Best Practice > Heating Systems
Nowadays homeowners can choose from a wide range of heating systems. Each comes with its own set of environmental credentials, outlined below.
The fuel/energy types most likely to be used in the house for heating and hot water are natural gas, oil, LPG (liquefied petroleum gas) and electricity (the latter alone, of course, being used for artificial lighting). Natural gas is the most widely-used fuel in the UK as a whole. In Ireland (north and south) however, oil is most widely-used, as much of each region is not supplied with natural gas. LPG is also found mainly in areas off the natural gas network.
Electricity can be used for heating and also for hot water, but generally is not recommended on energy and environmental grounds.
|Solid fuel||0.3 kg/kWh|
For further information on these emissions, click here.
The SEDBUK boiler database not only lists over 3000 boilers but gives details on the fuel (e.g. gas or oil), type (e.g. condensing, conventional, combi, modular), mounting position (floor or wall), rated output range (kW), efficiency (%) and energy rating (A to G – A being most efficient). You can also refine your search in terms of any of the above factors, as well as flue type and manufacturer and model name. The SEDBUK website also includes access to the BRE Boiler Sizing tool, to help you determine what output you require from your boiler.
The BRECSU Whole-house Boiler Sizing Method (which may be accessed via the SEDBUK database) and the HVCA/CIBSE Domestic Heating Design Guide. The latter, produced by the Heating and Ventilation Contractors Association in collaboration with the Chartered Institution of Building Services Engineers, is the most accurate method and the only one which addresses all building and boiler types. However, it is slow and requires a high level of technical knowledge. The BRECSU (Building Research Energy Conservation Support Unit) method is suitable for relatively simple designs incorporating natural gas, oil or LPG conventional boilers up to 25kW, though not for solid fuel or combi boilers. You may be wondering how you are supposed to know whether your boiler rating will be more than 25kW before you have used the sizing package. The answer is that 25kW would represent a very large conventional domestic boiler and would cover almost all conceivable dwellings – especially those with energy-efficient fabric!
It is compulsory, in most cases, to fit condensing boilers in UK and ROI. Condensing boilers differ from conventional ones in that they incorporate a condenser which recovers heat which would otherwise be lost up the flue. This recovered heat then contributes to the heating of water which is pumped round the heating/hot water circuit. As the boiler then needs to use less gas or oil, the system is more efficient and costs less to run. Additional advantages include the ability to install plastic flues, such is the reduction in flue gas temperatures due to the condenser.
Note that it is compulsory in UK for gas appliances to be fitted by a CORGI (Council Of Registered Gas Installers) trained installer. It is advisable to use an OFTEC (Oil-Firing Technical Association) or SNIPEF (Scottish and Northern Ireland Plumbing Employers Federation) approved installer for oil appliances.
Combination (or “combi”) boilers are so called because they provide not only heat to the heating circuit but also hot water direct, without the need for a hot water cylinder (HWC). This saves on the capital cost of the cylinder and associated piping – and also saves space. Running cost savings result from avoiding the cost of heat which is always lost from the HWC and piping (known as “standing losses”) – in effect, you heat only the hot water you actually use (barring standing losses from pipework to appliances).
These are the ultimate in high-efficiency quick-response boilers and, while costing a little more than the standard combi, will still pay for their over-cost within a few years. Again, these would be most suitable for smaller dwellings or those with a relatively low hot water requirement at any given time.
For those who wish to be green yet avoid the cost and intermittency of renewable energy technologies, wood-pellet boilers may be the answer. The pellets, which are manufactured from sustainably-managed or waste timber sources, can be used readily in a dedicated boiler to provide space heating and hot water. The best systems include spark ignition and adjustable feed-rate to provide a turn-down ratio (reduced boiler output in response to reduced demand) allowing control on a roomstat and timer. The uniform pellet size aids automatic feed as well as efficient combustion, while the low water content results in very little ash. The high density of the pellets reduces the volume needed for fuel storage and systems are designed to meet urban emissions requirements.
Wood chips also can be processed from sustainable or waste timber sources but are irregular in shape and have a higher water content and lower density than pellets, thus not lending themselves so well to automated systems. However, the cost is much lower and those in rural areas who can sources supplies readily, have the space required for storage and feed and can accommodate a little more in the way of maintenance (lighting, ash removal, etc) may find wood chips an economic and environmentally-friendly option.
A heat pump essentially is a refrigeration circuit where you are not so interested in the cooling effect, but instead make use of the heat rejected from the system. A heat pump takes heat from a low-grade heat source (such as the ground, water, or air) and produces useful heat. Due to the physics of the system, the electrical energy input to run the system pump/compressor is outweighed by the thermal energy produced.
The degree to which the thermal energy output exceeds the electrical energy input is expressed in terms of a ratio called Coefficient of Performance (CoP), typical values of which are of the order of 3.0 – 4.0. A CoP of 3.0 means that for every kWh of electrical energy used to run the system, 3 kWh of heat is produced.
Key advantages of a heat pump include reliability and low maintenance
Use BS 5449 to ensure the appropriate system design temperatures are specified.
Regarding central heating systems there are two basic types, those using water and those using air. “Wet” systems pump heated water round piping circuits, while “warm air” systems distribute heated air by way of ducts and grilles. The former are by far the most common and can be sub-divided into those using radiators as the heat emitters and those using under-floor pipes. Confusingly, radiators actually deliver most of their heat by convection, whereas the under-floor systems are fully radiant.
