Book - School of Science and Technology

46 The building in winterThe importance of air infiltration is that it may well account for as much as half ormore of the total heat loss and yet it remains the least amenable to logical and systematicprediction. With improvement to the thermal properties of the building structure throughadded insulation, air infiltration has increasingly become the dominant component inheat loss. Consequently, temperature guarantees become more difficult to sustain orchallenge since any performance test is as much related to the potential for faults in thebuilding as to those in the heating system.The heat needed to warm infiltration air is calculated using the specific heat capacity ofair (at constant pressure) and the specific mass, both at 20 C. Thus, from Table 1.1, thequantity required to raise unit volume through one kelvin is 1:012 1:205 ˆ1:219 kJ/m 3 K.There are two different methods of making an assessment of air infiltration. One isempirical and is based upon the number of times the air volume within a space will bechanged in one hour, y this being referred to as the air change rate. The second and morespecific approach is confined mainly to heavily glazed commercial buildings and, as willbe explained later, relates areas of openings such as assumed lengths of cracks aroundwindows and doors, etc., to rates of air flow. It is also appropriate to mention here theincreasing use of computational fluid dymanics (CFD) to evaluate airflow around andthrough buildings. This modeling tool can be used in complexcity centre locations to setup wind pressure coefficients for each facade of the building, it does however requirespecialist skills to perform this activity.Table 2.9 Natural air infiltration for heat losses (air change rate and ventilation allowance)Without weather strippingWith weather strippingRoom or buildingAir changeper hourNVentilationallowance(W/m 3 K)Air changeper hourNVentilationallowance(W/m 3 K)Large factory spacesHeavy construction300 to 3000 m 3 3 ¤ 4 0.25 ± ±3000 to 10 000 m 3 1 ¤ 2 0.17 ± ±over 10 000 m 3 1 ¤ 4 0.08 ± ±Unlined sheet construction300 to 3000 m 3 1 1 ¤ 2 0.50 ± ±3000 to 10 000 m 3 1 0.33 ± ±over 10 000 m 3 3 ¤ 4 0.25 ± ±Living spaces and officeswindows exposed on one side 1 0.333 ¤ 4 0.25windows exposed on two sides 1 1 ¤ 2 0.50 1 0.33windows exposed on more sides 2 0.67 1 1 ¤ 2 0.50Miscellaneousassembly and lecture halls1 ¤ 2 0.17 ± ±circulating spaces 1 1 ¤ 2 to 2 0.50 to 0.67 ± ±laboratories 1 to 2 0.33 to 0.67 ± ±lavatories 2 0.67 ± ±NoteSee DHSS and DES publications giving standards for Hospitals, Schools, etc.y Although the hour is an unacceptable time interval for calculations made in strict SI units, the use of rates(which are at best no more than informed guesses) in multiples of 0.0003 air changes per second wouldnot endear the concept to any practitioner!