Heating Systems for Low Energy Buildings Page 1 of 14

Heating Systems for Low Energy Buildings

Introduction ..................................................................................................................................................2 The design process........................................................................................................................................4 Energy efficiency ..........................................................................................................................................5 Heat Generation ...........................................................................................................................................7

i) Combined Heat and Power (CHP) ...................................................................................................7 ii) Multiple Boiler Arrangements ........................................................................................................7 iii) Heat Pumps .....................................................................................................................................7 iv) Electric Heating ..............................................................................................................................8

Heat Distribution Systems ..........................................................................................................................8 Heat Emitters ..............................................................................................................................................10

i) Radiators ..........................................................................................................................................10 ii) Natural Convectors ........................................................................................................................10 iii) Fan Convectors .............................................................................................................................11 iv) Underfloor Heating.......................................................................................................................11 v) Warm Air Heaters ..........................................................................................................................11 vi) Radiant Panels...............................................................................................................................11

Hot Water Plant..........................................................................................................................................12 i) Central Calorifier Systems..............................................................................................................13 ii) Central Self-Contained Systems ...................................................................................................13 iii) Local Storage Systems .................................................................................................................13 iv) Point of Use Water Heaters..........................................................................................................13 v) Solar Thermal Water Systems.......................................................................................................13

Summary......................................................................................................................................................14

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Introduction The heating of buildings accounts for over 40% of UK non-transport energy use (Figure 1). This is typically over 60% of domestic energy use, rising to ~80% if water heating included. Given the current targets for the reduction of energy use and associated CO2 emissions, a good design of heating system is essential to ensure efficiency and effective use of energy. A major problem is oversizing of heating systems, which can lead to reduced efficiency in part-load operation. The energy consumption of for oversized plants can be 50% more than necessary [source: CIBSE KS:8]. For low energy buildings this might become a greater problem because few engineers will have worked on a low energy building.

Figure 1 Pie chart showing the breakdown of non-transport related energy consumption in the UK.

There are many options available when designing a heating system. The fundamental components are:

• A means of generating the heat, the heat source. • A means of transporting that heat to where it is needed, the distribution network. • A means of delivering the heat into the space to be heated, the heat emitter.

Table 1 Examples of common heat sources, distribution networks and emitters.

gas CHP LPG solar oil biomass

coal off peak electricity electricity wind

Heat Source

Heat pumps, ground or air source Water: low, medium or high temperature

air steam Distribution network electricity

radiators ceiling panels fan convectors natural convectors panel heaters underfloor heating unit heaters storage heaters

Heat emitter

high temperature radiant panels

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The first thing to consider is why a building needs a heating system? • To keep the building up to temperature when occupied, • To pre-heat the building before occupancy.

These traditional answers revisiting when discussing low energy / low carbon buildings. Some of the options available are listed in Table 1. This gives rise to many different permutations. Whilst heating systems may seem simple, in practice there are many factors to be considered during the design process, in order to achieve a well-designed system that delivers the required level of thermal comfort for minimum energy usage. The airtightness and increased thermal insulation in a low energy building implies that for most buildings, most of the time, gains from occupants and equipment will account for most of the losses from the building including ventilation losses. However, because these gains may be localised within certain parts of the building there is a need to move excess heat around the building to where it is needed. This achieves two things, firstly it stops occupants having to dump heat outside of the building, which is wasteful, and secondly it will heat areas of low occupancy and stop these from triggering the heating system. So an important question to ask is:

How will heat be moved from spaces with high gains to those with low gains? An example: three models of primary schools, with identical U-values, occupancies and ventilation requirements. However, one has radiators (or other typical heaters) and opening windows provides ventilation, another has a mechanical ventilation system with heat injected into incoming air, exhaust air expelled via the corridors. Another has the same mechanical ventilation system but with the addition of a heat exchanger. Annual Heating Energy:

