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Boiler size needed?

Discussion in 'Plumbing and Central Heating' started by grahamw01, 27 Dec 2020.

  1. grahamw01

    grahamw01

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    I have a 3 story semi with 11 rads feeding 4 bedrooms, a large family bathroom. The largest bedroom has an ensuite. We're having an extension that will house a small WC, a laundry room and small office so # rads will increase to 14. What size boiler would suffice? Thanks, Graham
     
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  3. grahamw01

    grahamw01

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    It's actually 11 rads currently, increasing to 14. On the ground floor are 5 rads, 1 in hallway, 2 in living room, 1 in playroom and 1 in kitchen. The other 6 are upstairs
     
  4. oldbuffer

    oldbuffer

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    1. Size depends on what type of boiler you are having:
    1a. Combi - size for hot water requirements.
    1b. System or Heat Only / Regular - size for heating requirements and add 2 to 3 kW for hot water.
    2. Rule of thumb = about 1.5kW per radiator, so allowing for hot water ((14 x 1.5) +2) = 23kW, so probably a 24 or 25 kW boiler if not combi.
    3. If combi, work out maximum simultaneous hot water usage, and get a boiler which gets as close to supplying that as you can. Probably a 30 to 35 kW boiler. If going this route, make sure it can modulate down to around 21 kW - which shouldn't be a problem. Almost certainly won't get two simultaneous showers.
    4. If you want to do it properly, work out heating requirements for each room and sum them, then add for hot water.
    5. Any new boiler will be condensing. To maximise gas economy you need a lower return temperature, thus a lower "delta T" for the radiators, which may mean bigger radiators.
     
  5. grahamw01

    grahamw01

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    Thanks for response, hot water isn't as much an issue to me as feeding the rads. There's only myself, wife, 8 year old daughter and a new born. We'd always be in a situation where only one shower on at a time etc, mostly the dishwasher and washing machine on at same time as running bath or shower
     
  6. grahamw01

    grahamw01

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    Also will be a combi boiler
     
  7. Harry Bloomfield

    Harry Bloomfield

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    Washers and dishwashers these days, normally take in cold and heat the water themselves - so not a factor.
     
  8. grahamw01

    grahamw01

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    Ok so if the main issue is feeding 14 rads at same time, what size would I need for the CH?
     
  9. denso13

    denso13

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    Work out the heat loss of the building/rooms and size the radiators accordingly. This will then give you the required boiler output.

    Not being difficult but you can't expect a proper answer to..

     
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  11. grahamw01

    grahamw01

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    Ok how? And size of rads isn't what I'm after, it's what KW output I need for CH from Combi
     
  12. denso13

    denso13

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  13. grahamw01

    grahamw01

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    Yeah I plan on asking whoever I get to replace but I wanted some level on knowledge and confidence in what they're telling me so I know they're not BS'ing me
     
  14. grahamw01

    grahamw01

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    Where do I find size of rads? Most were here when we moved in and are different
     
  15. denso13

    denso13

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  16. grahamw01

    grahamw01

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    I tried and got site can't be reached
     
  17. grahamw01

    grahamw01

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    Got it now cheers
     
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