MS HVAC key terms

Heat that changes the temperature of a substance without changing its state. In air, it is the portion of total heat a thermostat responds to when the dry-bulb reading rises or falls.

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 the energy removed while dehumidifying air.

The ordinary air temperature measured by a thermometer not affected by moisture evaporation or radiation. It is the value a standard room thermostat senses and displays.

The lowest temperature air can reach by evaporative cooling alone, measured with a wetted wick and moving air. The gap between wet-bulb and dry-bulb indicates how much moisture the air can still absorb.

The temperature at which air becomes saturated and moisture begins to condense on surfaces. A dew point close to the room temperature signals high humidity and a risk of sweating ducts and coils.

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

The temperature of refrigerant vapor above its saturation temperature at the same pressure. Measured at the evaporator outlet, it confirms the expansion valve is feeding enough refrigerant to protect the compressor.

The amount of cooling of liquid refrigerant below its condensing temperature at the same pressure. Measured at the condenser outlet, it shows how much liquid is stacked in the condenser and whether the charge is adequate.

A metering device that regulates refrigerant flow into the evaporator by sensing suction line temperature and pressure, holding a set superheat. It opens on rising superheat and closes as the evaporator floods.

The indoor coil where low-pressure liquid refrigerant boils and absorbs heat from the air stream. Its job is to remove both sensible and latent heat while keeping the refrigerant fully vaporized before it leaves.

The coil where high-pressure refrigerant vapor gives up heat to outdoor air or water and condenses to liquid. Adequate airflow and clean coils are required for it to reject heat at the design condensing temperature.

The pump that raises refrigerant pressure and moves it around the circuit, separating the low side from the high side. It compresses vapor only; liquid slugging can damage valves and bearings.

The low-side pressure measured at the compressor inlet, which corresponds to the evaporator saturation temperature. It rises with warmer load or overfeeding and falls with a starved or restricted evaporator.

The high-side pressure at the compressor discharge, tied to the condensing temperature. It climbs with dirty condenser coils, restricted airflow, overcharge, or high outdoor ambient.

The quantity of refrigerant contained in a sealed system. Charge is verified by superheat on fixed-orifice systems and by subcooling on expansion-valve systems, not by pressure alone.

Removing refrigerant from a system into an approved container for reuse, recycling, or disposal instead of venting it. Recovery is required by regulation before opening a circuit to the atmosphere.

An instrument that measures electrical resistance to check continuity, open windings, or shorted components. Power must be off and the component isolated before testing, or readings will be false and the meter may be damaged.

A handheld meter combining voltage, resistance, and often current and temperature functions. Technicians use it to verify supply voltage, control circuit continuity, and component resistance during troubleshooting.

An electrically operated switch whose coil closes heavy-duty contacts to start a compressor or fan motor. Pitted contacts cause voltage drop and intermittent operation; a chattering coil points to low control voltage.

A capacitor connected in the motor circuit during operation to improve starting torque and efficiency. A bulged case or a microfarad reading outside its rated tolerance calls for replacement.

A high-microfarad capacitor switched in briefly at startup to boost motor torque, usually paired with a potential relay or start thermistor. It stays in circuit only for seconds and fails if left engaged.

A protective device that opens the motor circuit when current or temperature exceeds a safe limit. It guards the compressor against overheating from low voltage, high head pressure, or a failing winding.

The low-voltage wiring and devices, typically 24 volts, that decide when contactors and relays energize. Thermostats, safeties, and switches are wired in series so any open device stops the equipment.

A temperature-actuated control that starts and stops heating or cooling to hold a setpoint. Modern units add fan modes, staging, and deadband settings that affect comfort and cycling rate.

A small heater or resistor in a thermostat that warms the sensing element slightly as the circuit runs, ending the call early. It reduces overshoot so room temperature stays closer to setpoint.

A control that responds to moisture level rather than temperature, used to cycle dehumidification or ventilation. It is often set to prevent window condensation and mold growth in humid climates.

The outward force air exerts on duct walls, measured in inches of water column. Supply and return readings reveal whether the blower is moving design airflow or fighting a restrictive duct system.

The pressure created by the motion of air, measured with a pitot tube and used to calculate speed. It converts to feet per minute and, combined with duct area, gives volumetric airflow.

The volume of air moved per unit time, usually cubic feet per minute. Correct airflow is essential for refrigerant heat transfer, combustion safety, and comfortable room delivery.

Selecting duct dimensions so each run carries design airflow within acceptable friction loss and velocity. Undersized ducts raise static pressure, reduce airflow, and increase noise and energy use.

Air drawn back from conditioned spaces to the blower for filtering, heating, or cooling. Inadequate return path starves the system, lowering supply airflow and raising pressure in the building.

Conditioned air delivered from the equipment through supply ducts and registers into occupied rooms. Its temperature difference from room air, typically 15 to 22 degrees Fahrenheit in cooling, indicates system performance.

A gauge that measures small pressure differences in inches of water column, used for duct static pressure, gas pressure, and filter loading. Digital models read differential pressure directly and are preferred for diagnostics.

A diagnostic method that mounts a calibrated fan in an exterior doorway to measure building air leakage. Results guide air sealing and help size ventilation to meet indoor air quality needs.

Deliberate introduction of outdoor air to dilute pollutants, odors, and moisture indoors. It can be natural, exhaust-only, supply-only, or balanced, and must not compromise pressure balance or efficiency.

A grille with an integral damper and often adjustable deflectors that delivers or returns air at a room opening. Balancing dampers in registers trim flow between rooms on a single system.

Air supplied to a fuel-burning appliance so gas or oil burns completely and safely. Inadequate combustion air causes carbon monoxide production and flame roll-out, and is a common inspection failure.

The vent passage that carries combustion products from an appliance to the outdoors. Proper size, slope, and clearance protect against spillage of carbon monoxide into the living space.

The flow of combustion gases up a flue, created by temperature difference and stack effect. A blocked or backdrafting flue can spill exhaust indoors, so draft must be verified during service.

The difference between return and supply air temperatures across a furnace or heat pump. Comparing it to the nameplate range confirms whether airflow and firing rate are correct.