Heating of the house extension
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.
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.