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A building must provide a satisfactory thermal environment for its occupants as well as for the mechanical systems it accommodates. Energy within the human body produces uninterrupted heat at varying rates in order to maintain an ideal temperature of 37 °C in the internal organs. However, heat is lost by the body through radiation, convection and evaporation from the skin and the lungs, and there is a continuous process of adjustment to ensure a thermal balance between heat produced and heat lost. It’s particularly important that the brain temperature is maintained constant. The factors in the local environment which govern heat loss include not only air temperature but also air movement, relative humidity and the radiant temperatures from surrounding surfaces. The fabric of clothing and that of a building perform similar functions by maintaining temperature control through passive means which regulate natural flows of heat, air and moisture vapor. As a building involves a volume many times larger than that contained by clothing, and provides environmental conditions suitable for many occupants in different spaces, it must also provide active means for thermal comfort not unlike that achieved by the human body itself. --- Heat balance of building in relation to internal and external influences. --- Potential percentage heat losses from domestic buildings. Passive means Simply stated, the creation of thermal comfort within a building by passive means involves the reduction of the rate of heat losses from the inside to the outside in colder climates, and the reduction of heat gains from the outside to the inside in warmer climates. In both cases the transfer of heat is regulated by the external fabric of a building which can provide varying degrees of thermal insulation. The type and amount necessary to achieve ideal conditions varies according to resource availability, climate of locality and the degree of exposure. In the US the thermal performance of a building fabric is directly affected by such detailed criteria as seasonal changes and extremes of temperature; temperature differences between day and night (diurnal range); sky conditions (amount of sunlight and overshadowing); incoming and outgoing heat radiation; rainfall and its distribution; the effects of water absorption and repulsion on materials and their forms (weathering); air movements; and other special features influenced by location and orientation. A designer must interpret these requirements in conjunction with fashions in appearance for a building, together with the current legal requirements which Endeavour to control atmospheric pollution and conserve the use of fuel for space heating by regulating the amount of heat loss from a building to the external environment. [[ __Table of Thermal conductivity and thermal resistances of some materials__ Material Conductivity Resistance (W/m K) (m^2 K/W) Brickwork 105 mm 0.84 0.125 220 mm 0.84 0.262 335 mm 0.84 0.399 Plaster 15 mm hard 0.5 0.03 15 mm lightweight 0.16 0.09 10 mm plasterboard 0.06 Cavity Unventilated - 0.18 With foil face 0.3 Behind tile hanging 0.12 Tile hanging 0.84 0.038 Expanded polystyrene 25 mm 0.033 0.76 13 mm 0.033 0.39 Glass fiber 50 mm 0.035 1.43 75 mm 0.035 2.14 100 mm 0.035 2.86 Aerated concrete 100 mm 0.22 0.45 150 mm 0.22 0.68 Softwood, 100 mm 0.13 0.77 Weatherboarding, 20 mm 0.14; 0.14 External surface - 0.055 Internal surface - 0.123 * The thermal resistance of a structural component is expressed as: R = L ÷? where: L = thickness of element (m) ?= thermal conductivity (W/m K) ]] Heat transfer Heat may be lost from a building fabric to the external environment by convection (movement of heat by hot liquid or gas); conduction (transfer of heat through solids); and radiation (transfer of heat from a surface across a vacuum, gas- or liquid-filled space, or a transparent solid). Natural ventilation causes convection losses and takes the form of a stack effect, where warm air rises in a building, eventually escaping to be replaced by colder air. Controlled ventilation is desirable as one of the functions of a building, e.g. trickle vents in window frames. Air leakage through gaps where external components meet should be prevented with a silicone mastic seal, e.g. window frames to wall, and joists to inner leaf support. Airtightness with draught-proof seals must also be provided at openings, e.g. doors, window sashes and loft hatches. Heat energy losses through the building fabric should be minimized by continuity of insulation about the external envelope. Solid building materials used for a building fabric lose heat from warm to cold face by conduction. The ability of a material to conduct heat is known as its conductivity (?) value, and is measured by the amount of heat flow (watts) per square meter of surface area for a temperature difference of 1 K per meter thickness, i.e. W m/m^2 K or W/m K. When comparing the insulation properties of a material to that of others, it’s generally more convenient to use its resistivity value (r) because it takes no account of size or thickness. Resistivity is therefore expressed as the reciprocal of the conductivity, i.e. m K/W. In order to calculate the actual thermal