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Will a combi be ok?

Discussion in 'Plumbing and Central Heating' started by House7000, 28 Jun 2020.

  1. House7000

    House7000

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    Moved into a new house and currently have an ancient 50 year plus standard boiler with a tank on the landing and one in the loft.

    The pressure to the shower is pathetic as its gravity fed and all in all it needs replacing asap. My question is, will a combi be ok?

    House is currently a 3 bed with 9 radiators and 1 bathroom. Longer term plan we will likely add at least 1 extra shower/toilet and a downstairs extension which may add 2 or 3 radiators extra. Possible even longer term is a 4th bedroom but talking 10 years plus for that so less of an immediate concern.

    Have been recommended a 36kw and 40kw combi option which plumber has advised is more than what we need now but future proofing somewhat and will sort the pressure problem in the shower.

    Should I push for a system boiler given plans above or will that combi still be suffice?
     
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  3. bernardgreen

    bernardgreen

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    The 36--40kW is necessary to supply a decent flow of hot water to a shower, but to keep the house warm the heating load could be as low as 5kW.

    A combi boiler would have to be able to work efficiently at both 40kW and 5kW outputs. Not many can do that.

    If the combi boiler cannot modulate ( reduce its output ) low enough to match the heating load then the boiler has to cycle ON and OFF which adds to the wear and tear on the boiler and can mean temperatures in the house may not be constant.

    A smaller boiler with a hot water cylinder is often the better option for the customer.

    This page is old but still valid even though boilers have been improved.

    http://www.thnet.co.uk/thnet/BF/BFPages/overhist.htm

    With a cylinder you can have an electric immersion heater as a back up. Very useful when the boiler stops working and the service chap is not available.
     
  4. muggles

    muggles

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    As a very rough rule of thumb it's 1kW per radiator so your current heating system requires roughly 9kW maximum. As Bernard has said, a high power combi won't be terribly efficient on such a small system.

    First thing to do is check your incoming mains pressure and flow rate, as that will guide you on pressurised hot water options. I'd suggest looking at the Intergas Xclusive, which can happily run as either a combi or a system boiler, and has good modulation. You can therefore run it as a combi to suit your house as it is now, then it's a very simple changeover to make it a system boiler with an unvented cylinder later on as your hot water requirements increase.
     
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  5. House7000

    House7000

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    Thanks.
    In terms of not being as energy efficient what are we talking? Like an extra £50 a year in bills or more / less.
     
  6. Bernard.That is wrong...no debate,waffle,whatever,just wrong.
     
  7. bernardgreen

    bernardgreen

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    I accept that a 10kW electric shower provides enough flow and temperature of water to enable a person to comfortably wash themselves. Hence a gas boiler rated at 20kW would be enough for a good shower. ( maybe not in winter when the incoming water is colder )

    It is people who desire the luxurious experience of a hot drench or rainfall type of shower who would need considerable more than 10kW
     
  8. bernardgreen

    bernardgreen

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  10. Bernard.you are wrong...Quote as many kw calculations and theory as you like,you do not need 36kw gas combi to have a luxurious monsoon shower experience.
     
  11. bernardgreen

    bernardgreen

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    So why do these un-necessarily powerful boilers get sold and installed ?.
     
  12. Because people buy them.Why buy a mad fast car?.Some people seem to think of a shower as a sensory over load hypnotic cosmic event complete with coloured lights,music,body jets and a massive shower head....not a wash..hence as many kw as is freekin possible.
     
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  13. muggles

    muggles

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    Impossible to put a number on it as there are so many variables - size of house, insulation levels, draughtproofing, what temperature you want to be maintained, whether you're home all day or not, what the flow & return temps are etc etc.
     
  14. bernardgreen

    bernardgreen

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    That explains it, thanks.
     
  15. I am sure you would be happier with equations and delta T and thermodynamic laws,,but you seem to know everything about combis to start with so i will spare you..Shame I have been wasting my bleeding time for 20years fitting 24/28kw combi boilers(Sarcasm)
     
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