Boiler Installation and Replacement: A Complete Guide

The Three Main Boiler Types

The best boiler type depends mainly on hot-water demand, available space, mains water performance and the way the existing heating system is configured.

Combi boilers

A combi boiler heats the home and produces hot water on demand without a separate hot-water cylinder. It is a practical choice for many flats and houses where space matters and hot-water outlets are not routinely used at the same time.

System boilers

A system boiler works with a hot-water cylinder. Stored hot water can make it a better fit for homes with higher simultaneous demand, such as households where more than one shower or bath may be used close together.

Regular or heat-only boilers

A regular boiler works with a hot-water cylinder and, in many traditional systems, additional feed-and-expansion components. It can remain appropriate in larger or older properties where the existing system layout suits that arrangement.

For a direct comparison, read our combi boiler vs conventional boiler.

How Boiler Sizing Should Be Worked Out

Boiler sizing should not be reduced to bedroom count or radiator count alone. There are two separate questions: how much heat the property loses on a cold day, and how much hot water the household expects at one time.

• Space heating: the engineer should assess heat loss, taking account of floor area, insulation, glazing, exposed walls and the emitters in the system.
• Combi hot water: the domestic-hot-water output and flow rate must match the incoming mains supply and the household’s expected use.
• System or regular boiler: the boiler output and cylinder capacity/recovery should be selected together for the property’s heating and hot-water demand.

A larger number printed on a combi boiler does not necessarily mean the same output is available to the radiators. Manufacturers often quote separate central-heating and domestic-hot-water figures. Final selection should use the manufacturer data for the exact model and a property assessment.

Changing a Conventional Boiler to a Combi

A conventional-to-combi conversion is more involved than a like-for-like boiler swap. The hot-water cylinder and any redundant storage components are removed, pipework is altered, the new boiler is connected to the mains water supply, and the flue and condensate route are installed to suit the new position.

What the work normally includes

1. Assessing the existing heating system, mains water flow and proposed boiler position.
2. Draining and removing redundant boiler, cylinder or tank components where the conversion requires it.
3. Checking and, where necessary, altering gas, heating, hot-water and condensate pipework to suit the chosen appliance.
4. Installing the boiler and flue in accordance with the manufacturer’s instructions and Building Regulations.
5. Cleaning and treating the heating system as required before commissioning the new boiler.
6. Commissioning the appliance, setting controls and completing the required installation records and notification.

If system cleaning is needed, the method should match the condition of the system rather than being assumed in advance. For heavily contaminated systems, see our power flush service.

Will the Gas Pipe Need Replacing?

Possibly, but there is no universal pipe size that can be specified from the page. A Gas Safe engineer must calculate whether the existing gas supply can deliver the appliance’s required input while maintaining the correct operating pressure. The route, length, fittings and other gas appliances all affect the answer.

Where Can a New Boiler Be Installed?

Common locations include kitchens, utility rooms, suitable cupboards and, where the appliance and access arrangements allow it, loft spaces. The final location has to satisfy the boiler manufacturer’s instructions, flue-terminal clearances, condensate requirements, ventilation where applicable, frost protection and safe access for servicing.

During a conversion, placing the new boiler near existing services can simplify the work, but moving it elsewhere may be worthwhile if it improves access or frees useful space.

Moving a Boiler to Another Room

Boiler relocation is a separate design decision because it can require new gas pipework, heating and hot-water routes, a new condensate connection and a different flue position. The farther the move and the more finished surfaces that need opening and making good, the more involved the job becomes.

Our supporting guide explains the practical factors that affect the cost of moving a boiler without relying on generic fixed price bands.

Back Boiler Replacement

Replacing a back boiler normally involves removing the old appliance and choosing a completely new boiler position. Because the existing boiler sits within the fireplace/chimney arrangement, the project can involve more building work and pipe alterations than a straightforward wall-hung replacement. The survey should establish the new boiler type and position before the old system is stripped out.

How Long Does Boiler Replacement Take?

A straightforward like-for-like replacement can often be completed within a working day, but the actual programme depends on access, system condition and commissioning work. Conversions, major relocations and jobs involving extensive pipework or making-good usually take longer. The installer should confirm the expected duration after surveying the property.

How Much Does Boiler Replacement or Conversion Cost?

There is no reliable cluster-wide figure that applies to every London property. The final price depends on the boiler selected, whether the system type changes, the flue and condensate route, gas and water pipework, controls, system cleaning, access and any making-good work.

For a useful comparison, ask each installer to state exactly what is included rather than comparing only the headline total. A clear quote should identify the boiler model, controls, filter or water-treatment items, flue components, system cleaning, disposal, commissioning and any relocation or building work.

Choosing the Right Combi Boiler

Once you know a combi is the right system type, model selection should focus on the property’s hot-water demand, mains flow, central-heating requirement, installation space, controls, warranty terms and service support. Current model ranges change, so recommendations should be based on manufacturer data available at the time of the quote.

For current model shortlists by home size, use our dedicated guide to the best combi boiler for your home. If you have a three-bedroom property, see the more specific best combi boiler for a 3 bedroom house.

Hard Water, System Protection and Controls

Much of London has hard water, so scale and system cleanliness deserve attention during installation. The correct treatment depends on the appliance manufacturer’s instructions, local water conditions and the condition of the existing system. A competent installer should also specify suitable heating controls so the boiler can operate efficiently at the temperatures the system actually requires.

What Landlords Need to Know

For rented homes, boiler replacement sits alongside existing landlord safety and energy-efficiency duties. Gas appliances and flues provided for tenants must be maintained safely and checked at the required intervals, and the relevant gas-safety record must be provided to tenants. A replacement boiler is also notifiable building work in England unless it is self-certified through an appropriate competent person scheme.

A new heating system can affect an EPC assessment, but it should not be presented as guaranteeing a particular rating. Current minimum energy-efficiency rules for relevant privately rented homes depend on the property’s overall EPC and any applicable exemption, not on the boiler alone.

Before You Book an Installation

• Confirm whether you are keeping the same boiler type or changing system type.
• Have the incoming mains water performance checked before choosing a high-output combi.
• Ask how the heating load has been assessed rather than selecting by bedroom count alone.
• Agree the boiler position, flue route, condensate route and access for future servicing.
• Check the exact manufacturer warranty conditions for the proposed model.
• Make sure the quote clearly states what is included and what is excluded.

Our things to consider before boiler installation guide covers the planning questions in more detail, while our boiler reviews section helps compare current ranges.

Frequently Asked Questions

Is a combi boiler always best for a three-bedroom house?

No. Many three-bedroom homes work well with a combi, especially where one bathroom is used at a time, but simultaneous hot-water demand and mains water performance matter more than bedroom count alone. A system boiler and cylinder may be better where several outlets need hot water together.

Can a boiler go in an airing cupboard?

It can, provided the exact appliance can be installed there in accordance with its manufacturer instructions and the location allows safe flue routing, condensate drainage, clearances and service access.

Do all conventional-to-combi conversions need new gas pipework?

No. The existing gas supply must be checked and sized for the new appliance. Some installations require alterations; others may not. Pipe diameter on its own is not enough to decide.

Does a new boiler automatically cut bills by a set amount?

No. A modern condensing boiler can be more efficient than an older non-condensing boiler, but actual savings depend on the existing system, controls, insulation, energy use and the way the new boiler is set up.

Next Step

If you are ready to replace, convert or relocate your boiler, request a property-specific assessment for boiler installation in London The survey should confirm the system type, output, hot-water demand, mains flow and installation requirements before a final quote is issued.