Normal heating + windows = 36.8 MWh Mechanical ventilation = 34.8 MWh Mechanical ventilation + heat exchanger = 10.3 MWh

The system with the heat exchanger uses less than 30% of the energy of the standard system. Even just the ability to move heat form the classrooms to the unoccupied corridors as in the case of the mechanical ventilation only system uses less energy. Thought needs to be given to ventilation strategies in low energy buildings and what happens to the heat in the air. Returning to the question of pre-heat phase, a low energy building should need less heat because it will still have suffered a smaller reduction in temperature overnight due to the increased airtightness and insulation levels. Ignoring the question of boiler efficiency for the time being, it is generally more energy efficient to keep the pre-heat phase as short as possible, as the heat loss from a building is proportional to the internal-external temperature difference. However, for an airtight well insulated building this becomes less important as the heat cannot escape as easily. This implies that a long pre-heat phase can be used and potentially reducing the peak load. Hence we can say:

A low energy building can afford a longer pre-heat period and a smaller heating system.

Ideally a reasonable portion of the excess cost of producing a low energy building will be repaid through the reduction of running costs and installation of smaller systems. Another observation is that because the annual demand for heat will be much lower than usual the capital cost of the

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system per unit heat over the lifetime of the system is likely to be much higher. This suggests that capitally expensive systems (e.g. underfloor heating) might not be a sensible choice if working to a tight budget. In general we can conclude that the heating system for a low energy building needs to take into account the fact that it is sited within a low energy building, hence: A heating system for a low energy building is not just a normal system made smaller, there are

implications for changes to current practices.

The design process Design involves translating the needs of the client into proposals and specification of specific products. There are two main characteristics that define the design process. Firstly, evolution of a design through a series of stages during which the level of detail is increased. Secondly, iterative cycles of design where the proposed design is trialled, tested, evaluated and refined. Feedback is therefore a major contributor to the design process as shown in Figure 2.

Figure 2 Representation of the design process, adapted from CIBSE KS:8.

The aim of the design process is to deliver thermal comfort, most clients do not ask for a heating system as part of their design brief, their focus is on what the system delivers. Although there are many factors to consider, thermal comfort is fundamentally about how people interact with their thermal environment. In order to provide good levels of thermal comfort throughout the occupied areas of the building knowledge is needed of occupancy levels, internal / solar gains and building usage. The four main environmental factors that influence thermal comfort and hence productivity are:

• Air temperature • Relative humidity • Mean radiant temperature • Air velocity.

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Different people have different preferences with regards to temperature and ventilation. Comfort metrics such as percentage people dissatisfied can be used to check that heating systems are suitable for the required situation.

Figure 3 Example of temperature gradients created by different types of heating systems. Adapted from

CIBSE Guide F. To promote thermal comfort and productivity large temperature gradients (both vertically and across rooms) should be avoided. Figure 3 shows different temperature gradients created by different heat emitters. Ventilation strategies should also be chose to avoid the creation of draughts as this can have a negative impact on thermal comfort. Thought should be also given as to the effects of glazing and solar gains of the mean radiant temperature felt by occupants in the room. The placement of the heating systems requires some thought as the radiant temperatures and air velocities can vary for different locations within a space. Also as shown in Figure 3 some heating systems such as warm air can lead to temperature stratification in space. This mans that the inside temperature at high level can be higher than that used in the heat loss calculations and therefore the heat loss through the roof or ceiling can be higher than expected. A correction for the type of heat emitter and the height of the space will need to be applied to ensure estimates of heat loss are feasible, for example a 5-15% increase in the fabric heat loss is expected for a low level warm air system in a space 5-10m high. Further details can be found in CIBSE Guide A. This is part of the reason why roof U-values are lower than those for external walls: hot air rises. Energy efficiency

• Incorporate the most efficient primary plant to generate heat / hot water. But also take into account whether such a source leads to a high or low overall system efficiency.

