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  • FIRST POST
    • Arabanna
    • By Arabanna 13th Mar 18, 6:48 PM
    • 11Posts
    • 3Thanks
    Arabanna
    Combi or Conventional boiler?
    • #1
    • 13th Mar 18, 6:48 PM
    Combi or Conventional boiler? 13th Mar 18 at 6:48 PM
    I currently have a conventional heating system with a very old floor mounted Potterton boiler which is inefficient, and hence I am going to change it and qualify for a free new one under the government scheme.


    Should I get another conventional boiler or swap to a combi?


    My house has more than one bathroom.
    I have a tank which is heated by solar tubes in the summer hence I get this hot water for free.
    I have plenty of room for the tanks.
    From my understanding, if I go for a combi I will need to change all my pipes.
    I would like to put underfloor heating (off the central heating) in the kitchen when I re-do it.


    Which is more cost effective/ better/ more efficient?
    Thank you.
Page 1
    • CashStrapped
    • By CashStrapped 13th Mar 18, 6:52 PM
    • 1,293 Posts
    • 725 Thanks
    CashStrapped
    • #2
    • 13th Mar 18, 6:52 PM
    • #2
    • 13th Mar 18, 6:52 PM
    If you have solar tubes, these are not compatible with a combi boiler. You would normally need a hot water tank with a heating coil that can be used with solar tubes.

    All modern boilers are condensing boilers, people often get confused by this fact.

    You have three main options when considering a boiler:

    Combi boiler - This is what a lot of people install these days. Often because it is easy for the installer, and it saves a bit of space. It allows you to have hot water and heating on demand and it should not run out. It is a pressurised system so hot water is at mains pressure. The flow of hot water is slightly restricted by how fast the combi can heat and maintain that heat. Everything is combined into one unit so they are more complex, hence more to go wrong. Designed for smaller properties where space is an issue. Often struggle if more than one outlet is used at the same time or there is more than one bathroom. Due to more work required to fit and remove old installation, can be more expensive to install.

    *you can get combis with a mini storage tank inside. This is designed for slightly larger properties and are designed to mitigate the short comings of a standard combi but avoid having to install a full sized tank.

    System boiler - A modern version of a traditional boiler. Slightly more complex than a heat only boiler. A system boiler requires a hot water tank. This is a pressurised system so does not require open vented tanks in the loft. Everything is pressurised and pumped. Hot water is at mains pressure! Should give a higher hot water flow than a combi as hot water is stored. More suitable for larger properties with higher demand, more than one bathroom. Boiler is not as complex as a combi. It just heats water, valves external to the boiler control what to heat; heating, hot water or both. May be more expensive to install as it requires the old system to be removed and a pressurised tank to be installed at further cost. You can get a tank with a secondary coil for solar heating.

    Heat only boiler: This is a direct replacement for a traditional boiler where you have an unpressurised copper tank and cold water storage in the loft. As this is not a presssurised system, hot water is usually at low pressure (as per any traditional system) determined by gravity (how high up the cold storage tanks are). Very simple boilers, therefore should be very reliable as they just heat the water (few complex internals). Valves external to the boiler control what it being heated. Heating, hot water or both. Probably the cheapest option in terms of replacing an existing traditional boiler.

    ----

    Based on your existing set-up, I would expect a modern System boiler or Heat only boiler fits the requirements you have.If your existing set-up is the latter, it may just require a straight swap with a modern heat only boiler.

    -----

    A few questions:

    What is your current annual KWH use? What is the exact model number of your old Potterton? Does the grant cover the whole system? Are you having to pay anything towards it? Is your current system pressurised or does it have cold water storage tanks in the loft?

