Housing Retrofit: Gas fuelled space & water heating

• Refurbishment provides the opportunity to install a new or replace an existing heating system with a new more efficient type.

• There are a number of factors which need to be taken into consideration when choosing a new system:


Fuel type

In terms of CO2 emissions the best choice will be between gas (where available) and wood fuel. If choosing a wood-based system consideration should be given to availability of fuel supply.


Boiler size

The size of the boiler needs to be carefully calculated based on projected hot water and space heating demand – particularly where a building has undergone significant insulation and air-tightness improvements.


User needs

The pattern of use might determine the type of boiler selected. If, for example, a large family uses hot water at the same time, a system including hot water storage would be preferable on the other hand, a combination boiler is likely to be perfectly adequate for a small flat; Another consideration might be a boiler’s capacity to heat a space in reasonable time.



The amount of available space can also determine the boiler type. Where space is at a premium, or if locating a feed tank and its associate pipework in a loft is undesirable, then a form of combination boiler is possible choice whereas if hot water storage is important for example if the system is to be integrated with solar heating or a heat pump, then a system that includes a storage system will likely be preferred.



The inclusion of renewable sources of heating energy, particularly solar thermal, should be a consideration. The selection of a gas heating system will be determined in large part depending on whether ancillary sources of heating are to be accommodated.

gas heating flame


• The heating system examples illustrated below are for general guidance only. When specifying, confirm with the product manufacturer their installation methods and expected performance. For more detailed information, consult a heating engineer



Boiler types

• Boiler technology is in a state of technological development. This is particularly true in the case of developing systems that produce both heat and power (CHP). However, though it is important to understand the potential of these developments, the system specifier is currently much more likely to consider the selection of a more tried and tested traditional system.

• Choosing between a regular or combi/CPSU boiler depends on the heating and hot-water requirements of the household and the positions in which the boiler can be located. This must take into account requirements for flueing and ventilation. If the water pressure and pipe size are adequate, avoid using the cold loft space for water tanks and pipes, due to the risk of freezing. If this cannot be avoided, ensure that pipes are adequately insulated.

• If ‘renewable’ hot water provision (eg solar hot water or heat pump) is to be included, the choice of a normal combination boiler is precluded because the water source is from the mains. This can be overcome by the addition of a ‘thermal store’ that will provide pre-heated water to the combination boiler at mains pressure. With this combination of storage and boiler, it is essential to ensure that boiler and storage are compatible. Typically, the boiler will be of a ‘modulating’ type that is able to monitor the water temperature in the boiler and to adapt the flame with the solar water heating system demands


‘Regular’ or ‘Heat only’ boiler

• A regular boiler is one which provides space heating directly, but stores hot water in a separate storage system.
• Along with the boiler unit, it requires a feed tank, an expansion cistern, controls and a hot water cylinder.
• Regular boilers are more efficient than ‘Combi’ boilers, but require more space.

‘Regular’ or ‘Heat only’ boiler layout



• Suitable for larger homes, where there may be demands for hot water simultaneously.
• Where ample space is available for installation. (eg loft space)

Pro Provides a high hot water flow rate.
Pro More efficient than combination boilers
Pro Straight-forward design ensures easy maintenance
Pro Can be used with low mains pressure
Con Space requirement for feed tanks, hot water cylinder and pipework
Con Heat loss associated with hot water storage and pipework
Con Low pressure hot water
Con Poor flow rate for showers



‘System boiler’

A system boiler is a sub-type of regular boiler but designed for use in conjunction with a ‘Sealed system’ (see below). Components associated with the system, such as pumps and expansion vessels, are included within the boiler casing – so removing the necessity to locate the components elsewhere.

system boiler



• Suitable for larger homes, where hot water is required simultaneously, and where cold water tanks are undesirable.

Pro More efficient and responsive than ‘regular’ boilers
Pro They remove the need for an expansion cistern, feed and water tanks
Pro There are less separate components means that it is easy to install and maintain
Pro Can be used with low mains pressure
Con Space is required for hot water storage cylinder
Con Limited to use with a ‘sealed’ system only
Con Heat loss associated with hot water storage and pipework

boilers on the wall



‘Combination’ or ‘Combi’ boiler

• A combination boiler is one which provides space heating and hot water directly. In its simplest form (an ‘instantaneous combi’) water is provided ‘on-demand’ rather than via storage.
• The boiler is fed directly from the mains supply, obviating the need for cold water tank and associate piping.
• Combination boilers are available that include hot water storage within the appliance (see below)
• Showers must be of a ‘mains pressure’ type.

‘Combination’ or ‘Combi’ boiler


• Suitable for smaller homes where simultaneous hot water demand is unlikely.

Pro Instantaneous hot water
Pro High pressure hot water
Pro Space efficiency
Pro Good flow rate for showers
Pro Cheap to run
Pro Drinking water available at all taps
Con Unsuitable for anything other than small homes
Con Unsuitable for low pressure supplies
Con Ancillary water heating (eg solar) requires extra storage and compatibility
Con Poor flow rate for baths.
Con No back-up water supply if mains supply fails



‘Combined Primary Storage Unit’ (CPSU)

• This type of combination boiler incorporates very large hot water storage, 70 litres or more.
• This large water storage capacity creates a high hot water flow rate to taps and showers and heats up radiators quickly, making them good for larger homes. They also provide the ability to run baths and showers simultaneously with minimal pressure drop.



• Suitable for larger homes where the hot water demand is often simultaneous.