resistance (R) offered by a particular material of known thickness, the formula used is actual thickness (m) × m K/W, i.e. R has units of m^2 K/W. The higher the R value, the better the thermal resistance and the insulating performance of the material. and the lower a material's density, the greater its properties of insulation. It’s unfortunate that generally these relationships of density are counter to the noise control and structural qualities required from a material. A solid monolithic wall may be up to three times the thickness required for structural purposes in order to provide an acceptable level of thermal insulation. If a dense structural material is filled with pockets of air, its thermal insulation characteristics can be vastly improved while maintaining much of its strength characteristics. However, if these air pockets are allowed to become filled with water from exposure to rainfall or ground moisture, their insulation value will be cancelled altogether. This is because water is a much better conductor of heat than air; a saturated material could permit about 10 times more heat to be transferred through it when compared with its 'dry' state. Any heated material will radiate heat from its surface; bright metallic surfaces generally radiate least and dark surfaces the most. In this respect, radiation and absorption characteristics of materials correspond. Some construction methods incorporate air cavities which reduce the amount of heat transfer by conduction (and the passage of moisture) from inner warm materials to outer cold materials. Some heat, however, will be transferred across cavities by radiation, and the absorption (and reflection) properties of the adjacent surfaces across the cavity will be significant to the thermal insulation value of the construction. (The performance of the cavity as an insulator will be impaired if convection currents take place.) Radiation losses generally depend on the emissivity - the rate of radiant heat emission - from the surface and values depend upon roughness of surface, the rate of air movement across it, its orientation or position, and the temperature of the air and other bodies facing it. --- U value calculation for a one-brick-thick solid wall. --- Fully insulated cavity brick and block masonry wall. Thermal insulation values --- U value calculation for a one-brick-thick solid wall. ---Fully insulated cavity brick and block masonry wall. It’s unrealistic to rely on empirical rules to establish forms of constructions which provide satisfactory resistance to heat transfer as well as weather resistance, strength and stability and many of the other performance criteria. More over, current construction methods are generally less bulky than those used years ago, as greatly reduced thicknesses of materials are employed in combination with each other, each layer, leaf or skin perhaps fulfilling only one very specific function. The thermal comfort properties of these materials used in combination can only be assessed by calculating the amount of heat transfer from internal to external air (in cold climates). The thermal resistance properties of each layer must be taken into account, along with other factors relating to their surface texture and juxtapositions within the construction. The internal to external thermal transmission rate of all the layers of a construction is known as a U value and is more accurately defined as the number of watts transmitted through 1 square meter of construction for each single degree kelvin temperature difference between the air on each side of the construction, i.e. W/m^2 K. It’s calculated by taking the reciprocal of the sum of all the thermal resistance values (R values of all materials used and any air cavities) as well as the internal and external surface resistances. As the wall shown is consistent and continuous in its construction, i.e. the mortar and brick work have similar density and thermal properties, the U value calculation is relatively straightforward. Therefore the thermal transmittance of this element is: U = 1/R where R is the sum of all the separate resistances for different materials or structural components in the element, i.e. where: U = Thermal transmittance (W/m^2 K) Rsi = Internal surface resistance (m^2 K/W) R1, R2, R3 = Thermal resistance of structural components (m^2 K/W) Rso = External surface resistance (m^2 K/W) Note that Ra, the resistance for an air space, is included with the summation where a cavity or voids occur in the construction. Typical value is: 0.180 m^2 K/W. Walls with differing materials and composition will require more detailed calculation to determine the U value. The Proportional Area Method allows for inconsistent construction such as the relatively dense mortar between lightweight concrete blocks. Here the mortar will have a much higher thermal conductivity and a thermal bridging effect in the wall. However, the Combined Method (ISO 6946: Building components and building elements - thermal resistance and thermal transmittance - calculation method) is now preferred. This method provides a U value which represents the average of the upper and lower thermal resistance (R) limits. [...] The calculated U value for a particular construction is unlikely to provide an