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System Design

Specifying efficient systems

To help installers to specify efficient heating, a set of simple standards has been prepared known as “CHeSS” (Central Heating System Specifications).

It was first published in February 2001 and gives recommendations for current good practice, and best practice, for the energy efficiency of domestic wet central heating systems. It explains to purchasers and suppliers how the efficiency-critical components should be should be specified, using four simple names for reference:

CHeSS HR1 (2000) ; regular boiler and hot water store system – good practice
CHeSS HR1 (2000) ; combination boiler system – good practice
CHeSS HR1 (2000) ; regular boiler and hot water store system – best practice
CHeSS HR1 (2000) ; combination boiler system – best practice
CHeSS was produced in response to a request from the Heating Strategy Group of the Energy Efficiency Partnership for Homes, which recognised that one of the difficulties facing the domestic heating installation industry was a lack of common standards and understanding of what should be done to improve energy efficiency

It is intended that CHeSS should become widely used by installers, especially in the government policy initiatives and installation schemes for more efficient domestic heating.

Radiator's and their Location

The types and locations of heat emitters should be established, taking account of the probable layout of furniture, which could affect the height and length of the units. Placing radiators below windows is normally recommended to reduce cold down draughts, although this may be less important when windows are double glazed and inappropriate where radiators would be covered by full-length heavy drape curtains. Also, radiators placed near external doors may help offset the flow of cold air into the building.


Discuss the materials to be used, the different ways in which pipework can be routed and installed, and whether pipes should be concealed or exposed. Remember that all pipework run in solid floors must be put in purpose-built trenches with a removable cover. Also gas and oil pipework must be installed in accordance with the relevant standards.


The function and benefits of heating system controls should be explained to the client, including their effect on comfort and running costs. Find out if there are any special requirements, such as the need to heat part of the house to a higher temperature or at different times from the rest.

Domestic Hot Water

Establish the client's requirements for hot water and how they can be provided by the system. Average hot water requirements may be assessed from the number of occupants in the dwelling but the need for special requirements should be explored., including the need for balanced hot and cold water pressures or high water pressure where mixer fittings and showers are to be used. Check that the water supply provides adequate dynamic pressure and flow if a combination boiler or thermal store is to be considered.

Existing Systems

Where there is an existing heating system in the property, it should be established whether any of it is to be incorporated into the new system, taking account of the suitability and sizing of existing components.

The use of new equipment on a existing system should be carefully considered to ensure compatibility. A low water content boiler should not, for example, be used with gravity circulation unless specifically permitted by its manufacturer.

Existing systems should be flushed and cleaned before new equipment is installed and care should be taken with aggressive chemicals when cleaning older systems. Strainers should be fitted to ensure that sludge from existing system components is not transferred to new boilers. The client should be advised of any possible complications.

Energy Efficiency

Opportunities to improve energy efficiency should be considered and discussed with the client, as it may be more economical to carry them out at the same time as work on the heating ststem. Improvements may be made to thermal insulation, boiler efficiency and heating controls, generally resulting in both fuel cot savings and improved comfort. Improvements to thermal insulation should be considered before starting on detailed system design as they can substantially reduce design heat loads and affect the sizing of both radiators and boilers.