HVAC systems and equipment

Lesson 4 explained what HVAC systems must do: remove or add heat, bring in outdoor air and control moisture. This lesson explains how they do it. Buildings use a wide range of system types, from a single rooftop unit on a store to a central plant of chillers, boilers and air handlers serving a hospital. Each has different equipment, space needs, trades, lead times, costs and maintenance demands, and the choice made early in design shapes structure, roofs, shafts, ceilings and electrical service.

The construction manager should be able to recognize the main system types on drawings, understand what each piece of equipment does, anticipate the installation and coordination work it brings, and know what testing it needs before handover.

Ways to classify HVAC systems

One useful way to classify systems is by the medium that carries heat to the occupied space.

• All-air systems condition air in a central unit and deliver it through ducts: rooftop units, constant volume and variable air volume (VAV) systems.
• Air-water systems deliver treated outdoor air through ducts and handle most heating and cooling with water: fan coils, chilled beams and induction units.
• All-water systems use hydronic terminals such as radiators, radiant floors or panels, with separate ventilation.
• Refrigerant systems move refrigerant directly to indoor units: split systems and variable refrigerant flow (VRF).

Systems can also be classified as central (a plant serving the whole building) or decentralized (packaged units serving zones), and by heat source: gas, electric resistance, heat pump or district energy.

Air handling units, rooftop units and VAV

An air handling unit (AHU) is a large box containing fans, filters, coils for heating and cooling, dampers to mix outdoor and return air, and often energy recovery and humidification. A packaged rooftop unit (RTU) combines an air handler with its own refrigeration compressor and, often, a gas heat section in one factory-built unit.

In a constant volume system, the fan delivers the same airflow all the time and varies the supply temperature. In a variable air volume (VAV) system, the AHU supplies cool air at a roughly constant temperature, and VAV terminal boxes in each zone throttle the airflow to match the zone's need, often with a reheat coil for perimeter zones in winter. VAV saves fan energy and is the most common system in mid-size and large offices in North America.

Supply airflow is set by the zone's sensible cooling load and the temperature difference between supply and room air.

Ductwork

Ducts carry air between equipment and spaces. They are usually galvanized sheet steel, sometimes aluminium, stainless steel, fibreglass duct board or fabric, built to SMACNA standards for construction and sealing and to the pressure class the engineer specifies. Duct velocity affects noise and pressure drop: main ducts in offices typically run at about 5 to 10 m/s (1,000 to 2,000 fpm), lower near occupied spaces.

Ducts are bulky and are usually the first MEP element to fight for ceiling space. Field changes, such as flattening a duct to fit under a beam, raise velocity, noise and pressure drop and can starve downstream zones, so they need the engineer's review. Fire dampers and smoke dampers where ducts cross fire separations must be installed with access for inspection and testing.

Hydronic systems, boilers and chillers

Hydronic systems move heat with water, which carries far more heat per unit volume than air. A boiler heats water, often to 60 to 80 °C (140 to 180 °F) for conventional systems or lower for condensing boilers and heat pumps. A chiller cools water, typically to about 6 or 7 °C (42 to 45 °F). Pumps circulate the water through piping to coils in air handlers, fan coils, radiators or radiant floors.

Chillers are air-cooled, rejecting heat to outdoor air through fans and coils, or water-cooled, rejecting heat through a cooling tower. Water-cooled plants are more efficient but need towers, condenser water pumps, water treatment and Legionella management. Heat recovery chillers and heat pumps can move heat from cooling loads to heating loads at the same time.

Water flow is set by the load and the temperature difference across the coil.

In SI units, heat in kilowatts equals about 4.19 × flow in litres per second × temperature difference in kelvin.

Heat pumps, VRF and electrification

A heat pump uses a refrigeration cycle to move heat from a colder place to a warmer one. In heating mode it extracts heat from outdoor air, the ground or a water loop and delivers it indoors, using electricity to drive the compressor. Its efficiency is expressed as the coefficient of performance (COP): heat delivered divided by electrical energy used. A COP of 3 means three units of heat for every unit of electricity, which is why heat pumps are central to decarbonization policies.

Air-source heat pump capacity and COP fall as outdoor temperature drops. Cold-climate heat pumps keep useful capacity at −25 °C (−13 °F) or lower, but designers still check capacity at the winter design temperature and may add backup heat.

Variable refrigerant flow (VRF) systems connect one or more outdoor heat pump units to many indoor units through refrigerant piping, with branch controllers allowing some zones to heat while others cool. They save ceiling space compared with ducts but put large quantities of refrigerant inside the building, so refrigerant safety standards limit the charge that can serve a small room.

Refrigerant rules are changing. Canada's Ozone-depleting Substances and Halocarbon Alternatives Regulations phase down hydrofluorocarbons (HFCs) and set global warming potential (GWP) limits for new equipment in several categories from the mid-2020s, and CSA B52 governs refrigeration system safety, including the newer mildly flammable A2L refrigerants.

For the constructor, refrigerant transitions affect equipment availability, submittals, installer training and leak detection requirements in machine rooms.

Energy recovery and dedicated outdoor air

Heat recovery ventilators (HRV) transfer heat between outgoing exhaust air and incoming outdoor air; energy recovery ventilators (ERV) also transfer moisture. They are standard in Canadian houses and increasingly required in commercial buildings by energy codes. A dedicated outdoor air system (DOAS) handles all ventilation air in a separate unit, often with energy recovery, while fan coils, chilled beams, radiant panels or VRF handle the remaining heating and cooling. Separating ventilation from temperature control makes both easier to control and reduces fan energy.

Installation, start-up and testing, adjusting and balancing

HVAC installation brings heavy lifts, rigging and roof penetrations, structural supports and seismic restraints, vibration isolation, piping and duct pressure tests, insulation, control wiring and equipment start-up by factory technicians. Before handover, an independent testing, adjusting and balancing (TAB) contractor measures and adjusts airflows at every diffuser and water flows at every coil to match the design, and reports the results.

TAB agencies in Canada commonly follow AABC or NEBB procedures, which are used across North America, and the specification usually names the standard and requires a certified supervisor.