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”

  • British Gas Homecare:

    Worcester Bosch do sell cowls ( for vertical flue pipes) and elbows for horizontal flues. As you do not mention the boiler type, and ventilation must be correct for safe operation of the boiler, we suggest you contact Worcester Bosch direct to ask what parts they sell to remedy this situation.

    Worcester Bosch technical help can be contacted on 08705 266241

    If afterwards you require British Gas to complete any work on your boiler you can telephone 0808 178 2098 for a fixed price quote.

  • Shetty:

    I have a Glow worm Compact 100e combi boiler. The hot water operation seems to have stopped. Just before, it used to work when tap was opened at the lower level in bath. can you please tell me the solution for the problem…guess need to set some pressure switches.


  • British Gas Homecare:

    Combi boilers can be impacted by the water pressure to your property although that can not be diagnosed online and requires a home visit. There can be other reasons but they would be a guess on the information provided. If you want British Gas to investigate the problem for you we suggest you call the emergency heating repairs telephone number 0808 178 2098 where you can receive a fixed price quotation for any work you request.

  • Linda:

    my vokera 24he maxin boiler has a leak. How much would it cost to have someone to come out to look at it

  • British Gas Homecare:

    The cost of a boiler repair varies by region and as we do not know your postcode we are unable to give a quote in this comment. British Gas do have a heating repair service which could help you and if you telephone 0808 178 2098 you will be given a fixed price quote for the repair.

    If you want any further information on the emergency heating service there are details at

  • William:

    I have a Vokera Linea28 combi-boiler. I have hot water but no central heating.
    The timer (lcd) clock is on but is not incrementing in time so is probably from a back-up battery (?).
    The override doesn’t work so I’m presuming it is an internal electrical fault ? the wall thermostat clicks when passing the line but the boiler does not ignite for the pumping through the radiators, do you have any idea what the problem may be ?

  • British Gas Homecare:

    Sorry to hear that you are having a problem.

    The wall thermostat clicks when the room temperature is low and gives permission to the boiler to fire if the timer / programmer is on. From your description it sounds that the programmer has stopped. This could be a simple fault such as the electrical supply is off or a more complex one such as the programmer has failed.

    British Gas have a heating repairs service which you can telephone to see how much a fix would cost. If you telephone you will be given a fixed price quote so you know the cost before anyone comes to visit. The number is 0808 178 2098

  • Mike Smith:

    How do I relight the pilot on a Glowworm Hideawy FF boiler??

  • British Gas Homecare:

    That exact boiler designation is not a current boiler we are familiar with. The manufacturer should be able to supply you with a manual with the instructions although often the pilot lighting is automatic.

    Look at the manufacturer web site at for a set of manuals which may be the one for your specific boiler otherwise telephone them on 01773 828100

  • Jane Follett:

    I live in a victorian terraced house and my neighbour had a new combi boiler fitted about 6 month ago, however, instead of it being fitted on the outer wall as before, it is attached to the party wall with my bedroom and all I can hear is the noise it makes when it is used or at night when it cools and hums all the time.

    Is there anything I can do about it? It is driving my mad and I will have to move if not.

  • British Gas Homecare:

    We are sorry to read of the problem you are having.

    You could try fitting sound deadening materials to you adjoining wall but that disrupts your home and does not tackle the source of the problem. We must ask, have you discussed the problem with your neighbour and are they aware of the problem? Once they are aware, and if they are friendly, you can then start to explore the feasibility of amending their boiler installation to reduce the noise level.

  • Mr Steven Baker:

    I have a Heatline combi condensor boiler which is about 14 months old. The pressure is reading 2.4 bar which should not be a problem due to the built in safety features.
    I would like to know how to re-ignite the pilot light as the boiler is not firing up when the hot water tap is on.
    There are no instructions in the manual to guide me.

  • British Gas Homecare:

    We are sorry but we do not maintain a library of technical manuals for each boiler system.

    If you telephone the technical helpline of the manufacturer of your boiler they should be able to answer your question.

  • Ayo:

    I have just bought a new house and I need to have a corgi engineer inspect the boiler and gas in the property. Does british gas offer this service and how much will it cost for a three bedroom terrace house?

