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Discuss Choosing the right boiler - somewhere between 18kw and 42kw depending on where I look... in the Central Heating Forum area at

  1. Raydeeator

    Raydeeator New Member

    Evening all,

    Our boiler hasn't been doing very well recently. Just doesn't get the radiators hot at all. It was a relic of a smaller house, which was then extended. They didn't replace the boiler.

    We've decided to replace the boiler and sort out some pipework, install new valves etc, but I can't settle on the boiler size required. All radiators are already installed etc, so it's a question of finding the right sized boiler to heat it all appropriately;

    - 4 bed detached house
    - 1 bath, 2 showers
    - 2200 sq/ft
    - 15 rads, 2 towel rails

    We are going for a system boiler. My calculations are as follows

    - Current radiator output (Watts - ΔT = 50°) = 22620 Watts (includes towel rails and two rads in a glass conservatory)

    - House heat loss = 19000 Watts (17000 if remove conservatory)

    The boiler which has been suggested is the Worcester-Bosch Greenstar 30i. As far as I can tell it has 24kW CH output, and 11ish LPM flow, with a max BTU of 100,000.

    If I trawl the internet I find all sorts of stuff about 3kw per radiator, or any 4 bed houses needing 42kW boilers etc. I appreciate that in the past it's been common to massively oversize boilers, but it just makes me nervous that I'm missing a trick and the boiler won't be capable. With the numbers above would the 30i suffice and are there any apparent issues? I have the numbers but don't really know how to interpret them...

    My alternative is to have the 35CDI or something, at a marginally greater cost.

    Many many thanks.
  2. scott_d

    scott_d Plumber GSR

    Combi boiler?
  3. Raydeeator

    Raydeeator New Member

    Now we have a conventional boiler, we are changing to a system boiler and getting rid of the cold water tank.

    We need various bits of pipework doing at the same time so the added complexity of a system boiler doesn't really matter.
  4. scott_d

    scott_d Plumber GSR

    The 30i you are talking about is a combi
  5. Riley

    Riley Plumber GSR Top Contributor!!

    I’d suggest that you allow a competent gas safe engineer come out and carry out a full heat loss Calc of your property. All the rest is finger in the wind stuff
  6. Raydeeator

    Raydeeator New Member

    They do a system version I believe;

    Greenstar i System (27kW and 30kW) | Worcester, Bosch Group

    Perhaps not quite finger in the wind, it seems a fairly detailed estimate using this calculator;

    Calculate heat requirements
  7. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    so your looking at getting an unvented cylinder fitted

    as combis are only really designed for one bathroom / outlet at a time

    i would allow around 15kw for an unvented cylinder
  8. Riley

    Riley Plumber GSR Top Contributor!!

    I understand what you mean but there is a degree of engineers experience and judgement that goes into correct sizing and as you say your variation between KW has been vast. Why not get someone to size it all properly then you have comeback if it doesn’t meet your requirements.
  9. Raydeeator

    Raydeeator New Member

    System not a combi! They do both variants in the 30i.
  10. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    so you are looking at getting an unvented cylinder fitted
    • Funny Funny x 1
  11. gmartine

    gmartine GSR

    HW storage?
  12. Raydeeator

    Raydeeator New Member

    Hot water tank for the HW.
  13. king of pipes

    king of pipes Trusted Plumber GSR Top Contributor!!

    15 radiators and a cylinder 30kw wont be enough a system boiler will need a additional expansion vessel , so i would advise a heat only boiler expansion vessel Robo kit and unvented cylinder the final calculations would need to be calculated after a site vist . cheers kop
  14. Ric2013

    Ric2013 Plumber Top Contributor!!

    On a slight tangent, I can't really understand how we can ever size things reliably when we are the mercy of guessing the air changes per hour. I've seen the draughty/normal/airtight way of calculating before, but it seems fruitless to have to guess.

    For instance, the last time I calculated the heat loss for the ground floor of a house (including the heat losses to the first floor which is not heated to the same temperature), the ventilation heat loss for the area was a quarter of the total heat loss. ASSUMING 1ac/h, but the 1 ac/h was an estimate based on the fact that it was an old building that had been draughtsealed reasonably well. If the actual ventilation were 2 ac/h, then it would have needed 3kW heat, not 2.4kW.

    Of course, sizing the whole house, the actual fabric heat losses would be less as a proportion than sizing one floor (as internal walls heat transfer do not affect the boiler load, while they do affect the load on a radiator serving one room), but ventilation losses for the whole house might have been double those for the ground floor. While, at this point, 1.2kW or 2.4kW for the air changes isn't a great difference, it does start to mean that modifying a heat loss calculation for the thickness of loft insulation installed starts to feel rather academic.
  15. kris

    kris Trusted Plumber GSR

    I think a 30 kw system boiler will be perfect , your original thought of 3kw per rad is way off as many rads you'll find might below 1kw but depends on size and single or double , I was always taught to allow 13kw for hot water but I was using a boiler calculater this morning ( The energy saving Trust heating calculator ) and it stated allow 2kw for small unvented or 3kw for big so I'm gonna have to have a wee check as unvented does use less to heat and have massively less heat loss so quite possible ,,, But back to your boiler , 30kw won't be that far off the mark I don't think , too many people oversize boilers thinking oh it will modulate down but it doesn't get maximum efficiency / cond and there in lies the problem ,
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