Combi Boiler Installation

Combi boiler installation across the UK

If you are looking for a combi boiler installation, we are here to provide you with that. At Boilerhut, we have a large network of combi boiler installers who supply and install gas oil and LPG combi boilers to postcodes across the UK.

A combi boiler installation is usually a straightforward affair, but it requires expert installation and a qualified engineer. All of our engineers are Gas Safe Registered, OFTEC Registered and have oil and LPG certifications, enabling them to install gas, oil and LPG boilers.

Looking for a combi boiler installation?

The most popular type of boiler in the UK

Combination boilers – more commonly known as combi boilers, are the most popular type of boiler sold in the UK for central heating and domestic hot water.

Whilst first produced by Vaillant in the early 1960s and introduced to the UK by Worcester in 1971, combi boilers have gained massive popularity over the past 20 years or so, due to the excellent efficiency and heating capabilities they now provide. As they are compact, require no hot water cylinder or cold water tanks, over 70% of UK households now choose these over other boiler types.

Even if you have a larger property, there are still combi boiler options available for you.

Cost Calculation

How much does combi boiler installation cost?

The cost to install a combi boiler will vary depending on the boiler that it is replacing.

Below we have illustrated various installation scenarios and have detailed the typical costs for installing a new combi boiler in the same location and for installing a new combi boiler in a new location. It should be noted that the cost for moving the boiler will vary according to where it is moved to.

Combi BoilerReplacing an existing combi boiler with a new combi boiler

A combi to combi boiler swap is the easiest and the quickest combi boiler installation as it is replacing like for like. Below, we have detailed the average costs and installation time for this type of installation.

Type of installationAverage costInstallation time  
Replacing an existing combi boiler with a new boiler, in the same location£2000 – £25001 day
Replacing an existing combi boiler with a new boiler, in a new location£2500-£30001 to 2 days
Looking to replace your existing combi boiler with a new combi boiler?

System BoilerReplacing a regular/system boiler and a hot water cylinder with a combi boiler

Regular boilers are fed by two header tanks in the loft, an external pump is used to pump water around the radiators and hot water is stored in a cylinder. A combi boiler will replace all of these components. Below, we have provided the average cost and installation time to remove the hot water cylinder and redirect the hot water pipes to the new combi boiler location.

Type of installationAverage costInstallation time  
Replacing an existing regular boiler and hot water tank with a new combi boiler*£2,9002 days
Looking to replace your existing regular/system boiler and a hot water cylinder with a new combi boiler?

back boiler conversion to combi boiler installationReplacing a back boiler with a combi boiler 

Back boilers are a type of conventional boiler which were installed in open fireplaces in residential properties from the 1960s. They were phased out in 2005 when the new high-efficiency regulations were implemented, making it very difficult and expensive to fit this type of boiler. Providing central heating and domestic hot water, due to their high build quality and simplicity it is not uncommon for them to still be running today.

The main reason to swap a back boiler with a combi boiler is due to low efficiency, replacing a back boiler for a combi boiler could save you £300-£500 on your annual gas bill.

Below, we have provided the average cost and installation time for removing the back boiler and the hot water cylinder and replacing it with a combi boiler.

Type of installationAverage costInstallation time  
Replacing a back boiler and hot water tank with a new combi boiler£35002/3 days
Looking to replace your existing back boiler with a new combi boiler?
What is an LPG Combi Boiler?

What is a combi boiler?

Combi boilers are a space-saving boiler type that house all of the water heating and central heating components in one compact unit.

Heating directly from the mains, a combi boiler will provide you with an unlimited supply of instant hot water when you turn on a tap or shower. This water is delivered through your pipes at mains pressure where it arrives at your taps and showers.

Combi boilers only heat water when a hot tap or shower is turned on, this ensures maximum efficiency and means that it won’t cost you anything until this is used. To allow for quick delivery of hot water, the length of the pipe run from the boiler to the furthest outlet should be considered.

The central heating portion of a combi boiler is on a pressurised closed loop system, heated by the boiler as required. 

If you are short on space or you are looking to better utilise areas of your home, a combi boiler will be ideal because there is no need for a cold water tank in the loft nor a hot water cylinder in the airing cupboard.

Combi boilers deliver hot water through your taps or showers at mains pressure. This means that if your mains water pressure is good enough, you can enjoy hot showers without needing any extra pumps.

As no hot water is stored in an external cylinder and it is delivered as and when required, combi boilers are designed to maximise the amount of energy that is directly used to heat the water. This means there is no need to heat up and store more water than is required, reducing your carbon footprint and cutting down your energy bills.

Combi boilers are straightforward to install, economical to run and are typically installed in a kitchen, utility or airing cupboard. Flueing options should be considered when locating a new boiler (horizontal or vertical).

Gas Combi Boilers

How does a combi boiler work?

Whether your combi boiler runs on mains gas, oil or LPG, the fuel enters a sealed combustion chamber inside the boiler and an electric ignition sets it alight. The primary heat exchanger within the boiler then heats incoming water up to the required temperature, which then flows through all of the radiators in your home or is delivered to taps and showers, as needed.

