Combi boiler installation and replacement

A combi boiler provides central heating and heats water when you turn on a hot tap. A standard combi does not need a separate hot water cylinder, which can make it a practical replacement where space is limited. The right choice still depends on your heating needs, water supply and how much hot water you use at once.

This guide explains the decisions involved in combi boiler installation and replacement. For the wider service, see what is included in a JustFix boiler installation.

Is a combi boiler right for your home?

A combi can suit a household that wants hot water on demand without storing it in a cylinder. It is worth looking beyond the number of bedrooms: two homes of the same size can have very different hot water needs.

• Hot water use: think about whether showers, baths and taps are used at the same time, rather than simply counting bathrooms.
• Water supply: mains pressure and flow affect what the system can deliver. A higher-output boiler cannot compensate for an inadequate incoming supply.
• Heating demand: insulation, room sizes, windows and the building's heat loss matter when choosing the heating output.
• Space and access: the boiler position must allow suitable pipework, flue routing, condensate drainage and access for maintenance.

A home with high simultaneous hot water demand may be better suited to stored hot water. Compare the options in our combi, system and regular boiler guide, or read what size boiler you need.

Replacing an existing combi boiler

A combi-to-combi replacement in the same position is different from changing the entire system. Existing pipework and controls still need checking: keeping the same boiler type does not mean every connection or component can automatically be reused.

Tell the installer about unreliable hot water, pressure problems, cold radiators or changes to the home. Those details help distinguish an appliance replacement from work needed elsewhere in the heating system. A new boiler alone will not resolve every underlying problem.

Changing from a system or regular boiler to a combi

A conversion can involve changes to pipework and controls, removal of a hot water cylinder or redundant tanks, and checks on the incoming water supply. It needs a different scope from a straightforward like-for-like replacement.

Do not assume a combi is automatically an upgrade for every home. If your household uses several hot water outlets together, discuss whether keeping stored hot water is more suitable. Confirm the proposed work with the team before booking; answering questions about your current system is not confirmation that every conversion is suitable.

What to check in your combi installation quote

• Boiler specification: the exact model, heating output and hot water capability recommended for your home.
• Installation scope: the agreed position, flue and pipework changes, and removal of the old system.
• System condition: any cleaning or remedial work recommended for the existing heating circuit, rather than assuming a power flush is always necessary.
• Controls and extras: what is included and which smart controls or other options are additional.
• Warranty and aftercare: the manufacturer's terms and servicing requirements, separately from the installation workmanship guarantee.

Our boiler installation cost guide explains how the scope affects a quote, without treating an appliance-only price as the complete installed cost.

What happens during installation?

Gas work is carried out by a Gas Safe registered engineer. The installation involves making the old system safe, replacing the appliance and completing the agreed connections and checks. Commissioning and a handover are part of the process, not optional steps after the boiler has been mounted.

Ask when heating and hot water will be unavailable, what access is needed and how completion will be confirmed. Read how long boiler installation takes for the factors that affect disruption and timing.

Controls, warranty and ongoing care

At handover, ask how to use the heating controls, set a schedule and find the operating instructions. If you choose a smart controller, check the model and any app or connectivity requirements before deciding.

JustFix boiler installations include a 12-month workmanship guarantee, subject to the guarantee terms. This is separate from the manufacturer's boiler warranty, which has its own conditions. Check the terms for your chosen model and arrange servicing as required.

Start with your home, not a model number

You do not need to choose a boiler before starting. Begin your boiler quote with your fuel type and the questions about your existing system and household. JustFix currently installs mains gas boilers; the first step checks the fuel before you continue.

If you are still researching, browse our boiler guides for sizing, installation time and comparisons. If a fault needs diagnosing before you decide to replace, our boiler repairs service is the more relevant starting point.