Why heat historic buildings?
This section considers the three main reasons why we heat historic buildings: the comfort of the occupants, protection of other building services like water pipes being damaged by cold temperatures, and conservation of collections.
A good heating system can make the difference between historic buildings being cold relics of the past, to desirable and sustainable places, suitable for all manner of uses. Heating systems should therefore aim to provide appropriate comfort temperatures for the particular use of the building, seasons of use, and the historic significance of the building.
Thermal comfort is subjective. An individual’s ideal level of comfort is when they are not aware of their thermal environment. In a home, it is well understood that the requirements for thermal comfort vary depending on the room type and the occupant’s activity. For example, a living room will typically be heated to a warmer temperature than a bedroom.
The thermal comfort of a person is influenced by:
- Air temperature – this is well understood and measured by a thermometer
- Mean radiant temperature - the average radiant effect of the surface temperatures around an occupant
- Relative air speed – a measure of the net mean air speed across an occupant, often experienced as a cold draught
- Humidity - a measure of the proportion of moisture in the air to the amount of moisture that the air can hold.
There are also personal factors that affect thermal comfort:
- Metabolic heat production - the heat generated by an individual will depend on their activity
- The type and amount of clothing worn by an individual will vary across the seasons according to the weather and indoor thermal environment
Other factors can also play a part such as acclimatisation, state of health, expectations, and even access to food and drink. Trying to account for all of these variables to keep everyone happy is very complex. The Chartered Institution for Building Services Engineers (CIBSE) adds that there should be 'no local discomfort (either warm or cold) at any part of the human body due to, for example, asymmetric thermal radiation, draughts, warm or cold floors, or vertical air temperature differences’ (The Chartered Institute for Building Services Engineers (CIBSE) Guide A: Environmental Design 2019).
Thermal comfort is explained in our Technical Tuesdays webinar recording: Building Services: Principles and Decision-Making.
Minimal levels of heating should be provided to prevent frost damage to other services that contain cold water, such as cold-water supplies to kitchens and bathrooms. Central heating systems should be controlled by a frost thermostat that will get the heating to come on when the air temperature drops to a certain level.
The thermostat should be in the coldest part of the building, not in a boiler room, plant room or service riser where it will never operate. Frost thermostats should be set to operate at 7°C, and where there is a building management system (BMS) a warning alarm should be set a few degrees above at, say, 9°C. The minimum controls setting for most ‘occupation heating’ systems will be for frost protection.
Another method of providing frost protection, so you don't need to run the heating system, is to use what is called 'trace heating'. Trace heating is an electrical tape that is run along the length of any pipe that is at risk of freezing. The pipe is then wrapped in insulation.
Conservation heating is a system that is used to maintain the environmental conditions in a building required for the preservation of a historic collection or historic building fabric, such as wall paintings that are sensitive to changes in relative humidity. It is often controlled by a building management system (BMS).
The ideal conditions for the collection may not be ideal for the users of the building, and therefore special measures may be required such as wearing warmer clothing or limiting time in spaces with conservation heating. Alternatively, it may be an option to heat spaces for thermal comfort during occupied hours and then revert to conservation heating at other times. The viability of this alternative approach will depend upon the sensitivity of the historic fabric and/or collections, as well as the capabilities of the heating system.