Heating Systems for Historic Buildings

There are several ways in which you can heat buildings:

• central heating
• local heat emitters
• a combination of central heating and local heat emitters
• air based systems such as ducted warm air and air conditioning units.

Methods of heat transfer

Before looking at the different types of heating it is important to understand how heat is transferred from heat sources

There are three different methods of ‘heat transfer’: convection, radiation and conduction.

• Convection: Convective heating is the heating of the air in a space. Heat is transferred into the space and raises the temperature of the surrounding air. Heat is then transferred from this warmer air to the person. This type of heating is useful in counteracting draughts and raising internal surface temperatures to reduce the risk of condensation. Convective heating tends to have a fast response time, so is ideal for the occupants. However, the natural materials within historic buildings can be adversely affected by rapid temperature fluctuations. The warmer air encourages evaporation, resulting in an increased amount of moisture in the air that can condense when the heating system is turned off. Convective heating systems can result in high stratification of the heated air. Heat provided by a radiator is often assumed to be radiant, but 60-80% of the heat is transferred by convection.
• Radiation: Radiant heating is where high temperature heat sources radiate infrared energy directly to the object or person that absorbs the heat energy. This type of heating has a medium response time, although occupants will quickly feel the warm sensation. The air temperature is eventually heated by convection of the air coming into contact with the warmer surfaces. This results in less temperature fluctuation, which is beneficial to historic buildings. High output electrical radiant heaters require careful consideration as they pose a fire risk due to their high operating temperatures.
• Conduction: Conductive heating involves heat transfer from the hotter object to the colder object by direct contact with the hotter surface. This type of heating is not generally adopted or important in heating systems, although a small amount of conductive heating takes place when occupants sit directly on floors heated by underfloor heating systems.

Central Heating Systems

Central heating systems operate by having a centrally located plant that heats water that is pumped through pipework to heat emitters such as radiators, convectors, fan-assisted convectors, far infra-red radiant panels, underfloor heating, trench heaters and perimeter heaters. The heat emitters are located to heat spaces as required.

The central plant could be:

• boilers which could be powered by natural gas, oil, propane from a cylinder or tank, electricity or biomass
• heat pumps, air, water or ground
• hybrid heat pumps - heat pumps supplemented by gas or oil-fired boilers

Boilers

The boilers for a hot water-based heating system need to be sized to match heat losses to heat spaces. In historic buildings, the biggest challenges are identifying spaces for the boiler, ventilation and the flue.

Central heating flues

Flues discharge the fossil fuel combustion products to the atmosphere. Some types of flue also provide the plant with combustion air.

Flue runs may need to be lengthy. In historic buildings, the route of the flue needs to avoid damaging vulnerable fabric. Flue terminals may need to be painted so that their appearance blends in with the external fabric. The colour, size and detail of proposed flues should be developed at the stage when Listed Building Consent is applied for.

An old chimney is often a good place to run the flue from a new boiler in a traditional building. Depending on the boiler size it is possible to terminate some flues horizontally through the wall of the building.

Fitting a boiler to an existing chimney will normally require the insertion of a flue liner. It is important that this should be selected and installed by a competent certified professional to the European and British Standards for flues. A flexible corrosion-resistant metal liner is lowered down the chimney in a continuous length.

Not all chimneys are capable of accepting a new flue liner. Flues in traditional buildings are sometimes narrow, contorted or distorted by structural movement over the years. Attempting to force a flexible metal flue liner down such chimneys will not always be successful and risks damaging historic fabric. The space between the original flue and the new flue liner should be ventilated top and bottom. Flue liners that require backfilling between the liner and the chimney should be avoided in traditional buildings.

Flues may require access along their entire length for safety inspection purposes and flue systems for some condensing boilers will require a drainage connection.

Radiators and other heat emitters

Radiators

Radiators are the most familiar type of heat emitter and usually offer the best mix of radiant heat and convective heating of the air in spaces. Despite their name, they provide more heat by convection than by radiation.

Existing radiators, if of great age, and other heat emitters, such as pipe coils, especially those by Perkins and other early pioneers, may be of great historical importance as well as being quite suitable for re-use if free from leaks.

The style of radiator needs to reflect the setting. Generally use:

• Plain faced steel radiators against painted or plastered walls.
• Cast iron sectional radiators against brick or stone walls (or other patterned surfaces). There are poor quality items available that are one-piece cast replicas of the real cast iron sectional radiators and their use should be avoided.
• Domestic type steel radiators of the welded panel type are commonly used in modern domestic settings. The paint finish of the front surface can easily be damaged in busy commercial or institutional settings.