    One thing to consider is that older boilers, while more inefficient can be a lot more reliable. So what you may end up saving in efficiency you may end up spending on repairs with a new boiler, once the warrantee runs out.
    Last edited by CashStrapped; 13-03-2018 at 7:28 PM.
    • jk0
    • By jk0 13th Mar 18, 6:59 PM
    • 2,363 Posts
    • 25,131 Thanks
    jk0
    • #3
    • 13th Mar 18, 6:59 PM
    • #3
    • 13th Mar 18, 6:59 PM
    My combi is a damn nuisance when the water pressure is low. It won't heat the water. I would prefer a cylinder and cold water tank.
    • C_Mababejive
    • By C_Mababejive 13th Mar 18, 7:31 PM
    • 10,501 Posts
    • 9,444 Thanks
    C_Mababejive
    • #4
    • 13th Mar 18, 7:31 PM
    • #4
    • 13th Mar 18, 7:31 PM
    My combi is a damn nuisance when the water pressure is low. It won't heat the water. I would prefer a cylinder and cold water tank.
    Originally posted by jk0
    You can have a cylinder added to your system.
    Feudal Britain needs land reform. 70% of the land is "owned" by 1 % of the population and at least 50% is unregistered (inherited by landed gentry). Thats why your slave box costs so much..
    • Arabanna
    • By Arabanna 13th Mar 18, 7:51 PM
    • 11 Posts
    • 3 Thanks
    Arabanna
    • #5
    • 13th Mar 18, 7:51 PM
    Really helpful advice thank you
    • #5
    • 13th Mar 18, 7:51 PM
    What is your current annual KWH use?
    I have no idea as I only moved into the property in September.


    What is the exact model number of your old Potterton?
    It is a CF 150


    Does the grant cover the whole system? Are you having to pay anything towards it?
    The whole boiler is free. Not any pipework or new radiators.


    Is your current system pressurised or does it have cold water storage tanks in the loft?
    I have cold water storage in the loft and a tank in the airing cupboard upstairs.





    Will the pipes need replacing if I stick to the same system because they are old?


    So what would you do?
    • CashStrapped
    • By CashStrapped 13th Mar 18, 8:36 PM
    • 1,293 Posts
    • 725 Thanks
    CashStrapped
    • #6
    • 13th Mar 18, 8:36 PM
    • #6
    • 13th Mar 18, 8:36 PM
    The boiler you have has an indicated efficiency of 65%. While it is in-efficient by modern standards, it is probably a very reliable boiler. This is because it does not have a lot of the modern gubbins that make modern boilers less reliable.

    ----

    None of the pipework should need "replacing". The system will need a power-flush before the new boiler is installed. Any warrantee of a new boiler will normally stipulate this. But, the pipework should not need completely changing out.

    -----

    If your heating and hot water is fully pumped, and you have a small motorised valve to control this, there should not be any pipework changes required.

    If you do not have that, any pipework changes to fit a modern heat only boiler should be minimal anyway.

    ---

    In summary, in my view, you have two options.

    1) Stick with the current system. If it is not broke....and performs well, there should not be an issue.

    However, in your case, as you qualify for a free boiler replacement....

    2) Swap out the existing boiler for a modern heat only boiler. The boiler should be far more reliable than any modern combi, but may be less reliable than your old boiler. It will however be more efficient. You need to ensure that this new boiler is serviced every year to benefit from any warrantee.

    ---

    Option 3) is to replace the whole system with a pressurised system boiler. As this requires a pressurised tank, which is not covered, you may be expected to pay an extra 800 - 1000 on top for this set-up.

    ----

    Before you do this, you should get a handle on your energy use. Only by assessing this can you establish what you may save, and how efficient your current system is.

    If you moved in during september, you have a good few months of use you can assess. What was the gas meter reading when you moved in? What was the date the reading was taken? What is the meter reading today? From this you can work out your usage thus far. Your bills should also give an indication as to your annual use.

    You may save a lot more by just switching tariff. So doing this is useful.
    Last edited by CashStrapped; 13-03-2018 at 8:50 PM.
    • Arabanna
    • By Arabanna 13th Mar 18, 10:34 PM
    • 11 Posts
    • 3 Thanks
    Arabanna
    • #7
    • 13th Mar 18, 10:34 PM
    Combi or Conventional
    • #7
    • 13th Mar 18, 10:34 PM
    Thank you for your sound advice.


    My bills have been high but I have just had insulation put in the loft (there was only insulation from 1950s) and I know that I need to replace all the windows and doors as they are 5mm only and I have wind coming in through them and they let in water.


