BREDEM-12 Model description 2001 update

Transcription

1 BRE Garston, Watford, WD25 9XX 2002 BREDEM-12 Model description 2001 update B R Anderson, P F Chapman, N G Cutland*, C M Dickson*, G Henderson, J H Henderson*, P J Iles*, L Kosmina and L D Shorrock* *BRE Housing Centre BRE Scotland Independent Consultant constructing the future

2 Prices for all available BRE publications can be obtained from: CRC Ltd 151 Rosebery Avenue London, EC1R 4GB Tel: Fax: BR 438 ISBN Copyright BRE 2002 First published 2002 Printed from supplied camera-ready copy. BRE is committed to providing impartial and authoritative information on all aspects of the built environment for clients, designers, contractors, engineers, manufacturers, occupants, etc. We make every effort to ensure the accuracy and quality of information and guidance when it is first published. However, we can take no responsibility for the subsequent use of this information, nor for any errors or omissions it may contain. Published by Construction Research Communications Ltd by permission of Building Research Establishment Ltd Requests to copy any part of this publication should be made to: CRC Ltd Building Research Establishment Bucknalls Lane Watford, WD25 9XX BRE material is also published quarterly on CD Each CD contains BRE material published in the current year, including reports, specialist reports, and the Professional Development publications: Digests, Good Building Guides, Good Repair Guides and Information Papers. The CD collection gives you the opportunity to build a comprehensive library of BRE material at a fraction of the cost of printed copies. As a subscriber you also benefit from a 25% discount on other BRE titles. For more information contact: CRC Customer Services on Construction Research Communications CRC supplies a wide range of building and construction related information products from BRE and other highly respected organisations. Contact: post: CRC Ltd 151 Rosebery Avenue London, EC1R 4GB fax: phone: crc@construct.emap.co.uk website:

3 Contents 1. Introduction This Document Development of BREDEM What is BREDEM-12? Input Data Required 2 2. Basic Principles Approach Taken Occupancy Standard Number of Occupants Standard Heating Regimes 7 3. Water Heating Hot Water Demand Storage and Pipe Losses Storage Losses Primary Pipework Losses Distribution Losses Solar Panel Contribution Fuel Used Electric Water Heating Systems Hot Water Gains Lights and Appliances Electricity Demand Low Energy Lights Heating and Ventilating Electricity Gains Cooking Heating Systems Responsiveness Efficiency Controls Specific Loss Fabric Loss Ventilation Loss Infiltration Rate Wind Speed Occupant Ventilation Interzone Heat Transfer 28

4 8. Gains Solar Gains From Windows and Glazed Doors Solar Gains From Conservatories Internal Gains Total Gains Heating Season Length Heating Season Length Equation Solar Gain Total Gains Useful Gains Mean Internal Temperatures Demand Temperatures Background Temperature Background Temp. - Primary Responsiveness > Background Temp. - Primary Responsiveness < Mean Temperatures Space Heating Heating Equation Fuel Used Overall Energy Use 47 References 49 Appendix A Nomenclature 51 Appendix B Model Equations 57 Appendix C Calculating U-values 65 Appendix D Reference Tables 73

5 1. Introduction 1.1 This document This document describes the technical basis of BREDEM-12, the current annual version of the BRE Domestic Energy Model. The principles behind the model are discussed and the equations are listed. Sufficient information to implement the model is given. 1.2 Development of BREDEM BREDEM is a method for estimating the energy consumption in dwellings. It aims to provide an energy calculation that is substantially better than simple procedures such as design heat loss, but is considerably simpler to use than detailed simulation models. A full description of the background and philosophy behind the model is given by Anderson et al 1. BREDEM has undergone numerous developments since its inception in the early 1980s. It started as a simple single zone model that could be calculated by hand on a spreadsheet. The widespread use of personal computers has facilitated the implementation of more complex two zone models. Two such models are currently in use, BREDEM-8 3, which estimates monthly energy consumption, and BREDEM-12, which is an annual version. BREDEM-9 is a simplified (single zone) version of BREDEM-12, which can be calculated by hand on a spreadsheet. This forms the basis of SAP, the Government's Standard Assessment Procedure for producing an energy rating 4. A summary of the evolution of BREDEM is given by Shorrock and Anderson What is BREDEM-12? BREDEM-12 predicts the annual energy consumption in dwellings. Estimates of the following energy uses are made: space heating water heating lighting and electrical appliances cooking A number of different factors are taken into account, including building construction, heating systems and controls, and location. BREDEM-12 is generally implemented on a personal computer. This makes it easy to produce comparisons between different energy efficiency measures. It is used for the National Home Energy Rating (NHER) scheme. Alternatively, it can be used to carry out a more detailed study of a particular house and household, using specific occupancy -1-