  • British Gas Homecare:

    We are pleased to advise you that British Gas engineers can help you. The price varies by region and hence we cannot advise you of the exact cost in this email but if you would like to call 0808 178 2098 we will be pleased to provide you with a fixed price quote.

  • andrew:

    I have a combi boiler but when I turn on the hot taps, i get no hot water and the radiators do not heat up either when the heating is on. The bolier swicthes on and goes through the motions (as if it will heat the water) but nothing happens. What can I do?

  • British Gas Homecare:

    It is very difficult to diagnose central heating systems remotely, particularly on the limited information available.

    If you have taken out a British Gas Homecare service contract you should telephone the helpline number you have been provided with and an engineer will call to help remedy your problem. If you have not taken out any boiler service agreement British Gas have a heating repair service which could help. If you telephone 0808 178 2098 and explain your issue you will be provided with a no obligation fixed quote for fixing the problem.

  • S Godfrey:

    My neighbour has installed a boiler in his upstairs bedroom and the outlet is near my daughters bedroom window, I am worried about how close it is. Is there a legal distance it can be from a window.

  • British Gas Homecare:

    The minimum distance is 300mm.

  • Lou:

    We have a back boiler and it has started to make occasional big bang noises. We are due to have it serviced, but we were thinking about having it removed and replaced with a better boiler. Can you tell me approx how much this may cost at most and how do we arrange this.

  • British Gas Homecare:

    Advising you on cost is difficult as there are a number of outstanding questions we have, as such a quote could be very inaccurate.

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

  • Karen S:

    We curently have a non working combi boiler and to be honest it was never upto much in the water supply department. We want to convert back to a standard boiler, but we do not have room for an airing cupboard. Is it possible to have a hot water tank in the loft space? and as all radiators are insitu will these need to be replaced or will this fact reduce the cost of installation?

  • British Gas Homecare:

    You could have an unvented hot water cylinder fitted in the loft space as this is a sealed system and there are no need for tanks. It will also give you mains pressure hot water delivery to 2, 3 or 4 taps at any one time.

  • Karen S:

    We are currently building a granny flat onto the side of our 3 bed house, for my elderly mother. Can you advise of a combi boiler that would allow us to heat the extension with having to heat the main house? Many thanks!

  • Sevin Seydi:

    We installed a large Vaillant combi boiler two years ago. It works well and is adequate for the two of us Iin quite a large house). Now however two family members are moving in, with their own kitchen and bathroom. The plumber tells us that the capacity of the boiler is technically sufficient for the calls on it, but we might keep having a conflict if two hot taps are opened at the same time. He recommends a second small boiler for one of the kitchen + bathrooms. This sounds expensive, and there are problems in siting it. I thought of having an electric immersion heater for the new demand, but then I wondered if the existing boiler might not serve an indirect unvented cylinder to supply one kitchen + bathroom. No one seems to have heard of this being done. What do you say?

  • British Gas Homecare:

    A combi boiler cannot supply a cylinder or any kind including unvented, the flow rate will not cope with it. You should contact Valliant direct to see if the boiler model you have can cope with two draw offs – but we suspect the answer will be no. You could have a multi point water heater fitted to help with the extra bathroom/kitchen problem.

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

  • British Gas Homecare:

    We are sorry but you have supplied us with little information and we are unable to provide a better diagnosis via the Internet than the Gas Safe (was CORGI) registered engineer has provided through an on site visit.

    If you want British Gas to attend to the breakdown please call for a fixed price quotation.

  • Lisa Cooper:

    I ahev got a potterton Performa 28 combi bolier, we have had a corgi registered guy in who has changed the fan and the pcb but still no joy, this has been going for nearly 10 weeks with no heating or hot water, could you advise on what the problem might be?

  • c.lightowlers:

    My Baxi boiler is making a lot of noise, clicking, creaking and generally noisier than usual. The outside casing gets very hot and the hot water from the taps is much hotter than usual. Help please.

  • British Gas Homecare:

    If you have taken out an appropriate British Gas Homecare service contract you should telephone the helpline number you have been provided with and an engineer will call to help remedy your problem. If you have not taken out any boiler service agreement British Gas have a heating repair service which could help. If you telephone 0808 178 2098 and explain your issue you will be provided with a no obligation fixed quote for fixing the problem.