Last Updated on November 9, 2026
Boiler advice · Installation guide · Updated November 2026
What actually happens when a new combi boiler goes in: how long it takes, what the engineer does step by step, how to prepare and what to check before they leave.
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- ✔ Plain-English steps
Contents
- 1 How long does a combi boiler installation take?
- 2 Before installation day: how to prepare
- 3 What happens on the day, step by step
- 4 Where can a combi boiler go?
- 5 What to check before the engineer leaves
- 6 After your combi is fitted
- 7 Is a combi right for your home?
- 8 Signs it’s time for a new combi
- 9 Combi boiler installation day FAQs
How long does a combi boiler installation take?
Quick answer
A like-for-like combi swap usually takes 4 to 6 hours, so it’s normally done in a day. Converting from a system boiler to a combi usually takes around 2 days, and from a regular boiler with tanks in the loft up to 3 days. A back boiler replacement takes longer than a standard swap.
| Type of installation | Typical time | What makes it longer |
|---|---|---|
| Combi to combi, same place | 4–6 hours (one day) | Flue changes, a new gas run or a tricky position |
| System boiler to combi | Around 2 days | Removing the hot water cylinder and re-piping |
| Regular boiler to combi | Up to 3 days | Removing the cylinder plus the tanks in the loft |
| Back boiler to combi | Longer than a standard swap | Removing the old unit behind the fire, new flue and position |
| Moving the boiler to a new room | Adds pipework time | New gas, water and heating pipe runs |
Looking for prices rather than timings? Our combi boiler installation page lists the combis we fit and what they cost, and our combi boiler cost guide explains what drives the price.
Before installation day: how to prepare
- Clear the space around the boiler, the airing cupboard and, if you have tanks, the loft hatch and loft route.
- Clear a path from the front door to the boiler so the old unit can come out and the new one go in safely.
- Know where your stopcock and gas meter are, and make sure the engineer can get to them.
- Plan for no heating or hot water for most of the day on a swap, and longer on a conversion. Fill a kettle and a flask first.
- Move pets and valuables away from the work area, and park so the van can get close if you can.
- Have your old boiler manual or model to hand if you have it; it helps the engineer check the flue and gas supply.
What happens on the day, step by step
Arrival and checks
The Gas Safe engineer checks the job matches what was ordered, looks at the gas supply, flue route and pipework, and protects floors and surfaces.
Draining down
The water, gas and electrics to the boiler are isolated and the heating system is drained.
Out with the old
The old boiler and flue come off the wall. On a conversion, the cylinder and loft tanks come out too.
Pipework and flue
Gas, water, heating and condensate pipes are adapted for the new combi, and the new flue is fitted and sealed.
New boiler and filter
The combi goes on the wall with a magnetic system filter and the controls. The system gets a chemical flush and inhibitor.
Commissioning and handover
The engineer fills and pressurises the system, bleeds radiators, runs gas and flue safety checks and shows you how to use the boiler and controls.
If the job turns out to be different
Sometimes an engineer finds something nobody could see in advance, such as tanks in the loft on a “combi swap” or a flue that can’t go where planned. A good installer stops, explains and agrees any change with you before carrying on.
Where can a combi boiler go?
Combis are compact, so kitchen cupboards, utility rooms, airing cupboards (once a cylinder comes out) and some lofts are all common spots. The position has to allow a safe flue route to outside, a condensate drain and room for servicing. Moving the boiler is possible but adds pipework time.
What to check before the engineer leaves
- Hot water runs at every tap and the shower, and the heating comes on from the controls.
- The system pressure gauge reads in the normal range (often 1 to 1.5 bar when cold); ask what yours should be.
- You’ve been shown how to use the boiler, the controls and how to top up the pressure.
- A magnetic filter is fitted, and the system has been flushed and treated with inhibitor.
- The old boiler and packaging have been taken away and the area is clean.
- You know what paperwork is coming: the gas safety commissioning record, Building Control notification (via Gas Safe) and the manufacturer warranty registration.
After your combi is fitted
Over the first few weeks, keep an eye on the pressure and bleed any radiators that stay cold at the top. Register or check your warranty, keep the paperwork with the boiler manual, and book an annual service, because most manufacturer warranties need one to stay valid.
More detail in our guides to how long it takes to fit a new boiler and how often a boiler should be serviced.
Is a combi right for your home?
A combi heats water on demand from the mains, so it suits homes with one main bathroom and decent water pressure. Homes where several showers run at once are often better with a system or regular boiler. Our guide to what type of boiler you need walks you through it.
Signs it’s time for a new combi
- Leaks or drips from the boiler or its pipes
- Little or no hot water, or radiators with cold spots
- Pressure that keeps dropping or repeated error codes
- Banging, gurgling or whistling noises
- Frequent breakdowns, or parts that are no longer available
- A boiler that’s 15 or more years old and rising energy bills
Smell gas?
If you smell gas, turn off the gas at the meter if it’s safe to do so, open windows, don’t use switches or flames and call the National Gas Emergency number on 0800 111 999.
Combi boiler installation day FAQs
Can I install a combi boiler myself?
No. By law, any gas boiler installation in the UK must be carried out by a Gas Safe registered engineer. Fitting a boiler yourself is illegal and dangerous.
Do I need to be home during a combi installation?
Someone needs to be there to let the engineer in, agree the boiler position and flue route at the start, and go through the handover at the end. You don’t need to watch the work in between.
Will I have heating and hot water on installation day?
Not while the work is going on. Heating and hot water are off from when the system is drained until the new boiler is commissioned, which is usually most of the day on a straight swap.
Is a combi boiler installation messy?
There’s some dust and water when the old boiler comes off and pipes are cut, so engineers protect floors and surfaces. Conversions involve more work in the airing cupboard and loft.
What paperwork should I get after a combi installation?
You should get a gas safety commissioning record, confirmation that the installation has been notified to Building Control through the Gas Safe scheme, and the manufacturer warranty registration.
Is a combi boiler OK for a four-bedroom home?
It can be if hot water demand is low, for example one main bathroom. If two or more showers often run at the same time, a system or regular boiler with a cylinder usually copes better.
Can a combi boiler go in a loft?
Often, yes, if there is safe access, boarding, lighting and room for servicing, and the condensate pipe can be protected from freezing. Your engineer will confirm whether your loft is suitable.
