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Green Building Bible, Fourth Edition
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Products: What size boiler do I really need?

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  1.  
    • CommentAuthorWilliam
    • CommentTimeMar 14th 2007 edited
     
    My plumber tells me I need a 28kw boiler for my 100 square metre new build , 2 bedroomed detached hoose.
    Thing is, a few on-line calculators say I only need 14Kw. What am I missing , should I spend the extra £300 on capacity I don't need and waste energy too?

    PS: Ive got 280m of UFH pipe
    •  
      CommentAuthornigel
    • CommentTimeMar 14th 2007
     
    I would buy the smallest one you can get. I reckon you need under 10Kw if its built to current regs.
    If its a gas boiler most will modulate down to that sort of level.
    Plumber have a tendency to overspec so you never complain of being cold, they dont care if its less efficient.

    I suggest you find a smaller, modulating boiler.
    • CommentAuthorGuest
    • CommentTimeApr 18th 2007
     
    William if you have a SAP/NHER energy rating done on your new build house, you will get the information re approx size in Kw of your boiler as part of the printout. I must stress it needs to be done as an NHER assessment not just SAP. This takes into account the fabric heat losses and distribution heat losses based on your design, assuming you have an idea of the boiler efficiency. Heat gains from solar, cooking, metabolic (using standard occupancy for 2 bed house). Building/Planning control will not approve your build under the latest BR (6th April 2006) unless you have an energy rating done, so don't leave it too late.
    Regards Wessex Energy Services
  4.  
    Posted By: WilliamMy plumber tells me I need a 28kw boiler for my 100 square metre new build , 2 bedroomed detached hoose.
    Thing is, a few on-line calculators say I only need 14Kw. What am I missing , should I spend the extra £300 on capacity I don't need and waste energy too?


    Even 14kw sounds like a lot! I have a 109 year old house in Montreal that is not well insulated by any stretch of the imagination (though it is reasonably airtight) and at an outdoor temperature of -20C (i.e. vastly colder than the UK) the heatload with an interior temperature of 20.5C is only 14.6kw - and this house is 181 square metres. We build a new house that is over 325 square metres and it's heat load at -20C is of the order of 16kw. Even your on-line calculator figure of 14kw sounds very high - especially if it's a new build.

    Oversized boilers/heating systems become much less efficient than their rated steady-state figure.

    Paul.
  5.  
    Hi,

    I think you were specified a 28Kw unit as thats probably what the plumber usually fits and an gets from his supplier. I am being cynical because I keep finding this gap between what you actually need or want and what you are given. Its not a combi is it, they usually get specified as a big as you cen get. You need a boiler that meet your heating load (plus a margin). Find out what that load is an tell the plumbler boiler to suit. I'd be surprised if its anywhere near 28kw, more like 10-14Kw. Then look at arange of makers from the budget end upto the better like Viessmann and seek advice from others on the short list. I'm always suspicious of those who demand only one product but cannot come up with the rationale. You'll end up living with it for the next 10-20 yrs so its worth the effort.

    Cheers
    • CommentAuthorarthur
    • CommentTimeMay 1st 2007
     
    28Kw might make sense if its a combi. But whether a combi is a good idea in a 2 bedroom house is another story.
    • CommentAuthorMarkH
    • CommentTimeMay 2nd 2007
     
    Does using a thermal store for heating and hot water mean that the boiler can be smaller (kW) than a system where the boiler heats a "normal" HW tank and the central heating system directly? Would it depend on the size of thermal store and expected demand? If so it would seem to make sense to go for a larger thermal store to enable a smaller boiler to be used. Am I barking up the wrong tree? Any ideas?
  8.  
    Boiler size should be such that it is running continously at the design conditions of interior and exterior temperature. A thermal store may help in avoiding short-cycling, but it doesn't change the overall design capacity of the boiler. In my case, the design conditions are exterior temperature of -20C, interior temperature of 20.5C - this leads to a heat load of about 16 kw. The groundsource heatpump I use has a capacity of around 12kw so supplemental heating is required at the worst case design conditions - fortunately these worst case conditions are only hit around 2% of the time during the heating season. At an exterior temperature of a balmy -12C, the heatpump satisfies the load completely. The reason for this undersizing is to that it is not oversized for airconditions (all heatpumps in North America do both heating and air conditioning - it would be rather pointless if they didn't due to the hot summers).

    For combusion-based boilers, actual efficiency is very much related to duty cycle. Short cycles really hammer the efficiency and oversizing will give much higher fuel consumption than would be the case if the boiler was sized correctly in the first place. Of course, worst case conditions are not usually present so there may be a significant portion of the heating season where the boiler is effectively oversized. A thermal store could help mitigate this in allowing longer (but less frequenty) boiler firings. The thermal store won't change the overall worst-case load though.

    Hope this helps,

    Paul in Montreal.
    • CommentAuthorMarkH
    • CommentTimeMay 3rd 2007
     
    Paul in Montreal - thanks, I understand now. Just need to work out the boiler size now, so it's interesting to see your earlier comments on sizing. Will be interesting to see what different ratings I come up with.
    • CommentAuthorarthur
    • CommentTimeMay 8th 2007
     
    Combi boilers tend not to rated below about 24kW. That's likely to be at least double the capacity needed for central heating demands. Leaving aside questions of efficiency in the water heating side (i.e. is it better heating small amounts when you need them or heating a tank once or twice a day), that must result in a pretty significant efficiency loss on the central heating side.
    • CommentAuthorLizM
    • CommentTimeMay 8th 2007
     
    I thought condensing boilers were more efficient at part load than full load and that their efficiency curve peaked quite early 30 -40% peak load then tailed off slowly to about 95% load and dropped quite a bit over the last few percent of load?
    • CommentAuthorGuest
    • CommentTimeMay 11th 2007
     
    i have a detached 3 bedroom house 1955's , it has combined room cubic area of 282 and through using formula kw=area (cubic) /34 i get 8.29kw - however what would this be for a condensing boiler? ive been told i should get a 30kw condensing boiler?
    • CommentAuthorLizM
    • CommentTimeMay 11th 2007
     
    Guest - it doesn't matter what type of boiler you have - the 8.29 kW is the out put required. Therefore your boiler - condensing, non condensing, biofuel, electric panels etc. need to have an output of 8.29 kW.

    Again (as previous postings), it looks like you are being told to get a combination boiler which provides both heating and hot water. The high output required is for the instantaneous hot water, not the heating. Go back to whoever is "telling" you to get this boiler and ask what you are getting. If you don't understand, keep asking until you do! If you follow the rule of getting 3 quotes, you will probably get 3 different recommendations.

    For clarity, condensing boilers reclaim the heat that is normally lost through the flue gases and so are more effiecient than the non-condensing type. They can be combination boilers, described above, or system boilers which just provide heating directly and can be used to provide hot water via a cylinder.
    •  
      CommentAuthorJ..M
    • CommentTimeNov 15th 2007
     
    I recently purchased a 24Kw system boiler to service a 10Kw requirement. The reason for this was basically two fold:
    1. To cater for a up coming extension plan
    2. 24Kw was the smallest boiler I could find from that manufacturer with the features I wanted

    In fairness to all the stated rating of boilers is the maximum output. The majority if not all boilers can be adjusted to suit your individual requirements, mine is adjustable from 8.8Kw to 24 so in honesty it doesn't have to be like using a thermal lance to heat a cup of coffee ;)

    My car has a top speed rating of 135Mph ......... But I don't use all its speed capabilities traversing the Asda car park ;)

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