6 information, which would include the users heating requirements and living patterns. 1.4 Input data required The dwelling is divided into two zones, zone 1 is the living area, which is heated to a higher temperature than the rest of the house which is zone 2. The input data required to perform a BREDEM-12 calculation is summarised in Table 1.1. Table 1.1 Summary of information required to perform a BREDEM-12 calculation Information required Site definition Type of dwelling Degree day region Height above sea level (m) Number of sides sheltered from wind Mean site wind speed Level of overshading Number of storeys Definition of zones 1 and 2, noting whether zone 1 is all of a storey and whether the stairs provide a direct link between zone 1 and zone 2 Total floor area for each zone, volume for each zone Type of construction (eg timber frame, cavity) Building fabric Areas and U-values for roof, external walls for each zone Heat loss floor area for each zone, floor perimeter, type and amount of insulation Window areas, type of frame, type of glazing, level of leakiness (eg draught stripped, loose fitting), orientation, zone. Ventilation Pressure test result 1 Number and type of fans and vents Heating system Hot water heating Type (eg combi boiler with fan-assisted flue) Fuel Controls fitted (eg room thermostat) Level of independent control in zone 2 (eg TRVs) Secondary heating, type of appliance and fuel Number of pumps and fans Type of hot water heater eg from boiler, electric immersion Volume of hot water tank Thickness and type of tank insulation Whether primary pipework is insulated Whether there is a cylinder thermostat Location of tank (zone 1 or zone 2) Area of solar panel (if fitted) -2-

7 Information required Mechanical ventilation (if present) Cooking Efficiency of heat recovery Cooking system and fuel used (eg electric cooker, kitchen range) Location of kitchen (zone 1 or zone 2) Lighting Proportion of light bulbs that are low energy Occupancy Number of occupants 1 Demand temperature for each zone 1 Heating periods (for each zone if different) 1 Level of usage of hot water, lights and appliances and cooking (above average, average, below average or well below average) Conservatory Areas and U-values of components separating house and (if present & conservatory unheated) 2 Areas and U-values for external conservatory components Area and U-value of conservatory floor Area and type of glazing Whether a curtain or blind is present Orientation Zone adjacent to conservatory 1 Supplying this value is optional, a default can be used instead 2 Heated conservatories should be treated as part of the dwelling -3-

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9 2. Basic Principles 2.1 Approach Taken BREDEM-12 uses a mixture of analytical and empirical techniques. An analytical approach, involving balancing heat losses against gains, is used to calculate the space heating energy requirements. This incorporates empirical functions to estimate the utilisation of gains, demand for hot water and the energy use for cooking, lights and appliances. The overall energy balance is shown in Figure 2.1. Figure 2.1 Schematic illustration of the energy balance principle used in BREDEM The space heating equation is central to BREDEM. The annual heating requirement is expressed as the integral of the heat transfer equation for a house and its surroundings (see Anderson et al 1 for its derivation). The heat required depends on the temperature difference between the inside and the outside. However, if there were no heating the internal temperature would be higher than the external temperature due to solar and internal gains. The internal gains are due to water heating, cooking and the use of lights and appliances. Metabolic gains are also included. The space heating energy required, is that needed to raise the temperature that would result without heating, to the desired internal temperature. Thus, the mean rate of heat output over a period (sensibly longer -5-

10 than the daily temperature cycle so that heat storage effects can be neglected) is given by: Φ = H{T int - G/H - T ext } where in terms of the mean values over the specified period Φ is the mean daily heat output from the heating system (W) T int is the mean internal temperature ( o C) G is the mean useful gains (W) H is the specific heat loss for the dwelling (W/ o C) T ext is the mean external temperature ( o C) Integrating over the heating season gives: Q = 8.64 x 10-5.H DD{T int -G/H} where Q is the annual space heating energy requirement (GJ/year) DD{T int -G/H} denotes the degree days to the base T int -G/H The constant 8.64 x 10-5 is required to give the final answer in Giga Joules (GJ). The degree day function is calculated from mean monthly external temperatures. A two zone model is used for space heating. Zone 1 represents the living area, which is heated to a higher temperature than zone 2, the rest of the house. Zone 2 may be fully or partly heated or unheated. In a typical 2-storey house zone 1 will comprise about half the ground floor, or one quarter of the total floor area. The above equation is applied separately to each zone, taking into account the transfer of heat between the zones (see Section 11.1). The dependence of the space heating on the internal temperature, solar gains and the internal gains due to other energy uses, means that the space heating requirement needs to be considered after these quantities have been evaluated. -6-

11 2.2 Occupancy The occupants determine the heating regime, the temperatures and heating patterns. The use of hot water, lights and appliances and the contribution made by metabolic gains depend on the number of occupants. Actual heating regimes and numbers of occupants can be used. However, for calculating energy ratings, or when the number and behaviour of occupants is unknown, a standard occupancy is used Standard Number of Occupants The number of occupants is related to the number of rooms and hence the floor area. The relationship is only a rough indicator, as there is a large variability in practice. The standard number of occupants, N is given by: N = TFA N = TFA TFA 2 for TFA 450 m for TFA > 450 m 2 2 where TFA is the total floor area of the dwelling (m 2 ) Standard Heating Regimes An idealised heating regime is shown in Figures 2.2 and 2.3. The heating patterns are different for the two zones. However, as many heating systems cannot control the two zones independently, the standard heating regime assumes that the zone 1 heating times are applied throughout the dwelling. In the case where there is zone control, the differing heating times are applied for each zone. The demand temperature in zone 1 is the same (21 o C) for weekdays and weekends, and the desired temperature in zone 2 is taken to be 3 o C lower than that in zone 1. (In the standard case, all of zone 2 is heated, but a fraction that is heated may be specified). -7-

