The short answer
On installation day, a Gas Safe registered installer drains your heating system, removes the old boiler, fits the new one with its flue, filter and controls, then refills, tests and commissions it. A like-for-like swap usually takes one day, with heating and hot water back by the evening. A conversion or a boiler moving to a new spot can take up to two days. Before the installer leaves, they should show you how to use the boiler, complete the Benchmark checklist and explain how the warranty is registered. The Building Regulations certificate arrives by post afterwards.
The day before
A little preparation saves time and protects your home.
- Clear the area. Empty the cupboard around the boiler, the airing cupboard if a cylinder is involved, and a path from the front door.
- Clear the loft hatch if loft tanks are being removed or the flue goes through the roof.
- Move cars so the van can park close, and check any parking permit rules.
- Plan for no hot water. Shower the night before, fill a kettle and keep a few litres of drinking water aside.
- Keep pets and children away from the work area for the day.
- Have heaters ready in winter if you have them, especially if the job may run past one day.
Ask the installer the day before what time they will arrive and whether there are any changes to the plan.
How the day usually runs
| Time of day | What is happening | What you will notice |
|---|---|---|
| Arrival | Walk-through of the plan, dust sheets down | A short chat about where everything goes |
| Morning | Gas and water isolated, system drained, old boiler removed | No heating or hot water from here |
| Midday | New boiler, flue and filter fitted | Drilling if the flue position changes |
| Afternoon | Pipework connected, system cleaned and refilled with inhibitor | Radiators bled, some gurgling |
| Late afternoon | Commissioning, gas and flue tests, controls set up | Heating and hot water come back on |
| Handover | Walk-through of the boiler, controls and paperwork | Time to ask questions |
On a conversion from a system or regular boiler to a combi, there is more to do: the cylinder and loft tanks come out and pipework is altered, which is why that job often runs into a second day. The combi conversion page covers this in more detail.
Decisions that might come up
Even with a good survey, an installer sometimes finds something once the old boiler is off the wall. Common examples are a gas pipe too narrow for the new boiler, a corroded section of pipework, or a radiator valve that will not shut off. A good installer stops, explains what they have found, and gives you a price before doing anything not in the quote. You should never find unexpected extras on the final invoice without having agreed to them first.
The checks before handover
Before the installer leaves, they should:
- Test the gas supply for tightness and check the gas pressure at the boiler.
- Check the flue is sealed and the combustion readings are within the manufacturer’s limits.
- Confirm the condensate pipe drains properly and, if it runs outside, is protected against frost.
- Check every radiator heats up and the system holds its pressure.
- Set the flow temperature and programme the controls with you.
- Fill in the Benchmark checklist in the boiler manual, which records the commissioning details.
Ask the installer to show you the filling loop, the pressure gauge and the reset button before they leave. Knowing where these are saves a callout later.
The handover
Take ten minutes for this. Ask the installer to show you how to set the heating times and temperatures, what pressure the boiler should sit at when cold, how to top it up, and what the common fault codes mean. Ask what flow temperature they have set and why. Our guide on boiler efficiency ratings explains why that setting matters for your gas use.
Make sure you know who to call if something goes wrong in the first few weeks, and how long the installer’s own workmanship guarantee lasts.
The paperwork that follows
| Document | Who provides it | When |
|---|---|---|
| Benchmark checklist | Installer, completed in the manual | On the day |
| Invoice and guarantee details | Installer | On the day or soon after |
| Warranty registration confirmation | Manufacturer | Usually within a few weeks |
| Building Regulations compliance certificate | Gas Safe Register, after the installer notifies the job | By post, usually within a few weeks |
If the certificate or warranty confirmation has not arrived within a month or so, chase the installer. Our guide to boiler warranties explains why registration and the first annual service matter so much.
Before the day comes
Most of a good installation day is set up at the survey. Check your quote names the boiler model, output, warranty and everything included, as covered in our boiler replacement cost guide. City Boiler connects you with independent Gas Safe registered installers who give a fixed price before any work starts. You can check any engineer’s ID on the Gas Safe Register. Find out more on the boiler replacement hub.
Common questions
Do I need to be home during a boiler installation?
Someone over 18 should be there at the start to let the installer in and agree the plan, and at the end for the handover. In between, you do not need to stay, but you should be reachable by phone in case a decision comes up.
How long will I be without heating and hot water?
On a like-for-like swap, usually for the working day, with everything back on by the evening. A conversion or a boiler moving to a new position can take one to two days, so ask the installer whether you will have heating overnight.
Will the installer make a mess?
There is some disruption: draining water, lifting floorboards if pipes need rerouting, and drilling for a new flue. A careful installer uses dust sheets, protects floors and takes the old boiler away. Agree in advance who removes the old boiler and packaging.
What paperwork should I get after the installation?
You should get the completed Benchmark checklist in the manual, confirmation that the warranty is registered, and the installer's invoice. A Building Regulations compliance certificate follows by post once the installer has notified the job through Gas Safe.