Combi boilers have two heat exchangers inside them:

  1. A primary heat exchanger (for central heating and hot water)
  2. A secondary plate heat exchanger (for hot water)

The primary heat exchanger heats and recycles the water that goes through the pipes into the radiators.

The secondary plate heat exchanger uses that returning hot water from radiators, and heats up sealed pipes with potable water that goes into your taps.

Our guide will provide more detail about how a combi boiler works – How Does a Combi Boiler Work?

how a combi boiler works

Is a combi boiler suitable for my house?

The compact size of combi boilers makes them an ideal choice for properties with limited space. Combi boilers are not just a solution for flats and smaller houses though, combi boilers come in a variety of outputs making them suitable for properties of a range of sizes.

For instance, modern combi boilers now exist which are equipped to deal with the hot water and heating requirements of homes with 4+ bedrooms, 2+ bathrooms and 20+ radiators. Called storage combi boilers, these larger combi boilers house a thermal water storage tank within the unit itself. An example of one of these boilers is the Viessmann 111-W Storage Combi, which houses a 46-litre stainless steel thermal store within the boiler itself. This is a great option because even if you call for hot water in two bathrooms, you will still get it at mains pressure.

Generally, the larger the house and the higher the hot water demands, the higher the kilowatt rating the boiler should be. To illustrate this, we have matched combi boiler outputs to the property size that they are suitable for.

Combi Boiler kW OutputProperty Suitability
24-27kWProperties with 1-2 bedrooms, 10 radiators and 1 bathroom
28-34kWProperties with 3-4 bedrooms, 15 radiators and 1-2 bathrooms
35-42kWProperties with 4+ bedrooms, 20 radiators and 2+ bathrooms
It is worth considering the following too:

How much hot water do you use?

Hot Water Usage

If you require hot water from multiple taps at the same time, including showers, then perhaps a regular combi boiler won’t be enough. However, for properties with higher hot water demands, a storage combi boiler will be able to handle the hot water demands from several taps at once.

In the case of the Viessmann Vitodens 111-W, it utilises an integrated stainless steel thermal store with CLS (cylinder loading system). This makes sure that it refills the in-built thermal store within a few seconds without affecting the final flow rate. The Viessmann Vitodens 111-W can handle up to three bathrooms.

However, if you live in a larger house, it may be that a combi boiler isn’t the right option for you and you may want to consider a conventional boiler or a system boiler with an external hot water cylinder instead.

How close will the combi boiler be to the bathroom?

How close will the combi boiler be to the bathroom?

If you are planning to install your new combi boiler in the garage and you have a bathroom on the other side of the house, you could experience a delay in the time it takes for water to heat up and reach your taps and showers. For best results, your combi boiler should either be installed reasonably close to your bathroom, or equidistant to the kitchen taps and bathroom.

Advantages and disadvantages of combi boilers

Combi boiler advantages

  • Compact in size, meaning you have more options for where the boiler can be positioned in the property 
  • No need for a separate cold water tank or hot water cylinder ensures space in your property is saved and means there are fewer things to go wrong
  • Hot water on demand – instant and unlimited hot water that is provided as and when required
  • Energy bills are lowered – because of the way that hot water is supplied, no waste is created from unnecessarily heated water
  • Cheaper to install than system and conventional boilers as there is less pipework and components
  • No issues with air locks or low pressure as hot water is taken directly from the water mains

Combi boiler disadvantages

  • Combi boilers require good mains water pressure to operate properly 
  • Not suitable for use with power showers
  • As all components are combined, you will be left without heating and hot water if your combi boiler breaks down
  • If your house has too many bathrooms, a combi boiler may not be able to provide on-demand hot water to all of them simultaneously
Looking for combi boiler installation?

Points to consider when getting a combi boiler installed 

Do you have a 22mm gas supply?

All modern combi boilers require a 22mm gas supply. To determine whether you already have this, you will need to look at the gas supply pipe used for your current boiler. This is usually below the boiler to the middle, look for a yellow tap to identify where this is.

Gas Pipes

Where will your condensate pipe go?

Combi boilers need to emit condensate into an internal water waste pipe or an external drain or soakaway

Plume kit

This example shows a plume kit taking the flue away from an opening or a window, as per building regulations.

Flue position

Building regulations and Gas Safe requirements state that all flues need to be 300mm to 600mm away from a window opening

Popular gas combi boilers

Viessmann Vitodens 0-50-W Gas Combi Boiler


Ariston Clas ONE Gas Combi Boiler

Ariston Clas One Boiler

Worcester Bosch Greenstar 8000 Gas Combi Boiler

Worcester Bosch Greenstar 8000 Life Boiler
Looking for gas combi boiler installation?

Popular oil combi boilers

Worcester Bosch Greenstar Heatslave II Oil Combi Boiler

Worcester Greenstar Heatslave II Combi

Navien LCB700 Blue Flame Oil Boiler

Navien LCB700
Looking for oil combi boiler installation?

Popular LPG combi boilers

Viessmann Vitodens 200-W Gas Combi Boiler

Viessmann Vitodens 200-W

Worcester Bosch Greenstar CDi Compact Gas Combi Boiler

Worcester Bosch Greenstar CDi Compact Combi
Looking for LPG combi boiler installation?