Pro Combines the advantages of regular and combination boilers
Pro Enables rapid space heating.
Con Less efficient than a normal ‘combi’
Con Requires more boiler unit space than a normal ‘combi
Con Few models on the market.
Con Usually expensive


Heating systems

heating systems vented v unvented


'Unvented' / 'sealed' system

When installing a new system, it is more usual now to install an ‘unvented’ or ‘sealed’ system. An unvented system does not include a water expansion tank. Water is provided directly from the mains. A pressure vessel is included in the system to deal with any pressure variations in the water supply. Safety valves open if the interior pressure is too high. Other automated valves open to allow the interior water to be topped up if it drops to low.


'Vented' / 'open' system

The traditional form of heating system is a ‘vented’ system. This system incorporates a feed and expansion tank as well as, in the case of a regular boiler, a hot water storage cylinder. In addition to allowing for water expansion when heated the expansion tank also tops up the central heating system with water when called for. The expansion tank needs to be installed higher than the other system parts and most often this will be placed in the loft.



The controls regulating a modern heating system provide the user with the capability to minimise energy expenditure. A good and user-friendly control system will only provide heat when and where it is needed. It also ensures that the boiler does not operate unless there is demand for it to do so.

A capable control system will allow a user to:
• Match their occupancy pattern by providing timed water and space heating periods.
• Control individual room temperatures to comfort levels to reduce overheating.

controlled heating zones

• Divide larger houses into zones, each with dedicated time and temperature controls. Zones might reflect usage (eg upstairs / downstairs) or thermal dynamics (eg solar gain areas).


Commonly specified controls will include:

• A time switch which controls space and water heating
• A full programmer which allows the time settings for space and water heating to be fully independent
• Room thermostats which controls the switching on and off of the space heating according to desired temperatures
• Programmable room thermostats perform the above but with the ability to pre-set heating periods.
• A cylinder thermostat, where storage is installed, allows water heating to be switched on and off according to the desired cylinder water temperature.
• A thermostatic radiator valve (TRV) which adjusts the water flow according to room temperature.


Hot water storage

Where storage cylinders are included in a system, the water contained is heated either directly or indirectly.

hot water storage methods


The water that circulates around the boiler, pipes and radiators is kept totally separate from the stored water in the cylinder that is used for bathing.



The hot water from the boiler circulates directly through the cylinder. Used in open vented systems (see above).


Thermal stores

Address the problem of the temporal dislocation associated with heat sources that provide heat during certain times (eg solar heating) but when the need for heat is at other times (‘buffering’). For more information see ‘Thermal Storage’



Regulations and standards


Building Regs

• Building Regulations Approved Document ADL2 sets out the requirements for existing buildings.
• The regulations do not oblige the designer to perform DER/TER calculations, but where a new system is installed or an existing system replaced, the installation must follow the requirements of the ‘Domestic Heating Compliance Guide’ (DCLG, 2006).
• The guide requires that new regular and combination condensing boilers achieve a minimum efficiency of 86% as defined by SEDBUK (Seasonal Efficiency of a Domestic Boiler in the UK). A database of boiler efficiencies is maintained by the government at www.ncm-pcdb.org.uk/sap/


CHeSS (Central Heating System Specifications)

• CHeSS provides ready-made specifications covering heating system components that affect energy efficiency.
• The basic specifications HR7 (regular boilers) and HC7 (combination boilers) are sufficient to comply with Building Regs.
• The specifications HR8 and HC8 are best practice.
• The specifications are published in detail in ‘Central heating system specifications, Year 2008’ (EST CE51, 2008)



Domestic Heating Compliance Guide, DCLG, 2006
Domestic Heating Design Guide, CIBSE, 2007



• BS 1566:2002 Copper indirect cylinders for domestic purposes.
• BS 3198:1981, Specification for copper hot water storage combination units for domestic purposes
• BS EN 12897:2006 Specification for indirectly heated unvented hot water storage water heaters
• BS EN 12828:2003 Heating Systems in Buildings. Design for water based systems.
• BS 7671:2001, Requirements for electrical installations, IEE Wiring Regulations, Sixteenth edition.
• BS 7593:2006, Code of Practice for Treatment of water in domestic hot water central heating systems.
• BS 5440:Part 1 and Part 2:2000, Installation of flues and ventilation for gas appliances of rated input not exceeding 60kW (1st, 2nd and 3rd family gases.
• BS EN 483:2000 Gas-fired central heating boilers – Type C boilers of nominal heat input not exceeding 70kW.
• BS EN 12831:2003 Heating Systems in Buildings. Method for calculation of design heat load.
• BS EN 15316-1:2007. Heating systems in buildings. Method for calculation of system energy requirements and system efficiencies. General
• BS EN 15316-2-1:2007 (3 parts) Heating systems in buildings. Method for calculation of system energy requirements and system efficiencies.
• BS EN 15316-3:2007 (3 parts) Heating systems in buildings. Method for calculation of system energy requirements and system efficiencies.
• BS EN 15316-4:2007 (6 parts) Heating systems in buildings. Method for calculation of system energy requirements and system efficiencies.
• The Boiler (Efficiency) Regulations 1993, SI (1993) No 3083, as amended by the Boiler (Efficiency) (Amendment) Regulations 1994, SI (1994) No 3083.
• Domestic Heating Compliance Guide, (Compliance with Approved Documents L1A: New Dwellings and L1B: Existing Dwellings), First Edition, Communities and Local Government.


Further information

• Hot Water Association (www.hotwater.org.uk)
• Heating & Hotwater Industry Council (www.centralheating.co.uk)
• Association of Plumbing and Heating Contractors (www.competentpersonsscheme.co.uk)





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