Traditional central heating systems have a hot water storage tank (in an airing cupboard) and a feed and expansion tank (located in the loft). Having a hot water storage ability allows these systems to deliver large amounts of hot water at shorttraditional boiler layout notice. The hot water capacity is only limited by the size of the hot water tank and the ability of the boiler to renew the hot water when depleted.

The additional components and hot water storage also contribute to reduced energy efficiency, higher installation costs and a more complex system. You need more space if you have a traditional central heating system and the additional system components are all potential problems at some point in the future.

To counteract these deficiencies the combination or combi boiler was developed. This type of boiler heats hot water on demand whenever the hot water tap is turned on. This means that the supply of hot water is not limited and there are far fewer components in the central heating system.

combi layoutFewer components and no hot water storage all contribute to a lower cost installation with a better energy efficiency, resulting in lower ongoing heating costs. Fewer components mean that there are fewer parts which can break down at some inconvenient moment.

One disadvantage is that the hot water flow rate of a combi boiler will not match that of a conventional boiler. Water has to be heated on demand and hence it may take longer to run a bath although a shower will be a pleasant surprise as the hot water is delivered at mains pressure giving a high power spray. Heating water on demand has another downside in that there is a limited capacity for hot water production at any one moment. This is okay in houses with smaller families or single person occupancy but it can be a problem with larger houses and larger families where several hot taps may be turned on at the same moment and the demand for hot water is greater than the hot water production capability of the combi boiler.

Another issue to consider is the temperature of the hot water. This can be influenced by the temperature of the incoming cold water and the flow rate. As a result it is essential that the power of a new boiler is matched to your heating needs.

Some advanced combi boilers incorporate a small hot water cylinder. Installation is still easy with reduced components but the boiler stores some hot water to enable it to meet initial demands for hot water.

If you are considering getting a new boiler but do not know which type would be rightfully you there is the opportunity to get expert advice from the main boiler installation companies. They will insist upon sending ground one of their heating advisers to do an audit of your property and get their foot in the door so that they can give you a “no obligation” quotation. You may feel that this is a price worth paying, spending an hour of your evening with the heating adviser, as the end output will be a firm quotation based upon your requirements and knowledge of which type of boiler and what size is best suited to your home.

80 Responses to “Boiler Systems”

  • Mr Mandip Singh:

    Hi dear sir or madem i want to fit a new combi boiler . so if u can tell me how much is gona cost me .

  • British Gas Homecare:

    When quoting for a new boiler there are several items which need to be taken into consideration, such as, the heating and hot water requirements of the system, the positioning of the boiler and so on, all of which affect the type of boiler that would be most suitable for your property.

    As well as this, one also has to consider the cost of the installation which can vary significantly from one house to another.

    For these reasons, it is not possible to provide a costing without having a Heating Sales Adviser visit to assess your requirements. When suppliers provide a quick quotation online they often need to revise the figures quoted when the installer visits the property and the installation facts are established.

    Should you wish to read more about the factors which need to be taken into account in the selection of the right boiler for your home please visit our boilers site at

  • Mr J Bailey:

    Hi there,
    I have had a new vaillant boiler fitted, what paperwork should I have been left with?


  • British Gas Homecare:

    Can you please expand upon your question to help us to provide a satisfactory answer.

    Are you referring to sales paperwork, installation and user instructions or to energy efficiency / boiler type documentation.

    Could you also please advise who installed the boiler, was it British Gas ?

  • ade:

    why does the boiler system keeps cutting off each time the hot water tap is turned on.

  • British Gas Homecare:

    Diagnosing an issue such as this over the Internet is very difficult as there coule be several reasons for your problem.

    If you have previously taken out one of British Gas’s Homecare agreements We recommend you request a visit from a British Gas engineer who should be able to identify the cause and adjust / repair your system.

    If you do not have an agreement British Gas do offer an ‘on demand / emergency repair service. Telephone now on 0808 178 2098 for a Quotation. No matter how big the problem the price quoted includes all parts, labour and call out charges.

