MI HVAC key terms

A rating that expresses, as a percentage, how much of the energy in fuel a heating appliance converts to usable heat over a full heating season. The rest is lost up the flue or as jacket loss.

The efficiency measured while an appliance runs at stable operating conditions, without accounting for on-off cycling, warm-up, or standby losses. It is always equal to or higher than seasonal efficiency.

The difference between entering and leaving air temperature across a coil, furnace, or other component. It is used to verify airflow and equipment performance against the manufacturer's approved range.

The increase in air temperature as it passes through a furnace, calculated as supply air temperature minus return air temperature. It must fall within the range on the rating plate.

A high-efficiency gas furnace that extracts latent heat by cooling flue gases below their dew point, producing condensate. It typically achieves AFUE above 90 percent and uses PVC or CPVC venting.

Heat energy absorbed or released during a change of state, such as water vapor condensing to liquid, with no change in temperature. It is the energy recovered by condensing furnaces and released by evaporating coils.

Heat that changes the temperature of a substance without changing its state. In air conditioning, sensible load is the portion of the load that lowers dry-bulb temperature.

The ratio of sensible heat removal to total heat removal for a coil or space. A low ratio means a large share of capacity goes to dehumidification rather than temperature drop.

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

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.

A metering device that regulates refrigerant flow into the evaporator by sensing suction line temperature and pressure, maintaining a set superheat as load changes.

The temperature change that occurs while a zeotropic refrigerant blend boils or condenses at a constant pressure. Because components vaporize at different temperatures, charging and leak repair require care.

The resistance to airflow measured at the blower outlet and return inlet, excluding the internal resistance of the appliance itself. It is used with a fan table to determine delivered airflow.

The volume of air moved per minute, in cubic feet. Proper airflow is required for correct heating temperature rise, cooling capacity, and equipment longevity.

Air drawn back from the conditioned space to the equipment for reconditioning. Undersized return ducting is a common cause of high static pressure and low airflow.

Conditioned air delivered from the equipment to the occupied space through the supply duct system and registers.

A grille with an integral damper or deflector that covers a duct opening and directs supply or return air into or out of a room.

The accepted industry method for calculating residential heating and cooling loads from envelope, orientation, occupancy, and climate data. Michigan sizing must follow it rather than rules of thumb.

The industry procedure for selecting equipment that matches the calculated load, accounting for the structure's actual sensible and latent capacities at design conditions.

The industry method for sizing duct systems based on available static pressure, equivalent lengths, and friction rate so that each room receives its design airflow.

The outdoor and indoor conditions used for sizing equipment, selected so the system meets the load during severe but not record weather. Michigan winter design values are well below zero in many locations.

A measure of heating or cooling demand based on how far the average daily temperature falls below or rises above a base temperature. It is used for estimating seasonal fuel use and cost.

A control that energizes first-stage heating or cooling for normal loads and calls second stage only when the first cannot satisfy demand, improving comfort and efficiency.

A small heater in a mechanical thermostat that warms the sensing element slightly ahead of the setpoint so the system shuts down before overshooting the room temperature.

A programmed reduction in heating setpoint or increase in cooling setpoint during unoccupied hours to save energy. Recovery must be timed so comfort returns when the space is occupied.

A safety control that opens the burner circuit when the heat exchanger or plenum exceeds a safe temperature, usually caused by restricted airflow.

A temperature-actuated safety device set to open at a maximum safe plenum or bonnet temperature. It protects the heat exchanger and is a required safety control on forced-air furnaces.

The method by which a flame-sensing control confirms a burner flame by detecting current flowing through the flame from an energized electrode to ground. Loss of flame signal shuts off gas.

A device made of two dissimilar metals that generates a small voltage when heated, used in standing-pilot systems to hold the gas valve open and prove the pilot flame.

A group of thermocouples connected in series that produces enough voltage to operate a millivolt gas valve or control circuit without external power.

A device that interrupts power when it detects a small imbalance between hot and neutral conductors, protecting people from electric shock in wet or damp locations.

The intentional connection of non-current-carrying metal parts to earth through a grounding conductor, providing a low-impedance path that clears faults and protects users.

A safety procedure in which energy sources are isolated, locked, and tagged before service work so equipment cannot be re-energized by another worker.

The amount of heat needed to raise one pound of water one degree Fahrenheit. It is the standard unit for expressing heating and cooling capacity in North America.

The outward push of air against duct walls, measured in inches of water column. Supply and return static readings together reveal restriction in the air distribution system.

A hinged plate installed in a vent connector that opens to admit room air and stabilize draft, preventing excessive draft from pulling heat up the chimney.

A device on a natural-draft appliance that relieves flue gas spillage and maintains stable draft by mixing room air with flue products before they enter the vent.

The air supplied to a burner for complete burning of fuel. In tight modern homes it must be brought in from outdoors in adequate quantity to prevent spillage and carbon monoxide.

A colorless, odorless gas produced by incomplete combustion. It is a serious health hazard and must be tested for during service and addressed with proper venting and ventilation.

A short, capped drop leg installed in the gas piping near the appliance to collect dirt, scale, and debris before it reaches the controls.

The gas pressure delivered to the burner orifices, measured with a manometer and set to the value on the rating plate for correct input and combustion.

A device that reduces incoming gas pressure to a stable, lower delivery pressure for the appliance and maintains it as supply pressure or load varies.

The state-adopted code governing installation, alteration, and inspection of heating, ventilating, and air-conditioning systems in Michigan, enforced by local code officials.

The authorization a code authority issues before installing or altering regulated HVAC equipment. Work without a required permit can result in failed inspections and penalties.

The code official who inspects permitted mechanical work at required stages and approves or rejects it based on compliance with the adopted mechanical code.

A written agreement that modifies the scope, price, or schedule of a contracted job after the original contract is signed, authorized by the customer before work proceeds.

A documented projection of labor, material, equipment, and overhead costs used to price a job and establish a profitable selling price.

The portion of a selling price that covers fixed business costs such as insurance, vehicles, and administration, plus the return earned on the work performed.

A written list of incomplete or deficient items identified near the end of a project that the contractor must correct before final acceptance and final payment.