Boiler Sizing - Large 3 Bed House

Discussion in 'Plumbers' Talk' started by SJR88, Sep 16, 2021.

  1. SJR88

    SJR88 New Member

    Hi All,

    Hoping to get some sound advice regarding combi boiler sizing for a large 3 Bed House.

    We are refurbishing the house and plan to strip out the old boiler and pipework from the loft to relocate to the kitchen / diner whilst also renewing all copper pipework and installing new radiators.

    We are also keen to future proof for a potential loft extension to come in the next 5-10 years which could potentially add 2 more bedrooms and a small en-suite shower room.

    I've used 3 different BTU calculators online which based on my room sizes recommend the following BTU requirements per room

    Lounge & Dining Room (we've knocked through): 14691
    Hallway Downstairs: TBC - hard to calculate due to varying ceiling height and will also heat upstairs hallway - the neighbors have a radiator that gives out approx 6000 BTU
    Kitchen: 10958
    Our Bedroom: 5614
    Bedroom2: 4105
    Bedroom3: 2878
    Bathroom: 2900

    This equates to a total of 47.5 KW. Surely this doesn't mean that we would need a 47.5KW combi boiler? It seems excessive but as stated the BTU requirements are taken as an average from 3 different websites.

    We do have a 5KW wood burner stove going in the Lounge / diner which could reduce requirements accordingly?

    Plumbers that have quoted for the work are recommending between 33-38KW, Valiant Eco Tech or similar.

    I've attached the floor plan to give you an idea of the scale of the house,

    any advice you can give would be much appreciated,

    Thank you

    Attached Files:

  2. pppmacca43

    pppmacca43 Super Member

  3. SJR88

    SJR88 New Member

    Thanks. However, they recommend 3 different sizes 30, 35 and 40kW so quite a wide range!
  4. Severntrent

    Severntrent Screwfix Select

    3412 Btu = 1kw. Are you happy with your conversion ?
  5. rogerk101

    rogerk101 Screwfix Select

    I installed a 30kW W-B gas combi boiler in a 5 bedroom, 2 bathroom, extremely well insulated rental house that I refurbished 5 years ago.

    The first tenants were a 5 person Italian family, who really liked to be warm ... 25 degree people.

    The boiler worked perfectly for them, even allowing both showers to be used by teenagers simultaneously.

    I reiterate that I did an excellent job of insulation as I expect one of my kids to live in the house eventually, so that reduces the amount of heat needed from the boiler, but your goal and approach should be similar. Pay a little extra upfront for over-the-top insulation and you'll save every year with reduced energy bills and greater comfort.
    pppmacca43 likes this.
  6. SJR88

    SJR88 New Member

    SevernTrent…doh! Have just been informed the same, clearly haven’t done my homework!
  7. pppmacca43

    pppmacca43 Super Member

    I’d think a 33-35kw combi will be plenty large enough for your needs
    Creek likes this.
  8. SJR88

    SJR88 New Member

    In which case I’m looking at 15KW for radiators, with some additional allowance for extras in the loft, say a further 4kw, max 20KW taking into account rads for loft. Am I right in saying that the sizing that the plumbers are recommending is probably based on flow rate for hot water to serve 2 bathrooms? 35KW seems like it might be overkill?
  9. pppmacca43

    pppmacca43 Super Member

    Yes it is because u want to run two bathrooms off of it.
  10. Baxi Boy

    Baxi Boy Active Member

    Your radiator needs are almost 15kw,any combi can handle that,it’s the litres per minute to the taps that you need to consider.If you want a good flow,and especially if running a shower or 2 bathrooms I would aim for 35 kw approx,the difference in prices of bigger boilers isn’t a lot compared to the installation costs!
    I am assuming your ceiling heights to be 8’,if so I agree with most of your btu calculations roughly,except your main bedroom which is quite large at 1890 cubic feet approx and would really need 7560 btu or 2.2 kw. ( Always best to have bigger than too small and use controls)
    Last edited: Sep 16, 2021
  11. Severntrent

    Severntrent Screwfix Select

    Seems a bit OTT general rule is allow extra 3 - 5kw for each bathroom
  12. SJR88

    SJR88 New Member

    I hadn’t included the space behind the built in cupboards as part of the room size, would you consider this? Ceiling heights are all between 2.6m & 2.8m high, solid 220mm brick walls, some rooms have large window area sizes which I inputted into the calculator.

    Others are advising me that you will never get consistent hot water whilst two hot water outlets are open. I.e. 2 showers at the same time, is this true? If a hot water storage tank is advisable could this be bolted on to the system in the loft at a later date? I plan to have flow return hot and cold feeds going up to the loft, capped off with IVs low level in 2nd floor to avoid dead legs in the interim. If a hot water tank could be added to the loft later would I need priority feeds from the boiler? Whilst I have floor boards up open stud walls I want to future proof as much as reasonable possible,

    Thanks again!
  13. Mike83

    Mike83 Screwfix Select

    You’ve had good advice above but you also need to consider flow rates.
    If your incoming main is only 12lpm you will only get 12lpm maximum from the combi with your current set up.
    Adding an unvented cylinder won’t solve this issue.
    Work out what your mains flow is then explore your options.
  14. Starslikedust

    Starslikedust Active Member

    Air source heat pump under the renewable heat incentive. Free dosh from the Government

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