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Hi All,

I want to move my current boiler to allow me to convert the garage into a living room. The current boiler is an indirect heat system and I have a hot water tank with immersion in the airing cupboard. 

What size combi would I be likely to need for my property? It’s a 1999 build, three bed detached with 13 radiators and three bathrooms (one bath/two rainfall showers). 

I’m guess around a 40Kw boiler? I have a plumber coming to give me a quote this week but keen to double check the size of boiler required. 

Thanks 

Replies

  • jefaz07jefaz07 Forumite
    302 Posts
    Part of the Furniture 100 Posts Name Dropper
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    Depends what you want to do with it and what flow rates you already have. A combi will only work at max output on hot water. 
    A 30 would easily do those radiators
    but I’d say a 35 or 36 would be fine which will give you around 15lpm of hot water. If you haven’t got that pressure then no point in going higher. 
    Also depends on brand as some do 33/35/36/38/40
  • Jeepers_CreepersJeepers_Creepers Forumite
    3.4K Posts
    1,000 Posts Fifth Anniversary Name Dropper Combo Breaker
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    You need to have adequate pressure and flow for a combi - has this been confirmed? For a 40kW jobbie, you need very strong both.
    Pressure of 3bar minimum I'd suggest. Flow of 20lpm at a min (it has to supply hot and cold at the same time).
    Make sure the plumbers test both, and listen to what they say about boiler size. Come back with this info if needed.
  • Cameron1590_2Cameron1590_2 Forumite
    140 Posts
    Eighth Anniversary 10 Posts Combo Breaker
    You need to have adequate pressure and flow for a combi - has this been confirmed? For a 40kW jobbie, you need very strong both.
    Pressure of 3bar minimum I'd suggest. Flow of 20lpm at a min (it has to supply hot and cold at the same time).
    Make sure the plumbers test both, and listen to what they say about boiler size. Come back with this info if needed.
    They have recommend a 36kw Baxi which will come with a 10 year warrant. Just waiting on a price from them now. 

    Apparently the flow test showed it was well over 22l/min at the sink and reading 3.5bar of pressure. I’m assuming this will be ok for a combi. 

    Thanks 
  • Jeepers_CreepersJeepers_Creepers Forumite
    3.4K Posts
    1,000 Posts Fifth Anniversary Name Dropper Combo Breaker
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    Excellent flow and pressure, you lucky thing :smile:
    Cannot comment on the boiler choice. Vaillant and Veissmann seem to be the most highly regarded from what I hear and read, but expect to pay a £ew £undred more for them over the Baxi.  

  • southcoastrgisouthcoastrgi Forumite
    6.3K Posts
    Ninth Anniversary 1,000 Posts Name Dropper Photogenic
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    Still won't be able to successfully run two outlets at once, why do want to change your system which obviously works just to convert your garage 🤷‍♂️
    I'm only here while I wait for Corrie to start.

    You get no BS from me & if I think you are wrong I WILL tell you.
  • Jeepers_CreepersJeepers_Creepers Forumite
    3.4K Posts
    1,000 Posts Fifth Anniversary Name Dropper Combo Breaker
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    Good point.
    Cameron, a 35+kW combi will give you great showers, but only one at a time. You know this?
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