Boiler and Heating System Upgrade Guide for London Homes

A gas boiler in London typically lasts 10–15 years with annual servicing. Signs it may need replacement:

• Frequent breakdowns requiring engineer call-outs
• High gas consumption compared to similar households
• Boiler over 10–12 years old
• Yellow or orange flame rather than blue (indicates incomplete combustion)
• Noisy operation (banging, kettling, or rumbling)
• Radiators slow to heat or unevenly hot
• The system has had multiple component replacements in the past year

If your boiler is a pre-2005 model without condensing technology, replacing it with a modern condensing combi or system boiler will typically improve efficiency from 60–70% to 90–94%, reducing gas bills significantly.

Combi boiler: The most common type in London. Provides both central heating and instant hot water without a separate hot water cylinder. Suits 1–3 bedroom properties where hot water demand is not excessive. Modern combis are compact and efficient (90%+ efficiency).

System boiler with unvented cylinder: Better for larger homes with multiple bathrooms. The boiler heats a pressurised hot water cylinder, which stores hot water ready for use. Provides better flow rates for multiple simultaneous hot water outlets than a combi.

Heat-only (conventional) boiler with stored cylinder: Older format — requires a hot water cylinder and (in older systems) a cold water tank in the loft. Often being replaced with combis or system boilers during refurbishments.

Air source heat pump (ASHP): An electricity-powered alternative to gas boilers. Extracts heat from outside air and transfers it inside. Very efficient (300–400% efficiency, known as Coefficient of Performance). Works best with underfloor heating or larger radiators. More viable in well-insulated properties. Upfront cost is significantly higher than a gas boiler, but the gap is narrowing with government support (Boiler Upgrade Scheme grants of £7,500 as of 2025).

Ground source heat pump: More efficient than ASHP but requires external ground loops or boreholes — not practical for most London terraces. More relevant for larger properties with outdoor space.

If you are doing a larger refurbishment, the heating system is one of the first things to consider because:

• Replacing pipework is far easier when walls are stripped back
• Upgrading from 15mm to 22mm pipework improves flow rate and efficiency
• Adding underfloor heating requires a screed layer — plan this before flooring decisions
• Magnetic filters, chemical inhibitor dosing, and powerflushing should be done before new radiators are fitted
• A new boiler installed in a system with 20-year-old pipework will underperform until the system is balanced and cleaned

Plan the heating upgrade in coordination with the wider refurbishment programme, not as an afterthought.

Modern refurbishments often update radiators alongside the boiler. Considerations:

Sizing: Radiators must be correctly sized for each room's heat loss. Undersized radiators mean the system runs for longer at higher temperatures, reducing efficiency.

Designer radiators: Vertical column radiators are popular in London refurbishments. They work well where wall space is limited and can be a design feature.

Low surface temperature (LST) radiators: Required in properties with young children or vulnerable occupants. Reduced surface temperature reduces burn risk.

Towel rails: A heated towel rail in each bathroom is standard in a quality refurbishment. Electric towel rails (independent of the central heating) are useful for summer use when the boiler is not running.

Underfloor heating (UFH) is increasingly common in London extensions and refurbishments. It provides even, radiant heat from the floor up — comfortable, invisible, and compatible with heat pumps (which work best at lower flow temperatures).

Wet UFH (hydronic): Connected to the central heating system via a manifold. More efficient for larger areas. Requires a screed to embed the pipes.

Electric UFH mats: Easier to install, no wet trades needed. Better for smaller areas (bathrooms, utility rooms). Running costs are higher than wet systems connected to a heat pump.

UFH is ideal in kitchen extensions with stone, tile, or polished concrete flooring, where it provides warmth without radiators taking up wall space.

• Combi boiler replacement (like-for-like): £1,800–£3,200 installed
• System boiler with unvented cylinder: £3,000–£5,500 installed
• Full system replacement (boiler, radiators, pipework): £5,000–£12,000
• Air source heat pump (full system, single-zone): £10,000–£18,000 (after £7,500 BUS grant: £2,500–£10,500)
• Wet underfloor heating (per m2): £80–£150 installed
• Electric underfloor heating (per m2): £50–£90 installed

Replacing a pre-2005 non-condensing boiler with a modern condensing combi typically improves the Energy Performance Certificate (EPC) by 5–15 points. Adding insulation alongside the boiler upgrade amplifies the improvement.

An ASHP (rated in the EPC as more efficient than a gas boiler at current energy factors) can move a property into a higher EPC band — important for properties in the London rental market, where minimum EPC C will likely become mandatory in the coming years.

Boiler Upgrade Scheme (BUS): £7,500 off the installation cost of an air source or ground source heat pump. Available to homeowners in England. Installer must be MCS-certified.

ECO4 scheme: Government-funded insulation and heating upgrades for eligible households. Available based on means testing and property condition.

VAT: Heat pumps and associated works are currently zero-rated for VAT (0%), compared to 5% VAT on boiler installations. This improves the cost comparison.

Gas work must be carried out by a Gas Safe registered engineer. Always check the engineer's Gas Safe card or verify online at gassaferegister.co.uk before any gas work begins.

Heat pump installation should be carried out by an MCS-certified installer to qualify for grant funding.

rcbGroup coordinates heating upgrades within wider refurbishment and extension projects, ensuring the heating system is designed and installed in sequence with the rest of the works.