  • Mr C Sidwell:

    Do you use Brisih gas Corgi qualified installers and engineer’s to carry out any works if I upgrade?

  • British Gas Homecare:

    We promote British Gas Central Heating systems. All British Gas Boiler Installation and Servicing work is carried out by British Gas Engineers who have been trained to the highest standards at the British Gas Engineering Academy.

    We trust that this gives you confidence in any work carried out on your heating system. In addition British Gas will send a trained heating advisor to your property to complete a free, no-obligation quotation for any work required so that you can be confident that you have an accurate quotation that will not change once the engineers arrive to install your system.

  • alan white:

    I had a leaflet delivered by the post office from you–it came today offering £250 off a new boiler and free installation—Today is the 1st May–your offer expired 27th April 2007 ?? Offer No AR02

    Was this a gimmick ?

  • British Gas Homecare:

    No – no gimick at all and you will be pleased to learn that the offer of up to £934 savings has been extended to the end of May.

  • k owen:

    i stayed in for an appointment between 12 6pm no one turned up lol not too impressed can you give me another time slot

  • British Gas Homecare:

    We are sorry to hear that and have passed your comment to British Gas online enquiries department for them to email you with a response and arrange another appointment.

  • becks:

    i have a potterton puma 80e…about 9 yrs old, had 1 of the boards changed 2 months ago. now another problem, hot water working ok, if heating is off, heating not working at all, if i try to use water when heating is on timer(even if it’s off)boiler goes onto lock out within minutes. reset does nothing,have been told it’s a switch i need, not sure which, or how much, been quoted £60 for one part and £120 for another without fitting or vat, is this something i should consider or has my boiler had it??

  • British Gas Homecare:

    Reading your comments it sounds that the system is starting to have electrical problems and could start to incur ongoing costs to rectify the situation. Without seeing the system it is very difficult to advise whether the repair will fix your issues and you will have years of trouble free operation or whether there are other components that will soon start to fail. This is the same situation as with an older car … at what point do the maintenance costs rise and purchase of a replacement becomes cost effective / a necessity.

    Boilers tend to have a 15 plus year life so this is rather early to have problems but the 15 years is an average figure. You also need to take into account the operating efficiency of the boiler. Modern boilers are very efficient and installing a new boiler can lead to lower heating bills year on year as less gas is consumed.

    We suggest you contact Baxi Potterton direct to see if they are able to give you a better view.

  • j. thomas:

    i want to install gas and put in a central heating system. Neighbouring properties have gas but mine doesn’t, how much could i expect to pay?

  • British Gas Homecare:

    British Gas’s boiler installation service requires a gas supply to be available at the property and the installation engineers do not connect properties to the gas mains. We therefore are not aware of the current charge for gas connection.

    The responsibility for this is with Transco and they will be able to advise what their costs would be to connect your property if you contact them direct on 0845 6056677.

  • D. Ridley:

    Is it more cost effective to keep the hot water on constant or set to timer to come on/off as required?

  • British Gas Homecare:

    We assume you have a system with hot water stored in a hot water cylinder.

    For best energy efficiency you should: –
    – Ensure that the cylinder has the recommended insulation to prevent heat loss
    – Set the boiler hot water thermostat to the recommended level
    – Set the central heating room thermostat to the recommended levels to save energy
    – Set the timer to heat water at times to match your family’s hot water usage requirements
    – take showers in preference to baths as they use less hot water

  • Michael O'Neill:

    I am looking to expend my existing central heating system.
    I want to add two radiators to a conservatory (recently built) and one radiator to the lounge.
    I need advice as to whether my existing boiler( Ariston Micro-Combi 23MFFI/27MFFI) would be upto the job or whether I would need to replace it.

    Any advice or comments would be appreciated.

