Book - School of Science and Technology

664 Running costsTable 23.3 Part L maximum allowable carbon intensities of heating systemsMaximum carbon intensities (kgC/kWh) at stated % of themaximum heat output of the heating systemFuel 100% 30%Natural gas 0.068 0.064Other fuels 0.088 0.088The proposed new Building Regulations includes a Carbon Index for dwellings which isbased on estimated energy consumption. The rating is given as a number from 0 to 10based on the annual CO 2 emissions per unit floor area from fuel consumption. TheCarbon Index is used to demonstrate compliance with the Building Regulations.Estimating energy useFor residential buildings, the BRE Domestic Energy Model (BREDEM) provides thestandard method of estimating energy use in dwellings and, to assess the effectiveness ofdesign solutions, increasing use is made of Energy Ratings. Two national rating schemes,supported by the Energy Efficiency Office of the Department of the Environment, are theNational Home Energy Rating (NHER) scheme and the MVM Starpoint scheme. Both arecomputer models which provide a simple numerical energy rating, from 1 to 10 for NHERand from 1 to 5 for Starpoint, based upon the energy cost. The higher the rating, the moreenergy efficient the design.For all other types of building the degree-day method which has been in use for manyyears is still the standard manual calculation for heated and naturally ventilated buildingsand provides results of acceptable accuracy. Particular care must be taken, however,where the systems vary significantly from the accepted traditional designs, in which caserecourse to computer modelling methods would be the preferred alternative. The degreedaymethod is a function of heat loss, the ratio of normal-to-peak load which will applyover the period considered, hours of use, internal heat gains, thermal characteristics of thebuilding fabric and system efficiencies.Heat lossesThe totals calculated for design purposes will be in excess of those used as a basis for anestimate of energy consumption. This is due to the fact that if one considers any building,heating design must be such that on each external aspect, sufficient warmth may beprovided to maintain a satisfactory internal temperature. In practice, air infiltrationresulting from wind will occur only on the windward side; other aspects, i.e. the leewardside, will exfiltrate.The improvement in the thermal transmission properties of the building fabric in recentyears has reduced heat losses by conduction and, in consequence, infiltration losses are ahigher proportion of the total; in excess of one-third of the total in the example in Chapter2. For such application, the heat loss used for energy calculations may thus be taken at15±20% lower than the total calculated for design purposes.Any losses from piping in a central system which do not contribute to the building heatrequirements must be calculated as a separate exercise and added to the net heat loss