entirely accurate thermal transmittance rate because the heat flow conditions through the construction will vary with the amount of solar radiation, moisture conditions and the effects of prevailing winds. The quality of workmanship and supervision will also have an effect. Nevertheless, calculated U values are a reason able guide for comparing the thermal insulation values of different forms of construction and as an indication as to whether or not the heat energy losses from a building are within the limitations of building legislation. The higher the U value, the poorer the thermal insulation properties of an element of construction. Also, a U value refers to the total constructional thickness of an element, whether consisting of a single layer of material or a combination of materials, with or without separating cavities. The Building Regulations: Conservation of fuel and power, aims to increase the energy efficiency of buildings and thereby to reduce the emission of burnt fuel gases into the atmosphere. These are generally known as greenhouse gases and they include:
Carbon dioxide (CO2) is the least powerful but it’s the most prominent, accounting for about 80 per cent of all greenhouse gas emissions globally. Therefore, current thermal insulation and boiler efficiency calculations are designed specifically to reduce its contribution to the greenhouse effect. This term is widely used and includes changes to atmospheric conditions such as global warming and ozone depletion, leading to the physical effects of polar melting and rising sea levels. Requirements for new buildings in the United States generally include an enclosing envelope of insulation, double glazing throughout and draught sealing of all doors, opening windows, loft hatches and other gaps at interfaces in the building fabric. The effectiveness of these provisions and the efficiency of hot water and heating systems can be calculated by a standard assessment procedure outlined. Objective U values for elements of construction, but variations can apply to these depending on the contribution made by many other energy-related factors. For dwellings some of the most significant contributory factors are considered. --- Objective and Limiting U values. Criteria for appraising the thermal efficiency of dwellings As previously indicated, U values of the external elements are a very important factor and will have a contribution to the calculations which determine the CO2 emission potential for a dwelling. This measure of CO2 is known as the Dwelling Emissions Rate (DER). The DER is compared with, and should be not greater than, a Target Emissions Rate (TER). The TER is the energy performance requirement for a particular dwelling, obtained by calculating the likely mass of CO2 emissions in units of kg per m^2 of floor area per year. Calculation sheets and formulae to determine the DER and TER are published in the government's Standard Assessment Procedure, known as SAP. For dwellings, a checklist is provided in the appendices to the Federal Building Regulations. Principal criteria:
Incidental solar gains and glazing orientation The effects of solar gain can be a beneficial energy supplement in the winter months, but quite the opposite in summer if not controlled. The effects of solar overheating are considered. Window areas are limited as explained. This is not just to restrict the heat losses from within, but to regulate external gains. Table below provides a comparative guide to average seasonal solar gain data measured from October to April in the London area. Units are expressed in GJ per m^2 of external construction. [[ Table of Solar gain potential: Construction Orientation Dwelling thermal capacity Light Medium Heavy Double glazing N 0.162 0.189 0.216 S 0.513 0.621 0.711 E 0.306 0.369 0.423 W 0.306 0.369 0.423 Opaque N 0.005 0.006 0.007 fabric/structure S 0.017 0.021 0.024 E 0.011 0.012 0.015 W 0.011 0.012 0.015 Definitions: Light - timber or steel frame inner leaf with brick cladding Medium - brick and lightweight block cavity wall Heavy - solid brick or cavity brickwork inner and outer leaf ]] [[ Table of Dwelling ventilation heat losses Location Construction Ventilation rate Heat loss rate (a/c per hour) (W/m^3 K) Sheltered Well-sealed modern 0.5 0.17 Average 1.0 0.34 Drafty/not-modernized 1.5 0.51 Exposed Well-sealed modern 1.0 0.34 Average 1.5 0.51 Drafty/not-modernized 2.0 0.68 Note: a/c per hour refers to the volume air changes per hour. ]] Solar overheating Solar gains through the outer fabric of a building can be advantageous in the winter, but provision should be made to limit these gains in summer. Otherwise, to maintain reasonable levels of comfort, expensive fuel consuming air-conditioning will be necessary, thereby opposing the concept of fuel energy saving and reduction in atmospheric pollutants. Solar controls can be passive or active, and include limiting window areas, particularly those with southerly exposure, designing in thermal capacity or installing shading devices such as automatic or manually controlled awnings. A compromise between glazed area and desire for natural lighting may be necessary, to moderate the use of internal lighting. Ventilation characteristics Ventilation is essential for internal