  • British Gas Homecare:

    Before a boiler is selected and installed its power output should be sized taking into account the heat losses for the house. If, at that time, you advised the heating assessor that you were considering extending the property and fitting additional radiators they may have increased the boiler power. If you did not the boiler output would have been sized to your property and heating needs at the time, boilers which are over rated for the property will be inefficient and this will increase running costs.

    Without knowing how much capacity is being taken up by the existing system it is impossible to say whether the new radiators will fit in with the current boiler or whether you need to upgrade the boiler. As a general rule though, if the boiler was installed a few years ago it would have been sized to work with the existing heat losses – so adding 3 radiators would generally mean a bigger boiler is required.

    This is a difficult question that customers often meet and British Gas are able to send a heating advisor to your home to review the installation options, work out the correct heat loss for new boiler, and help to select the right boiler for your home if an upgrade is required. This visit is free with no-obligations on your part and will also enable an accurate quotation to be developed for a new boiler.

  • j. brown:

    hi our boiler is a little bit old so i just wondered how much it would cost for a new one?

  • British Gas Homecare:

    There are many options available in selecting a new boiler that prevent a ‘quick price quote’ being given: –
    – what type, regular, system or combi
    – what power rating ( determined by your homes heat loss calculation)
    – where is it to be installed

    Often the options appear complex. To ensure that you select the right boiler British Gas are able to send a heating advisor to your home to review the installation options . This visit is free with no-obligations on your part and will also enable an accurate quotation to be developed for a new boiler

  • Panos Stam:

    I have got a combi boiler and it started leaking water around the attachment site of the air vent. Do you have any ideas regarding the nature of the problem and how can I solve it?


  • British Gas Homecare:

    It is very difficult to diagnose faults remotely. Obviously this could be a boiler leak. Alternatively, you mention that the leak is from the vent and it could be condensate from the exhaust gases particularly as the weather has been colder recently.

    In any circumstance the system is not working as installed and you should ask a trained plumber to investigate. Make sure that they do not charge you a large call out fee.
    If you have previously taken out one of British Gas’s Homecare agreements we recommend you request a visit from a British Gas engineer who should be able to identify the cause and adjust / repair your system.

    If you do not have an agreement, British Gas do offer an ‘on demand / emergency repair service. Telephone now on 0808 178 2098 for a Quotation. No matter how big the problem the price quoted includes all parts, labour and call out charges.

  • M Stevens:

    My house is a mid-terrace,ex-local authority property. The hot water/heating is part of a communal bolier system. I would like to come off this system. I need advise on what sort of system would be approriate for me. Do you have experience in dealing with conversions of this type? Can I arrange to have someone visit and then give me a quote, please?

  • British Gas Homecare:

    British Gas have experience of converting the heating systems in ex-local authority properties. The new installation will require disconnection from the existing system and then a brand new boiler, cylinder and controls adding as it is assumed that you already have radiators. The only decision you need to make is whether to purchase a combi boiler or a regular / system boiler.

    There is a considerable amount of background information on our site at britishgas .boiler-systems .info which will enable you to read of the options available to you. In summary a combi boiler gives you instantaneous hot water and feeds the central heating through a pressurised system so there is no need for hot water tanks or a cylinder. A regular condensing boiler feeds the hot water through the cylinder and your radiators through a fully pumped system and is able to deliver a greater hot water flow through several taps simultaneously.

  • A Taylor:

    I have recently had a new combi-boiler installed, the work was sub-contracted out to a local firm even though I was told it would be done by British gas engineers. The work carried out is of a poor standard and the new 22mm copper pipe work to the boiler has been installed outside my house in total exposure to the elements with no protective tubing. The location of the pipe (across a step) makes damage inevitable. A local heating engineer also commmented on the unsafe and poor practice of the installation. Please can you give me advice about external copper gas piping and whether its acceptable to be left exposed?

  • British Gas Homecare:

    Reading your comments it can be seen that you are not happy with the installation you have received and British Gas need to give you satisfaction that you have a safe installation that has been installed to their standards rather than respond just to the question on the exposed pipe work.