• Optimise the use of renewable energy sources (FITS and RHI can help finance this). • Ensure that heat / hot water s distributed effectively and efficiently. • Include effective controls on primary plant and distribution systems to ensure that heat

is only provided when and where it is needed and at the correct temperatures. • Be responsive to changes in climate, internal and solar gains, occupancies and usage.

CIBSE guide F states that designers should:

• Select fuels and tariffs that promote efficiency and minimise running costs. • Consider de-centralised heating and hot water generation plants on large sites to reduce

standing losses and improve load matching. • Locate plants to minimise distribution losses.

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• Ensure distribution systems are sized correctly to minimise pump and fan energy consumption, also to reduce potentially increased maintenance costs.

• Insulate pipework, valves etc to reduce distribution losses. • Ensure the base-load is provided by the most efficient plant. • Utilise condensing boilers where feasible and appropriate, these have the benefit that

they can be more efficient at part load than at full load and are therefore useful in small installations.

• Consider heat / energy recovery where feasible.

Remember: In a low energy building people and incidental gains should be considered a main

source of heat.

Figure 4 Illustration of typical efficiencies for different low temperature hot water (LTHW) boilers at

different loads. Adapted from CIBSE Guide F. With ever tightening building regulations and the drive for more energy efficient buildings, the levels of insulation present in walls and roofs is increasing. As the level of insulation in building elements increases heat loss through those elements is decreased, and hence other heat loss routes become more important. In highly insulated buildings heat loss through infiltration is a major heat loss path accounting for up to 50% of heating load in small buildings. Ventilation is also a major consideration and in buildings where high levels of ventilation are required; offices, schools etc the use of heat recovery should be considered as a possibility. As we have discussed oversizing can cause system inefficiencies, increased maintenance and shortened lifespan. Therefore there is the need to address load diversity, not all peak loads will necessarily occur at the same time so thought needs to be given to the correct sizing of heating systems. Thought should also be given to the use of the building in question, the attire of the people within the spaces and the level of activity of the occupants as these will have an impact upon how the space should be heated and hence the peak loading calculations.

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Peak loads may only occur for short periods of time, hence there are benefits to several smaller systems rather than one large system. The primary system should be as efficient as possible such as a condensing boiler if appropriate. Modular systems also have the benefit that extra modules can be added at a later date if more heat is required due to changes in building use. Heat Generation There are several different ways heat can be generated for transportation to the space where it is required. Some of the more common ways are detailed below.

i) Combined Heat and Power (CHP) Combined heat and power (CHP) has a wide range of applications in buildings and can be used to provide a large reduction in CO2 emissions. An important consideration when thinking of CHP in a building is when the heat will be needed. Typically in the summer months there is little demand for heat compared to the winter. This can have an impact of the sizing of the CHP plant if the plant is too large energy is wasted (see Figure 5).

Figure 5 Example of possible implementation of CHP and loading when combined with a typical boiler

plant. Source: CIBSE guide F.

ii) Multiple Boiler Arrangements Multiple boiler arrangements can be used to better match the demand for heat more closely and hence improve energy efficiency (Figure 4). These may comprise an integrated package of modules or independent boilers including CHP or Biomass (Figure 5). As load increases individual modules are independently switched on. Since each boiler is operating close to its own design load, efficiency is maintained. The overall plant can therefore provide an improved part load efficiency. Careful sequence control is fundamental to the correct operation of this system. In some instances such as for low temperature systems, it can be economic to specify all the boilers in a multiple arrangement as condensing. However, in most instances it is more economic to specify the lead boiler as condensing with high efficiency boilers to top up. This minimises capital cost while keeping overall plant efficiency.

iii) Heat Pumps Heat pumps can in theory produce a high coefficient of performance (CoP) when operating at low temperature differentials (see Figure 6). Heat pump have found wide spread use in applications where low-grade heat is available as a resource. Heat pumps are available in a number of different forms and exploit different sources of low-grade heat.

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Figure 6 Effect of temperature on heat pump CoP. Source: CIBSE guide F.