comfort. Particular requirements apply to all habitable rooms and especially to sanitary accommodation, bathrooms and kitchens. Internal air movement to the exterior is a potential heat energy loss and must be regulated as defined in the Building Regulations Code: Ventilation. As airtightness by sealing of the external envelope is now standard for new construction, it’s important to establish the right balance with con trolled ventilation. The Building Regulations recognize the simplest of facilities from trickle vents let into the window head, to ducted passive stacks and mechanical assisted energy recovery systems. A general comparison between likely ventilation rates for dwellings of various construction standards and potential heat losses is given. Table of values indicates typical ventilation rates for specific rooms. Type and efficiency of boiler Domestic hot water and heating boilers are rated on an alphabetic scale from A to G. This classification is based on an independent testing facility to establish the Seasonal Efficiency of a Domestic Boiler in the United States, known by the acronym SEDBUK. Google for a comparison of manufacturers' products. The Building Regulations require that only boilers of the highest grades (A and B), i.e. greater than 86% efficiency, may be installed in new premises and as replacements in existing dwellings. These high-efficiency boilers are otherwise known as condensing boilers, because the burnt fuel that primarily heats the water is directed by a fan around the boiler heat exchanger to create a secondary heat transfer before being discharged via the flue. This secondary heating produces some condensate for draining to a suitable outlet. Type of fuel The type of fuel chosen for hot water and heating by the end user will depend on availability and comparable costs. The Building Regulations are principally concerned with the amount of carbon dioxide (CO2) emitted per unit of fuel energy supplied. A measure of this is the factors listed for a variety of fuels in Federal Code. Figures range from 1.00 for mains gas to 1.47 for grid electricity. The relatively high factor for electricity allows for the carbon emissions at power station generating source, as when consumed, the efficiency is close to 100%. The appropriate factor is included in the formula for determining the carbon DER as part of the SAP assessment. Type of hot water and heating system Traditional hot water systems have a gravity or convection circulation between boiler and hot water storage vessel. This is slow and inefficient in the use of fuel. All new systems should have pumped circulation with the heating system. Conventional radiator heat emitters require a boiler hot water flow temperature of about 80 °C, whereas under-floor panels or coils of pipe can be served with water at about 50 °C. Under floor systems have the effect of heating up and storing energy in solid floors, slowly dissipating heat so that internal temperature swings are less pronounced and less frequent, reducing boiler activity. Other considerations: [] Decorative effect log or coal, gas-fuelled fires should have limited or restricted use. These appliances are an aesthetic feature that may be used as a supplementary heat source, but they are fuel inefficient when com pared with a central source system. [] Insulation of hot water and heating flow and return pipes to prevent unnecessary heat losses. Hot water storage vessels are usually insulated as a manufacturing standard. [] Thermostats applied to both hot water and heating systems, preferably to control a motorized diverter valve to each provision from a single pumped boiler flow pipe. [] Separate thermostats required for heating to upper and lower floors, each connected to a motorized valve to regulate the hot water to separate circuits. This is known as zone control or zoning, and is a requirement for all dwellings exceeding 150 m^2 living space/floor area. Separate time controls may also apply. [] Thermostatic radiator valves may be fitted to each heat emitter for individual control. These are standard fittings to all bedroom radiators. [] Programmer required to provide overall system management. This is basically a 24-hour clock controller, traditionally set to provide hot water and heating twice a day, or bypassed when either or both are needed all day. More advanced versions can provide pre-settings for 7 days or 28 days. [] Solar panels may be considered as a supplementary energy source in the United States. They generally occupy an area of about 40 m^2 of south-facing roof space for a typical house. The optimum roof pitch is about 40°. [] A boiler interlock is a feature provided by most equipment manufacturers. This facility prevents the boiler water temperature thermostat switching the boiler on when the water jacket temperature drops below its pre-setting, even when the time controller is on. Instead, the boiler fuel valve and circuit pump are controlled by the room or hot water thermostats. Energy management systems The quality and sophistication of these advanced programmers varies considerably. There are basically two types: 1. Compensated circuit: A computerized controller receives information from both internal and external air temperature sensors. From these it can regulate the boiler water supply temperature. The warmer the external air, the cooler the delivery hot water to emitters, and vice versa. 