Calorific value
Heat content per unit quantity of fuel. Gross calorific value (GCV, also known as higher heating value) accounts for total energy content. Net calorific value (NCV, or lower heating value) excludes energy which will be dissipated as latent heat in water vapour in products of combustion. See table of gross calorific values of various fuels. For public gas supply in the UK, daily GCVs can be looked up here
CCL
Climate Change Levy. UK energy tax imposed on non-domestic users.
CFL
Compact Fluorescent Lamp. Low-energy replacement for filament lamp, often plug-compatible with existing fitting.
Chillers
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CHP
Combined Heat and Power. Scheme for supplying electricity with recovery of the resultant waste heat for useful purposes.
CMVP
Certified Measurement and Verification Professional. Qualification awarded by the Efficiency Valuation Organisation to appropriately-educated and experienced practitioners who have attended an approved training course and passed an examination set by the Association of Energy Engineers.

You can download a list of people who have qualified as CMVPs in the UK

CO2
Carbon dioxide - a product of combustion, and persistent greenhouse gas.
Coefficient of Performance
Ratio of work or useful output to the amount of work or energy input, most commonly in relation to refrigeration equipment
Combustion
Conversion of fuels into heat provides opportunities for energy savings for virtually every user, simply through improved maintenance. Read more about the principles and for a specific examination of bogus claims for magnetic enhancers read this.
Compensator
Temperature control on heating circuit (usually when feeding radiators) which limits heat output by varying circulation temperature according to prevailing weather
Compressed air
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Condensate
Water returned from steam-heated equipment
Conductive losses
Heat lost from a building by transmission through solid elements such as walls, roofs, glazing, etc
Controls, automatic
One of the most significant aspects of energy management. Read more on automatic controls in buildings and controls in other applications.
Convective losses
Heat lost from a building through ventilation and uncontrolled air ingress
Controls, heating ventilating and air conditioning
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Controls, other
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Cusum chart explained
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CV
Calorific value (see also GCV, NCV)