The Complete Guide to New Boiler Installation in London

Why the process starts with a survey, not a price

A boiler that's the wrong size or wrong type for a property causes real problems afterwards — weak hot water, a system that short-cycles, or an install that doesn't suit the existing pipework. A proper survey looks at radiator count, bathroom count, incoming mains flow rate, flue routing and the condition of existing pipework before recommending anything.

This is also where boiler type gets decided honestly: combi, system or regular each suit different properties (see our combi vs. system vs. regular guide), and a surveyor who recommends the same type regardless of what they find isn't doing the job properly.

What a fixed quote should include

A written quote after survey should state the boiler make and model, its output, the installation scope (including any pipework, flue or control changes), old boiler removal, and the total fixed price — not a range that can move once work starts.

• Boiler make, model and output (kW)
• What happens to your existing pipework, flue and controls
• Old boiler removal and disposal
• Total fixed price, in writing, before work starts
• Expected timescale on site

Gas Safe registration and Building Regulations

Work on a gas boiler in the UK is legally restricted to Gas Safe registered engineers — this isn't optional or a marketing claim, it's the law. A registered engineer notifies the installation through the Gas Safe scheme, which satisfies the Building Regulations requirement, and completes the manufacturer's Benchmark commissioning checklist, which most manufacturers require to validate the warranty.

Before any installer starts work, you can check their registration directly on the Gas Safe Register — it takes a minute and it's the single most important check in the whole process.

What to expect on installation day

Timescales depend on scope: a like-for-like combi swap in the same location is often a single day, while moving the boiler, converting between boiler types, or upgrading pipework and controls at the same time typically takes one to two days. Your quote should state which applies before you book a date.

What a like-for-like replacement typically costs

To provide useful context, industry-average installation prices for straightforward, like-for-like combi boiler swaps are often quoted. For a 24-27kW combi boiler replacement, the typical UK price range (industry average) is between £1,050 and £2,250. For a slightly more powerful 28-30kW combi, the typical range is £2,000 to £2,800. These ranges are based on national averages and assume the new boiler is fitted in the same location with minimal changes to pipework or system components.

These figures are for guidance only. Prices for larger combi boilers, system boilers, or conventional boilers vary more widely and are not reliably reflected in a single range. Your final, fixed quote will only be accurate after a survey of your home, which determines the correct boiler output, any necessary system upgrades, and the specifics of the installation work required. A genuine quote is based on your property, not a published price list.

Common reasons installation takes longer than a day

While a straightforward swap may be completed in a day, several practical factors can extend the timeline. Moving the boiler to a new location, such as from a kitchen to a loft, adds significant work for flue and gas pipe rerouting. Upgrading old, narrow pipework to meet the demands of a modern boiler is another common reason, as is converting from a system or conventional boiler to a combi, which alters the central heating and hot water setup.

Poor access to the existing boiler or its location can also cause delays. If the boiler is in a cramped cupboard or requires extensive work to remove an old, heavy model, the initial stages take longer. These are all standard parts of a proper installation, and a good engineer will factor them into the project plan from the outset after the initial survey.

Questions worth asking before you book anyone

Before you confirm any booking for a boiler installation, asking the right questions can help you choose a competent installer and set clear expectations. A reputable company will be transparent and provide straightforward answers. Your decision should be based on more than just price, as the process starts with understanding your specific requirements through a survey.

Use this practical checklist when speaking to prospective installers. Their responses will clarify their qualifications, the scope of work, and how the project will be managed, helping to avoid misunderstandings later on.

• Can I see your Gas Safe registration details? This is the legal requirement for anyone working on gas appliances.
• Are you accredited by the boiler manufacturer? This ensures the warranty will be fully valid.
• Is the quote you've provided a fixed price? A fixed quote should clearly include all labour, materials, and any applicable Building Regulations charges.
• What is your plan for removing and disposing of my old boiler? A professional service should handle this responsibly.
• What is the expected timescale for the complete installation? While a standard replacement often takes a day, be aware of common reasons it might take longer, such as system upgrades or unforeseen complications.