BPI Heating key terms

The continuous layer of insulation, air sealing, and building materials that separates heated indoor space from unconditioned or outdoor space. It controls heat loss and marks where insulation and air control should align.

The upward movement of indoor air caused by warm air being lighter than cold outdoor air. It creates pressure differences across the building shell, drawing air in low and pushing it out high.

A continuous building layer that blocks uncontrolled air movement through the enclosure. It is separate from insulation and must connect across walls, ceilings, floors, and penetrations to work.

A measure of a material's resistance to heat flow. Higher numbers mean better insulation performance; the value depends on material type, thickness, and installed condition.

A weather-based measure of how much heating is needed. It is calculated from the difference between a base indoor temperature and the average outdoor temperature over a day.

Heat energy that changes the temperature of air or another substance without changing its phase. It is the part of heating load that thermometers directly indicate.

The estimated heat loss of a building at a selected cold outdoor design temperature and desired indoor temperature. It is used to size heating equipment and distribution components.

Air supplied to a burner, appliance, or combustion zone to provide oxygen for burning fuel. Insufficient combustion air can cause incomplete combustion and carbon monoxide production.

The pressure difference that moves air and flue gases through a vent or chimney. It is produced by temperature differences and by fans, and it must be adequate without being excessive.

The mixture of gases and water vapor produced by burning fuel and carried away through the vent. It contains products such as carbon dioxide, water vapor, nitrogen, and sometimes carbon monoxide.

A colorless, odorless gas produced by incomplete combustion. It is toxic because it binds to hemoglobin and reduces the blood's ability to carry oxygen.

Flue gases escaping from a draft hood or vent connector into the room instead of going up the vent. Spillage can introduce carbon monoxide and moisture into indoor air.

A condition where flue gases are pulled down or reversed through a vent into the building. It is often caused by depressurization or by a blocked or poorly designed venting system.

Air supplied to combustion beyond the amount theoretically needed for complete burning. Some excess air is normal and helps ensure complete combustion, but too much lowers efficiency.

A device at the outlet of some appliances that provides dilution air and stabilizes draft. It also allows flue gases to spill if venting is obstructed or building pressure is unfavorable.

A hinged flap installed in a flue connector that opens to admit room air and stabilize draft. It is common on oil appliances and some gas appliances, and it must be adjusted correctly.

The minimum safe distance between an appliance or vent and materials that can catch fire. It is specified by the manufacturer and by code and must be maintained during installation.

A gas appliance that operates with negative vent pressure and produces flue gases cool enough to avoid condensation in the vent. It typically vents into a vertical chimney or B-vent.

A high-efficiency furnace that cools flue gases enough to condense water vapor and recover latent heat. It uses a secondary heat exchanger and must be vented with compatible materials.

The metal assembly that separates combustion gases from indoor air while transferring heat. Cracks or holes can allow flue gases to enter the supply air stream.

The gas pressure measured at the appliance manifold, downstream of the gas valve. It must match the manufacturer's rating for the fuel type and is adjusted during setup.

A safety control that opens the burner circuit when temperature or another limit exceeds a set point. It shuts down heating to prevent overheating or unsafe operation.

A fan that pulls flue gases through the appliance and creates negative pressure in the vent. It is common on fan-assisted furnaces and must prove airflow before the burner stays on.

The measurement of flue gas composition and temperature to evaluate combustion safety and efficiency. It typically includes oxygen, carbon dioxide, carbon monoxide, and stack temperature readings.

The outward force exerted by air against duct walls, measured in inches of water column. It is used to check fan performance, duct restrictions, and airflow problems.

The difference between supply-air and return-air temperatures across a heating appliance or system. It shows whether airflow and heating output are within the manufacturer's specified range.

Annual fuel utilization efficiency, a rating that estimates how much fuel energy becomes useful heat over a typical heating season. Higher values indicate greater efficiency.

A field method of measuring gas input by timing the meter dial while other appliances are off. The result is compared with the appliance rating to confirm proper input.

A carbon monoxide reading calculated as if no excess air were present in the sample. It allows comparison of CO levels from different combustion conditions and appliances.

A procedure that creates the strongest realistic negative pressure in a combustion appliance zone. It checks whether exhaust fans, ducts, or other forces can cause backdrafting or spillage.

A box or chamber attached to the furnace outlet or inlet that distributes air to ducts. Supply and return plenums must be sealed and sized to support proper airflow.

Ductwork that carries air from conditioned rooms back to the furnace or air handler. Leaky returns can draw in unconditioned air or create pressure problems.

A movable plate or blade inside ductwork or a vent that adjusts airflow. Dampers can be manual or motorized and are used for balancing, zoning, or shutoff.

Dividing a building into separate control areas with individual thermostats and dampers or valves. Zoning improves comfort but requires correct bypass or airflow management.

A control that senses room temperature and signals the heating system to start or stop. It may include setback schedules, fan control, and anticipation settings.

A temperature control used on hydronic boilers to start and stop the burner or circulator. It helps maintain water temperature within a safe and useful range.

A boiler control strategy that lowers supply-water temperature as outdoor temperature rises. It improves efficiency and comfort by matching heat output to building load.