The use of reclaimed or refurbished radiators to match existing emitters or the period style may be worth exploring. The old radiators will need to be appropriately pressure tested, cleaned and repainted, and radiator valves overhauled or replaced.

Trench heaters

Trench heaters use a piped (or electrical) heat emitter located within the floor to heat the space above via convection. Fan assisted trench heaters are also available. This type of heater is commonly found around the perimeter of buildings and can be used to offset the downdraughts from glazing and high external walls. Installation requires some disruption to the floor unless existing trenches are available, and location is important to minimise the ingress of dirt. Sensitive historical fabric and objects can be at risk from the 100% convective heat output and fast response time (the time taken to heat a space). Regular cleaning is also required.

Fan convectors

Fan convectors use a fan to draw the room air across a pipe coil or electrical element to heat the space by convection. These heaters can be wall, floor and ceiling mounted or recessed. Natural convectors without fans are available; however, their heat output is reduced. The noise of fan convectors needs to be considered, with sensitive historical fabric and objects at risk from the 100% convective heat output and fast response time (the time taken to heat a space). Regular cleaning of the air filters is also required.

Wall mounted convector heaters with no moving components

These heaters are commonly electric and they heat the room air by convection. They need regular cleaning to stop dust building up on the heater elements and grilles that will reduce their effectiveness.

Far infra-red radiant panels

These usually have flat metal surfaces, which may be mistaken for flat panel radiators or not even noticed at all. Their discrete appearance may thus be of use within historic buildings. They provide most of their heat by radiation and a little by convection.

Underfloor heating

Underfloor heating uses pipe coils operating at low temperatures or electrical elements below the floor surface. Adding underfloor heating to a historic interior may be challenging, but where flooring has to be lifted and replaced for other reasons, it may be a discrete method of adding heating. The limited heat output needs to be noted. Underfloor heating alone may not be sufficient and additional heat emitters may need to be provided.

Church Pew heaters

These heaters can be convective or radiant. They are intended to provide local electric heating to warm the occupants rather than heat the entire church. The advantages of this type of heating are likely lower running costs and associated carbon emissions, and some flexibility in the switching off of individual pews. Installation costs are also usually lower.

Some considerations with pew heaters:

• They do not provide the level of comfort as churches with conventional heating systems.
• Check installation with the church’s insurers. Some have exclusions relating to this type of heating.
• The surface temperatures (most are similar to radiators) and there is a risk pew occupants can get burns. Some heaters have guards to protect occupants. It may be possible to mitigate risks through careful design and management.
• These types of heaters are not usually designed to provide frost protection. Cold water pipes and similar may need separate protection.
• Confirm sufficient electrical capacity is available.
• Carry out a detailed review of the heating if condensation is an issue at the church.
• They should not be affixed to seating of historic or aesthetic interest. The heaters may also dry out the wood of the seating.

Locating radiators and heat emitters

Deciding whether to locate heat emitters is a balance between:

• the historic sensitivity of the space to be heated
• the effectiveness of the heat emitter with respect to its location
• the practicality of installation, and
• avoiding installation below objects of historical significance which could be damaged.

For example, radiators are often sited under windows as these spaces are often not used for other purposes, not because these are the only locations where radiators would work.

The use of heat emitters of various sorts to reduce troublesome cold downdraughts, especially in tall spaces, is commended. However, smoke tests to determine the extent of the problem may be required. The type of smoke needs to be carefully chosen so that smoke residue does not contaminate vulnerable historic surfaces. Fire detection systems may need to be temporarily isolated to allow the tests to take place.

Local electric heat emitters including underfloor heating

These operate by means of electricity and include:

• electric versions of central heating heat emitters such as convector heaters, fan convectors. plinth heaters, and unit heaters
• electric-only heat emitters such as electric fan heaters, electric oil-filled (or water-filled) radiators, electric panel heaters of various types (including far infrared), under-carpet heating mats, and underfloor heating.

It may be possible to convert existing radiators to electric operation, which may mean that an existing radiator can be retained for re-use. Some heaters can even be over-painted to blend in with the background surfaces upon which they are mounted, although any manufacturer’s warranty would be void.

Electric underfloor heating uses electric heating elements embedded below the floor surface. Adding underfloor heating to a historic interior may be challenging, but where flooring has to be lifted and replaced for other reasons, it may be a discrete method of adding heating. Whilst the heat output of electric systems is slightly greater than the central heating version, it is still limited, and underfloor heating alone may not be sufficient and additional heat emitters may need to be provided. The running cost may also limit its use to small areas that would otherwise be difficult to heat.

