Gas Boiler Installation
Timeline, Hidden Costs & What to Expect
A standard boiler swap takes one day. A conversion takes two to three. Here is exactly what happens and what to budget for.
Updated April 2026
Installation Timeline
| Scenario | Duration | What is Involved |
|---|---|---|
| Like-for-like combi swap | 1 day (4-6 hours) | Remove old boiler, fit new one in same position, connect gas/water/flue, test and commission |
| Combi relocation | 1.5 - 2 days | All above plus rerouting gas, water, and flue pipes to new location. May need flue extension or external condensate routing. |
| Conventional to combi conversion | 2 - 3 days | Remove cylinder and loft tank, reroute all hot water pipework, upgrade mains feed, fit new boiler. Most complex standard job. |
| System boiler install | 1.5 - 2 days | Fit boiler and connect to existing or new cylinder. May need to upgrade controls and zone valves. |
| Full system change (e.g. gravity to sealed) | 2 - 3 days | New boiler, new cylinder, reroute pipework, upgrade radiator valves, possible radiator replacements. The most disruptive option. |
Hidden Costs and Extras
The boiler price in your quote may not include these extras. Always ask your installer whether each item is included or charged separately.
| Item | Cost | When Needed |
|---|---|---|
| Powerflush (system clean) | £300 - £500 | Recommended for systems over 10 years old. Removes sludge from radiators and pipes. |
| Magnetic filter (MagnaClean) | £100 - £150 | Recommended for all new installs. Catches debris before it reaches the boiler. |
| TRV upgrade | £150 - £300 | For a 3-bed home. Thermostatic valves on all radiators for zone control. |
| Smart thermostat | £150 - £250 | Optional but recommended. Nest, Hive, or Tado. Some boiler deals include one free. |
| Flue extension | £50 - £100/metre | If the boiler is not on an external wall or the flue needs to route through a loft void. |
| Condensate pipe routing | £50 - £150 | If the condensate needs to be routed externally. Should be insulated to prevent freezing. |
| Scaffolding | £150 - £300 | If the flue exits at height (loft conversion, upper-floor boiler). Unusual but possible. |
| Asbestos flue removal | £200 - £400 | Older properties (pre-1990s) may have asbestos flue pipes. Must be removed by a licensed contractor. |
| Gas supply upgrade | £200 - £500 | Rare. Only needed if your gas meter/supply cannot handle the new boiler's demand. |
Gas Safe Requirement
All gas boiler installations must be carried out by a Gas Safe registered engineer. It is illegal for an unregistered person to work on gas appliances. The penalty is an unlimited fine or imprisonment.
How to verify: Ask to see the engineer's Gas Safe ID card. Check the registration number on the Gas Safe Register website (gasaferegister.co.uk). The card has an expiry date and lists which gas work the engineer is qualified for.
Red flags: No Gas Safe card, cash-only payment, no written quote, vague cost breakdown, pressure to decide immediately, no mention of Building Regulations notification.
How to Get Quotes
- 1. Get at least three written quotes. Never accept the first quote. Prices vary by 30-50% between installers for the same job.
- 2. Make sure each quote includes: boiler make and model, supply cost, labour cost, all extras (flue, filter, powerflush, controls), VAT, warranty terms, and estimated completion date.
- 3. Check Gas Safe registration for every installer. Note their registration number and verify it online.
- 4. Ask about manufacturer warranty. A Worcester Bosch accredited installer can offer up to 12 years. A non-accredited installer may only get you 5 years on the same boiler.
- 5. Consider online boiler companies. Heatable and BOXT offer fixed national pricing with manufacturer-accredited installers. Often 10-15% cheaper than local quotes, especially in London.
Preparing Your Home
- Clear at least 1 metre of space around the current boiler location
- Move furniture away from radiators if a powerflush is being done
- Protect flooring near the boiler with old sheets or dust sheets
- Ensure clear access to the loft if flue routing or tank removal is needed
- The engineer will need access to water stopcock, gas meter, and all radiators
- You will be without heating and hot water for the duration. Plan accordingly (kettle, electric heater if winter)
Frequently Asked Questions
How long does a boiler installation take?
A straightforward like-for-like combi swap takes 4 to 6 hours (one day). Relocating the boiler adds half a day. Converting from conventional to combi takes 2 to 3 days. A full system change with new pipework and cylinder takes 2 to 3 days.
Do I need a powerflush with a new boiler?
Not always, but it is recommended if your heating system is over 10 years old or if your radiators have cold spots. A powerflush removes sludge and debris that can reduce your new boiler's efficiency and shorten its lifespan. It costs £300 to £500 and adds 2-4 hours to the job.
What is a magnetic filter?
A magnetic filter (like MagnaClean) attaches to the pipework near the boiler and catches metallic debris from the heating system before it reaches the boiler's heat exchanger. It costs £100 to £150 fitted and is recommended for all new installations. It needs cleaning once a year during the annual service.
Can I install a boiler myself?
No. It is illegal to install a gas boiler unless you are Gas Safe registered. Even if you are a competent DIYer, gas work requires certification, specialist training, and Building Regulations notification. The penalty for illegal gas work is an unlimited fine or imprisonment.