Heating of the house extension

Boiler capacity

For houses equipped with gas- or oil-fired central heating, one of the key issues associated with an extension is whether the existing boiler has adequate capacity to heat the enlarged house. In many cases, the additional heat loss associated with the extension makes it necessary to install a new boiler with appropriate additional capacity, at significant cost.

However, in some cases a well insulated extension may reduce the overall heat loss of the house (by covering up some of the original, less well insulated roof or walls), or leave it almost unchanged, so that a new boiler is not needed. Sometimes the boiler in the original house has been over-sized, and it may be able to cope perfectly adequately with an increase in heat loss of the order of 10 per cent. It is therefore worth calculating the effect of the proposed conversion on the heat loss of the house, at an early stage of the design. Adopting the recommended Best Practice U-values (in Table 1) will reduce the heat loss of the extension, and may help to avoid a requirement for a new boiler, thus reducing the overall cost of the project.

For more information see the CIBSE Domestic Heating - Design Guide.

Replacement boilers If a new boiler is required, the UK building regulations require that it achieves a minimum seasonal efficiency. Further to this, from 1 April 2005, all gas boilers installed in England and Wales are required to be condensing boilers (aside from a small number of exceptions). The Best Practice standard is to install a boiler of seasonal efficiency grade A or B (i.e. at least 90 per cent).3 In addition, if the boiler is replaced the building regulations require the following.

• The existing heating system must be upgraded to ‘fully pumped’ circulation (i.e. not ‘gravity feed’) if it is not already fully pumped.

• The heating controls must be upgraded to include a programmer, a room thermostat and a thermostat on any hot water storage cylinder.

• The room thermostat must be ‘interlocked’ to the boiler so that the boiler does not fire when there is no demand for heat.

Rooms with internal or solar heat gains (i.e. bathrooms, and rooms with south-facing glazing) should also have responsive heating controls such as thermostatic radiator valves (TRVs), so that the heat input is reduced when ‘free’ heat gains are available instead.This improves efficiency and reduces the risk of overheating.

Condensing boilers

The most efficient type of boiler is the condensing boiler, which is becoming the standard type of boiler in most parts of the UK.

Further to this, from 1 April 2005, all gas boilers installed in England and Wales are required to be condensing boilers (aside from a small number of exceptions by the building regulations).

Condensing boilers have larger heat exchangers than regular boilers, and achieve seasonal efficiencies between 86 and 91 per cent. The efficiency of a condensing boiler remains high even when it is working at a low level of output (e.g. providing hot water only, in summer).

Where a new boiler is required to cope with the additional heat load of an extension, the improved efficiency obtained from a condensing boiler (compared with an original, conventional boiler)

will often offset the additional demand, resulting in little or no increase in fuel cost.