MD HVAC key terms

The temperature at which a refrigerant boils or condenses at a given pressure. Liquid and vapor coexist at this temperature, and each pressure has its own unique saturation temperature.

The number of degrees a refrigerant vapor is warmer than its saturation temperature at the same pressure. Measured at the evaporator outlet, it shows whether the metering device is feeding the right amount of refrigerant.

The number of degrees a liquid refrigerant is cooler than its saturation temperature at the same pressure. Measured at the condenser outlet, it is used to check refrigerant charge and condenser performance.

A metering device that controls how much liquid refrigerant enters the evaporator by opening and closing in response to temperature and pressure sensed at the evaporator outlet, holding a steady superheat.

The pump at the heart of a refrigeration circuit. It draws low-pressure vapor from the evaporator, squeezes it into high-pressure, high-temperature vapor, and pushes it toward the condenser.

The indoor coil where low-pressure liquid refrigerant absorbs heat from the air or water passing over it and boils into vapor, producing the cooling effect.

The outdoor coil where high-pressure refrigerant vapor gives up heat to the surrounding air or water and condenses into a liquid so it can be reused in the evaporator.

A four-way valve in a heat pump that switches the direction of refrigerant flow, swapping the roles of the indoor and outdoor coils so the same system can heat or cool.

Air that must be supplied to a gas or oil burner so the fuel burns completely. A starved appliance can produce carbon monoxide, so openings or ducts sized by code must feed the space.

The movement of combustion gases from the burner up and out of the appliance. Natural draft relies on warm gas rising, while induced and forced draft use a fan to pull or push the gases.

The passage or pipe that carries combustion products from the appliance to the outdoors. It must be sized, sloped, and terminated so gases leave the building safely and do not spill back inside.

The gas pressure delivered to the burner orifices, usually measured in inches of water column. The rating plate lists the correct value, and it is set by adjusting the gas valve regulator.

A safety device placed in a pilot flame. The heat generates a small voltage that holds the gas valve open; if the pilot goes out, the voltage drops and the valve closes to stop unburned gas.

A temperature control on a boiler or water heater that starts and stops the burner to keep water within a set range; it may also act as a high-limit safety.

A tank connected to a closed hydronic system that absorbs the extra volume created as water heats up, keeping pressure within safe limits as the system cycles.

The outward push of air against the walls of a duct, measured in inches of water column. It shows how hard the blower must work to move air through the system.

The total pressure the blower must overcome in the supply and return ducts, including filters and coils, measured at the equipment. It is used to confirm that airflow matches the design.

How many times the full volume of air in a space is replaced in one hour. It is used to judge ventilation, infiltration, and how quickly contaminants are diluted.

The amount of water vapor in the air compared with the maximum the air could hold at that temperature, stated as a percentage. Comfort and mold growth depend on keeping it in range.

The temperature to which air must be cooled for the water vapor in it to begin condensing. Air at or below its dew point forms condensation on cool surfaces.

Minimum Efficiency Reporting Value, a number that ranks how well an air filter captures particles of different sizes. Higher numbers capture more particles but also add resistance to airflow.

A heavy-duty relay whose contacts close to send line voltage to a compressor or fan motor when the low-voltage control circuit energizes its coil.

A capacitor wired into a motor circuit continuously to shift phase and improve running torque; a weak capacitor makes the motor draw higher current and overheat.

A protective device inside or on a motor that opens the circuit when temperature rises too high, then resets after the motor cools, preventing a burned-out winding.

The current a motor draws the instant it is energized with the shaft unable to turn. Comparing measured current to the nameplate LRA helps diagnose seized bearings or a failed start component.

The relationship among voltage, current, and resistance in a circuit: volts equal amps times ohms. Technicians use it to predict what a meter should read while troubleshooting.

A device that compares current leaving and returning on a circuit and trips in milliseconds when some current leaks to ground, protecting people near water from shock.

An official authorization from the local code authority to install or alter HVAC equipment. Work done without one can fail inspection, void warranties, and create liability when a property is sold.

A widely adopted residential standard that sets minimum whole-house and local exhaust ventilation rates and addresses kitchen and bath exhaust, duct tightness, and infiltration.

The required practice of capturing refrigerant from a system into a container instead of releasing it to the atmosphere. Technicians must hold EPA Section 608 certification to buy refrigerant and perform this work.

A procedure for shutting off and locking the energy source to equipment before service, with a tag naming who locked it, so no one can start the unit while someone is working.

The gloves, eye protection, respirators, and other gear worn to reduce injury from hazards such as refrigerant frostbite, electrical shock, sharp edges, and airborne particles.

Flowing dry nitrogen through piping while brazing to displace oxygen, which prevents soot and scale from forming inside the lines and lowers the risk of fire.

An accepted method for sizing a system by calculating heat loss and heat gain room by room from windows, insulation, and climate data, rather than guessing from square footage.

The start-up process of verifying that a new system is installed as designed: checking charge, airflow, combustion, controls, and safety devices, then recording readings for later comparison.

A written agreement signed before the work is done that changes the contract's scope, price, or schedule; it protects both the contractor and the customer from later disputes.