Heating for a loft conversion

For houses with gas or oil fired central heating, a key issue is whether the existing boiler has adequate capacity to heat the enlarged house once the loft conversion is completed. In many cases, the additional heating load makes it necessary to install a new boiler with extra capacity, at significant cost.

A well insulated loft conversion, though, may reduce (or at least leave virtually unchanged) the overall heat loss of the house by improving insulation levels in the exposed roof or walls – thus avoiding the need for a new boiler. Sometimes the original boiler was over-sized, and it may be able to cope perfectly adequately with an increase in heat demand in the order of 10 per cent. It is therefore worth calculating the effect of the proposed conversion on the overall heat requirement of the house at an early stage of the design.Adopting the Best Practice insulation standards in Table 1 will reduce the heat loss of the conversion. This may avoid a requirement for a new boiler, with the associated additional costs. For more information see CIBSE's (The Chartered Institute of Building Services Engineers) Domestic Heating – Design Guide.

If a new boiler is required, building regulations require it to have a minimum seasonal efficiency.The Best Practice standard is to install a boiler of seasonal efficiency grade A or B (i.e. at least 90 per cent).

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). In addition, if the boiler is replaced, the regulations require that:

• the heating system must be 'fully pumped' circulation (i.e. not 'gravity feed');

• the heating controls are graded 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 (preventing the boiler from firing if there is no demand for heat).

Rooms with internal or solar heat gains (bathrooms, and rooms with south-facing glazing) should also have responsive heating controls such as thermostatic radiator valves (TRVs).These ensure that the heat input is reduced when 'free' heat gains are available.They improve efficiency and reduce the risk of overheating.