12 Figure 2.2 Weekday heating pattern Temperature C zone 1 zone Time of day (hours) Figure 2.3 Weekend heating pattern Temperature C zone 1 zone Time of day (hours) -8-

13 3. Water Heating 3.1 Hot Water Demand The hot water demand is based on a formula derived by British Gas: Hot water demand (litres/day) = N where N is the number of occupants. Assuming a 50 o C temperature rise within the hot water cylinder, and a 15% loss of energy between the tank and tap, the hot water energy at the tap, Q u (W) is given by: Q u = N The above demand function applies to an average household, but the following adjustments to Q u can be made to account for different levels of usage: Above average +20% Below average -20% Well below average -40% 3.2 Storage and Pipe Losses Sources of heat loss are from the hot water cylinder, the primary pipework and the distribution pipework. For certain types of hot water system, some of these losses will not be present, as detailed in the following sections Storage Losses Losses from hot water cylinders depend on: the type and thickness of insulation on the cylinder the temperature of the stored water the volume and shape of the cylinder the length of time before hot water is drawn off The cylinder shape, stored water temperature and the length of time before hot water is drawn off are all assumed to be fixed. The basic form of the equation describing the heat lost from a cylinder is: Heat lost = constant x cylinder volume/insulation thickness This equation has been calibrated using a combination of field data and data from the corresponding British Standard for cylinder insulation. The constant is different for loose jackets and for spray foam insulation, as they behave differently. This equation is obviously not valid for cylinders with little or no insulation, in which case a linear -9-

14 approximation is used: Heat lost = cylinder volume x (a - b x insulation thickness) where a and b are constants, which have been evaluated using the losses of a completely uninsulated cylinder, and the changeover point between the two relationships. The resulting equations for heat loss, Q t (W), are: Q t = 3 V HW for an uninsulated cylinder Q t = V HW ( I HW ) for loose jacket < 25 mm Q t = 40 V HW / I HW for loose jacket 25 mm Q t = V HW ( I HW ) Q t = 12.5 V HW / I HW for spray foam < 10 mm for spray foam 10 mm Where V HW is the volume of the hot water cylinder in litres I HW is the insulation thickness in mm If appropriate, the following factors should then be applied for modern boilers (where storage volumes are obtained from SEDBUK or a manufacturer s statement). For older boilers, these do not apply. For storage combination boiler with primary store, multiply by 1.5 For storage combination boiler with secondary store, no modification For hot water only thermal store in airing cupboard, multiply by 1.5 For hot water only thermal store not in airing cupboard, multiply by 2.1 For integrated thermal store or CPSU in airing cupboard, multiply by 2.8 For integrated thermal store or CPSU not in airing cupboard, multiply by 3.92 If no cylinder thermostat present, multiply by 1.3 For all cases, this is then multiplied by the volume factor, VF, given by VF = (120 / V HW ) 1/3 Such that Q t = Q t x VF Q t is reduced by 20% for lower than average occupancy and to 16.6% of its value for well below average occupancy. This latter reduction is to take account of field evidence which suggests that some households tend to use the water heater in a different manner to usual, turning it on just before hot water is required. For storage boilers, the above storage losses are only to be included for those boilers where the volume and insulation properties can be obtained from the SEDBUK database or a manufacturers declaration (i.e. not those who s efficiency is obtained from table D.2, appendix D). Otherwise, assume storage loss is zero. -10-

15 3.2.2 Primary Pipework Losses If the water heating is supplied by a central heating boiler, heat losses from primary pipework can also be significant, particularly if the pipes are uninsulated or the system keeps pipes hot for long periods of time. This is the case if there is no cylinder thermostat; such systems are not uncommon and are usually based on gravity feed of hot water from boiler to cylinder. The primary pipework losses (W), Q pp, are set out in Table 3.1. Table 3.1 Primary circuit losses System type Watts Electric immersion heater 0 Boiler with uninsulated primary pipework and no cylinder thermostat 140 Boiler with insulated primary pipework and no cylinder thermostat 70 Boiler with uninsulated primary pipework and with cylinder thermostat 70 Boiler with insulated primary pipework and with cylinder thermostat 40 CPSU (including electric CPSU) 0 Boiler and thermal store within a single casing and connected by less than 1.5 metres of lagged primary pipework (cylinder thermostat present). Boiler and thermal store not within a single casing and connected by more than 1.5 metres of lagged primary pipework (cylinder thermostat present). - uninsulated primary pipe work - insulated primary pipe work Additional losses for combi-boilers: - without keep-hot facility* - with keep-hot facility, controlled by time clock - with keep-hot facility, not controlled by time clock Community heating 40 * "keep-hot facility" means a combination boiler with an internal hot water store of less than 15 litres, where the water may be kept hot while there is no demand. If unknown, assume no keep-hot facility. The keep-hot facility can be maintained solely by burning fuel or by electricity. If the keep-hot facility is maintained hot solely by burning fuel, use an appropriate loss for combi boiler from the above table. If it is maintained by electricity and the power rating, P, of the keep hot facility is known from the Boiler Efficiency Database, the primary loss is equal to 0.032P. If P is unknown, refer to table D8, appendix D. -11-