2. Optimum start controller: This is a variation using an external air sensor to provide information for the programmer to determine the optimum startup time of the boiler. On milder days, fuel savings will result in a system start later than provisionally programmed. Openings in the external envelope Windows, doors, roof windows and roof lights in combined areas should not exceed 25% of a dwelling total floor area. Factory-sealed double-glazed units are standard in modern buildings. Triple glazing is also possible, but with each pane of glass, light transmittance reduces by about 85%. Light transmittance is considerably reduced by solar reflecting tinted glass. The glass thickness and void width will have some effect on the thermal efficiency of the unit. Table 5 provides some comparisons for 4 mm double and triple glazing, separated by a 16 mm air or argon-filled void. Also included is low-emissivity or Low 'E' glass. This type of glass is used for the inner pane and has an outer surface coating of microscopically thin metal oxide, designed to reflect long-wave heat radiation back into a room, whilst being permeable to short-wave solar heat radiation and sunlight. An objective area weighted average U value of 1.8 W/m^2 K. An alternative acceptable measure is the European Window Energy Rating Scheme (EWERS). This establishes the thermal efficiency of a window system and rates it on an alphabetical scale of A (> 0) down to G (<-70). A rating of D (-30) is acceptable. Table -- Indicative U values for u PVC or wood-framed glazing systems Glazing Void U value (W/m2 K) Single - 5.6 Double, float glass ×2 Air 2.7 Double, float glass ×2 Argon 2.6 Double, float glass + Low 'E' Air 2.0 Double, float glass + Low 'E' Argon 1.7 Triple, float glass ×3 Air 2.0 Triple, float glass ×3 Argon 1.9 Triple, float glass ×2 + Low 'E' Air 1.4 Triple, float glass ×2 + Low 'E' Argon 1.3 Notes: 1. Thicker glass and an increased void width will reduce the U value. 2. Metal frames will increase the figures given by up to 20% unless insulation is incorporated into the hollow sections and a thermal break positioned to reduce conducted heat loss. Quality of construction In terms of energy conservation, construction of the external building fabric should be to a high degree of thermal insulation and airtightness. Vulnerable areas at construction interfaces, such as wall and eaves, and window or door openings and wall, should be built to the standards prescribed in the Robust Details accompanying the Building Regulations, Approved Document L. Alternative construction is acceptable, but whichever is used, sample buildings are subjected to air pressure testing. The objective is to attain an air permeability of less than 7 m^3h/m^2 of envelope area, the worst acceptable being 10m^3/h/m^2. Leakage can be detected by using smoke pellets. Lighting External lighting applies to fixtures attached to the exterior of a dwelling and any others associated with that dwelling. It does not include lighting to common areas in flats and communal accesses. Provisions for economic use are an individual lamp capacity not greater than 150 W, with an automatic facility to switch off when there is adequate natural light and when not required at night. Alternatively, the light fitting should be of the socket type that will only accept lamps with an efficacy greater than 40 lumens per circuit watt, i.e. compact fluorescent lamps. Efficacy can be defined as the efficiency of lamps in lumens per watt (lm/W), where a lumen is a measure of visible light, often expressed as lumens per square meter (lm/m^2 ) or lux, i.e. luminance. Internal lighting compliance is achieved where fixed lighting positions in living rooms and other most used locations in a dwelling are provided with lamps of a luminous efficacy greater than 40 lumens per circuit watt. These should be disposed at not less than 1 for every 25 m^2 of floor area and not less than one for every four fixed light fittings. Fluorescent and compact fluorescent lamps satisfy this requirement, tungsten filament lamps do not. Measurement of carbon emissions The carbon index method is an established measure for carbon assessment which provides a numerical comparison between dwellings. The assessment criteria are based on calculated carbon dioxide emissions in kilograms or tons per year relative to dwelling floor area. The following formulae may be used: CF = CO2 ÷ (TFA + 45) CI = 17.7 - (9 log10 CF) where: CF = Carbon factor CO2 = Carbon dioxide emission in kg/year TFA = Dwelling total floor area in m2 CI = Carbon index log10 = Logarithm to the base of 10 e.g. a dwelling of 180 m2 total floor area producing CO2 emissions of 2 500 kg/year: CF = 2 500 ÷ (180 + 45) = 11.11 CI = 17.7 - (9 log10 11.11) = 8.29 (8.3) The carbon index is represented between 0 and 10, rounded to one decimal place. All new dwellings should have a value of at least 8.0. SAP worksheets from Approved Document L can also be used to determine the annual CO2 emissions. 