Heating, Ventilation, and Air-Conditioning (HVAC) Systems

Most HVAC systems have two cycles: the refrigeration cycle and the air distribution cycle.

Refrigeration Cycle

Refrigeration is a process that removes heat, rather than adding cold. The process of refrigeration transfers heat from one place to another. Heat flows from an object or substance of a higher temperature to one of a lower temperature. In refrigeration, optimizing the rate of heat transfer is important.

Click here to view the Refrigeration Cycle Diagram.

A mechanical refrigeration system consists of four main components:

1. The compressor compresses the refrigerant gas and sends the compressed gas to the condenser.
2. The condenser is a heat exchanger that removes heat from the hot compressed gas and allows it to condense into a liquid. The liquid refrigerant is then routed to a metering device.
3. The metering device restricts the flow by forcing the refrigerant to go through a small hole that causes the pressure drop in the liquid and allows it to begin to evaporate into a gas.
4. The evaporator is where the refrigerant evaporates. It is then routed back to the compressor, completing the cycle. The refrigerant is used repeatedly, absorbing heat from one area and relocating it to another, generally through a cooling tower in a large central system and a condenser in a small apartment unit.

Air conditioners are rated by the number of British Thermal Units (BTUs) of heat they can remove per hour. Another common rating term for air conditioner size is the "ton." One ton of refrigeration extracts or absorbs 12,000 BTUs of heat per hour.

Air Distribution Cycle

While the refrigeration cycle works to remove heat, the air distribution cycle works to move air evenly throughout a building or space. The air distribution system involves some type of air handling unit that includes fans, ducts, filters, dampers, and heating and cooling coils. The process of air-conditioning involves bringing outside air in to provide a positive, higher-than-outside pressure in the building. The building becomes a pressurized box, which prevents infiltration of unfiltered, unconditioned outside air through any openings such as doors or cracks.

Air brought into a building from the outdoors (often through the ventilation system) that has not been previously circulated through the system is known as "Make-Up Air." Make-Up Air is brought into the building through fresh air dampers. The amount of fresh air brought into a building can be regulated manually or automatically with the dampers.

HVAC System Management

Energy management systems are available that regulate HVAC systems to conserve energy. These systems can be programmed to maintain an optimum temperature level by taking readings throughout the day. The system raises and lowers temperatures at certain times of the day (when energy costs are lower) and turns systems off and on. The use of energy management systems can help prevent people from tampering with thermostats, which can waste a tremendous amount of energy.

• Know the operating parameters, hours of operation, and levels of tenant comfort for the property.
• Identify maintenance objectives as specifically as possible.
• Determine the maintenance responsibilities to be handled by the staff and outside contractors.
• Budget carefully to support maintenance objectives, including the purchase of software upgrades for monitoring equipment.
• Refer to equipment manuals for preventive maintenance routines. These routines tell the property manager which work the staff can do and which work needs to be contracted.
• Sometimes, a total service contract for HVAC maintenance is appropriate. The contract then becomes a budgeted monthly payment based on a negotiated price. The contractor has an incentive to keep the HVAC system in top condition to avoid the cost of equipment failures. A contractor may be able to offer energy-saving recommendations and equipment retrofits to keep the HVAC system efficient and up-to-date. In-house personnel often do not have this expertise.
• Consider special insurance, such as Boiler and Equipment Coverage, for sophisticated HVAC systems.
• Test and Balance, using a HVAC contractor, to maintain peak HVAC efficiency. This helps measure air flow through dampers to ensure even air distribution.