This topic is concerned primarily with space heating. Any fundamental modifications to a home heating regime should wait until after the programme of draught sealing and insulation has been completed. This is because the heating load should have been considerably reduced.
Taking first the conventional open fire, this, as has been stated, is an effective means of removing heat from rooms. Not only does it draw in air for combustion, it is also a thermal chimney, pulling air from the home through the stack effect. Some fireplaces have a flap which can close the flue when the fire is not in use.
In Germany, the answer has been to provide ducted air from the outside to the fireplace. The front of the fire is sealed by a glass door. In the UK some proprietary open fires incorporate ducted air. The most efficient alternative is a closed stove with glass doors which provides both radiant and convection heat with ducted access to outside air. An advantage is that such stoves can burn a variety of fuels.
For a gas fire, the most thermally efficient is that which is glass fronted and has a balanced flue. This is a single horizontal duct to the exterior which provides combustion air, whilst also expelling exhaust gases. Some open gas fires offer the facility of a duct to the external wall. This type draws combustion air from the living space whilst exhaust air is mechanically expelled through the horizontal duct. The problem with this type is that, when the fire is not lit, the duct provides a clear passage for the ingress of cold air and when it is lit it forcibly extracts the hot flue gases without the option of providing convection heat. The final downside is that it requires a permanent vent to the outside for combustion air.
In the domestic sector, most systems consist of either the wet or ducted warm air variety. Some examples of electric underfloor heating still persist in the UK despite being the least economical and environmentally responsible of all systems. As regards the wet systems there are three aspects which can benefit from upgrading treatment. The first is the boiler. If a boiler is at or near the point of being replaced, it is advisable to install a condensing boiler. This exploits the flue gases to give an efficiency improvement of about 20 per cent over a conventional boiler. How this works is that combustion gases contain steam from the burning of natural gas which is cooled by the return water from the central heating. This condenses releasing its latent heat. In the UK it is important that the appliance is installed by a CORGI registered operative. On a cautionary note, it has been claimed that ‘after a very short time in operation cheap condensing boilers do not condense’ (Liddell and Grant, 2002: 12). Since such changes should take place after the property has been insulated and draught proofed, it is important that the boiler should be sized according to the revised rate of heat loss.
The second aspect concerns the radiators. Modern systems use small bore flow and return pipes of 15 or 22 mm diameter. Hot water is pumped under pressure to each radiator through flow and return pipes connected to a common manifold. Not only does this maximize efficiency, it also enables radiators to be isolated for maintenance. Older systems are of the single pipe variety and work by convection or gravity. The water in the system passes through each radiator in turn before returning to the boiler. If a system is due for replacement it is advisable to select a small bore or even a micro-bore system (12 or 6 mm diameter) with radiators which offer the maximum heated surface area. Where pipes are under the floor it is important for them to be insulated.
In older systems, high priority should be given to replacing simple on-off valves controlling radiators with the thermostatic variety. This enables the temperature of an individual radiator to be set at a level appropriate for its location. Foil-backed insulation board placed behind a radiator will significantly improve its efficiency.
It has long been the practice to place radiators below windows, since this is the situation which is most obviously cool. It is also the place which ensures that a radiator gives off much of its heat to the outer atmosphere. Far better to place radiators against inner walls. Where radiators do exist below windows it is essential to ensure that curtains fall behind the radiator.
The third factor concerns the room thermostat. It should be positioned in the main living area and away from direct sunlight. The conventional wisdom dictates that the system should be switched off overnight. Unless the construction of the house offers high thermal mass, this is not an ideal situation. It is important that the temperature in bedrooms should be maintained at a comfort level of about 18°C, especially for the elderly and infirm. This could be achieved by manually changing the thermostat setting each evening and morning. However, the smart solution is to fit a thermostat in the main bedroom coupled to a time clock which automatically switches from the living room thermostat to the night thermostat and then back to the day thermostat at appropriate times for the particular householder. This ensures that it is the bedrooms which have the optimum temperature.
Developments in technology now make available controls which can perform a variety of functions and which are able to learn to operate the system to maximum efficiency according to the habits of the occupants. It is also desirable to have a control system in which the central heating system operates in harmony with the heat recovery ventilation system. It is important to check that an installer is aware of the latest developments in boiler and control technology.
There is also a boiler thermostat which controls the running temperature of the unit. It is usually best to select a high setting for the sake of efficiency. It would be well to consult the manufacturer’s user manual or seek specialist advice as to the optimum setting for the boiler in question.
When installing a new central heating system, the following may be a useful checklist.
• Ensure that the system is not over-designed, but matched to the size and thermal efficiency of the property.
• Select a boiler with electronic ignition; the traditional pilot light is wasteful.
• Ensure that the radiators and controls are ‘state of the art’ in terms of thermal efficiency and CO2 rating.
• Spend time to become conversant with the instruction manuals in order to refine the system to best suit specific needs.
• Maintain the system regularly. Both the boiler and radiators need regular maintenance, the latter to ensure that air is bled from the system periodically.
• In the UK, use a CORGI registered installer.
This is a feature which is becoming increasingly popular, more for its amenity value than saving energy. If it is an addition, say, to a living room and therefore part of the living space it must conform to building regulations unless it can be fully closed off. The warmth generated in a conservatory can be used to supply warm air to the cold side of the house. The warm air must obviously be ducted from the highest point of the glazing. Alternatively water heated in a pipe painted matt black at the apex could supplement the domestic hot water supply. If there is no fan extract of warm air, then buoyancy circulation will occur with hot air rising and cooler air moving in to take its place. An advantage of a conservatory is that it contributes to the thermal mass of the building which can be particularly helpful if there is sunshine during winter months.
Figure 6.1 Conservatory with ducted solar heated air.
However, there are two notes of caution. First, there may be excessive solar gain so it is important to ensure that adequate solar blinds are fitted. Portable air conditioning units to provide cooling should be avoided at all costs. Second, in many cases the energy-saving benefits of a conservatory can be more than offset by the installation of heating to enable it to be a space for all seasons (Figure 6.1).
• When replacing a central heating boiler choose a high quality condensing boiler.
• Installation should be by a CORGI registered fitter.
• All radiators should be fitted with thermostatic valves.
• Reflective foil behind a radiator with its reflective face against the radiator improves performance.
• Ensure curtains fall behind a radiator.
• Consider replacing controls/thermostats to ensure that temperatures are appropriate for particular rooms at specific times, especially bedrooms.
• Conservatories should be regarded as sun spaces and not extensions of a living room to be heated in winter.
• Heat from a conservatory can be ducted to the cold side of a house, reducing the load on central heating.