16 3.2.3 Distribution Losses The distribution losses between the cylinder and the tap are taken to be 15% of the energy leaving the tank. This is equivalent to 17.6% of the energy leaving the tap. Thus, the distribution heat loss, Q d (W) is given by: Q d = Q u 3.3 Solar Panel Contribution The energy saved by a solar water heating system depends on several factors: The collector panel (its area, positioning, and collection efficiency) The availability of solar radiation (varying by region) The hot water demand In this model it is assumed that the panel is south facing and angled at 30 degrees to the horizontal, consistent with it being positioned on a sloping roof. The proportion of the energy reaching the panel that can actually be used depends not only on the panel itself, but also on the amount of hot water that is drawn off. The collector will work more efficiently if more hot water is used, since the water in the collector is more likely to be hotter than the water already in the hot water tank if this is the case. Thus, the amount of useful energy available from the panel depends on the ratio of demand to supply, called the load ratio (LR). The load ratio is defined as follows: 1 LR (0.5 f = (Q sp u S 30 A + Q ) t sp ) Where f sp is the collector panel efficiency (default is 0.5) S 30 is the radiation falling on a south facing 30 o inclined plane for the degree day region (W/m 2 ) (Table D.16, appendix D) A sp is the area of the solar panel (m 2 ) Q u is the hot water demand (W) Q t is the tank loss rate (W) From the load ratio, the proportion of the total energy incident on the panel that is useful, UT, is calculated as follows: 1 UT = 0.61* * LR or 1 UT = 0.65 / ( ) LR 2 1 LR for 1/LR for 1/LR <

17 To then calculate the useful solar energy, Q s, the formula is simply the product of the available energy and the proportion of that energy which can be used: Q = f s sp S 30 A sp UT 3.4 Fuel Used The fuel required to supply the hot water requirement, Q w in GJ/year, is given by: Q w = { Q u + Q t + Q d + Q pp - Q s } / (31.71ε w ) where ε w is the efficiency of the water heating appliance (Table D.1 or D.2 Appendix D) and the factor converts from W to GJ/year. As a large solar input term could theoretically cause a negative value for the fuel required, Q w is constrained to have a minimum value of 0.5 GJ/year Electric Water Heating Systems In houses where off-peak electricity is used, it is important to predict the fraction of onpeak usage, as it will have a significant impact on the overall running costs. A common form of electric water heating in these cases is the dual immersion system, where one heater heats the whole cylinder overnight, and a second smaller heater placed near the top of the cylinder provides additional daytime heating if required. Other systems use a single immersion heater connected to a time clock for controlling day and night rate charging. The equations for calculating the on-peak fraction, OnP HW, have been derived from field trial data from the Electricity Association. The fraction depends on the number of occupants, N, and the hot water tank volume V HW (litres). 7-hour off-peak tariff For dual immersion systems: OnP HW = { N ( V HW ) V HW } / 100 For single immersion systems: OnP HW = ( N) / V HW N 10-hour off-peak tariff For dual immersion systems: OnP HW = { N ( V HW ) V HW } / 100 For single immersion systems: OnP HW = ( N) / (1.5V HW ) N If in any of the above equations OnP HW is evaluated to be 0.01 or less, then it is reset to

18 3.5 Hot Water Gains The sources of internal gains resulting from the use of hot water are: storage losses primary pipework losses distribution losses heat lost from the hot water at the point of use The first three have already been discussed (Section 3.2). The gains arising from these losses occur both inside and outside the occupancy period. It is estimated that only 80% of these gains make a useful contribution. The gains arising from the use of hot water coincide with the periods of occupancy and contribute to the useful gains. However, a large proportion of the energy is lost as hot water down the drain. The useful contribution is estimated as 25% of the hot water demand at the tap. Thus, the hot water gains, G w (W), are given by: G w = 0.25 Q u (Q t + Q d + Q pp ) -14-

19 4. Lights and Appliances The estimation of electricity used by lights and appliances is important because: In well insulated houses, the electricity expenditure is a large proportion of the total fuel expenditure. The level of electricity use affects the internal gains. As households make use of more electrical appliances (eg dishwashers, tumble dryers), the effects of electricity usage increase. 4.1 Electricity Demand One use of BREDEM-12 is in the calculation of energy ratings based on the total fuel costs per square metre of dwelling. The aim of the rating is to indicate energy efficiency. In order to avoid the rating being dominated by electricity consumption, a linear approximation based on the floor area can be used: E LA = x TFA where E LA is the average electricity consumption for lights and appliances in GJ/year TFA is the total floor area in m 2 However, if a more accurate prediction of electricity use is required, an alternative definition can be used. This is based on actual occupancy data. The formula used depends on the value of the product of the total floor area (TFA) and the number of occupants (N): E LA = TFA x N for TFA x N < 710 E LA = TFA x N x 10-6 (TFA x N) 2 for 710 TFA x N < 2400 E LA = for 2400 TFA x N Both the electricity demand functions are modified if low energy lights (Section 4.2) or heating and ventilating fans (Section 4.3) are used. The overall electricity consumption for lights and appliances is given by E L (GJ/year) where: E L = E LA - reduction due to low energy lights + electricity for pumps and fans The above functions relate to average appliance usage. -15-

