How do I calculate the boiler size needed for my home?

Discussion in 'Plumbers' Talk' started by DS99, Aug 12, 2019.

  1. DS99

    DS99 New Member


    Can anyone point me in the direction please as to where I may calculate the heating requirements / boiler output size for my home? I'll be going for a Regular boiler.

    I'm totally happy in measuring room volumes etc. I assume that there are formulae available somewhere?

    Someone suggested matching the existing but, I know for sure that the property has been extended since built and the boiler is likely, the original still. 1983 build.

  2. Jimbo

    Jimbo Well-Known Member

    Either work out each room individually using an online radiator calculator, or consider each major section of the structure as a whole and do the same. Then add enough to service the hot water cylinder - 7 to 11kW unless you have a quick recovery cylinder.

    Or call someone in to give you some quotes, maybe British Gas as a stick in the sand.
  3. Mike83

    Mike83 Well-Known Member

    Your average system boiler is probably 15-16kw.
    This won’t be far out.
    If you oversize the boiler if will probably cycle slightly more often.
    If you undersize it then it will probably take a little longer to heat both the heating and HW when on at the same time in cold temperatures.

    Just our of interest how were others taught to size a boiler for a new installation. I mean from tradesmen during your apprenticeship not textbooks?
  4. Heat

    Heat Well-Known Member

    Just by calculating each room and adding for cylinder.
    Most houses obviously didn’t have to calculate boiler output as were standard size semi house
    Mike83 likes this.
  5. Mike83

    Mike83 Well-Known Member

    What did you add for the cylinder though.
    The full kw amount or a different figure.

    I was taught to add up all the rads then add 10%. Then add 4kw for the cylinder (average size vented)
    Heat likes this.
  6. Hans_25

    Hans_25 Well-Known Member

    Rule of thumb, 1.5kw per room (1.5kw is average size radiator), add 3kw for hot water cylinder.

    Have an 18Kw here for 3 bed semi FWIW.
    Mike83 and Heat like this.
  7. Heat

    Heat Well-Known Member

    Used to allow 12000 btu for average cylinders (about 3.5 kw) but then upped that to 15000 (about 4.4kw) - so similar to what you were taught.
    I now add 6kw due to cylinders being larger nowadays in many average homes.
    I added typically 10% to most rooms, especially any that would be on colder side of house or large rooms or with 2 or more outside walls etc.
    I tend to think of extreme weather requirements
    Last edited: Aug 12, 2019
    Mike83 likes this.
  8. Hans_25

    Hans_25 Well-Known Member

    Always best to go larger than needed, same with radiators that way rooms will heat up more quickly.
    Mike83, Heat and KIAB like this.
  9. Heat

    Heat Well-Known Member

    Very true. You never will get a customer complaining that they have a good functioning radiator.
    To have plenty of output in a rad has the advantage that the boiler can run at lower temperature to achieve required room temperatures.
    Also when we get extreme winter weather of say, minus 10 and below, the rads will still cope.
    Condensing boilers need rads to be slightly oversized.
    Personally I try to have most rooms in a home to have accurate sized rads with equal addition percentage output. But in living rooms or rooms that would be difficult to keep warm I make sure rads are well oversized.
    With modern convector rads they tend to not look oversized thankfully
    Baxi Boy and Mike83 like this.
  10. The Teach

    The Teach Well-Known Member

    This heat loss calculator is easy to use
    As mentioned by Jimbo,if hot water heating required some more boiler kw's will be required.It depends on the cylinder manufactures instructions.

    Mear calculator ;) or what ever boiler the installer had spare & sometimes guess work. Them days boilers cold be range rated to suit varying demand s:)
    Mike83 likes this.
  11. Baxi Boy

    Baxi Boy Active Member

    Assuming you are keeping existing radiators,add up their outputs by their type and size,add 10%,then allow another 3.5 kw for a tank.You can heat a reasonable sized home with a 15kw boiler!
    Mike83 likes this.
  12. kiaora

    kiaora Well-Known Member

    I’m not in the game anymore, but when I started it was 5 btu per cubic foot !

    It may help you ?

    Good luck

    Baxi Boy likes this.
  13. Baxi Boy

    Baxi Boy Active Member

    5 btu per cubic foot is actually a very good guide,though there are considerations for insulation,glazing types etc,and different rooms have different design temperatures! Now they talk more in kw more than btu.
  14. The Teach

    The Teach Well-Known Member

    By the time DS999 comes back,gas will be a banned fuel or the greeny politicians will have taxed it out of existence.

    its happening in proper euroland,poxy brussels Sprouts.

    been there seen it :D

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