Air source heat pumps may be used to extract heat from outside air or from ventilation exhaust air. When outside air is used as the heat source, the CoP tends to decline as the air temperature drops (i.e. winter). Also problems in high humidity areas such as the UK can include icing of the heat exchanger often requiring the heat pump to be run in reverse to de-frost, this reduces the CoP. Air to Air heat pumps supplying heating only (typically only winter use), using outside air as the heat source in the UK can have a relatively low CoP. Their main advantage is that they are available for relatively small capacities compared to say Biomass boilers. They are also simple to control and unlike pellet stores for don’t constituent a safety hazard. Ground or water source heat pumps extract heat from the ground, bodies of water such as lakes at ambient temperature. These heat sources have greater specific heat than air and provided there is sufficient mass, the temperature of the heat source should remain fairly constant over the year.

iv) Electric Heating Various systems are available with outputs up to 5 kW; these include panel radiators, natural draught and fanned electric convectors. These systems are generally:

• Inexpensive to install • Can reduce space requirements • Require little maintenance • Provide quick response to controls • Are highly efficient but can have high CO2 emissions • Are suitable for intermittently heated areas • Have high running costs.

Due to the high unit cost of electricity, high levels of insulation and good central/local control of the system is required for the efficient use of electric heating. Electric storage heaters can take advantage of low electricity costs at night. They also have a low capital cost, are easy to install and are maintenance free. However, their main disadvantages are the limited charging capacity and the difficulty of controlling output, often leading to expensive daytime re-charging. Heat Distribution Systems The characteristics of a heat distribution system can have a profound effect on the thermal performance and energy consumption of the system. The main sources of inefficiency are:

• incorrectly sized pump and fans resulting in high energy consumption.

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• The position of plant rooms and the subsequent length of pipework influences both capital and running costs by increasing heat losses.

• Inadequate insulation on pipework / ducting. • Part loads resulting in reduced efficiency.

As heating demand reduces due to increased levels of insulation and air tightness, the importance of heating distribution losses increases. There is a need to choose the heat distribution system carefully depending upon the type of emitters that will be used, the size of the distribution network and the presence of any other processes that require heat or hot water, a comparison between different heat distribution media is shown in Table 2.

Table 2 Characteristics and comparisons of different heat distribution media. Medium Characteristics Pros Cons

Air

Low heat capacity, low density, small temperature difference between supply and return so large volume

needed.

No heat emitters needed

Direct heat, no other medium or heat

exchanger needed.

Large volume of air required, hence large ducts, fans can have high energy usage.

Water

High heat capacity, high density, large temperature

difference permissible between supply and return. Smaller volume required

than air.

Small volume required so small ducts, requires

less space.

Required heat emitters to transfer heat to the

space.

LTHW

Low temperature hot water systems operate below

~90°C and at low pressures that can be generated by an open or sealed expansion

vessel.

Generally recognised as simple to install and

safe to operate. Use with condensing boilers

to maximise energy efficiency.

System temperatures limit output.

MTHW

Medium temperature hot water systems operate

between ~90-120°C with a greater drop in water

temperature around the system. Pressurisation up to

5 Bar.

Higher temperatures and larger temperature

drops give smaller pipework, which may be advantageous on

larger systems.

Pressurisation requires additional plant,

controls and safety requirements.

HTHW

High temperature hot water systems operate at

temperatures over 120°C, sometimes up to 200°C. Even greater temperature drops around system, with pressurisation up to 10 Bar.

Higher temperatures and temperature drops

give even smaller pipework

Safety requires all pipework must be

welded to the standards required for steam

pipework. Unlikely to be a cost-effective

method of heat transfer except for over long

distances.

Steam

Exploits the latent heat of condensation to provide very high heat transfer capacity. Operates at high pressures.

Principally used in hospitals or kitchens where steam is

required.

Use of latent heat of condensation allows large transfer of heat.

Higher maintenance and water treatment requirements, extra safety requirements.

