Glossary

Key terms used in heat loss calculations and heating design, with plain-English definitions. Each term links to a deeper guide where the concept is explained in full.

ACH (air changes per hour): The number of times the entire volume of air in a room is replaced with outside air in one hour. Higher ACH means more ventilation heat loss. Read more →
Air permeability: A measure of how leaky a building is, expressed as the volume of air leaking through the envelope per hour per square metre of envelope area at a standard test pressure. Lower air permeability means a tighter building. Read more →
Delta T: The temperature difference between the mean water temperature in a radiator and the room air temperature. Radiator output depends heavily on Delta T — a lower Delta T means less heat output from the same radiator. Read more →
Design heat loss: The calculated rate of heat escaping from a room or building under design conditions — typically a cold winter day at the local design outside temperature. This is the figure used to size heating systems and check emitter output. Read more →
Emitter: Any device that delivers heat to a room — radiators, underfloor heating, fan convectors, towel rails. Each emitter has a heat output that depends on its size, type and the water temperature flowing through it. Read more →
Fabric heat loss: Heat conducted through the solid parts of the building envelope — external walls, windows, doors, floors and roof or ceiling. Calculated from the area, U-value and temperature difference of each surface. Read more →
Flow temperature: The temperature of the water leaving the boiler or heat pump and entering the radiator circuit. Higher flow temperatures mean more radiator output, but lower boiler efficiency and much lower heat pump efficiency. Read more →
Heat output: The rate at which an emitter delivers heat to a room, measured in watts. Catalogue outputs are usually quoted at Delta T 50 and must be corrected for the actual flow and return temperatures in use. Read more →
Heating margin: The difference between a room's design heat loss and the corrected output of its installed emitter. A positive margin means the emitter has spare capacity; a negative margin means the room may not reach its target temperature on a cold day. Read more →
Infiltration: Uncontrolled air leakage through gaps, cracks and openings in the building envelope. Infiltration is one component of ventilation heat loss and is driven by wind pressure and the stack effect. Read more →
Intermittent heating allowance: An uplift added to the steady-state heat loss to account for the extra capacity needed to reheat a building after a period with the heating off — typically the overnight setback or daytime vacancy. Read more →
Return temperature: The temperature of the water returning to the boiler or heat pump after passing through the radiator circuit. The difference between flow and return temperatures indicates how much heat the emitters have extracted from the water. Read more →
Room-by-room heat loss: A heat loss calculation performed individually for each room, rather than as a single whole-house figure. Essential for checking that each emitter is correctly sized and for identifying problem rooms. Read more →
Thermal bridge: A localised area in the building envelope where heat flows more easily than through the surrounding construction — typically at junctions, lintels, corners and window reveals where insulation is interrupted. Read more →
U-value: A measure of how easily heat passes through a building element, expressed in W/m²K. A lower U-value means better insulation. U-values are used to calculate fabric heat loss through each surface. Read more →
Ventilation heat loss: Heat carried away by air movement — draughts, extract fans, open vents and general air leakage through the building envelope. The heating system must warm the cold replacement air that enters the building. Read more →
Weather compensation: A control strategy where the boiler or heat pump automatically adjusts its flow temperature based on the outside air temperature. Colder outside means higher flow temperature; milder outside means lower flow temperature and better efficiency. Read more →
Y-value: A simplified factor used to account for thermal bridging heat loss at junctions and construction details. Applied as a blanket uplift to the total fabric heat loss, because detailed junction modelling is rarely practical for existing domestic buildings. Read more →