HI HVAC key terms

Heat that changes the temperature of air or a substance without changing its state. In HVAC load work it drives dry-bulb temperature rise and is measured with a thermometer.

Heat absorbed or released when a substance changes state, such as water evaporating or vapor condensing, with no change in temperature. In comfort cooling it is tied to moisture removal.

The air supplied to a fuel-burning appliance so the gas or oil can burn completely and safely. Shortage causes incomplete combustion, carbon monoxide, and flame rollout.

Air drawn into a vent or flue alongside flue gases to reduce their concentration and temperature before discharge outdoors. It is distinct from the combustion air fed to the burner.

The pressure difference that moves flue gases up a chimney or vent and out of the building. Natural draft depends on warm gas rising; induced draft uses a fan to create the pull.

A hinged plate in the flue connector of an oil or gas appliance that opens to admit room air and holds a steady negative pressure in the vent. It stabilizes draft over firing rates.

The ratio of heat delivered to the conditioned space to the energy content of fuel consumed over a heating season, expressed as a percentage. It accounts for steady-state efficiency plus cycling and jacket losses.

A gas furnace designed to cool flue gases enough that water vapor condenses, recovering latent heat and reaching efficiencies above roughly 90 percent. It needs a drain for condensate and often PVC venting.

A control strategy that raises boiler or supply water temperature as outdoor air gets colder, matching heat output to building load. It improves efficiency and reduces overheating during mild weather.

A fuel-fired or electric appliance that heats water or steam for space heating or domestic hot water. It is not a furnace, because the heat is distributed by piping rather than ducted air.

The number of times the total volume of air in a space is replaced in one hour. It is used to judge ventilation adequacy, infiltration, and required exhaust for a room.

Unintentional outdoor air leaking into a building through cracks, joints, and penetrations driven by wind and pressure differences. It adds sensible and latent load and can upset pressure balance.

Outdoor air brought in to replace air exhausted from a building by fans, hoods, or dryers. Without enough of it, the building goes negative and appliances may backdraft.

Mechanical removal of air from a space using fans, such as bathroom, kitchen, or laboratory exhaust. It controls pollutants at the source and must be balanced against supply and makeup air.

Reversal of normal flue flow, causing combustion products to spill into the living space instead of going up the vent. It is commonly caused by depressurization from exhaust fans or blocked flues.

A colorless, odorless gas produced by incomplete combustion that binds to hemoglobin and can be fatal. It is measured in parts per million, and low-level monitors alert occupants before symptoms appear.

Minimum Efficiency Reporting Value, a 1-to-16 scale (higher with HEPA extensions) describing how well a filter captures particles of a given size. Higher ratings remove more fine particles but add static pressure.

An organic chemical that vaporizes at room temperature from paints, adhesives, cleaners, and furnishings. VOCs are an indoor air quality concern addressed by source control, ventilation, and filtration.

The repeating process in which refrigerant is compressed, condensed, expanded, and evaporated to move heat from a cooler space to a warmer one. It is the basis of most air conditioning and refrigeration.

The number of degrees a refrigerant vapor is above its saturation temperature at a given pressure. Measured at the evaporator outlet, it confirms the expansion valve is feeding enough refrigerant.

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

The coil where low-pressure refrigerant absorbs heat and boils, cooling the air or fluid passing over it. It is the indoor coil in cooling mode and the outdoor coil in a heat pump's heating mode.

The coil where high-pressure refrigerant vapor rejects heat and condenses to liquid. It is the outdoor coil in cooling mode and must shed both sensible and latent heat picked up by the system.

Thermostatic expansion valve, a modulating metering device that maintains a set superheat at the evaporator outlet by varying refrigerant flow. It responds to bulb temperature and evaporator pressure.

The temperature difference between the dew point and bubble point of a zeotropic refrigerant blend at a given pressure. It affects charging and coil design and must be considered when setting superheat.

The outward push of air against duct walls, expressed in inches of water column. In duct design it represents the resistance the blower must overcome to deliver design airflow.

The straight-duct length that would produce the same friction loss as a fitting such as an elbow or tee. Adding it to measured run length gives the total effective length used in duct sizing.

The pressure lost per unit length of duct, usually inches of water per 100 feet. It is the target used in equal-friction duct sizing and depends on airflow, duct size, and roughness.

A box or chamber attached to the supply or return side of air-handling equipment that distributes or collects air before it enters ducts. It must be sized and sealed to limit turbulence and loss.

A device that keeps a small amount of water flowing into a floor drain or condensate trap so the seal does not evaporate. A dry trap lets sewer gas into the space.

The pipe that carries water removed from the air by a cooling coil to a drain. It needs a trap and slope so it drains freely without air being pulled through it.

A tank connected to a closed hydronic loop that accepts the increased volume of water as it heats, limiting pressure rise. It may contain a diaphragm separating water from compressed air.

A control loop that combines proportional, integral, and derivative actions to hold a setpoint. Proportional responds to error, integral removes offset, and derivative anticipates change to reduce overshoot.

The range around a setpoint in which no control action occurs, preventing short cycling and hunting. A wider deadband gives steadier operation but larger temperature swings.

The target value a control system tries to maintain, such as 72 degrees Fahrenheit or 50 percent relative humidity. It is compared with the measured value to produce a control signal.

Rapid on-off operation of a compressor, burner, or fan with run times too brief for stable performance. Causes include oversized equipment, a bad thermostat, low charge, or a tripping safety.

A control state that prevents an appliance from operating after a fault or repeated failed ignition attempts, often requiring manual reset. It protects equipment and occupants from unsafe conditions.

A probe that proves a burner flame is present by sensing current through the flame or ultraviolet light. If it does not detect flame, the control shuts the gas valve for safety.

A pair of dissimilar metal junctions that generates a small voltage when heated, used in standing-pilot systems to hold the gas valve open. Loss of flame drops the voltage and closes the valve.

An industry-accepted method for calculating residential heating and cooling loads from envelope, orientation, and climate data. It produces room-by-room loads used to size equipment and ducts correctly.

An industry-accepted method for sizing residential duct systems based on the room loads from load calculations. It selects duct sizes and blower settings to deliver required airflow within available static pressure.

The outdoor and indoor temperature and humidity values chosen for load calculations, representing a severe but not extreme weather day. They anchor equipment sizing and airflow targets.

The temperature of air measured by an ordinary thermometer, with no regard to moisture content. It is the value used for thermostat setpoints and sensible load calculations.

The temperature reached by air when water evaporates into it until it is saturated, reflecting both heat and moisture. It is used to find enthalpy and to set up evaporative and charging procedures.