8.6 Condensation and thermal bridging The consistency of construction is a very important factor. Here most of the construction is of layered form but is 'bridged' at intervals by a material or an air space providing less thermal resistance. This results in variations in thermal transmittance values along the construction and the occurrence of cold bridging. The slightly cooler internal surface at the point of the cold bridge provides a dew-point temperature at which the warm internal air cools, causing droplets of water (condensation) as well as dust to be deposited locally, i.e. pattern staining. --- Cold bridge and pattern staining. --- Cold bridging by incomplete cavity insulation. --- Forms of condensation. In practice, it’s also necessary to prevent the occurrence of condensation at the surface or within material(s) used for a building fabric. This takes place when atmospheric temperature (dry bulb) falls below the dew point temperature - a property that depends upon the water vapor content of the air and therefore upon the vapor pressure. The amount of water vapor contained in the atmosphere will fall according to temperature and relative humidity (ratio of vapor pressure present to completely saturated air). Surface condensation is likely to occur when air containing a given amount of water vapor is cooled by coming into contact with a cold plane. High relative humidity, more than 80 per cent, will cause mould growths on the surface of organic materials or droplets of water on non-organic surfaces. This risk can be reduced by keeping internal surfaces at a higher temperature than the dew-point of the internal air, which involves carefully balancing the building fabric insulation and internal heating and ventilating requirements. Simply to increase the insulation value of the building fabric may only move the dew-point of the air into the body of the material and pro duce interstitial condensation, causing a loss of insulation and deterioration. One solution to the damage caused by surface and interstitial condensation is to use a construction method which makes the condensation or dew-point coincide with a cavity which separates external weathering layers from internal insulating layers. This cavity should be ventilated, and the moisture formed by condensation (together with any which penetrates the external weathering layers) must be adequately collected and diverted to the outside. When a cavity cannot be incorporated in the correct position relative to the point of condensation, then a vapor control layer or membrane must be used. This consists of a thin impermeable sheet (e.g. plastics or reinforced aluminum foil) and should be placed on the warm side of the insulating material to prevent transference of moist air from the warm interior of a room into the fabric of the building. Care must be taken to ensure that the sheets are adequately lapped and folded at their edges to prevent moisture penetration. A vapor control layer must also be incorporated for composite construction methods employing different layers of materials providing varying thermal and vapor resistance properties. The problems associated with the practical application of vapor control layers are discussed. Modern forms of construction and living patterns have greatly increased the risk of condensation, not only upon internal surfaces, but also within wall, floor and roof constructions. Traditional construction allowed moisture vapor to pass more easily through its fabric and through air gaps around doors and windows. There was often less moisture vapor to be vented, owing to lower temperature requirements and greater ventilation rates at moisture sources (air bricks and chimney flues, etc.). See also section 10.4 for a summary of current legislation governing the ventilation of rooms in domestic buildings. Thermal capacity The selection processes for a building fabric to provide satisfactory thermal comfort conditions must include consideration of the form of space heating to be employed. When the heat source remains virtually constant, it may be considered expedient to adopt a building fabric which will store heat or have a high thermal capacity. For example, masonry walling material will gradually build up a reservoir of heat while it’s being warmed. When the heat source ceases for a short period (i.e. overnight) or when the external temperature drops below normal, the stored heat will be slowly given back. At any event, the internal wall surfaces will feel relatively warm and comfortable, and the possibility of condensation occurring will be less likely, except during the period when the heat source is initially resumed. In hot climates, very thick and heavy construction buffers the effect of very high external day time temperatures on the internal climate of the building. When intermittent heat sources are used, or other marked fluctuations from steady temperatures occur, dense construction will be slow to warm up, and lower surface temperatures may be present for some time. This will give rise to discomfort for the occupants and condensation. In rooms used occasionally, and therefore requiring only intermittent heating for comfort, lining the interior surfaces with material of low thermal capacity, incorporating a vapor control layer, will reduce the amount of heat required and produce a quick thermal response. Alternatively, a construction