    Do you think I should stick with the old Potterton or risk having the new one putting in (reliability v efficiency)???!!! Should I store the Potterton in the garage in case the new one goes wrong - although the new ones on offer are Valliants and Worcester so they are good makes.


    Thanks for your patience!
    • CashStrapped
    • By CashStrapped 13th Mar 18, 10:48 PM
    • 1,293 Posts
    • 725 Thanks
    CashStrapped
    • #8
    • 13th Mar 18, 10:48 PM
    • #8
    • 13th Mar 18, 10:48 PM
    They won't be able to put the potterton back on once it is down. It is a non-condensing boiler, so I don't think anyone would legally be able to re-install it.

    When you say high use....it depends on your KWH use.

    ------

    Just to clarify, modern heat only boilers should still be very reliable. They are very simple, therefore less to go wrong. Especially compared to a modern combi.

    They are just less robust than your old potterton so to speak.

    As the boiler is paid for by a grand, I may well be tempted to. A new boiler should be covered by a 5-7 year warrantee anyway. Vaillant actually have a 10 year warranty if you use one of their approved installers!

    I should have said, heat only boilers are also called "Regular Boilers".

    But....

    Do some maths first. Work out what it could save you. Knowing your annual KWH use or estimated use will be super useful, so it is good to know. Your bill may already give you an indication. If it is not on your bill you can ask your supplier for information based on your address.

    Even working out what you have used since you moved in to know will be useful.
    Last edited by CashStrapped; 13-03-2018 at 11:33 PM.
    • Arabanna
    • By Arabanna 14th Mar 18, 9:44 AM
    • 11 Posts
    • 3 Thanks
    Arabanna
    • #9
    • 14th Mar 18, 9:44 AM
    Meter reading
    • #9
    • 14th Mar 18, 9:44 AM
    METER READINGS:
    28/08/17: 0081
    (I swapped supplier in October to the best deal)
    10/10/17: 0163
    07/03/18: 1013
    Total used in just over 6 months: 932 units

    If I am right, you multiply that by about 31.75 to get kWh= 29591 (my annual prediction from the energy company was 31112)


    Breakdown:
    Oct - Dec: used 239
    Dec - Jan 234
    Jan - Feb 177
    Feb - March 200

    Like I said, I had poor insulation, and the windows/ doors need attention.
    Seems like I should go for the new boiler as I have the chance.

    Last question, sorry to take your time, my radiators are all old. Some definitely need replacing as don't work properly/ are very rusty. Will it help with efficiency by replacing all the radiators?

    Thank you again for all your advice.
    • Hengus
    • By Hengus 14th Mar 18, 10:11 AM
    • 6,059 Posts
    • 3,735 Thanks
    Hengus
    Modern condensing boilers are designed to be at their most efficient when running at a low return temperature. It follows that some of the existing radiators in a home may in fact be too small as they were designed to be used with an old boiler with a 85C flow temperature.

    New boilers should never be added to an existing system without some form radiator/system cleaning. Over time, water and metal in radiators produce rust which settles in the bottom of each radiator. This sludge can be moved around the system by the pump and it can block the boiler. Blockages will invalidate the manufacturer's warranty.

    The new boiler also needs to be sized for your home. I have a new ATag boiler which was installed last April. It replaced a 24kW Ideal Classic. My installer and I had long discussions about the size of the boiler that was needed. For CH, a 18kW boiler would have been sufficient; however, our hot water heating requirements dictated a larger boiler as we have hot water priority unvented cylinder heating. The boiler output modulates in the range 5 to 23.8kWs. The radiators are sized so that when the house is up to temperature the radiators are barely warm to the touch. As I post, the boiler flow temperature is 47C with a return of 35C. Condensing occurs whenever the return temperature is below 54C. My graph shows that a boiler with a gross efficiency of 88% at a 54C return flow increases to 96% efficiency at a flow return temperature of 35C..

    My advice would be to talk to installers who understand CH systems and only those who are prepared to do comprehensive heat loss calculations. Sadly, a lot of installers just want to sell you a big boiler.
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