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How much does a replacement combination boiler cost?

The most common question I get asked as a plumber and gas engineer is about cost.

It's human nature to want to know how much you're going to spend. My aim here is to outline the main factors when considering a new combination boiler. Please keep in mind that the prices below are there to give you a rough idea of the cost of materials and do not include labour costs.


a) The make of the boiler

I put boiler manufacturers into three broad categories:

  1. budget boiler brands i.e. Heatline, Ravenheat, Ferroli
  2. mid-range boiler brands i.e. Baxi, Glow Worm, Ideal
  3. top-end boiler brands i.e. Worcester Bosch, Vaillant

Of course, the above categorisation is very much a generalisation. I am not here making any judgement on quality. It's much like car brands I suppose. For example a budget brand could be Dacia, mid-range might be Ford, and top-end might be Mercedes. It does not mean that just because one is less expensive than the other that it is any less reliable or doesn't do the job that is required of it. So cost wise you can get a Heatline combi boiler for less than £500, Baxi combi boilers start at around £600, and Worcester combi's start at around £900.

b) The size (kw rating) of the boiler

The size of your boiler in terms of KW rating is a really important factor. This will be determined by two factors:

  1. size of your property (the number and size of the radiators you have)
  2. the mains water supply and hot water demand

This second factor is usually the most important. The greater your hot water demand the greater your boiler KW rating should be. You will get a better flow from your mixer shower and a quicker filling bath with a higher rated boiler. A word of warning here though. Because combi boilers rely on heating your incoming cold mains water supply, your hot water flow from your boiler will  be governed by this. So if your cold main only supplies 10 litres per minute of cold water to your boiler you won't get any more than 10 litres per minute of hot water out of your boiler. A typical 40KW combi boiler can provide over 16 litres per minute of hot water but would really be wasted if your incoming mains was only 10 litres per minute. It is really important that the size of the boiler you get is suitable for you and your property.

c) The flue

The cost of the flue (chimney) will depend on where your boiler is located and where the flue terminates. If your flue is a standard horizontal one that goes straight through the wall behind (or to the side) of the boiler then your looking at between £50 and £100 (dependant on make). If your boiler is further away from the outside wall then you are going to need extension pieces and possibly elbows, offsets and brackets. All of which add to costs.

If your flue needs to terminate vertically i.e. through the roof, then this is most likely going to increase the cost (not only for the flue itself but also as the installer will need to access the roof).

Because condensing boilers now plume (looks like steam coming from the flue), a 'plume kit' may also need to be fitted to direct this 'plume' away from windows, passage ways or even a neighbour's boundary.

If your flue is less than 2 meters above ground level or is a place where it could potentially get blocked or damaged the a terminal guard will need to be fitted. 

d) Gas pipework

Gas pipework to a new combi boiler will need to be a minimum of 22mm, and may even need to be 28mm depending on the KW rating of your boiler and how far it is from the gas meter. Keep in mind that many old boilers were installed using only 15mm pipe which will need upgrading, and could add quite substantially to the installation cost.

e) Condensate pipework

New boilers produce condensation which needs to be disposed of. This is done be running plastic pipe from your boiler to a drain (usually and preferably to a waste pipe inside your property i.e. to your kitchen sink waste pipe). Sometimes access to the waste may not be possible. For example if your boiler is located on the opposite side of the house to the kitchen or bathroom then a drain may not be easily accessible. In these instances a soakway may need to be installed, or sometimes a pump may be needed to pump the condensate away. Again, if necessary, these are going to add to your installation cost.

f) Cleaning the central heating system

As a minimum your system will need a chemical cleaner added and a good flushing through. This is a minimum requirement of all new boiler installations and should already be part of your quote. However, if you have an old system that has never been treated with inhibitor and your radiators have been slow to heat up or have coldspots, then you could heave problems with sludge build up. If this is the case then a powerflush may be necessary to thoroughly clean the system and prevent future problems. If done properly a powerflush can take up to a day and add upwards of £250 (depending on the size of your system) to your final bill.

As an addition, a good quality central heating filter should also be fitted to maintain the water quality in your system. These retail at around £100.

g) System controls

If your radiators don't have Thermostatic Radiator Valves (TRV's) already fitted then these will need adding and cost between £10 and £20 each.