20 The electricity consumption, E LA, can be adjusted to take account of differing consumption levels: Above average +20% Below average -20% Well below average -40% 4.2 Low Energy Lights Field evidence has shown that lighting accounts for about 16% of the total electricity consumption in lights and appliances. Low energy (fluorescent) lights typically consume only 20% of the electricity for the same lighting level. In order to calculate the energy saved by low energy lights, a weighted average of the proportion of low energy lighting in each room is calculated. The proportion is calculated on a five point scale: None 0.00 Some 0.25 Half 0.50 Most 0.75 All 1.00 The weighting is allocated according to the room: Lounge 2 Kitchen 2 Hallway 2 All other rooms 1 The overall fraction of low energy lights in the dwelling, LEL, is used to calculate the reduction in electricity use, E red (GJ/year): E red = 0.8 x 0.16 x E LA x LEL 4.3 Heating and Ventilating Heating and ventilating systems make use of electricity in pumps and fans. The electricity consumption is estimated from typical ratings and the annual use of equipment (Table D.8, Appendix D). These consumption values are added to the previously calculated electricity consumption. -16-

21 4.4 Electricity Gains The gains from lights and appliances are calculated from the final figure for electricity use (after corrections for low energy lights, fans and pumps). However, heat generated by fans that exhaust air outside of the dwelling, does not contribute to internal gains. These include: - extract fans - fans in fan assisted boilers - the extract fan in mechanical ventilation heat recovery systems (MVHR) For MVHR (Mechanical Ventilation with Heat Recovery), or for a gas room heater with fan flue, the extract fan is assumed not to contribute to internal gains whereas the warm air circulation fan does contribute. Thus in these two systems the electricity is halved to calculate the gains. The electricity gains for lights and appliances, G L (in Watts), in any given month are: G L = 31.7 x (E L - electricity used in extract fans - electricity used in fans in fan assisted boilers x electricity used in MVHR x electricity used in fan assisted gas room heater) -17-

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23 5. Cooking It has been found that cooking use varies with the number of occupants. The general form of the equation describing the fuel used in cooking, E k, is: E k = A + B.N where N is the number of occupants A and B are constants which depend on the cooking fuel. Less fuel is used for electric cooking than for gas, which in turn uses less than kitchen ranges. It is difficult to determine the values of A and B as cooking fuel use has changed significantly over the last few decades. The introduction of electric toasters, kettles, fryers and microwave ovens has significantly changed cooking patterns. There is no definitive field data on which to base the comparative values. The current version of BREDEM-12 assumes a ratio of energy use of 1.75:1 between gas and electric cookers. The gains from cooking coincide with occupancy, but some will be deliberately ventilated due to the creation of smells and local overheating. For electric cooking, 90% of the energy used is assumed to contribute to the gains. Gas cooking generates an additional water load, and so requires extra ventilation. It is assumed that the requirement is to maintain the same relative humidity as for electric cooking. This means that only 75% of the gains from gas cooking is counted, the rest being deliberately ventilated. The resulting cooking fuel use and gains are in Table 5.1. Table 5.1 Cooking fuel use and associated gains Cooking system E k, Cooking fuel (GJ/year) G k, Cooking gains (W) Electric N N Gas N N Kitchen range N N Gas hob and electric oven N (Gas) N (Electricity) N The above consumption levels relate to average usage, these can be adjusted to take differing consumptions into account: Above average +20% Below average -20% Well below average -40%. -19-

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25 6. Heating Systems BREDEM takes account of dwellings which have more than one heating system. Portable heaters are excluded from the calculations, unless they are necessary to achieve adequate heating. Up to two different space heating systems can be specified. Secondary heating systems are likely to be gas fires, coal fires or electric heaters. The contribution that each heating system makes to the overall heating requirements of each zone needs to be estimated. Table D.7, Appendix D gives typical percentage contributions for various combinations of heating systems. The type of heating system and the way it is controlled have a significant effect on the internal temperatures and hence energy requirements. Each space heating system is characterised by its: responsiveness efficiency controls Hot water heating may be provided by the space heating boiler, or by an independent system. The water heating system is characterised by its efficiency. 6.1 Responsiveness The responsiveness of a heating system is a measure of how quickly the heat output drops when heating is no longer required. A responsive system is one where the heat output falls to zero soon after the end of a heating period. However, there are many systems for which this is not the case. These include coal fired appliances, under floor heating systems and electric storage heaters. These unresponsive heating systems result in a significantly higher internal temperature, during periods when no heating is required. This in turn gives a higher mean internal temperature. This is shown in Figure 6.1. Responsiveness is measured on a scale from 0 to 1, where 1 represents a fully responsive heating system and 0 a completely unresponsive system. The responsiveness of different heating systems is shown in Tables D.1 and D.4, Appendix D. Separate values are required for the primary and secondary systems. -21-

26 Figure 6.1 The effect of heating system responsiveness on internal temperature Internal temperature background temperature (unresponsive system) background temperature (responsive system) Time of day Responsive Unresponsive -22-