    Contractors are sometimes used to install systems but this should not matter to the customer as the system is being supplied and guaranteed by British Gas. The workmanship should be to British Gas standards and should also be inspected post installation by the quality process.

    We cannot answer fully as this site is an affiliate of British Gas and works with them to promote the sale of their systems, although we do have access to British Gas technical experts to respond to customer questions.

    The first advice we would give you is to not sign the installation acceptance form and to not settle the remainder of the bill until British Gas have reviewed your complaint and given a full and satisfactory response. If you have already accepted the work and paid the bill we would still recommend that you contact British Gas and ask that their local installation manager visits your property to inspect the work to either confirm that it is properly installed or to make arrangements to remedy any poor work.

    To initiate your complaint we suggest you telephone British Gas Installation complaints on 0845 600 0352

    We hope that you receive a satisfactory response and that any issues are quickly remedied.

  • Rob Waldron:

    I have recently bought this property and it has a SIME super 102 combi-boiler installed. Are you able to supply me with a handbook/manual for this?

  • British Gas Homecare:

    We are sorry but we do not hold a stock of manuals to supply to visitors. However we suggest you contact the manufacturer direct and they should be able to assist you.

    SIME’s details are: –

    SIME Ltd.
    Unit D2 Enterprise Way, Bradford Road
    Bradford, West Yorkshire, BD10 8EW

    Phone : 08709911114
    Fax : 08709911115
    web www sime ltd uk

  • baxi bermuda 45/4E:

    i have just moved into my new house and it has the boiler that i have mention above.but i can not get the pilot light to come on so i can not get any hot water.can you give me any advise on how to get it lit thank you A waiter gloucester ITS THE BAXI BERMUDA 45/4E

  • British Gas Homecare:

    If having consulted the manual you are unable to ignite the pilot light then there could be several reasons why the pilot will not remain on, all of which are hard to diagnose via the Internet.

    We suggest you call british Gas Emergency line on 0845 026 2438 to get assistance, when calling you will be provided with a fixed price quotation.

  • peter walton:

    Hello, we have an oil fired harmony coachman boiler (thermical) we are experiencing a lot of water hammer and kettle type noises in the piping system. the noises are loud, several large bangs followed by whooshing sound?
    The vent line to the expansion tank is HOT. when the boiler system bangs the vent line releases water into the expansion tank. this hot water is then replaced from the expansion tank. the expansion tank then tops up with new cold water. this cycle continues every 20 mins? obviously untreated water is entering the system eveytime it tops up – what can be done to fix the problem – please help.

  • donal mac manus:


    I wonder can you help.

    I have just moved into a new flat that I have bought. There is a boiler but therer does not seem to be ANY timer even thought I have searched everywhere for this.

    This is a new build and I seem to have continuous hotwater.
    Is this normal? Will I be be paying for this?

    How do I get a timer?

  • British Gas Homecare:

    A central heating timer is an important component for controlling the energy usage of your boiler and a missing timer will lead to higher operating costs. We are assuming that the boiler is in your property and serves only your property – Have you confirmed that you are not on a community heating scheme with hot water provided to a whole area from a central boiler?

    A timer can be purchased from central heating suppliers but it will need to be fitted by a competent engineer. If you wish British Gas to provide a quotation please telephone 0808 178 2098

  • Andy Wright:

    I have bought a new combi boiler and radiator pack from a large retail store, I can fit all of the radiators and pipe work but would like British gas to fit the boiler, is this possible and who do I contact to arrange this?

  • British Gas Homecare:

    British Gas supply and fit, they do not install only or supply only.

  • Currie:

    Hello, is it true that I would need a new flue system/pipe installed because I am changing from a combi boiler to a condensing boiler? I understand why new heat exchanger materials are required to avoid corrosion. However I don’t understand why this would apply to the flue as the flue gases are similar in both systems (but cooler and lower in nitrogen oxides in a condenser boiler).

