Ask most people what makes a boiler installation expensive and they will name the boiler. In practice the appliance is often less than half the figure, and the variation between quotes comes almost entirely from the other side of the ledger: hours on site, materials consumed, and how much of the existing system has to be corrected before a new appliance is safe to run.
So this guide sets the appliance aside. Below is what the labour and materials portion is buying, hour by hour.
Where the hours go on a typical replacement
Indicative time split for a like-for-like combi swap in a reasonable position.
| Task | Rough share of the time | What makes it longer |
|---|---|---|
| Protection, isolation, drain down | About a tenth | No accessible drain point; upstairs installations |
| Removing old appliance and flue | About a fifth | Floor-standing units, back boilers, asbestos-era flue seals |
| Pipework and gas alterations | The largest single share | Undersized gas run, imperial pipe, changed connection centres |
| Hanging and flueing the new boiler | About a fifth | Long flue runs, roof terminations, awkward bracket fixings |
| Flushing, dosing, filter fitting | Around a tenth | Heavily sludged systems needing a full power flush |
| Commissioning and handover | Around a tenth | Balancing every radiator, configuring unfamiliar smart controls |
Why a conversion costs so much more than a swap
Converting from a cylinder system to a combi is a different job wearing the same name. The cylinder comes out, the loft tanks come out, the cold feed and vent pipework gets capped off or removed, the hot water pipework is reconfigured to run at mains pressure, and the boiler frequently moves to a new position with a new flue and a new condensate route.
That is two or three days of work with a materials list several times longer, plus making good where tanks and cylinders once sat. It is why a conversion quote sitting alongside a straight-swap quote looks unreasonable until you see what is inside it.
Line items worth asking to see priced separately
Not because they are suspicious, but because they are the items quotes silently differ on.
- Gas supply pipe upsizing, with the length and route stated
- Chemical cleanse versus full power flush, and which one your system needs
- The magnetic filter — make, and whether it is a serviceable unit
- Flue kit and any extensions, plus scaffolding or access equipment
- New thermostatic radiator valves, and how many
- Controls: what is being reused and what is being replaced
- Making good, decorating and disposal of the old appliance and tanks
Getting comparable quotes
- 1
Insist on a survey
A price given over the phone cannot know your gas pipe run or your mains flow, so it will change.
- 2
Fix the appliance across quotes
Ask each installer to price the same model, or the labour differences disappear into brand differences.
- 3
Ask what is excluded
Exclusions are where cheap quotes live. Get them written down.
- 4
Check the compliance items are in
Notification, commissioning record and warranty registration are part of the job, not extras.
Questions people ask
Is a two-day installation a bad sign?
Usually the opposite. Two days often means gas pipework is being upsized or the system is being properly cleaned rather than skipped to finish by teatime.
Why does moving the boiler cost so much?
Because everything follows it — gas, water, condensate, flue and electrics all need new routes, and each route may involve lifting floors or boxing pipework.
Can I supply my own boiler to save money?
It is possible, but warranty registration, correct output selection and any faulty-out-of-box issues then sit with you, and many installers will not warrant the appliance side of the work.
Are evening or weekend installations dearer?
Installations are normally planned within working hours because commissioning and merchant support need daytime access. Emergency repair work is a different service to a planned installation.
Who does the work
- Gas Safe registered engineers — registration 554328
- Worcester Bosch accredited installer
- John The Plumb Ltd, company registration 10026483
Read a boiler quote as a shopping list of hours and materials rather than a single price, and the cheapest one often stops looking like the best value within about ninety seconds.
Ask us to itemise a boiler installation quote
We will set out the appliance, the gas and pipework, the system cleaning and the compliance items separately so you can see exactly what you are comparing.