How to Choose a New Boiler for Your Home Renovation
A new boiler during a home renovation is an opportunity to upgrade the heating system properly — specifying a unit that meets the demands of the modified property, improves energy efficiency, and reduces running costs. Here's how to make the right decision rather than defaulting to a like-for-like replacement.
Boiler Types
Combi boiler. The most common type in London. A combi (combination) boiler provides both central heating and instant hot water from a single compact unit, without the need for a separate hot water cylinder or cold water tank. Well-suited to smaller properties (up to 3–4 bedrooms) with one or two bathrooms. Not suitable where high simultaneous demand for hot water exists (e.g. two showers running at the same time in a large house) as a combi's flow rate is limited.
System boiler with unvented cylinder. A system boiler heats water that is stored in a separate unvented hot water cylinder, which provides mains-pressure hot water to multiple outlets simultaneously. Well-suited to larger properties with high hot water demand. The cylinder requires additional space and must be installed with a pressure relief valve and tundish in a suitable location.
Heat-only (conventional) boiler with vented system. Found in older properties with gravity-fed systems (cold water storage tank in the loft, hot water cylinder in the airing cupboard). These are increasingly being replaced with combi or system boiler conversions, but in some older large properties the conventional system may be retained.
Output Sizing
The output of the boiler (measured in kW) must be matched to the heat demand of the property. An undersized boiler will struggle to heat the property adequately; an oversized boiler will cycle on and off inefficiently and wear out more quickly. A heat loss calculation for the specific property — carried out by the heating engineer — is the correct way to specify boiler output. Do not accept a like-for-like replacement without a calculation, particularly if the property has been extended or if insulation has been upgraded.
Efficiency
All new gas boilers in the UK must be condensing boilers — which recover heat from flue gases that would otherwise be lost. Modern condensing combi boilers operate at 89–94% efficiency (SEDBUK rating A). Specifying an A-rated boiler versus a lower-rated one makes a meaningful difference to annual running costs over a 12–15 year boiler lifespan.
Alternative Heating Technologies
Where a major renovation is underway, this is the ideal moment to evaluate whether an air source heat pump (ASHP) is appropriate for the property. An ASHP is most effective in a well-insulated property with underfloor heating or upgraded radiators. The Boiler Upgrade Scheme (BUS) currently provides a £7,500 grant towards ASHP installation. Consider the whole-system cost and running cost comparison before defaulting to a gas boiler replacement.
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