27 6.2 Efficiency The efficiency of each space and water heating system is used to calculate the quantity of fuel required for heating. It is defined by: Efficiency = Useful energy Energy delivered The efficiencies of different space and water heating systems are listed in Tables D.1 and D.2, Appendix D, but if possible, efficiencies should be taken from the SEDBUK boiler efficiency database ( An average efficiency is used, although in reality the efficiency of a heating system will vary on a monthly basis according to the load. 6.3 Controls The control of the heating system can influence the quantity of energy required to heat the dwelling in a number of ways: A lack of temperature control effectively increases the demand temperature. This applies to heating systems with neither thermostats nor thermostatic radiator valves and to storage heaters without automatic charge controls. If the system does not switch off when the dwelling has reached the desired temperature, then the appliance or boiler efficiency is reduced. This can occur with systems that rely solely on TRVs. A lack of independent control in zone 2 increases the amount of overheating in zone 2. If zone 2 heaters can be controlled independently then a shorter zone 2 heating period can be assumed, as occupants do not normally require zone 2 to be heated for as long as zone 1. If there is a lack of controls to turn a heating system off this in effect makes the system unresponsive. The effect of different heating control systems is listed in Table D.5, Appendix D. -23-

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29 7. Specific Loss This section describes the calculation of the specific heat loss for each zone. The heat transfer between zones 1 and 2 is also discussed. The specific heat loss is the rate at which heat is lost from the dwelling per degree temperature difference between outside and inside. It strongly influences the amount of heating required in a dwelling. It comprises two parts, the loss through the fabric of the building and the losses through ventilation. The overall specific loss for each zone, H i, is given by: H 1 = (ΣA i U i ) 1 + (ΣL tbi ψ i ) n T V 1 (W/ o C) H 2 = (ΣA i U i ) 2 + (ΣL tbi ψ i ) n T V 2 (W/ o C) where A i is the area of an external element (m 2 ) U i is the U-value of an external element (W/m 2 o C) L tbi is the length of a linear thermal bridge (m) ψ i is the linear thermal transmittance (W/m o C) n T is the total air change rate (ach) V 1 is the volume of zone 1 (m 3 ) V 2 is the volume of zone 2 (m 3 ) 7.1 Fabric Loss In principle, the fabric loss for each zone should include terms to take account of area heat loss, linear and point thermal bridges. However, as the effect of point thermal bridges is usually small, these can be neglected giving: Fabric loss (W/ o C) = ΣA i U i + ΣL tbi ψ i All areas and lengths are measured to the interior surface of the boundary walls. U- values for composite elements should be calculated according BS EN ISO 6946 for walls, BS EN ISO for floors and basements, BS EN ISO and for windows and doors. The U-values of elements next to an unheated space incorporate the buffering effects of the unheated space and so have an improved U-value (see Appendix C.1). The ground floor U-value calculation is based on the ratio between the perimeter and area of the floor (see Appendix C.2). Window (and door) U-values need to take into account the U-values of both the frame and the glazing (see Appendix C.3 for details). Linear thermal bridges exist at the corners of the dwelling. The calculation of the term ΣL tbi ψ i is described in Appendix C

30 7.2 Ventilation Loss BREDEM requires a reasonable estimate of the air change rate in order to calculate the overall heating requirement. The actual ventilation rate depends on a large number of factors that cannot be assessed from a site survey or from plans. The algorithm estimates the air leakage rate, and then adds on an estimate for occupant ventilation. It assumes that when the air leakage rate is low, the occupants will deliberately open windows to increase the air flow. Conversely, when the leakage rate is high, the windows are kept shut. The air leakage rate (or infiltration rate) will also depend on the local wind speed and exposure to the wind. The total air change rate, n T (ach), is the sum of the occupancy ventilation, n o, and the infiltration, n I, which has been adjusted to take the exposure into account: n T = n I + n o The heat loss through ventilation for each zone (taking into account the density and specific heat capacity of air) is then 0.33 n T V i (W/ o C). The calculation of each of these terms is described below Infiltration Rate The infiltration rate can be evaluated either from a pressure test result, or by adding together estimates of the leakage effects of the various characteristics of the dwelling. There is leakage through the building fabric (walls, doors, windows etc) and through forced ventilation from fans, flues and chimneys. The leakage values for the building fabric are in Table D.9, Appendix D. An additional 0.1 ach is added to the ventilation rate for each storey above ground level to allow for the stack effect. The flow rates for fans, flues and chimneys (in m 3 /hour) are in Table D.10, Appendix D. The total flow rate is calculated by assessing the number of items of each type and multiplying by the corresponding air flow rate. The total is then converted to an equivalent air change rate, by dividing by the volume of the dwelling. Thus, the infiltration rate, n i, in air changes per hour is given by: n i = Σ L i + ΣW i p i + Σ N i F i /V T where L i is a fabric leakage component (ach) W i is a window leakage component (ach) p i is the proportion (by area) of windows of a given type N i is the number of items (fans, vents etc.) of a given type -26-

