Flap or similar used to regulate, divert or stop flow of air or gases in a duct

Darkness, hours of
Measurable factor which may act as a good driving factor for management of lighting demand. Download Excel table of monthly and weekly standard figures (for latitude 52°N but suitable for most of England and Wales)

Direct Current Mode of electricity transmission in which the polarity is fixed (contrast AC).

Discounted Cash Flow. Method of evaluating future cash flows (for example investment in energy-saving measures and resultant savings) in a manner which gives progressively less weight to savings and costs occurring further in the future. Download an Excel workbook which explains and illustrates the principle and can be used for your own evaluations.

Dead band
Either deliberate separation of (say) heating and cooling control set points to inhibit simultaneous operation or switching differential of a thermostat or other similar control

To reduce the carbon dioxide emissions associated with energy derived from fossil fuels

Degree days
Measure of how cold (or hot) the weather was at a given location over a particular interval of time (usually monthly or weekly), expressed as a single numerical index. See data and related articles or download tables of annual regional totals for heating or cooling.

Removal of water vapour from the air, usually to achieve a comfortable relative humidity. Achieved either by dessicant drying system or by cooling the air to condense surplus water and then reheating it. Read an article about how to account for variable energy demand driven by humidity

Elimination of the blanket of warm air that otherwise collects under a high ceiling when the heating system is running

Domestic Hot Water, i.e. provided for washing, bathing, kitchens etc

Draught proofing
Reduction or prevention of unwanted air ingress into a building. Incoming air displaces heated air and then itself needs to be heated. Read an article about building fabric

Driving factor
An independently-measurable factor (e.g. production throughput, mileage, degree-day value) that determines or explains the required quantity of energy or other consumable resource.

Distribution use of system: (UK) charges for local transportation of electricity from a grid supply point to the consumer