Help? Is this a good boiler set up?

Discussion in 'Plumbers' Talk' started by JaneyJane, Feb 6, 2020.

  1. JaneyJane

    JaneyJane New Member

    Hi! I'm renovating an old house. 4-beds, potentially 8 people staying there and all wanting a shower etc in the morning. Teenagers involved so lots of hot water required!
    My contractor has recommended the following:
    the 30kw model of this: https://www.worcester-bosch.co.uk/products/boilers/directory/greenstar-i-system-27kw-and-30kw
    with this cylinder: https://www.builderdepot.co.uk/rm-180l-stelflow-180hi-indirect-horizontal-unvented-cylinder
    I haven't got a clue! What do you think? Reliable, efficient system that'll keep us in plenty of hot water even at busy times?
    Many thanks
     
  2. JaneyJane

    JaneyJane New Member

  3. Bob Rathbone

    Bob Rathbone Screwfix Select

    How many shower cubicles? if you have more than one then a combi will not be suitable given that they may both be used at the same time. What is suggested seems reasonable.
     
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  4. andy48

    andy48 Screwfix Select

    1. The principle is sound. The following assumes 3 showers (normal, not ran head) running simultaneously:
    2. However:
    2a. Make sure there is enough pressure and flow at the incoming main, and at the unvented hot water cylinder to make an unvented system work effectively. I'd look for a minimum of 1.5 bar dynamic pressure and a flow rate of around 25 to 30 litres / minute. If you haven't got, or can't get, to those figures, you are likely to be gravely disappointed with the results.
    2b. A 180 litre unvented cylinder sounds a little small to me for the number of people involved. I'd suggest 250 litres or more. I note the one you are looking at is a horizontal cylinder. RM do not quote recovery times (time to heat a full cylinder from cold) for these, yet with a potential for 8 showers in quick succession I think you'd need a fast recovery cylinder.
    2c. An unvented cylinder MUST be installed by someone with a "G3" qualification. The installation must be noted to local building control, and a G3 qualified installer should complete the necessary notification.
    2d. Unvented cylinders require an annual inspection. This is not a legal requirement, but it would be foolish not to conform. Failure to service will probably void any warranty.
    2e. If your contractor is builder rather than a plumber / heating engineer, then in my view there is a fairly high chance he or she will not truly understand the requirements for a space and water heating system. You would be well advised to ensure that the contractor is taking advice from someone who knows what they are doing, and that the contractor will take responsibility for the final system meeting its specification. Its for you to specify what you want.
     
    JaneyJane likes this.
  5. Mike83

    Mike83 Screwfix Select

    To maximise the efficiency I would opt for something less than 30kw.
    How many radiators is there?
     
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  6. rogerk101

    rogerk101 Screwfix Select

    I have a 26kW combi which more than adequately serves two showers. A 30kW combi would serve 3 showers simultaneously.
     
    JaneyJane likes this.
  7. rogerk101

    rogerk101 Screwfix Select

    A 30kW boiler is no less efficient than a 20kW one. (In fact it's probably more efficient.) It'll also service 3 simultaneous showers, which a 20kW boiler would struggle with.
     
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  8. Mike83

    Mike83 Screwfix Select

    He’s not looking for a combi.
    System boiler is what he wants.
    A lower output boiler will have a better modulating ratio. This should make it more efficient.
     
    JaneyJane likes this.
  9. JaneyJane

    JaneyJane New Member

    Thanks for replying. It's a system boiler and there are 2 shower and 1 deep bath that takes about 200l when a 60kg person is in it.
     
  10. JaneyJane

    JaneyJane New Member

    Thanks Andy! There are two showers and 1 deep bath with c.200l capacity when a 60-70kg person is in it.
    2a. Good point on the water pressure, any idea how I would find that info out?
    2b. I agree 180l sounds a bit small so if we can squeeze a larger cylinder in we will. What would you consider a fast recover time - 20 mins?
    2c. My contractor is getting his subcontracted plumber in to do the job and thanks, I will ask re G3 qualification.
     
  11. JaneyJane

    JaneyJane New Member

    Thanks Mike. There are 11 rads in all. Would you mind explaining how less than 30kw is more efficient? I just don't know about these things!
     
  12. JaneyJane

    JaneyJane New Member

    Thanks - yes I'm after a system boiler.
     
  13. Mike83

    Mike83 Screwfix Select

    All boilers have a modulation ratio.
    The higher the ratio, then the better the boiler can lower its output.
    A 30kw boiler with a 5:1 ratio will be able to operate as low as 6kw before the boiler has to turn off.
    A 20kw would be able to go down to 4kw potentially.
    The longer a boiler can run at minimum rate instead of turning on and off is better for efficiency and wear and tear.

    A lot of the time your boiler/system may only require a few KW of heat output. If your boiler can’t go as low as what’s required then it will simply turn off. This could result in the boiler condensing less.

    A 30kw system boiler with 11 rads and cylinder will rarely be running at maximum.
     

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