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General: Specifying boiler power

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  1.  
    • CommentAuthorRustychain
    • CommentTimeNov 13th 2007
     
    We are getting a new combi fitted in the next week or so. We live in a 50's three bedroom semi, with one bathroom and 11 radiators (due to an extension providing a kitchen, utility room and downstairs WC).

    If I have understood what I have read on this forum, under worked high power boilers are less efficient than fully worked less powerful boilers. Have I got that correct?

    The reason I ask is that I was going to spec a Vaillant ecoTec plus 837, due to the reasonable flow rate on the HW (needed for a shower) and the stainless steel heat exchange. But I wonder if this is overkill for my house and whether I could get away with a smaller model. The central heating outputs are as follows:
    ecoTEC plus 824 - 6.7 - 19.0kW
    ecoTEC plus 831 - 8.7 - 24.0kW
    ecoTEC plus 837 - 12.0 - 28.0kW

    Thanks

    Sean
    • CommentAuthortony
    • CommentTimeNov 13th 2007
     
    middle one is the one to go for Unless you do some insulating and draft-proofing.
    •  
      CommentAuthorfostertom
    • CommentTimeNov 13th 2007
     
    Posted By: Rustychainunder worked high power boilers are less efficient than fully worked less powerful boilers
    Generally correct - but don't combis get round this? - they have accumulators and stuff so they work flat out and then stop for a decent while, instead of cycling on/off all the time, as conventionals do under part load? Meanwhile with a bigger combi you've got the benefit of approaching-barely-adequate amounts of piping hot water, instead of the inefficiencies of hanging about waiting for the usual combi trickle. Am I wrong?
    • CommentAuthortony
    • CommentTimeNov 13th 2007
     
    Even the smallest combi above gives a good flow of hot water. But we should not be using excessive amounts of water any way.
    •  
      CommentAuthorfostertom
    • CommentTimeNov 13th 2007
     
    But is it true that big combis also perform well on low load, for reasons given?
    •  
      CommentAuthornigel
    • CommentTimeNov 13th 2007
     
    Most boilers now modulate which is why they show a range of outputs.

    It is important to size the boiler on the expected heat demand and make sure it is within the modulation range otherwise the boiler will cycle.

    The problem with combis and modern housing is that they are sized for DHW and therefore generally oversized for heat output.
    •  
      CommentAuthorfostertom
    • CommentTimeNov 13th 2007
     
    Right, so combis, despite all their buffer tanks and sophisticated controls, don't get round the inefficiencies of oversizing, better than ordinary modulating boilers?
    • CommentAuthorRustychain
    • CommentTimeNov 13th 2007
     
    Thanks for the responses.

    I've had the cavity walls filled on the existing house and the extension has 100mm dritherm slab cavity walls with 120 Kingspan (not as green as I would have liked, but as is often the case it came down to cost). The house should be fairly well insulated on that basis (oh yes, the loft of the main house is also fully insualted). There are negligible drafts.

    Tony - fair comment on the water usage. We had a Vokera in our last house which gave plenty of water (wish I could remember which model it was!). I would rather down spec the boiler to avoid the misses enoying her showers too much!!!

    As I want the boiler to be future proof (in case we convert the loft in the future and get another shower in the process), could I spec the larger boiler and put an in line restrictor at the current shower to reduce flow?

    A follow on query from the comments. With a lower power model I assume it will take longer for the radiators to reach full operating temp (and therefore longer to bring the house up to a comfortable temp?). If this is true, would using a higher power boiler to bring the house up to the desired temperature more quickly be more/less/the same in efficiency terms. My gut reaction is the same.
  9.  
    I believe you can 'range rates' the gas flow of your combi to suit your required central heating output

    the installation manual usely has a graph, with gas pressure/output

    though the plumbers I know never bother with it and laugh when I mention it

    Jim
    •  
      CommentAuthorfostertom
    • CommentTimeNov 13th 2007
     
    Posted By: Rustychainto avoid the misses enoying her showers too much!!!
    How many misses do you have?
  11.  
    Have you considered the possibility of adding solar hot water in the future? If you are considering "future proofing" perhaps that needs thinking about when choosing your boiler type. I believe you can combine solar with a combi but not sure how well it works and you still need to allow for a space for a tank somewhere.

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