31 F i is the flow rate for items (fans, vents etc.) of a given type (m 3 /hour) V T is the total house volume (m 3 ) Where pressure test results are available, these measure the leakage rate at an excess pressure of 50Pa. The dwelling infiltration is estimated as Q 50 /20, where Q 50 is the pressure test result in m 3 per hour. Fans, chimneys and flues are sealed during pressure tests, so their contribution needs to be included. Thus, the infiltration rate is given by: n i = Q 50 /20 + Σ N i F i /V T Wind Speed The ventilation for a dwelling is affected by the local wind speed. In determining the local wind speed, three factors are taken into account. Firstly the regional windspeed, v r, which is shown in Figure D.1, Appendix D. This wind speed typically applies to areas 50 miles across, and there is little variation over distances of several miles. Secondly there is the site exposure factor, Sh E, which relates to the general land use of the local area, this factor applies to areas of a few miles across. It varies from 1.0, for hill and coastal sites, to 0.8 for urban environments and sheltered valleys. The different categories of exposure and the corresponding value of the site exposure factor are listed in Table D.11, Appendix D. The third factor, dwelling exposure (Sh DW ) is classified according to the number of sides of the building that are sheltered by an obstacle. An obstacle shelters a surface of a dwelling if it effectively prevents the wind from hitting most of that side of the building. A dividing wall between adjacent dwellings counts as a sheltered wall, so a mid-terraced house is sheltered on two sides. In a high density estate, houses will often shelter each other, resulting in a house in the middle of an estate being sheltered on all four sides. The values of the dwelling exposure factor are in Table D.12, Appendix D. The site windspeed v site is then: v site = v r Sh E and the wind factor, f wind is: vsite f wind = 4 The overall infiltration rate adjusted for exposure, n I (ach), is given by: n I = f wind Sh DW n i -27-

32 7.2.3 Occupant Ventilation When there is no mechanical ventilation system, it is assumed that occupants deliberately ventilate the dwelling if the infiltration rate is less than one air change an hour. The occupant ventilation, n o, is given by: 2 n o = n I n I for n I < 1.0 n o = 0 for n I 1.0 If there is full mechanical ventilation, it is assumed that there is a fixed ventilation rate of 0.4 ach. Where there is heat recovery, the ventilation term n o is reduced by an amount dependent on the heat recovery efficiency, ε hr, to give an equivalent heat loss through ventilation. The occupant ventilation is then given by: n o = 0.4 (1 - ε hr ) 7.3 Interzone Heat Transfer The value of the interzone heat transfer coefficient is significant because it affects the temperature difference between zone 1 and zone 2. In most dwellings, the temperature in zone 1 is well controlled, while the temperature in zone 2 is not directly controlled. If there is a large transfer of heat from zone 1 to zone 2, this will increase the temperature of zone 2, resulting in an increase in the total energy loss from the building. The interzone heat transfer coefficient, H 3, can be estimated from the geometry of the building. It is assumed that the internal elements have a U-value of about 1.7. The wall and floor area separating zone 1 from zone 2 is estimated using simple geometrical arguments. This depends on the number of storeys and the proportion of the ground floor that is zone 1. Additionally, there will be an exchange of air between the two zones. This air flow will be significantly increased if the stairs enter zone 1 directly from zone 2. The value of the interzone heat transfer coefficient, H 3 (W/ o C), is calculated from Table

33 Table 7.1 Interzone heat transfer coefficient H 3 (W/ o C) A z1 < A z2 A z1 > A z2 1 storey dwelling 0.8 A z A z1 0.8 A z A z2 Stairs do not link zone 1 and zone 2 directly Zone 1 is part of one storey and stairs link zone 1 and 2 directly Zone 1 is all of one storey and stairs link zone 1 and 2 directly 2.53 A z A z A z A z2 4.2 A z A z1 4.2 A z A z2 4.2 A z1 4.2 A z2 A z1 is area of zone 1 (m 2 ); A z2 is area of zone 2 (m 2 ) -29-

34

35 8. Gains Useful gains reduce the amount of heating required. There are two types of gains: internal gains solar gains 8.1 Solar Gains from Windows and Glazed Doors Solar gains need to be calculated provisionally at first, as the final figure will depend on the length of heating season, the calculation of which requires a first estimate of the gains. The solar gains through a window depend on: the incident solar flux the area of the window the transmission factor for the glazing The incident solar flux depends on the region of the country, the orientation of the opening and the degree of over-shading. The provisional solar flux incident on a horizontal surface for each region is given in the final column of Table D.15, Appendix D. This figure is then multiplied by a factor to convert it to vertical flux. Fx i = Fx PH OF where Fx i is the solar flux for an element with the orientation of element i (W/m 2 ) Fx PH is the provisional horizontal solar flux for the region (Table D.15) OF is the orientation factor (Table D.13) The overshading of an opening has a major effect on the solar gain. Greater weight needs to be given to the amount of shading on south facing openings. An overshading factor is used, which has a value ranging from 0 (for total shading) to 1 (for no shading). The assessment of overshading is made separately for zone 1 and zone 2. The factors for different levels of overshading are given in Table D.17, Appendix D. The area of a window or door is defined as the area of the hole in the wall. In order to account for the area covered by the frame, a frame factor is used, this takes different values according to the type of frame (Table D.18, Appendix D). There is also a transmission factor that represents the proportion of the incident solar flux that is transmitted through the glazing into the house. A value of 1 corresponds to perfect transmission, and a value of 0 to complete opacity. The transmission factors for different types of glazing are listed in Table D.19, Appendix D. The provisional solar gain is given by: -31-