Electric under-carpet heating mats may also be a discrete and useful means of providing heating to areas that would otherwise be difficult to heat, but the limitations of heat output, running cost and thickness of the system under the carpet should be noted.

All types of underfloor heating must be designed so as not to cause damage to historical floors. Floors in contact with the ground that have known or suspected damp issues should be investigated by a specialist prior to installing underfloor heating in these areas.

Heating controls

Controls and methods of operation

Heating controls vary widely in functionality and sophistication. The basic provision will include electrical isolation of the heating plant, whereas most heating systems including modern domestic installations will have programmers and thermostats. Historic buildings should have a Building Management Systems (BMS), with a sophisticated controller where there is a large number of heating plant or the complexity, cost or functionality of the heating system will benefit from this provision.

Building regulations require the controls to be effective and the requirements vary depending on whether the historic building is a dwelling or other type of building.

Some of the contemporary thermostats and controllers available will not be appropriate for historic spaces. With careful co-ordination it is often possible to discretely locate thimble sized thermistors to measure the internal temperature. Controls that do not sense the internal room environment can often be located away from historically sensitive areas in places like cupboards. It is recommended that all visible components of the controls system in the ‘front of house’ areas are considered at the design stage of any project.

Thermostatic radiator valves allow local temperature control from individual radiators and there are many options available that include a traditional appearance which are well suited to older radiator styles.

In some situations where controls wiring to sensors and controllers will adversely affect the historic environment, it can be possible to install a wireless controls system and achieve the same functionality.

As with all building services installations, consideration needs to be given to maintenance access for all control equipment.

Heating systems with limited potential for use in historic buildings

Direct-fired gas warm air heaters

Heaters with individual wall flues are visually intrusive both within and without buildings; and gas piping and power cabling needs to be installed. Along with the need to make multiple penetrations through historic fabric, direct-fired warm air heaters are most unlikely to be suitable for use in historic buildings. Heaters without flues supplied by natural gas or gas bottles (usually propane) should never be used inside a historic building for any reason due to the amount of harmful water vapour they emit.

Hot air curtains

These are typically installed over external doorways and operate intermittently when doors open to provide a curtain of warm air that prevents infiltration. They should not be confused with small fan convectors that are sometimes installed above entrances. Hot air curtains, if designed properly, are very effective but they are very energy intensive. It may be more appropriate to create a weather lobby instead.

Near-infrared electric heaters

These work by emitting near infrared radiation generated by a special type of lamp or longitudinal heating element. They usually have a polished metal reflector to help direct the heat and glow red or orange. The heat emitted is usually quite directional (and, as the reflectors are usually not asymmetric, often quite wastefully). This, along with their poor appearance, especially when operating, means that their use in historic buildings where appearance is important will be limited or most unlikely. They might however be useful, for some ‘back-of-house’ situations where their visual impact is not significant and especially where those who are to benefit from this type of heating are mostly static.

Fireplaces, hearths, wood-burning and multi-fuel stoves

Fireplaces and hearths can be functional in historic buildings, as well as being an important part of the character and often used as a focal point in rooms. Wood, house coal, smokeless coal, and manufactured solid are the main solid fuels. Some historic fireplaces are fitted with natural gas burners which require less maintenance than the solid fuel alternatives.

Most fireplaces, hearths and stoves are used as an additional heat source. The heat output from fireplaces is largely radiant and a large proportion of the heat is lost via the chimney. As such, fireplaces are not considered to be an efficient form of heating.

Because of the growth of the use of open fires and stoves (and biomass boilers), domestic burning of wood and coal now makes up 38% of the UK’s primary emissions of fine particulate matter. Coal burned in open fires also releases harmful sulphur dioxide. The Government’s Clean Air Strategy 2019 sets out actions to reduce these emissions from homes. Legislation is planned to prohibit the sale of the most polluting fuels, and new stoves will have to be cleaner.

The strategy also highlights the impacts of indoor pollution. The safe operation of open fires and wood-burning and multi-fuel stoves requires a suitable chimney and sometimes ventilation provision, the requirements for both will vary depending on the fuel type and heating capacity.

More briefing on historic chimney design and use of open fires is given in our Energy Efficiency and Historic Buildings. Open Fires, Chimneys and Flues guidance.

There are some other safety considerations of note for historic buildings:

• Thatched roof buildings - Evidence suggests the growing popularity and use of wood burning and multi-fuel stoves have been responsible for an increasing number of fires in thatched roof buildings. Our web page provides guidance on reducing such fire risks.
• Visitor displays and ‘hot works’ - Lit fires in kitchens and drawing rooms can add to the visitor experience in historic buildings open to the public. Such fires come under ‘hot works’ regulations as explained in our advisory note.

Related publication: