Sensible Heating
Balancing Energy Consumption, Comfort and Conservation
Tim Bowden
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Nostell Priory, Wakefield uses a conservation heating approach. By heating the
internal space so that it is a few
degrees above the external ambient air temperature,
the internal relative humidity can be maintained within a
suitable range for the
preservation of the historic interior and the significant collection of Chippendale
furniture. |
Balancing the conflicting requirements of occupant comfort, protection of historic fabric and energy consumption can be challenging. This article considers the key issues and gives examples of how they can be addressed in a number of historic building types.
PROTECTION OF HISTORIC FABRIC AND CONTENTS
Historic buildings are sensitive to changes in environmental conditions. This is particularly true in those that contain organic materials in their construction or contents such as furniture, pictures, timber panelling and leather components such as those found in church organs.
The most important environmental parameter in a historic building is relative humidity (RH), which should ideally be in the 40-65 per cent range. When RH is too low, cracks can form in organic materials and furniture joints tend to become loose. When RH is too high there is an increased risk of mould growth, dry rot and insect infestation.
Understanding the prevailing conditions in a building is key to providing effective control. Therefore, monitoring temperature and humidity in the main spaces, ideally over at least a full year, is important to provide an understanding of the current conditions and allow the impact of any proposed changes to be monitored against a base case. Factors such as moisture entering the building due to poor maintenance of rainwater disposal systems will affect the internal environment. Only when the existing conditions in the building have been established can the impact of operational changes such as increased use or the addition of a café, with its associated moisture generation, start to be understood.
TEMPERATURE, HUMIDITY AND COMFORT
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| Remains of a Roman heating system in Chester which used the radiant effect of warm walls and floors to heat ‘leaky’ and poorly insulated buildings. Probably the first services engineers, much can be learned from Roman approaches to providing comfort conditions in historic buildings. |
Temperature and humidity are linked. If a volume of air is cooled, its relative humidity will rise up to the point of 100 per cent RH, which is known as its dew point, and further cooling will cause the water vapour to condense out. This is demonstrated when warm air touches a cold single glazed window in the winter months and condensation forms on the surface of the glass.
The external environment changes during the seasons, being generally cold in winter with lower RH and warm in summer with higher RH.
When buildings are heated in winter to make them more comfortable for the occupants, the general RH in the space is reduced below the ideal humidity range for conservation and this presents an increased risk to organic contents and fabric.
CONTROLLING RISK TO CONTENTS
The temperature and humidity in a space can be controlled by mechanical systems (air conditioning) but this is expensive in terms of capital cost, space requirements, maintenance and energy consumption. Air conditioning is therefore only appropriate for very specific conservation applications, such as in archive stores that meet British Standard BS5454 and some museums and galleries where temperature and humidity must be kept within narrow parameters.
One approach that is often used in historic buildings is ‘conservation heating’. It has been found that heating the internal space of a building to a few degrees above the external ambient air temperature generally maintains the internal relative humidity within the ideal range.
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| Liverpool Cathedral has recently undertaken improvements to the heating system to provide enhanced comfort conditions at events held in the winter months. An original underfloor labyrinth system which had not been used since the 1950s was brought back into operation. |
Using this method, the heating devices are required to raise the temperature by only a few degrees (rather than the 20°C for a conventional heating system, assuming 0°C external temperature and 20°C internal comfort temperature). The heat output from the heating devices is low and therefore energy consumption is minimised.
To fully influence the internal humidity this type of heating must be employed constantly. Also, in summer when the external RH is high (>65%RH), to reduce the internal RH levels the temperature in the space needs to rise further. Applying heating when external temperatures are high should only take place when it is essential. For normal applications a fixed maximum temperature should be agreed above which no further heating will be applied. RH levels outside the ideal range are likely to occur and are acceptable for limited periods.
THERMAL INERTIA
Buildings themselves can also moderate changes to the internal environment. According to the principle of thermal mass, a building acts as a sponge absorbing thermal energy when the surroundings are higher in temperature and releasing it when the surroundings are cooler. This is also applicable to moderating changes to the building’s moisture content. Smaller items such as furniture and pictures that are not part of the building structure are likely to show signs of unfavourable conditions more quickly due to their low thermal/hygroscopic mass. Sustained heating of a building to occupant comfort levels will gradually reduce the RH in the building. Intermittently heating a space and having wide swings in internal temperature (and therefore humidity) with items of low thermal/hygroscopic mass is likely to lead to cracking.
Generally the building temperatures that give the ideal conservation conditions are lower than the comfort requirements of occupants.
COMFORT CONDITIONS
Heat is transferred by conduction (from the heat source to a body in contact with it), radiation (from the heat source to another body or surface without heating the medium in between) and convection (from the heat source to the surroundings through the mixing of heated air). Radiation is a major influence on comfort in buildings with exposed internal masonry surfaces. Occupants lose heat by radiation to the surrounding cold stonework of a building. Even during periods of warm weather when the internal air temperature in spaces is higher, occupants can feel cold due to the cold radiation effect. This heat transferred by radiation does not heat the air; heat is exchanged between the body and surrounding surfaces. Under these conditions the occupants are often more tolerant of lower internal air temperatures with higher air leakage when the surface temperatures of the building envelope are higher.
The diagrams below show the mechanisms affecting comfort involved.
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The mechanisms to maintain thermal comfort these can be summarised as:
- occupant clothing
- occupant activity
- air temperature
- surface temperatures (radiant effects)
- air movement (drafts)
- humidity (impacts on the ability to sweat).
The combination of these mechanisms will dictate whether occupants are comfortable. Comfort is very subjective and normally conditions are said to be comfortable if 80 per cent of the occupants have that view.
Usually it is comfort conditions in the cold winter months that are of concern, but occasionally where historic buildings are used for offices, high levels of occupancy and associated IT equipment can result in high internal summertime temperatures.
ENERGY CONSUMPTION
While considering energy consumption, we have assumed that only heating is provided to the building since comfort cooling or air conditioning are only rarely used in historic buildings.
Energy consumption will be influenced by the following factors:
- the relative importance of conservation of the building/contents and occupant comfort
- activity within the space and comfort levels expected
- hours of use
- thermal performance of the building fabric including airtightness
- volume/scale of the spaces being heated
- efficiency and mode of heating distribution to the building
- efficiency of the heating energy source
- heating control including management by the occupants.
APPROACHES TO HEATING HISTORIC BUILDINGS
(a) Conservation trust property
This class of building primarily consists of
‘show spaces’ with significant fabric such
as wallpaper, curtains and other valuable
furnishings. Conservation of the fabric and
objects is the primary focus.
Conservation heating is provided to show spaces to maintain suitable RH levels. Low internal temperature in winter to show spaces with associated poor occupant comfort is acceptable but adequate humidity levels are maintained for conservation. Ideally offices and rest rooms where good occupant comfort are required should be located in less sensitive areas of the building where the provision of heating is likely to cause less risk to fabric. These spaces should be limited in area and, where possible, have improved fabric performance. The provision of separate heating systems for show spaces and back-ofhouse should be provided due to the different demands/output requirements.
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| Having shutters to windows and closing them at night reduces heat loss and improves security. |
(b) Converted georgian town-house
In this example, the building is assumed to have been converted to
offices. These might be occupied during the normal working week but
not at weekends. The significant historic fabric is non-organic, such as
fireplaces and tiled floors. Occupant comfort is the primary focus.
Improvements to the fabric should be made as far as possible. After the walls and roof, generally the most significant source of heat loss will be the single glazed windows, which should be improved if possible. Original timber shutters are often available internally and arrangement should be made to close these at the end of the day to reduce heat loss. External doors may open directly into the building and so a draught lobby might be provided, if possible. The provision of carpets within the building will reduce the cold radiant effect from stone or tiled floors and improve comfort, but on a solid ground floor they must still allow the floor to breathe.
(c) church converted for community use
There are often intermittent and variable hours of use in this type of
building. The significant historic fabric may include timber elements
such as choir stalls.
The best approach is to minimise the number of areas where continuous heating is required for occupant comfort and provide these as insulated spaces with controlled ventilation. The main space is likely to have poor fabric performance, airtightness and radiant cold surfaces. Consideration should be given to options which improve fabric performance, although these may be limited, and which provide a barrier between occupants and cold surfaces such as the provision of carpet. The occupants may have to accept a compromise in comfort conditions, with colder rooms, but a radiant heating system is likely to be the most effective and will minimise drying of timber elements if suitably positioned.
CONCLUSION
Maintaining an appropriate environment for people, building fabric and objects is a delicate balance. In relation to fabric and objects the effect of poor environmental conditions often takes time to manifest itself whereas people are usually quick to complain if they are uncomfortable. The monitoring of environmental conditions, particularly identifying and responding to trends, enables sensible approaches to be adopted which take into account the effect on historic fabric and objects but at the same time allows the sustainable use of existing buildings.