method of entirely low thermal capacity can be adopted (timber walling incorporating vapor control layer and thermal insulation in spaces between structural members), and often this is the chosen form where the need to conserve fuel resources is paramount. In both cases -- internally insulated dense construction or lightweight construction -- less heat will be stored and the room will cool more rapidly when the heat source is curtailed. One acceptable compromise is to adopt a dense construction with thermal insulation on the external face. In this way, once a wall has been warmed, there will be a long period before it’s finally dissipated completely, during which time the heat source may be resumed again. Heat gains Unwanted solar heat gain into a building can be a source of considerable thermal discomfort and interrupt the working of normal methods of space heating. However, heat gains of this nature can be modified by careful adjustment of the amount of glazed areas (either fixed or openable); the type of glass used for windows; building and configuration; thermal character of the building fabric; surface colors and texture; and the degree of absorption permitted by exposed materials. The usual methods adapted are shading or screening devices, increasing the thermal capacity of the building fabric, and adopting a reflective outer surface. Nevertheless, with special reference to colder climates like the United States, the beneficial physiological and biological effects upon humans, animals and plants should be considered before deciding to eliminate solar heat gains altogether. Besides solar sources, considerable heat gains can be encountered when the function of a building requires the use of large amounts of energy. Office lighting systems and mechanical ventilation plant, exhibition display cases and lamps, and even people crowded together in super markets, cinemas, swimming-pools and disco halls can all create a considerable amount of heat. In large buildings the heat generated in this way may sometimes mean that cooling or ventilation plant is needed instead of heating plant, even during the cold winter season. The designer of such a building must ensure the correct thermal balance between heat requirement, heat inputs or gains and the required insulation standards for the building fabric. Considerable financial benefits may also be obtained if the heat gains can be transformed and stored for later use. Active means To arrive at a suitable method of space heating for a building requires examination of the fuel to be used as a source of heat, the methods of distributing the heat source to the heat emitters, and the appropriate means of heat output. Detailed analysis of these factors is beyond the scope of this guide, but it will be realised from the immediately preceding paragraphs that they exert a profound influence on the thermal comfort standards achieved by a building. With few exceptions, any form of heat emission can be powered by any fuel. The choice currently available includes fossil fuels (wood, coal, gas and oil); electricity; and, to a lesser extent, renewable resources (solar, animal wastes, geothermal, tidal, wind, wave, and ambient energy from light fittings, mechanical plant and even human beings). Fossil fuels are being consumed at an increasingly rapid rate to keep up with comfort standards for today. In order to regulate consumption of these finite resources and to control the atmospheric pollution from their combustion, the following items now have high priority:
--- Principles of a CHP installation. Wood is a renewable resource, but as materials for building purposes are also being depleted, it’s becoming an extremely valuable commodity which is far too important to burn. Electricity is generated mainly from fossil fuels, and for this reason ideally should not be used as a direct heating source because its production by this means is both inefficient and expensive. The increasing use of nuclear reactors for the production of electricity, using naturally occurring resources, certainly makes a more viable heating source. Developments in nuclear fusion (there is still an ample supply of uranium) could make electricity the major future source for thermal comfort, provided conversion to a useful form can be accomplished in safety and without danger of radiation or pollution to people and the countryside. Fear of these happening is one of the reasons for increasing interest in the use of certain renewable resources and ambient alternative energy sources for generating electricity, e.g. wind- and wave-powered turbines. Combined heat and power (CHP) systems have enjoyed far more success. Size of plant can vary from small units applied to hotels, schools, etc., to larger-scaled district heating/energy systems. The principle shown uses the surplus energy in flue gases and cooling water from conventional oil- or gas-fired electricity generators, directed through heat exchangers for use with hot water storage and heating. One of the most important decisions, as far as user comfort is concerned, is the choice of the form of emitter (radiator, convectors, ducted warm air, etc.). The selection of emitter is influenced by the response of a building fabric to changes in external air