If the site requiring heat is a disparate collection of buildings or perhaps one large building complex there is the option of have a centralised heating system and distribution network or a de-centralised collection of independent heating systems. Both types of system have benefits and drawbacks a comparison between the two is show below in Table 3, the most efficient

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combination of heat distribution media and system will depend upon the site under consideration.

Table 3 Comparison of centralised and de-centralised systems. Centralised De-centralised

Capital cost

Capital cost per unit falls with increasing capacity of central plant.

Capital cost of distribution is high.

Low overall capital cost, savings made on minimising the use of air and water

distribution systems.

Space requirements

Space requirements of central plant and distribution systems are

significant, particularly ductwork.

Large, high flues needed.

Smaller or balanced flues can often be used.

Flueing arrangements can be more

difficult in some locations.

System efficiency

Central plant tends to be better engineered, operating at higher

system efficiencies, due to higher more stable loads.

As load factor falls, total efficiency falls and loses can be significant.

Energy performance in buildings with diverse patterns of use is usually better.

System operation

Convenient for some institutions to have a centralised plant.

Distribution loses can be significant.

May require more control systems.

Zoning of the systems can be matched more closely to the occupancy patterns.

System maintenance and lifetime

Central plant tends to be better engineered, more durable.

Less resilience if no standby plant

provided.

Can be readily altered and extended.

Equipment tends to be les robust with shorter operational life.

Plant failure only affects local services.

Fuel choice

Flexibility in choice of fuel, boilers can be dual fuel.

Better utilisation of CHP, etc.

Fuel needs to be supplied throughout the site.

Boilers tend to be single fuel.

Heat Emitters Different types of heat emitter have different output characteristics in terms of the ratio between radiant heat and convection. Where there is a high ventilation rate, the use of radiant heating systems results in lower energy consumption. Fully radiant systems heat occupants directly without heating the air to full comfort temperatures.

i) Radiators Radiators have a convective component of 50-70%, they are cheap and easy to install. Control is simple but can be slow to respond. Local control is available through the use of thermostatic valves.

ii) Natural Convectors Natural convectors have a convective component of ~80% and tend to produce a more pronounced vertical temperature gradient, which can result in inefficient use. These are best used in well-insulated rooms with a low air change rate and relatively low ceilings. The convectors themselves are often unobtrusive being incorporated in to the floor or skirting or a room.

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iii) Fan Convectors Fan convectors like natural convectors are suitable for well insulated rooms with a low air change rate. These can be directly incorporated into mechanical ventilation systems. Fan convectors respond very rapidly to control and can have a variable output allowing for greater control. The high cost of electricity means that these are not the most economical heating system, they also have a higher maintenance requirement than other systems and can be noisy.

iv) Underfloor Heating Underfloor heating usually consists of a low temperature warm water distribution system set into the floor slab. This type of system gives a slow thermal response and it better suited to spaces where there is continuous occupation. As the heat source (the water) is at a relatively low temperature compared to other systems there is a level of self-regulation. As the room warms up the temperature difference between the air and the floor decreases as does the heat output. Generally operating between 35-45 °C, these systems provide an ideal opportunity to use condensing boilers due to the low return water temperatures. Heat pumps are equally advantageous in underfloor systems due to the low temperature of the water required. There is a high thermal inertia with underfloor systems and therefore are less responsive to local heat gains. Underfloor heating is difficult to control in buildings with fluctuating gains such as a school. If the building is up to temperature at occupancy the underfloor heating system can be turned off but the high thermal inertia of the building slab in which it resides will continue to emit heat until it reaches the same temperature as the room. There is unlikely to be any use for this heat and the room temperature will require the occupants to dump the heat by opening windows etc. If this occurs then low energy aspirations will not be realised. Underfloor heating systems are therefore best suited to situations where there is consistent internal gains and preferably constant occupancy, otherwise a sophisticated control system is required to predict when systems need to be turned off prior to reaching heating set points.

