NC HVAC key terms

Heat that changes the temperature of air or another substance without changing its phase. In comfort cooling it is the portion of the load that shows up on a dry-bulb thermometer.

Heat absorbed or released when a substance changes phase, such as water evaporating or vapor condensing, with no change in temperature. In buildings it is tied mainly to moisture removal and humidity control.

The total heat content of moist air per pound of dry air, combining sensible and latent energy. Technicians use it on a psychrometric chart to find coil loads and airflow requirements.

The ordinary air temperature measured by a thermometer exposed to the air but shielded from radiation and moisture. It is the number most people mean when they say room temperature.

The lowest temperature air can reach through evaporative cooling, measured with a thermometer whose bulb is kept wet. It indicates how much moisture the air can still absorb.

The amount of water vapor in air expressed as a percentage of the maximum the air could hold at that temperature. It changes as air is heated or cooled even when the actual moisture content stays the same.

The temperature at which air becomes saturated and water vapor begins to condense. Surfaces colder than the dew point will sweat, which drives decisions about insulation and coil temperatures.

A graph that plots the properties of moist air, including dry-bulb, wet-bulb, dew point, humidity ratio, and enthalpy. It is used to analyze air-handling processes such as heating, cooling, and humidifying.

The repeating sequence in which refrigerant absorbs heat at low pressure, is compressed, rejects heat at high pressure, and expands back to low pressure. These four steps move heat from a cooler space to a warmer one.

The pump in a refrigeration system that raises refrigerant pressure and temperature so it can reject heat. It is the component that consumes most of the system's electrical energy.

The heat exchanger where hot, high-pressure refrigerant vapor gives up heat to outdoor air or water and condenses into a liquid. Its ability to reject heat depends on airflow and cleanliness of the coil.

The indoor heat exchanger where low-pressure liquid refrigerant boils and absorbs heat from the air stream. Its temperature must stay below the room dew point to remove moisture.

A restriction such as a thermostatic expansion valve or fixed orifice that drops refrigerant pressure between the condenser and evaporator. It also regulates how much refrigerant enters the evaporator coil.

The number of degrees a refrigerant vapor is above its saturation temperature at a given pressure. Measuring it at the evaporator outlet shows whether the metering device is feeding the coil correctly.

The number of degrees a liquid refrigerant is below its saturation temperature at a given pressure. Measuring it at the condenser outlet indicates how much liquid is stacked in the condenser.

A system that uses a reversing valve and refrigerant cycle to both heat and cool a space by reversing the direction of heat flow. In heating mode the outdoor coil becomes the evaporator.

A timed or demand-initiated mode in which a heat pump temporarily reverses or stops heating to melt frost from the outdoor coil. Auxiliary heat usually runs during defrost so the space does not cool off.

The outdoor temperature at which a heat pump's heating capacity exactly equals the building's heat loss. Below that point supplemental heat must make up the difference.

A room-by-room method for estimating heating and cooling loads from envelope, window, infiltration, and internal gain data. Sizing equipment from a load calculation prevents short cycling and poor humidity control.

The outdoor winter and summer conditions a system is sized to handle, chosen from local weather data. North Carolina practice uses published winter and summer design values for the project location.

A duct design method that sizes ducts from room airflow and available static pressure so each branch delivers its required cfm. It accounts for friction loss and fitting losses in the distribution system.

The resistance to airflow, expressed in inches of water column, that the blower must overcome in the duct system and components outside the unit. It is compared to the blower's rated performance to set airflow.

The friction loss of a duct fitting expressed as the length of straight duct that would create the same pressure drop. Adding equivalent lengths to measured runs gives the total effective length used for sizing.

The volume of air moved per minute, usually stated in cubic feet per minute. Proper airflow is required for both capacity and safe combustion in fuel-fired equipment.

Air drawn from conditioned spaces back to the air handler for reconditioning. Undersized or restricted returns are a common cause of high static pressure and low delivered capacity.

The air supplied to a fuel-burning appliance for burning gas or oil. Codes require a permanent, adequately sized source so burners operate safely and do not backdraft flue gases.

The passage or pipe that carries products of combustion from an appliance to the outdoors. It must be properly sized, sloped, and vented to prevent spillage of carbon monoxide into the building.

The flow of air and combustion gases through an appliance and vent system, created by natural buoyancy or a mechanical inducer. Proper draft keeps flue gases moving up and out of the building.

A colorless, odorless gas produced by incomplete combustion that can be fatal at low concentrations. Installation of alarms and verification of proper venting are core safety requirements.

A device that reduces and holds fuel gas pressure at a safe, steady level for the appliance burner. Manifold pressure is checked with a manometer during startup and service.

The gas pressure measured at the appliance burner manifold, typically a few inches of water column for natural gas. It is set by the appliance regulator and verified with a manometer.

A temperature-sensing control that cycles heating or cooling equipment to hold a setpoint. Modern programmable and communicating models also manage fan modes, schedules, and staging.

The documented order in which a control system starts, stages, and stops equipment in response to conditions. Understanding it is essential for troubleshooting and for confirming safe operation.

A control state in which the equipment is prevented from running after a fault or safety trip until it is manually reset. It protects the compressor and burner from repeated unsafe starts.

A safety control that opens the compressor circuit when discharge pressure exceeds a set limit. It protects the system from damage caused by blocked airflow, overcharge, or failed condenser fans.

The process of removing refrigerant from a system into a recovery cylinder for reuse, recycling, or disposal. Venting refrigerant to the atmosphere is prohibited under federal clean air rules.

The fluid that circulates through a refrigeration system, changing phase to absorb and release heat. Its pressure-temperature relationship determines system operating pressures and safety hazards.

Official authorization from the local code authority to perform regulated work such as installing or replacing mechanical equipment. Work must generally be inspected and approved before it is covered or placed in service.

A code official's on-site review of permitted work at defined stages to confirm compliance with adopted codes. Failed inspections must be corrected and reinspected before the job can proceed.

The adopted set of regulations governing installation, alteration, and maintenance of HVAC and mechanical systems in a jurisdiction. It sets requirements for clearances, ventilation, venting, and equipment approval.

A written modification to the contract scope, price, or schedule agreed to by the owner and contractor. Unpriced or undocumented changes are a leading source of disputes in HVAC projects.

The written description of the work, materials, and responsibilities a contractor is obligated to perform under a contract. It defines what is included and, by omission, what is excluded.

A legal claim against property filed by a contractor or supplier to secure payment for labor or materials provided. Notice and filing deadlines are strictly set by state statute.

A safety procedure that isolates hazardous energy sources and marks them so equipment cannot be energized during service. Only the person who applied the lock may remove it.

Wearable gear such as gloves, eye protection, and respirators that reduces a worker's exposure to hazards. It is the last line of defense after engineering and administrative controls.