How to Install a Combi Boiler
A combi installation runs from survey to signed Benchmark: size the appliance, upgrade the supplies, set the flue and condensate correctly, then flush, fill and commission. It is notifiable work for a Gas Safe registered engineer.
A combined heating and hot-water (combi) boiler is the most common installation in UK homes, heating water on demand with no stored cylinder. Fitting one properly is a sequence of decisions — sizing, supply upgrades, flue and condensate routing, system cleaning and commissioning — each of which affects how well the finished job performs.
This guide walks through that sequence so the work is understood end to end. Installing a gas boiler is notifiable work that must be carried out and commissioned by a Gas Safe registered engineer, who notifies the installation and completes the Benchmark checklist — this is a professional overview, not a DIY instruction.
Key takeaways
- Size the combi to the property's hot-water demand and check the incoming mains flow and pressure will support it.
- A modern combi usually needs a 22 mm gas supply — size the pipe run and gas-rate the appliance to prove it.
- Flue and condensate must be routed and terminated to the manufacturer's instructions and current regulations.
- Always flush or power flush the system and add inhibitor and a magnetic filter to protect the new appliance and its warranty.
- Commission the boiler, complete the Benchmark checklist and register the appliance — notifiable work for a Gas Safe engineer.
Survey, sizing and planning
Every good install starts with a survey, not a boiler order. Size the combi to the property's hot-water demand — the number of bathrooms and the expected simultaneous use — while checking the incoming mains flow and pressure can actually feed it, because a combi heats water on demand with no stored buffer. An oversized boiler short cycles; an undersized one disappoints on hot water.
Plan the position, the flue route and termination, the condensate run, and the gas and water connections before lifting a tool. Confirming the gas supply can deliver the required rate — often meaning an upgrade to 22 mm pipe — is part of planning, not an afterthought discovered halfway through.
Check the mains before you promise the hot water
A combi's hot-water performance is capped by the incoming mains flow and pressure. Measure them at survey — promising a strong two-bathroom performance the mains can't support is a callback you booked yourself.
Gas and water supply
The gas supply must be sized to deliver the boiler's full input without excessive pressure drop — commonly a 22 mm run to a modern combi. Undersized pipework is a frequent cause of an appliance that gas-rates low and underperforms, and it cannot be fixed at the gas valve, so size the run properly to the pipe-sizing rules before you commit.
On the water side, connect the mains cold to the boiler, the heating flow and return, and the domestic hot-water outlet, with isolation valves so the appliance can be serviced without draining the house. Fit the filling loop and expansion arrangements to the manufacturer's layout.
Doing this on a job? Benchmark commissioning checklist — part of TradePlanr. Free to start, no card needed.
Flue and condensate
The flue must be installed and terminated to the manufacturer's instructions and the current regulations — correct clearances from windows, doors, boundaries and openings, the right terminal, and inspection points where the flue is concealed. Getting terminations right is a safety matter, not a cosmetic one.
The condensate drain carries away acidic condensate and must fall to a suitable drain, ideally internally to avoid freezing. Where an external run is unavoidable, it should be in larger-bore, well-lagged pipe to reduce the freeze risk that causes so many winter cut-outs. Both flue and condensate should follow the manufacturer's guidance exactly.
Mounting, flushing and filling
Mount the boiler securely and level, on a wall that will take its weight, positioned for the planned flue and connections and with the clearances needed for servicing. Make the gas, water and electrical connections, taking care with the gas tightness of every joint.
Before firing, the system must be cleaned. On a new-build the pipework is flushed to clear debris and flux; on an existing system that a new boiler is joining, a power flush or thorough clean removes the sludge that would otherwise foul the new heat exchanger. Then dose with inhibitor, fit a magnetic filter, and fill to the correct cold pressure. Skipping the clean is the fastest way to a warranty problem.
Quote, commission and certificate in one flow
With TradePlanr you can build the install quote, then complete the Benchmark commissioning checklist and the gas safety paperwork on site and hand over signed PDFs — the whole install, from quote to certificate, running from a single job record.
Commissioning and handover
Commissioning is where the install is proven. Carry out the gas tightness test, gas-rate the appliance to confirm it fires at its rated input, check the burner pressure and combustion with a flue gas analyser, and confirm the safety devices, controls and hot-water performance all work as intended. Any shortfall is corrected before the customer takes it on.
Complete the Benchmark commissioning checklist — the manufacturer's record that validates the warranty — register the appliance, and hand the customer their paperwork with a demonstration of the controls. The installation is notifiable, so it must be notified through the Gas Safe registered engineer's scheme. A properly commissioned and documented install is what protects both the customer and the warranty.
From guidance to action
Frequently asked questions
Can I install a combi boiler myself?
No. Installing a gas boiler is notifiable work that legally must be carried out and commissioned by a Gas Safe registered engineer. This guide is a professional overview of the process, not a DIY instruction — unqualified gas work is illegal and dangerous.
Do I need to upgrade the gas supply for a combi?
Often, yes. Modern combis usually need a 22 mm gas supply to deliver their full input without excessive pressure drop. An undersized run makes the appliance gas-rate low and underperform, and it cannot be corrected at the gas valve — so the pipe is sized properly at the planning stage.
Why does the system need flushing before a new combi?
To protect the new appliance. Sludge and debris from an existing system will foul a new heat exchanger and can void the warranty. A power flush or thorough clean, plus inhibitor and a magnetic filter, is standard practice and usually a manufacturer requirement for the guarantee.
What is the Benchmark checklist for?
Benchmark is the manufacturer's commissioning record. Completing it at install validates the boiler's warranty and documents that the appliance was set up correctly — gas rate, combustion, pressures and safety checks. Alongside registering the appliance, it is part of a proper handover.
Related guides
Combi vs System vs Regular Boiler
How the three boiler types differ, which suits which property, and what each means for the installation.
How to Price a Boiler Installation
Break a boiler installation quote into its parts, with indicative UK ranges and how to price from your own costs.
The Benchmark Certificate and Boiler Warranties
What Benchmark is, how it protects the warranty, and completing it correctly at commissioning.