Irrespective of which type of boiler and heating system you choose and which fuel, the overall efficiency of your system will be greatly influenced by the level of control. Relevant information may be found in the Central Heating System Specification (CHeSS) publication.
The controls should provide what is termed “boiler interlock” – that is, the boiler will not fire unless there is a demand for heat in the house. The demand should be signalled from a room thermostat (for heating) or a hot water cylinder thermostat (for hot water), which has registered that the room/water temperature has fallen below its setpoint.
The other essential component is a programmer/timer – make sure you get at least a 2-channel 7-day programmer. This will allow you to set a number of on/off periods for each day of the week, separately for heating and hot water.
Room thermostats (or “Roomstats” for short) must be located somewhere which is representative of the temperature requirements and conditions of the rest of the house, since the whole heating system is usually controlled on this one thermostat. Places to avoid are; living rooms with secondary heating sources such as open fires, electric fires or coal-effect gas fires, as the heat from these will influence the roomstat; places subject to direct solar gains or excessive draughts, which again will influence the reading – close to windows and doors is therefore not suitable; other areas of higher heat gain, such as kitchens. In fact, unless your main living room has no secondary heating, in which case that is a good location for the roomstat, the only other suitable place tends to be in the hall, as close as possible to the centre of the house. If you have two heating circuits you will need a roomstat for each.
Thermostatic radiator valves (TRVs) incorporate a thermostat which sense the air temperature in the room causing the hot water flow to the radiator to be reduced progressively as the room temperature rises.
The living room is often regarded as a separate zone, as comfort temperatures are generally a little higher here. Alternatively, bedrooms, not occupied during the day, could be seen as a separate zone. TRVs can give effective zone control but do not provide boiler interlock (see above) – that requires a roomstat.
Another option is to install a system which incorporates additional 2-port valves, which may be opened or closed from a central control panel. Wiring needs to be installed between each valve and the controller and to the 12V power supply unit.
Full zone control is required for dwellings over 100 m² (Republic of Ireland) and 150 m² (UK).
Positioning of radiators –
Radiators have tended to be located beneath windows, for various reasons. Two thermally-related reasons are as follows;
The obvious disadvantage is that heat rising from the radiator (by convection) can be lost straight through the glazing, without ever contributing to heating the room. The ideal location for a radiator, in thermal terms, is on a wall at right angles to the window wall. In this way, convection currents can be set up which allow air movement parallel to the plane of the window, which will minimise heat loss through the window.
Radiators located on an external wall, especially in older solid-walled houses, may be fitted with reflective panels (foils) behind, to reduce heat loss to the wall.
Under-floor systems, by introducing heat at floor level, can provide a more even temperature gradient from floor to ceiling. More heat at lower level, where it is often most needed, can also give improved comfort conditions.
Under-floor heating works well with condensing boilers as it operates at lower water flow temperatures, which improves the efficiency of the boilers
Under-floor heating can also be used on upper floors, which will require some modification compared to normal suspended timber floors. The pipes can be laid in a dry mortar screed resting on timber or concrete formwork, which is then sheeted over and the flooring laid on top.
The main down-side with under floor heating is the response time of the system. Response time simply describes the time taken between introducing heat into a room and comfort temperatures being reached.
See section on floors for advice on floor coverings for use with underfloor heating.
A warm air heating system incorporates a heating unit, which essentially does a similar job to that of a wet system boiler, though it heats air instead of water. Systems fired on natural gas or LPG can be direct or indirect (that is, involving a heat exchanger), with oil-fired systems being indirect only. Systems may also be supplied with integrated water heaters.
Advantages of warm air systems include very quick start-up times from cold and low running costs. This can be a significant advantage in an intermittently-occupied dwelling with thermally lightweight internal surfaces. The system fan may also be run without the burner during the summer to circulate air through the building.
Electric storage heaters should never be chosen unless all other options are technically unfeasible. In this case you should opt for the highest possible fabric insulation levels in order to bring the heat requirement down to a minimum. Storage heaters are uncontrollable at point of use, the amount of heating capacity available during the day being determined by the charging rate the previous night (although systems have been developed which use weather-forecasting to determine the charging rate). Even if the somewhat rudimentary input/output controls are used correctly, the amount of heat output available will decrease throughout the day, reaching a minimum when you need it most, in the late evening.
While being relatively cheap to install, storage heating systems are also expensive to run (even though off-peak electricity is used) especially taking into account supplementary heating (which is almost invariably peak rate electric). Finally, where the electricity is generated via fossil fuels, electric storage heating is also environmentally unfriendly. Where storage heaters exist, serious consideration should be given to their replacement with a wet or warm-air system, fed from an efficient boiler/heating unit.
Focal-point fires have become more popular in recent years as more houses are built without open fires, or as people use less the open fires they have. Electric focal-point fires are often found placed in front of an unused fireplace, while gas-fired coal-effect fires have become increasingly common in new houses as well as being retrofitted into existing ones.
Open fires are really not practicable these days, they can result in very high heat loss through air being drawn up the chimney – equivalent to a window being left open permanently.
Houses are being built all over Ireland (North and South), which do not require a central heating system. By using very high levels of insulation and good air tightness, combined with mechanical ventilation and heat recovery systems the whole house can be heated by, say, a wood burning stove in the main living area.