v) Warm Air Heaters Warm air systems have a quick thermal response but can promote temperature stratification. The units can be directly fired so distribution losses are reduced, but the systems are often noisy and require large lengths of ductwork, which increase fan power requirements.

vi) Radiant Panels Radiant panels can be supplied with medium or high temperature water or steam and have a radiant component of ~65%. These are best suited for use in large spaces with high air change rates. They have the benefit that they heat the occupants and building fabric rather than the air within the building. Due to the temperature of the panels care needs to be taken to ensure the occupants cannot burn themselves. The height of the panels is important, if they are positioned too low then overheating can occur, whereas locating them too high can result in increased energy consumption due to reduced comfort levels and longer running times.

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Table 4 Comparison between different heat emitters. Design points Pros Cons

Radiators

Output up to 70% convective.

Occasionally limit on surface temperatures.

Good temperature control.

Balance of radiant and convective heat gives good thermal comfort.

Low maintenance, cheap

Fairly slow to respond to control.

Slow thermal response.

Natural convectors

Quicker to respond to control.

Skirting or floor trench convectors can be

unobtrusive.

Can occupy more floor space.

Can get higher temperature stratification

in space.

Fan convectors Can also be used to deliver ventilation air. Quick thermal response.

Can be noisy. Higher Maintenance. Occupies more floor

space.

Underfloor heating

Check required output can be achieved with

acceptable surface temperatures.

Unobtrusive. Good space temperature distribution with little

stratification.

Heat output limited. Slow response to control.

Warm air heaters Can be direct fired units. Quick thermal response.

Can be noisy. Can get considerable

temperature stratification in space.

Low temperature radiant panels

Ceiling panels need relatively low

temperatures to avoid discomfort.

Unobtrusive. Low maintenance. Slow response to control.

High temperature radiant panels

Can be direct gas or oil fired units.

Check that irradiance levels are acceptable

for comfort.

Quicker thermal response.

Can be used in spaces with high air change

rates and high ceilings.

Need to be mounted at high level to avoid local high intensity radiation

and discomfort.

Hot Water Plant As we can see from Figure 1 the heating of hot water is a large proportion of the total energy use of buildings in the UK. In a low energy building where heating energy usage has been minimised this proportion grows. Once we reach Passivhaus-style efficiencies it could well be greater than the space heating demand. As a major contributor to the total energy use of a building and potentially a stumbling block towards achieving a truly low energy building equal thought should be given to the design of the hot water system as is given to the heating system. The key will be an accurate idea of the true hot water demand. It is worth comparing the effort that is undergone to estimate heat loss from the building via a thermal model against that used to predict hot water demand – usually just a value plucked from a table! Hot water plants should always be sized correctly thus minimising capital and running costs. Reducing temperatures saves energy and reduces the risk of scalding, but this should not be achieved at the expense of safety. To avoid multiplication of legionella, hot water should be stored at a temperature of 60 °C and should be distributed such that a temperature of 50 °C is achieved at the tap within 1 minute. This requires thought when considering low energy design. In practice for all but domestic properties this means that either water is circulated centrally around the building to maintain its temperatures, electric or trace heating is placed along the pipework. Both these approaches will use substantial amounts of energy and this indicates that unless hot water use is high, point of use might be better.

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The main types of hot water system are:

• Central calorifiers, supplied by the main heating system (see previous sections). • Central self-contained gas or electric. • Local storage, gas or electric. • Local point of use, usually electric.

i) Central Calorifier Systems

Where hot water loads are high and distribution systems are compact, a well controlled central boiler / calorifier plant can operate reasonably economically. However, where hot water loads are not substantial, separate heating and hot water plants will be more energy efficient especially during the summer months. Combination boilers can provide an energy efficient way of achieving both heating and hot water in a small (typically domestic) centralised system. Hot water is heated almost instantly on demand, although interaction between the heating and hot water may occur in winter if the demand is high.

