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Choosing the correct size of combi

Discussion in 'Plumbing and Central Heating' started by goosander, 20 Sep 2017.

  1. goosander

    goosander

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    Now that nearly all boilers can modulate their output over a wide range, is there any good reason not to go for a larger output model (within reason) in order to get better a better hot water flow rate?

    I am planning on replacing my old Ideal Mexico Slimline heat only boiler with a combi in the next couple of months. I am looking at the Intergas Eco RF range, but am not sure which size would be best.

    The 30 model is rated between 7.2 and 23.7KW for CH and can supply 10.5l/min of HW whereas the 36 model is rated between 7.6 and 27KW for CH and 13l/min for HW.

    I have no experience of combi boilers but would like enough HW flow to guarantee a similar standard of shower to what I get now - the current system is pumped and very good though the pump is very noisy.

    The property is a large 2 bed flat over 2 floors with 8 radiators, property is 1990 ish so insulated but not to modern standards and is located in chilly scotland! Currently there are two showers both pumped that deliver about 14 l/min flow, it is unlikely that both showers would be used simultaneously. Not measured the mains water pressure but seems fine, I get about 20 l/min flow of cold water from the kitchen tap.

    So would the 30 model be adequate for a good shower, or should I go for the 36 model? Is there any downside to having the larger boiler for heating - it appear to modulate down almost as low as the smaller model.
     
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  3. You want Dan Robinson for Intergas advice, but i'd say that a 23.7Kw isn't going to give you a very good shower. The boiler will modulate down to the amount of heat required, but best to get the heating requirements calculated first.
     
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  4. flameport

    flameport

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    Don't get a combi boiler then.

    Although all pumps will make some noise, some are quieter than others and your old one may be worse now than when new.

    What was the reason for getting a combi?
     
  5. goosander

    goosander

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    The 23.7 is the CH output, HW output is nearer to 30 Kw (hence the 30 model designation)
     
  6. goosander

    goosander

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    Partly the efficiency of HW on demand and partly being able to get rid of the noisy pump, also getting rid of HW tank would free up storage space. Would have though a decent combi would be OK for a single shower but as I said, I've never used one - surely better than an electric shower though?
     
  7. flameport

    flameport

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    That is a myth - most losses are in the pipework, losses from an insulated cylinder are insignificant (only a few pence a day).

    A combi is far better than an electric shower, but that's only because electric showers are the most feeble you can get.
    It will certainly do one reasonable shower, but not two.
    Another factor to consider is that for any given output temperature, combi flow rate will be lower in the winter, as the incoming water will be cooler.
     
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  9. Dan Robinson

    Dan Robinson

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    Any combination boiler will be fine for heating. If you're only using a single shower at a time, the 30 will be OK.

    Other than that Flameport has covered most things.
     
  10. bernardgreen

    bernardgreen

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    The technical department of a boiler manufacturer was asked this question on the 29th August, they have yet to reply.
    ~~~~~~~~~~~~~~~~~~
    How was actual effficiency of the [your combi] measured ? The large mass of aluminium heat exchanger has to be heated before any heat energy reaches the water. If the test was for example how much gas was required to raise the temperature of 100 litres of water 40 degs Celcius then two very different results are obtained.
    1) the 100 litres are drawn off in one continuous stream. ( heat exchanger heated only once )
    2) the 100 litres are drawn off in 20 separate 5 litre flows. ( the heat exchanger cools down while no water is flowing and then has to be reheated )
    Which method was used for the test and measurement of efficiency for [your boiler]
    Method 1) would give results that the manufacturer would like to see but method 2 is more typical of the operation of a combi gas boiler in a domestic situation.
    ~~~~~~~~~~~~~~~~~~

    Which ( combi or hot water cylinder ) is more efficient does depend on the way you use hot water. The quoted "efficiency" is not the same as performance. A boiler with a high "efficiency" calculated from laboratory test results may perform poorly in a real life installation

    The concept of the combi boiler was developed to provide gas fired heating and hot water in flats and very small houses where there was no space for a hot water cylinder and header tanks. They are frequently sold to people for whom a combi is not the best option.
     
  11. Dan Robinson

    Dan Robinson

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    Feck off Bernard. Jesus. Go find another drum to bang. Or contact trading standards and see how far you get.
     
  12. :mrgreen: You need to make an appointment with the manufactures technical department and expect to pay a fee.
     
  13. goosander

    goosander

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    OK thanks Dan (and FP).
     
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