  • British Gas Homecare:

    Each boiler has its own individual flue with works the same in principal but is designed in shape/diameter differently. This is part of the overall efficiency/ safety design of the boiler. You will need to contact the manufacturer of you new boiler to see if they have something similar to your current flue pipe to cause the least amount of disruption. Please note that we did send you an email but the delivery failed as you had not supplied a valid email address.

  • Tom Goodman:

    A helpful information section, thanks.


    Glow worm ultima f40 boiler (about 7 years old) heat exchanger has failed and the leak has corroded the internals badly.
    A replacement is called for. (The cost of replacing the heat exchanger etc is £500 or more) The system has a large traditional tank indirect cylinder as shown in your illustration. One bedroom flat, double glazed.

    Boiler is mounted on external wall.

    What modern condensing boiler would you recommend to replace the existing and interface with the existing system and controls.

    Installation is in a small store room with no size constraints.

    What would be typical cost for the boiler excluding labour and sundry parts but VAT inclusive ?

  • British Gas Homecare:

    The Worcester 18ri would fit in perfectly, it is an 18kw boiler.

    The typical cost will be influenced by the installation requirements and hence it would be wrong to quote a price in this email as it could change when the facts are established.

    To establish the full installation requirements and ensure that this is the right boiler for your home British Gas are able to send a heating advisor to your house to review the installation options and review the boiler options with you. This visit is free with no-obligations on your part and will also enable an accurate quotation to be developed for a new boiler.

  • E Almond:

    My tank in the airing cupboard is leaking. Is this covered by my 3 star cover?

  • British Gas Homecare:

    3 star cover is an old term and the new product descriptions are Homecare 100,200,300 and 400. Homecare 100 does not cover ‘tanks’ but Homecare 200,300 and 400 do. If you check your latest statement it should advise which product you have.

  • miss pamela wilkinson:

    I have a worceter combi boiler which is five years old and is constantly clicking when fired up or the water is turned on. Can you service it and how much will it cost? I havent the confidence to trust local plumbers.
    Thanks,please respond!

  • British Gas Homecare:

    We are pleased to advise you that British Gas are able to service and repair Worcester Combi Boilers.

    There are two options available to you
    1. Use the Heating Repairs service – telephone 0808 178 2098 for a fixed price quote (we are unable to give a quote on the information you supplied as the price varies by region and there are discounts if you are a British Gas customer).

    2. Ask for the repair to be completed but take out a boiler service contract and the cost of the repair will be spread over the year. The telephone operator will be happy to discuss these options with you.

  • Amanda:

    I am about to exchange contracts on a house which has a Baxi Solo Boiler which was fitted in February 2005. My local plumber says that this type of boiler should not have been fitted after 1 April 2004 due to a change in the regulations- is that correct? It was fitted by a registered Corgi fitter. I will need to speak to the vendor asap if this is the case. Help please!

  • British Gas Homecare:

    We believe you are referring to the change in the Building regulations which did not start until 1st April 2005 – so your plumber is slightly wrong with his dates. Under these regulations all new boilers must be condensing boilers unless the installer is able to confirm that would be too difficult to install a condensing boiler in your home.

  • Oz:

    Ive just had an boiler installed. My Hot water turns on ok but when i switch on the CH i get an error message on my boiler which says ( No water in the primary circuit ) WHAT DOES THIS MEAN ????

  • British Gas Homecare:

    It means that there is a problem in your heating system.

    You should immediately call back the plumber that installed your boiler and ask them to review the installation, until you are happy you should also advise them that you are not making the final payment on the installation.

  • C Mcveigh:

    We have a worcester combi boiler installed with a horizontal flue and when it rains extremely heavy we get water running through the flue and out the bottom of the boiler. The boiler then fails to ignite until dried out. Is there anything we can have fitted to stop this happening?