50/70 Oil boiler

We are converting a run down ( large) cottage - 3/4 bedrooms and hopefully a large open plan kitchen- which we want to put UFH into- this is a open vaulted ceiling with original oak beams which we have uncovered Oil is only option ( have tank etc etc. and space for one- and it previously had one.
Had the offer of a Worcester Bosch condenser.. but it’s 50/70 KW and all the internet searches say I need a 35/43 boiler.
what’s people views - should I take the bigger one - is it going to be a problem/ eat the oil ?? We have two log burners also as room heaters.
What would you do ?! 


  • When you say offer of a 50/70, is it a second hand one? It's not a massive issue to fit a bigger boiler than necessary, you just generally wouldn't as bigger boilers are more expensive to buy in the first place. It can be slightly less efficient as an oversized one can cycle more, but conversely you can potentially run it at a lower temperature so it condenses more and.can be more efficient
    A bigger boiler will also heat the house up quicker and you can increase the pump speed too. So you can turn the heating on later than you would.

  • QrizB
    QrizB Posts: 13,663
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    I'm going to disagree with the previous poster.
    If you need a 35/43 (43kW) but fit a 50/70 (70kW) you're going to be hugely overpowered.
    It can be slightly less efficient as an oversized one can cycle more
    Quite a lot less efficient, I'd suggest.
    ... but conversely you can potentially run it at a lower temperature
    That really doesn't follow.
    A bigger boiler will also heat the house up quicker
    Nor does that. The rate at which your house heats will be limited by the size of your radiators.
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  • Well the rate at which your house heats up is controlled by the size of your radiators and how hot they are.  A big boiler can make them very hot very quickly but then it's liable to turn itself off for a bit as the return water temperature gets too high.

    So you may find the boiler stays on for very short periods and then off until it has cooled down a bit.  This isn't good for the boiler and it isn't good for efficiency and thus running cost.  
  • Many thanks - the boiler is a new one - ordered by a company that the customer didn’t want . Does the under floor heating effect the calculations ??
    would you buy it - it retails at £3300- wants £1750 ? - still in box - with flue - Help
  • lohr500
    lohr500 Posts: 925
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    It sounds way over the top for the size of your property.

    To put it into context, we live in an old 5 bedroom, three bathroom converted farm house in Yorkshire spread over three floors with 17 conventional radiators. 9 of those radiators are large double panelled types and the house isn't the best when it comes to insulation. We also use a lot of hot water.

    Our oil boiler is a 2 1/2 year old Grant condensing 26/36 conventional model, jetted to run at 30kW..

    It is well on top of the job at 30kW.

    So a 50/70 sized boiler sounds overkill to me based on the description of your property.
    As others have commented, such an oversized boiler is likely to result in it cycling on an of very often as the boiler output will exceed the rate at which the UFH/radiators can dissipate heat into the rooms.

    It may be a cheap buy in the short term, but highly likely to cost you more on the running costs because it won't be running efficiently.  
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