A commercial HVAC system does three jobs: it adds or removes heat, it brings in outdoor air and exhausts stale air, and it moves conditioned air or water to the spaces that need it. Every system, from a single rooftop unit to a central plant with chillers and boilers, is a combination of equipment that does those three jobs, held together by controls.
The three jobs: heat, ventilation and distribution
It helps to separate what the system does from the equipment that does it.
- Heating and cooling changes the temperature, and often the humidity, of air or water. Heat comes from burning gas or oil, from electric resistance, or from a heat pump moving heat from one place to another. Cooling almost always uses a refrigeration cycle.
- Ventilation replaces some indoor air with outdoor air to dilute odours, carbon dioxide and contaminants. Building codes set minimum ventilation rates, commonly by reference to ASHRAE 62.1 for commercial spaces.
- Distribution delivers heating, cooling and fresh air to rooms through ducts, pipes, fans and pumps.
When a space is uncomfortable, the cause is usually a failure in one of these three jobs, and knowing which one narrows the search.
Common system types
Packaged rooftop units
Many low-rise commercial buildings in Canada are served by rooftop units. Each unit contains a fan, filters, a gas or electric heating section, a refrigeration circuit for cooling and an outdoor air damper, all in one box. Each unit serves one zone or a group of zones through ducts. They are simple to replace one at a time, but a large building may have dozens of them to maintain.
Central air handlers with a central plant
Larger buildings often use air handling units in mechanical rooms. The air handler contains fans, filters and coils, but the heating and cooling come from a central plant: boilers that make hot water or steam, and chillers that make chilled water. Pumps move that water through pipes to the coils. This arrangement is more complex but can be more efficient and easier to maintain in one place.
Terminal units and zoning
A central air handler supplies many rooms, and each room needs different amounts of heating or cooling. Variable air volume (VAV) boxes adjust how much air each zone receives, often with a reheat coil to warm air for perimeter offices in winter. Other buildings use fan coil units, baseboard radiators, radiant panels or heat pumps in each zone.
Split systems and heat pumps
Smaller spaces may use split systems, with an indoor fan coil and an outdoor condensing unit joined by refrigerant piping. Variable refrigerant flow (VRF) systems extend this idea to many indoor units on one outdoor system. Heat pumps, which can both heat and cool, are increasingly common, including in colder parts of the country. See our guide to heat pumps in cold climates.
Key components and what they do
| Component | What it does | Common problems |
|---|---|---|
| Filters | Catch dust and particles before they reach coils and occupants | Overdue changes, poor fit, air bypassing the filter |
| Coils | Transfer heat between air and water or refrigerant | Dirt build-up, freeze damage, leaks |
| Fans and motors | Move air through ducts | Worn belts and bearings, wrong speed settings |
| Dampers | Control outdoor air, return air and zone airflow | Seized linkages, failed actuators |
| Pumps | Move hot or chilled water | Seal leaks, bearing wear, air in the system |
| Controls | Decide when and how hard equipment runs | Overridden schedules, failed sensors, poor sequences |
Controls hold it all together
Equipment does what the controls tell it to. In small buildings that may be a thermostat on each unit. In larger buildings, a building automation system schedules equipment, resets temperatures based on outdoor conditions, and coordinates the plant with the air handlers. Many comfort and energy problems are control problems: a schedule overridden years ago, a sensor in the wrong place, or an economizer that never opens. Our guide to building automation systems goes into more detail.
Signs your system needs a closer look
- Hot and cold complaints that move around the building or change with the season.
- Stuffy rooms, lingering odours or condensation on windows.
- Equipment running at night or on weekends when the building is empty.
- Heating and cooling running at the same time in the same area.
- Rising energy bills without a change in use.
- Noisy fans or pumps, water around equipment or frequent breakdowns.
Leave the equipment to qualified trades. Gas-fired equipment, refrigeration circuits and electrical components must be serviced by licensed technicians. Building staff can change filters and report problems, but should not open burners, refrigerant lines or energized panels.
How ehZee can help
Our mechanical system inspections look at your heating, cooling, ventilation and controls together, and explain how the pieces are working as a system. We start by asking why you need the inspection, then give you a written report with findings ranked by priority.
Common questions
What is the difference between an air handler and a rooftop unit?
A rooftop unit is packaged equipment that contains its own heating and cooling. An air handler usually relies on hot and chilled water from a central plant and sits indoors in a mechanical room.
Why does the system bring in cold outdoor air in winter?
People need fresh air whatever the weather, and codes set minimum ventilation rates. The system heats the outdoor air, and heat recovery equipment can reduce the energy that takes.
How long does commercial HVAC equipment last?
It varies widely with the type of equipment, how hard it works and how well it is maintained. An inspection that records age and condition is a better guide than a rule of thumb.