# Glossary

ACH
  Air changes per hour – a measure of ventilation rate.

coefficient of performance
  The ratio of electricity used to the heat or cooling delivered by a heat pump or refrigerator.

colour rendering index (CRI)
  An index of lighting quality that is applied to lamps, indicating how wide and even a spectrum of visible light they produce.

community heating
  Another term for district heating.

degree day
  A unit of ‘heating requirement’ calculated from the outside/inside temperature difference and used in designing heating systems.

district heating
  The large-scale distribution of heat, sometimes called community heating.

energy services
  The ultimate aims for which energy systems are built: warm homes, cooked food, illumination, mobility and manufactured articles.

fuel cells
  A device for producing an electric current by means of what is essentially the reverse process to electrolysis – combining two gases (typically hydrogen and oxygen) to produce electricity.

fuel poverty
  A household is considered to be in fuel poverty if in order to achieve an acceptable degree of comfort: (a) they have required fuel costs that are above average level and (b) were they to spend that amount, they would be left with a residual income below the official poverty line.

geothermal energy
  Heat drawn from the Earth’s internal heat.

gross space heating
  The total heat lost by a building, measured in kilowatt-hours per day.

heat loss coefficient
  Total fabric and ventilation heat loss of a building; the sum of the products of the U-values of all of the elements of the external fabric (walls, roof, floor, windows, doors) and their respective areas, plus the ventilation heat loss. It may also be called ‘heat transfer coefficient’. It has units of W K-1.

infiltration
  Unplanned and unwanted ventilation.

latent heat of vaporization
  The energy absorbed by a liquid when it evaporates (or released when it condenses).

lumen
  The SI unit of perceived light intensity.

luminaire
  Technical term for what is more informally called a ‘light fitting’.

luminous efficacy
  the amount of light emitted in lumens by a lamp per watt of electricity used. It has units of lumens per watt (lm W-1).

mechanical ventilation with heat recovery
  A method of reducing the heat loss due to ventilation in buildings. The system allows the warm outgoing air to pre-heat the cold incoming air by passing both streams through a heat exchanger. This requires a properly airtight building to work effectively.

net space heating demand
  The amount of energy needed to be supplied by a heating system to maintain the internal temperature.

pressure test
  A method for testing the airtightness of a building.

seasonal or system performance factor (SPF)
  the ratio of the useful heat produced by a heat pump heating system to its total electricity consumption (including pumps, fans and top-up heating).

thermal conductivity (λ or k)
  A material property defined as the rate of heat flow (in watts) across a one-metre cube of the material with a temperature difference of one degree (K or °C). Thermal conductivity is usually expressed in units of W m−1 K−1. A good thermal insulator has a low thermal conductivity.

thermal efficiency
  the ratio of the useful heat produced to the fuel energy input for a heating device such as a gas boiler.

thermal insulator
  a material with a low thermal conductivity which does not allow heat to flow easily.

thermal resistance
  The amount of resistance to heat transfer offered by a particular slab of building material. It has the unit m2 K W-1.

ventilation
  The flow of air through a building, both planned and unplanned.

ventilation rate
  The average rate at which air flows through a building, usually specified as air changes per hour (ACH).

window energy rating
  Windows are now sold with an energy rating on a scale ‘A’ to ‘G’ in a similar manner to domestic appliances. The rating takes into account the thermal heat loss, transparency to solar gains and air leakage.

winter design temperature
  The air temperature for the coldest weather likely to occur on the worst days at a given location.

OpenLearn - Energy in buildings
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