temperature, to temperature variations caused by the sun, and to the heat gains from changing occupancy, cooking or the use of other heat-producing equipment. If discomfort is to be avoided, either from underheating or overheating, the response of the heating system must be equal to or shorter than the response of the building fabric. The response of the fabric is determined by its mass, the degree and position of insulation, the reflectivity of external surfaces where exposed to the sun, and the area and orientation of windows. Furthermore, controls have a vital role to play in achieving economy of operation; the generally available controls are not yet able to provide immediate response to changes in temperature, so there may be a time lapse, creating discomfort. --- Circuit diagram for heater Table of Typical internal design temperatures and air infiltration rates Location Temperature (°C); Air changes per hour; Bathroom 22 2.0; Bedroom 18 1.0; Bed/sitting room 21 1.5; Dining room 21 1.5; Hall/landing 18 1.5; Kitchen 18 2.0 Living room 21 1.5 WC 18 2.0 Space heating and hot water controls -- general requirements Provisions in the Building Regs as outlined are intended for systems with centralized boiler heat energy sources and not individual solid fuel, gas or electric heaters. Electric storage heaters are included, with an expectation that units have an automatic charge control mechanism (thermostat) to regulate the energy consumption relative to room temperature. Conventional gas- or oil-fired boiler systems of hot water or warm air circulation satisfy the regulations if they contain the following:
Zone control: a means of controlling individual room temperatures where different heating needs are appropriate. This may be achieved by fitting thermostatic radiator valves to each emitter, or by using room thermo stats to control zoned circuits. For instance, kitchen and workroom temperatures need not be the same as in living areas. Some examples with minimum air change rates are listed. Large dwellings should be zoned into maximum floor areas of 150 m^2 with each zone having separate time controls. Timing control: systems should be capable of being programmed to provide both hot water and space heating independently at predetermined times. This applies specifically to gas- and oil-fired installations, and solid fuel boiler systems with forced air draught electric fans. Separate timing is unnecessary for boilers with an instantaneous draw-off facility, e.g. combination boilers and natural draught solid fuel boilers. Boiler control: gas- and oil-fired boilers should be controlled within the programmed cycle by room thermostats for space heating and a storage cylinder thermostat for hot water. Where thermostatic radiator valves are deployed, overall control from a room thermostat should disconnect the boiler and pump when there is no demand for heat. A thermostatic pipeline switch would be suitable to control stored hot water. Additionally, manufacturers provide a manually controllable working thermostat within a boiler and a limit (high-temperature) thermo stat as a supplementary safety cut-out on boilers applied to larger non-domestic installations. Additional requirements for domestic hot water storage systems include:
Solar energy The reuse of solar energy is becoming increasingly important as an alternative means of both space and water heating, and various forms of construction have been devised which 'capture' this free resource. For example, a glazed conservatory on the south side of a building will create an accumulation of heat which can be absorbed into an interior wall of high thermal capacity. The heat stored in this manner will be emitted into the interior spaces at night or during other periods of no sunshine. The amount of heat absorbed by an internal wall of this nature can be regulated by louvers opening in the conservatory so that excessive heat gains won’t occur and cause discomfort. This system is known as a passive solar energy resource. Active solar systems have been the subject of considerable research over the past 50 years. Basic systems consist of an exposed glass collector panel, behind which are located pipes containing water circulated through a heat exchanger in a storage vessel. More recent developments are far more sophisticated and include collectors inside clear glass vacuum-sealed cylinders, as well as power generation from photovoltaic fuel cells. In the United States, the water heated by solar means can provide a useful supplement to hot water and space heating systems, whereas in many parts of the world it’s the principal energy resource due to the high levels of solar radiation. The potential for solar energy is considerable as solar radiation can be quite effective even on cloudy days. In the US and Canada, the average amount of solar radiation on a south-facing roof inclined at an optimum angle of about 40° is around 1000 kW/m^2 . To date, there has been some reluctance to accept these systems in the United States, as the capital outlay may take several years to recoup in fuel savings. Also, the exposed panels can be perceived as a visual intrusion on the appearance of a building. Previous: Sound/Noise
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