The short answer
On installation day, a Gas Safe registered engineer will isolate gas and water, remove the old boiler, fit and connect the new one, commission it and issue you with a Gas Safe certificate and Benchmark Checklist. The process typically takes 4–8 hours for a combi swap. For timescales for more complex jobs, see how long installation takes.
Many homeowners feel uncertain about what a boiler installation day involves, what the engineer should do, and what they should receive at the end. Knowing the sequence of events helps you recognise a proper job and gives you the confidence to ask questions if something seems to be skipped.
Installation day at a glance
- Start Gas isolation, drain down, old boiler off
- Main work Hang new boiler, connect gas/water/flue/condensate
- Completion Fill, pressurise, commission, test
- Documents issued Gas Safe certificate + Benchmark Checklist
- Controls Engineer walks you through new programmer/thermostat
- Duration (combi swap) 4–8 hours
Before the engineer arrives
Clear a working space around the boiler location and any routes the engineer will need to access — removing items from under the boiler, clearing the area in front and noting where cupboard doors open. Know where your stopcock (cold-water mains isolation valve) and gas emergency control valve are; the engineer will need to use them. If you have a combi boiler with a filling loop, locate it. If the engineer has not done a pre-installation survey, confirm the scope of work before they start to ensure there are no surprises about what is or is not included.
Step 1: isolation, drain-down and removal
The engineer will isolate the gas supply at the boiler service valve or the emergency control valve and turn off the water supply. The heating system is drained down — the water is removed from the radiators, pipes and the existing boiler to allow the old unit to be disconnected safely. The flue terminal is removed from outside, and the old boiler is disconnected from gas, water and electrics before being lifted off the wall. The engineer will inspect the visible pipework and note any issues — corroded fittings, undersized pipes, or evidence of sludge in the drained water — before proceeding.
Step 2: fitting and connecting the new boiler
The new boiler is hung on the wall, usually on a mounting frame or bracket. The engineer connects: the flow and return (hot and cool water pipes to and from the radiators); the cold mains supply (for a combi); the gas supply via a gas isolation valve; the condensate pipe (leading to an internal drain wherever possible); the electrical supply (usually from a switched fused spur); and the thermostat or programmer wiring. The new flue terminal is fitted through the wall and sealed. At each stage the engineer works to Gas Safe standards, using appropriate fittings and sealing compounds.
| Stage | What happens | Typical duration |
|---|---|---|
| Isolation & drain-down | Gas off, water off, old boiler removed | 30–60 min |
| New boiler fitting | Hang, connect all services, fit flue | 2–4 hours |
| Fill & pressurise | Refill system, check for leaks | 30–60 min |
| Commissioning & test | Fire up, balance, set controls, test safety devices | 1–2 hours |
| Handover & paperwork | Walk-through of controls, Gas Safe cert, Benchmark | 15–30 min |
Step 3: commissioning and what you receive
Commissioning is the stage where the engineer sets the boiler to its correct operating parameters, balances the system, tests the gas pressure, checks all safety devices (the pressure-relief valve, the overheat thermostat and the gas valve) and confirms the flue is drawing correctly. The Benchmark Commissioning Checklist is completed at this stage, recording the measured values. At handover, the engineer should walk you through the new controls — setting the clock, adjusting the hot-water temperature and operating the programmer or smart thermostat. You should receive the completed Benchmark Checklist and the Gas Safe installation certificate before the engineer leaves. Keep both documents safe; you will need them for warranty claims and when you sell the property. See our guide on choosing a boiler installer for what to look for before booking. This is general information; all gas work must be carried out by a Gas Safe registered engineer.
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Frequently asked questions
What documents should I receive after a boiler installation?
You should receive a Gas Safe certificate of installation and a completed Benchmark Commissioning Checklist. Both are required — the Gas Safe certificate confirms the work is safe, and the Benchmark Checklist is needed to validate most boiler manufacturers’ warranties.
Will the engineer show me how to use the new boiler?
A professional engineer will always walk you through the new controls at the end of the installation day, including how to adjust the temperature, set the programmer or smart thermostat, repressurise the system and read any fault codes.
What happens if the engineer finds a problem on installation day?
If the engineer finds unexpected issues — corroded pipework, an undersized gas supply or evidence of heavy sludge — they should stop, explain the issue and discuss the options and any additional cost before proceeding. Do not allow work to continue without understanding what has been found and what it means for the job.
Do I need to be at home all day for a boiler installation?
You should be present at the start and end of the day. During the installation you do not need to supervise, but someone should be in the property in case the engineer needs access to a stopcock, meter or other area of the home.
Sources & further reading
- Gas Safe Register — installation obligations, certification and commissioning requirements
- HHIC — Benchmark Commissioning Checklist and installation process
- GOV.UK / Building Regulations Approved Document J — combustion appliance installation standards
- Energy Saving Trust — what to expect from a boiler replacement
This is general information, not advice for your specific property or installation. Costs, timescales and outcomes vary with your home, system condition and chosen engineer. All gas boiler work must be carried out by a Gas Safe registered engineer.