You will also need a room thermostat and separate programmer, or alternatively a programmable thermostat. Consider also if you want to connect your heating to wifi so you can control it remotely on your mobile phone. Either way you are looking at between £50 and £200.


What I have tried to do here is help you appreciate all the different  elements that need to be taken into account when considering a new combi boiler installation. The above factors need to be considered when comparing quotes. Remember, labour costs will need to be factored in. This information will hopefully assist you in making an informed choice.

Factors to consider when replacing your boiler

When you get a quote these are all things that the installer will take into account and advise you on. This is just to give you a 'heads-up' so you are prepared before a site visit.


  • How much do you want to spend on the boiler itself? How long do you want the warranty to be? Does the boiler brand have a good reputation?


  • What size (KW rating) boiler is suitable for the size of your property and your needs?


  • Will I be able to have my new boiler in the same location as the old one? Do I need a boiler that is small enough to fit in a kitchen cupboard? Do I want my boiler in a completely new location (i.e. loft or airing cupboard)?


  • Will the flue run and terminate in a location that is in line with manufacturer's instructions? Will a plume kit need installing? Will a terminal guard be needed?


  • Is the existing gas pipe big enough to supply the new boiler?


  • Is there an accessible drain in close proximity to the boiler?


  • Are my existing radiators efficient and in good condition? Are my radiators fitted with Thermostatic Radiator Valves (TRV')?


  • Have your radiators been slow to heat up or have cold spots?


  • Do I need new a new programmer/room thermostat fitting? Do I want to upgrade to a smart control?

Does my new boiler meet the warranty requirements?

In recent years boiler manufacturer's have competed by offering longer and longer warranties.

Some manufacturer's now offer 12 year warranties on certain boiler models. This is pretty incredible when you consider that a boiler is something that is in use more or less every day of the year, and during cold snaps can be on continuously for log stretches of time.

Most of us never read the terms and conditions when it comes to warranties (not just boilers!). It is not as simple as just getting your boiler installed and then expecting the manufacturer to honour the warranty.

If you are getting a new boiler then you need to make sure the installation meets the requirements below (as a minimum).

  1. The installation MUST be registered with Gas Safe.
  2. The boiler MUST be registered with the manufacturer (usually within 30 days of installation).
  3. The Benchmark commissioning checklist (found at the back of the installation manual) MUST be properly filled in at the time the boiler was installed.
  4. The boiler MUST be installed according to manufacturer's instructions. This includes factors like clearances and accessibility, but also the requirement that the boiler is fitted on a central heating system that has been properly cleaned with inhibitor added.
  5. The boiler MUST be serviced by a Gas Safe engineer (in line with manufacturer's instructions) every year. You will need proof of this (there is a service record part for the service engineer to fill in the Benchmark section).

If your  installation does not meet the criteria above then the manufacturer could very well refuse to honour the warranty.

What are advantages and disadvantages of a combi boiler?


  • saves space as there is no need for storage tanks or hot water cylinders.
  • no long wait times to heat up hot water.
  • continuous/endless supply of hot water.
  • lower energy costs - only heats the amount of water that is needed.
  • lower maintenance costs - the integral heating and hot water functions are covered by the boiler manufacturer's warranty (i.e. no separate pump or diverter valve).


  • do not work well in properties with poor flow rates and/or poor incoming water pressure.
  • Hot water flow rates are reduced when providing hot water to two or more outlets simultaneously. Therefore may not be the best option for larger houses with more than one bathroom where there is a high demand for hot water.
  • no hot water back-up i.e. if boiler fails to work there is no immersion heater as an alternative heat source for hot water.

Does my central heating system need power flushing?

Signs that your central heating system may need a powerflush

  • radiators seem to take ages to warm up.
  • the water in your central heating system is dirty (black/brown colour).
  • radiators don't fully and evenly warm up (have cold spots {usually at the bottom}).
  • repeated failure of system components like pump and diverter valves.
  • boiler knocks and bangs (especially on start up).

Any of the symptoms above could indicate that your central heating system has circulation and flow problems due to the formation of rust, sludge and scale deposits.

What is a power flush?

A power flush machine is connected to your central heating pipework and a chemical cleaner is added. The water is then pumped around your system to loosen and mobilise harmful corrosion deposits like rust, magnetite and scale. Once loosened, the unwanted debris and cleaning chemicals are purged from the system with clean water. At the end of the process your system is filled with clean water and central heating inhibitor which will help reduce future corrosion.