open office.

Because actual occupancy in many offices now varies widely from day to day with hybrid work, demand-controlled ventilation in meeting rooms and large open areas can save considerable heating energy in winter.

Common office system types

The dominant system in Canadian office towers remains central air handlers feeding VAV terminals, with perimeter heating from hydronic radiation or reheat coils. Our guide to constant volume versus VAV systems explains why VAV became the standard.

The shift toward decoupled systems

Newer office designs increasingly decouple ventilation from heating and cooling. A dedicated outdoor air system with energy recovery handles ventilation and dehumidification, while VRF, fan coils, radiant panels or chilled beams handle space loads. This lets the central fans shrink considerably and makes electrification easier, since heat pumps can serve the terminal systems.

Comfort targets

ASHRAE Standard 55 defines thermal comfort conditions. Typical office design setpoints fall in these ranges:

• Winter: 20 to 22 C (68 to 72 F) at 20 to 40 percent RH, with lower RH typical in Canadian winters unless humidification is provided.
• Summer: 23 to 25 C (73 to 77 F) at up to about 60 percent RH.

Air movement, radiant asymmetry from cold glass and drafts matter as much as dry-bulb temperature. Many perimeter complaints in winter come from cold downdrafts off tall glazing, which perimeter heating directly beneath the window is designed to counter.

Tenant fit-outs

In multi-tenant buildings, the base building system is installed first, then each tenant's space is designed and built later. This is where office HVAC most often goes wrong.

Common fit-out problems include:

1. New walls built without adding thermostats or terminals, leaving enclosed offices served by a sensor in another room.
2. Diffusers relocated without rebalancing, leaving some rooms starved and others overcooled.
3. Meeting rooms and server closets added without accounting for their higher loads.
4. Return air paths blocked by full-height partitions in plenum-return buildings.
5. Base building VAV boxes reused far outside their intended airflow range.

Landlords can avoid most of these with clear tenant design standards, a base building engineer review of each fit-out, and a requirement for air balancing and controls verification before occupancy.

Energy performance

Offices have well-understood energy saving opportunities, most of them in controls rather than equipment:

• Schedules: systems running far beyond actual occupied hours are the most common waste found in retro-commissioning.
• VAV minimums: zone minimum airflows set too high drive reheat energy and overcooling.
• Supply air temperature and static pressure reset: both reduce fan and reheat energy when applied correctly.
• Economizer operation: stuck or poorly controlled economizers waste free cooling or overload heating coils.
• Simultaneous heating and cooling: perimeter radiation fighting overhead cooling in the same zone.

New offices must meet the energy code adopted by the province, typically NECB 2017 or 2020, or in some provinces ASHRAE 90.1 or a municipal step code.

Frequently asked questions

Why is my office too cold in summer?

The most common causes are VAV boxes with minimum airflows set too high, thermostats located in the wrong room, and supply air too cold for the actual load. Measuring airflow and reviewing trend data usually identifies the cause. Our guide to hot and cold complaints covers this in detail.

Should we reduce ventilation when the office is half empty?

Ventilation can safely track actual occupancy using CO2 or occupancy-based control, provided the area-based component of ASHRAE 62.1 is still delivered. Reducing ventilation without measurement or control is not recommended.

Key takeaways

• Separate perimeter and interior zones, and separate each exposure, to avoid chronic comfort complaints.
• VAV with reheat remains common, but decoupled DOAS-based systems are increasingly used in new offices.
• Actual plug and lighting loads in modern offices are often lower than older design assumptions.
• Tenant fit-outs are the biggest source of HVAC problems in multi-tenant buildings, so set standards and require balancing.
• Most energy savings in existing offices come from schedules, VAV minimums and resets rather than new equipment.