ii) Central Self-Contained Systems Self-contained central hot water systems are typically more efficient than systems combined with the main heating. This is because standing losses are lower and the poor part-load efficiencies characteristic of boilers sized for the full heating duty are avoided during summer operation. Relatively high efficiency storage water heaters are commonly used and condensing versions offer a further high efficiency option. Electric immersion heating is generally only economic (due to high electricity costs) where there is adequate capacity for off-peak storage. The system needs to be controlled and sized to prevent expensive daytime top-ups.

iii) Local Storage Systems Small-decentralised gas fired storage water heaters close to the point of use can significantly improve efficiency since the standing and distribution losses are greatly reduced. The problems associated with legionella are also normally reduced with localised systems. Capital and maintenance costs are likely to be higher, but this is usually more than compensated for by the increased efficiency. This type of system is ideally suited to situations where there are short periods of high demand such as catering and sports facilities.

iv) Point of Use Water Heaters Evidence suggests that point of use water heating is extremely economical where the demand is low, e.g. offices without catering facilities. Capital cost and delivered energy consumption is generally low. Water softening may be required to prevent scaling of units, adding to cost but reducing maintenance.

v) Solar Thermal Water Systems The addition of solar thermal water systems to a building can reduce the fossil fuel energy used to heat water. It is unlikely that a solar thermal system will be able to provide sufficient heat all year round but it will dramatically reduced energy costs in the summer months and pre-heat water in the winter months thereby reducing water-heating costs. Capital cost is high but the renewable heat incentive (RHI) provides a financial incentive for such systems. Solar thermal units require space that could be used for other renewable systems such a photovoltaics therefore thought needs to be given to the use of the building and which system will give the greatest benefit. One important issue for non-domestic properties is that solar hot water will still require the use of trace heating or continuous pumping so as to guard against legionella as described above, greatly reducing its carbon credentials.

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Summary Heating is often considered a simple, basic system, however there are many options and many different permutations to be considered. Most buildings require heat but different building types, uses and locations have different heating requirements. A holiday cottage on Dartmoor has different heating requirements to a city centre ground floor flat. A department store has different heating needs to a large warehouse DIY store. The construction of the building is also important as is the level of internal and solar gains. A school of heavy weight construction with long unoccupied periods will require a different heating solution to a school of lightweight construction. It is important when designing the system to understand the needs of the client and the type and usage of the building in order to choose and appropriate heating system. The choice of internal and external design conditions can have a substantial impact on the initial system loads and subsequent system performance. These are fundamental part of heating load calculations and the choice should be very carefully considered. For example the difference between assuming an internal-external temperature difference of 21 K (-1 °C to 20 °C) and one of 25 K (-4 °C to 21 °C) for a particular building is nearly a 20% increase in the heat loss an hence an increase in heating requirement. It is also important to consider what system performance criteria are required and the level of control that is acceptable. Establishing the required system performance criteria at the briefing stage is one of the most crucial tasks in the design process. It is vital that clients and designers have a thorough understanding of what conditions are required and what can practically be delivered. For example the difference between a specified internal condition of 21 °C ± 1 °C and 21 °C ± 2 °C can have a considerable impact on the buildings energy consumption, control choice and system performance. The closer the control the more expensive the system, if the design conditions can be relaxed (within reasonable limits) the system can be simpler, cheaper and use less energy. Therefore it is important to have a dialogue with the client to understand their specific requirements and make them aware of the possible need to change current practices in order to achieve a low-carbon building. One key question is, how will the system be controlled. In any modelling it might well have been assumed that all rooms experience perfect control. But in the real building if a room does not have a temperature sensor how will this control be achieved? In reality occupants may well open windows to dump heat while the system is still injecting heat into a space due to insufficient control (the thermostat may be in a different cooler space). If this happens then it is unlikely you have a low energy building.

The heating system, the controls and the occupants are not adjuncts to the low energy building:

they are as important as the insulation in the walls.