36 G sprov = Σ Aw i Fx i Fr i Tx i Ov I where G sprov is the provisional solar gain (W) Aw i is the gross area of the glazed element i (m 2 ) Fr i is the proportion of the element that is glass Tx i is the transmission factor for the type of glazing in element i Ov i is the overshading factor for the glazed element i The calculation of G sprov includes any windows that are covered by an unheated conservatory. Note: for a roof window in a pitched roof with a pitch of less than 70 o, treat as a Northfacing vertical window for the calculation of solar flux if its orientation is within 30 o of North. For all other orientations use the horizontal flux. In all these cases the overshading factor is 1.0. For a roof with a pitch of more than 70 o, treat as a vertical window. 8.2 Solar Gains from Conservatories For the purposes of this model, a conservatory is defined as an unheated enclosed space adjacent to one or more external elements of the dwelling, with at least 50% of its exposed vertical surface glazed. This definition allows for the inclusion of partially glazed sheds and covered in balconies. If the space is heated to a comfortable temperature, then it should be treated as part of the dwelling. Conservatories make the following useful contributions to the heating of dwellings: They shelter the building elements that they cover, reducing the heat loss. They capture solar gains and heat up. Some of this heat is then conducted into the dwelling through the wall separating the conservatory from the house. Some of the ventilation air that infiltrates the dwelling will have passed through the conservatory, and hence will have been pre-heated. The overall effect of the conservatory is calculated using a simple heat balance. The heat inputs to the conservatory are the solar gains and the heat conducted from the dwelling to the conservatory. Heat is lost through the external elements of the conservatory, and through ventilation losses to the external environment and the house. This information allows a mean conservatory temperature to be calculated. From this, the reduced heat loss and ventilation pre-heat can be calculated. The following assumptions are made: The proportion of ventilation air that is pre-heated before entering the dwelling is proportional to the fraction, F c, of the total wall area that is covered by the conservatory. The conservatory ventilation loss is split between the dwelling and the external environment in proportion to the area of the elements separating the conservatory from the dwelling and from the external environment respectively. -32-

37 The solar gains for the glazed elements in the conservatory, C s (W), are calculated in the same way as for windows: C s = Σ Aw i Fx i Fr i Tx i Ov i The net solar gain for the conservatory, S c, is calculated by subtracting the solar gains (calculated ignoring the conservatory) through any windows that are covered by the conservatory, from the conservatory solar gains: S c = C s - Solar gains through any windows covered by the conservatory The thermal resistance of the elements separating the conservatory from the rest of the dwelling, R w, is given by: R w = (ΣA i U i ) -1 where the sum is over the elements concerned. The thermal resistance, R c, of the external elements of the conservatory is calculated in the same way. The resulting formula for calculating the conservatory gains, G c (W), is: 1 Gc = Rw F c n T Sc k1 VT k1+ k2 where and 1 1 Ah k1 = Fc nt VT and k2 = Fc nt VT Rw Rc Ae A h is the area of the elements separating the conservatory from the dwelling (m 2 ) A e is the external area of conservatory elements (m 2 ) F c is the fraction of the total external wall area covered by the conservatory V T is the dwelling volume (m 3 ) S c is the net solar gain into the conservatory (W) n T is the total air change rate (ach) The factor 12 in the equation for G c represents the average temperature difference between the mean internal temperature and the mean external temperature. It is set to this constant to simplify the calculations and avoid the need for iteration. 8.3 Internal Gains The internal gains due to water heating, lights and appliances, and cooking have all been discussed earlier (see Sections 3, 4 and 5). Additionally, there are metabolic gains, G m (W), due to the occupants: G m = 60 N -33-

38 where N is the number of occupants. 8.4 Total Gains The total gains equal the sum of the following: Solar gains Conservatory gains Water heating gains Cooking gains Gains from electricity used in lights and appliances Metabolic gains The total gains, G prov (W) are given by: G prov = G k + G L + G w + G m + G sprov + G c The subscript prov indicates that the gains are provisional at this stage. The gains will be recalculated later when the solar gain taking account of the heating season length has been calculated. -34-

39 9. Heating Season Length The length of the heating season needs to be calculated because it varies significantly across the UK and has an effect on energy use. In cooler regions, the heating may need to be on for nearly the whole year, whereas dwellings in warmer regions will require less heating. The characteristics of the building must also be considered a poorly insulated house may require heating for a greater portion of the year than one that is well insulated. For these reasons it is logical that the heating season length should vary significantly from dwelling to dwelling, affecting energy use. Solar gain needs to be considered over this same portion of the year, as it is only during the heating season that it will be useful. 9.1 Heating Season Length Calculation The length of the heating season needs to be looked at on a monthly basis and depends on both the external temperature for the degree day region for the month and the gain to loss ratio (GLR) of the dwelling. GLR = G prov /(H 1 +H 2 ) Using this, a threshold temperature can be defined as follows: T Thresh = 17.4 GLR This effectively calculates the external temperature below which the heating will need to be on. To then calculate the number of days in each month for which heating will be required, the following formula is used: HD(m) = DD Thresh+1 DD Thresh DD Thresh =N(m)(T Thresh -T ext (m)) / [ 1-exp{-5(T Thresh -T ext (m))} ] for T Thresh T ext, or DD Thresh =N(m)/5 for T Thresh =T ext and DD Thresh+1 =N(m)(T Thresh+1 -T ext (m)) / [ 1-exp {-5(T Thresh+1 -T ext (m))} ] for T Thresh+1 T ext, or DD Thresh+1 =N(m)/5 for T Thresh+1 =T ext where HD(m) is the number of days in the month requiring heating N(m) is the number of days in the month T Thresh is threshold temperature above which heating is not required ( C) T Thresh+1 is threshold temperature plus 1 degree ( C) T ext (m) is the external temperature for the month for that region -35-