GasBoilerCost.com / UK 2026
Installation / Page 11

Gas boiler installation. Timeline and every hidden cost.

A standard boiler swap takes 1 day. A conversion takes 2 to 3. Here is exactly what happens, day by day, and the full list of extras that quietly add £200 to £1,500 to the headline price.

What happens, day by day

Like-for-like combi swap

1 day

08:00 engineer arrives, drains old system, removes old boiler. 11:00 fits new boiler in same position. 13:00 connects gas, water, condensate. 15:00 fills, vents, commissions. 16:00 hands over Benchmark certificate, registers warranty. Done.

Combi relocation

1.5 to 2 days

Day 1 morning: drain, decommission, remove old unit. Day 1 afternoon: chase new pipework, fit new wall plate. Day 2 morning: hang boiler, connect, test. Day 2 afternoon: fill, commission, hand over. Adds £250 to £500 over a swap.

Regular to combi conversion

2 to 3 days

Day 1: drain entire system, remove cylinder, remove cold water tank, cap pipework. Day 2: install new combi, reroute hot water mains, fit pressure-reducing valve. Day 3: commission, balance radiators, possibly add magnetic filter and TRVs. Adds £600 to £1,500 over a swap.

Full system change

2 to 3 days

Day 1: drain system, remove old boiler and cylinder. Day 2: install new boiler, new unvented cylinder, new pipework. Day 3: commission, balance, certify. Adds £700 to £1,800 over a swap.

Extras itemised

The headline price your installer quotes rarely includes everything. These are the line items that quietly add £200 to £1,500.

ItemCostWhen you need it
Powerflush£300 to £500Recommended for systems 10+ years old, mandatory for some warranties.
Magnetic system filter£100 to £150Recommended for every new install, often warranty-required.
TRV upgrade (per radiator)£25 to £40 fittedWhere existing radiators have plain valves only.
TRV pack (3-bed home)£150 to £300Whole-house upgrade, 6 to 8 radiators.
Smart thermostat£150 to £250Hive, Nest, Tado, Drayton Wiser. Often discounted in installer quotes.
Flue extension£50 to £100 per metreWhere new boiler position changes flue run length.
Condensate pipe routing£50 to £150If condensate runs externally and freezing risk requires insulation.
Scaffolding£150 to £300If flue exits at height (typical for 2-storey rear extensions).
Asbestos flue removal£200 to £400Pre-2000 installations. Specialist contractor required.
New gas supply pipe (28mm)£250 to £600Where the existing pipe is undersized for the new boiler output.

Gas Safe is non-negotiable

Every gas appliance installation in the UK must be completed by a Gas Safe registered engineer. Verify before booking:

  1. Visit gassaferegister.co.uk and search by company name or postcode.
  2. Ask the engineer to show their Gas Safe ID card on arrival. Front shows the licence categories. Back shows the work types they are qualified for (must include "Boilers" for a boiler install).
  3. Match the registration number on the card to the one on the website.
  4. Check the licence has not expired. ID cards display the renewal date clearly.

Working with an unregistered installer is illegal, voids the manufacturer warranty, invalidates your home insurance and creates real safety risk.

Getting good quotes

Three written quotes is the rule. Each quote should include: brand and model, warranty length, every extra (filter, TRVs, thermostat), labour days, total VAT-inclusive price.

Red flags to walk away from

  • No Gas Safe registration number visible on quote or website
  • Cash only, or refusal to provide a written quote
  • No itemised breakdown, just a single 'all-in' figure with no detail
  • Pressure to sign on the day with limited-time discount
  • Quote significantly below market for your region (often a deposit-vanish scam)
  • Promises of 'cashback in 6 weeks' from a finance scheme you have not researched

Preparing your home

  • Clear at least 1 metre access in front of the boiler and to the cylinder if applicable.
  • Lay dust sheets over carpets along the engineer's path. Most engineers will but bring your own.
  • Allow loft access if the flue routes upward. A folding ladder helps.
  • Keep pets in a separate room. Boiler decommissioning is loud.
  • Plan for no heating or hot water for the duration. Stock kettle water, schedule showers at the gym.

What you get at the end

  • Gas Safe Building Regulations Compliance Certificate, posted to you within 30 days.
  • Benchmark logbook completed by the engineer, kept in the boiler manual.
  • Manufacturer warranty registration confirmation.
  • A walk-through of your new controls (thermostat, app, programmer).
  • Receipt and invoice with itemised work, payment method evidence, VAT details.

FAQ

  • How long does a boiler installation take?
    A like-for-like combi swap is a 1-day job for an experienced Gas Safe engineer. Add half a day if the boiler is moving more than a metre. A regular-to-combi conversion runs 2 to 3 days because the cylinder and cold-water tank come out and pipework reroutes. A full system change (different type, new cylinder) is 2 to 3 days.
  • Do I need a powerflush with a new boiler?
    Sometimes. A powerflush clears sludge and limescale from the heating system, protecting the new boiler's heat exchanger. Mandatory under some manufacturer warranties for systems older than 10 years. Skip it on a near-new system or where a chemical clean is sufficient. Costs £300 to £500. Many engineers fit a magnetic filter as a cheaper preventive alternative for systems where powerflush is borderline.
  • What is a magnetic filter?
    A small canister fitted to the heating return pipe that captures iron oxide sludge using a magnet. It keeps the boiler heat exchanger clean and is required by some long-warranty offers. Costs £100 to £150 fitted. Should be cleaned at every annual service, which adds 5 minutes to the visit.
  • Can I install a boiler myself?
    No. UK law requires all gas appliance installations to be completed by a Gas Safe registered engineer. DIY installation is illegal, voids any warranty, and invalidates building insurance. Even fitting a thermostatic radiator valve while the boiler is running is best done by an engineer if pipework changes are involved.

For a step-by-step guide framed around boiler replacement specifically, see the sister site BoilerReplacementCost.com.