Help understanding KW rating for boilers

kittennose
kittennose Posts: 142
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Just about to order a new boiler and cylinder and want to make sure I understand this corrrectly as I want to maximise our Cylinder recovery time which is 24mins at 24kw from 10degrees according to the manufacturer. 24kw is the max it takes.

Our new boiler is a Vaillant ecoFIT Pure 418 which obviously has a max output of 18kw (actually looking at the spec it's 19.1)

However it also says for domestic Hot Water it is 25.2KW which appears to be perfect for us.

What I can't get my head around is why this is different. Surely the boiler simply heats water, so if it can heat 25kw which is this not badged as a 25kw Boiler?

I called Vaillant and they were useless, with one saying that's not true and saying I must be on the combi page, and one saying yes it's true but he simply check the link I was looking at and gave no further advice.

 https://professional.vaillant.co.uk/for-installers/products/ecofit-pure-open-vent-boiler-26116.html
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  • Albermarle
    Albermarle Posts: 21,231
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    I would strongly suspect a simple printing error.
    If you look along the hot water KW line, the last two both say 35. If you removed one of these and moved the rest of the numbers rightward one slot, then they would all match as expected.
  • Typhoon2000
    Typhoon2000 Posts: 1,122
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    Isn’t that a combi boiler? If you are fitting an new unvented cylinder, you want a system boiler, or you won’t be able to have the combi supply the same taps as the cylinder.
  • FreeBear
    FreeBear Posts: 14,274
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    kittennose said: Our new boiler is a Vaillant ecoFIT Pure 418 which obviously has a max output of 18kw (actually looking at the spec it's 19.1)

    However it also says for domestic Hot Water it is 25.2KW which appears to be perfect for us.
    Is this "new" as in being ordered, or have now installed ?

    The specs on the 418 says a maximum heat output for CH is 19.1KW (with a 50°C/30°C flow/return) - That is way more than most houses really need. The minimum output being 5.2KW - A very poor modulation range and you may well find the boiler short cycling quite a bit...
    Putting out 25KW to heat the DHW tank is perfectly doable - It just means running the burner at 130% for the short time that that much heat is needed (the boiler will modulate down to 5KW if needs be). When I use my combi boiler for DHW, it initially peaks at 127%, and then drops to 105-110%.

    If it is not too late to change your mind about boilers, I'd suggest either Viessmann or Worcester Bosch - Both come with 10+ year warranties and a much better modulation range (typically 10:1). Viessmann also use stainless steel for the main heat exchanger and is OpenTherm compatible.

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  • QrizB
    QrizB Posts: 13,685
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    Isn’t that a combi boiler? If you are fitting an new unvented cylinder, you want a system boiler, or you won’t be able to have the combi supply the same taps as the cylinder.
    The link provided by the OP is for an open vented boiler (ie. a regular boiler), suitable for an open vented DHW system. Not a combi or a system boiler.
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  • ThisIsWeird
    ThisIsWeird Posts: 4,458
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    edited 5 January at 3:34PM
    Hi Kitten.
    My advice - whoa!
    I don't mean that in a bad way - that boiler could be perfectly fine, and Vaillant are generally considered decent - but to listen to the advice given above.
    For a start, if Vaillant cannot give you clear answers aimed at a laypeep, then imo they barely deserve your custom.
    Next, why Heat-Only? Why not 'system' - ie do away with that nasty wee tank in the loft that will give you bovver over time? Ok, you don't have to, but have you considered it?
    And then consider alternatives like wot FreeBear has pointed to.
    As for your actual Q - no idea :neutral:



  • Rodders53
    Rodders53 Posts: 2,090
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    These are questions and explanations that the OP should be posing with his installer - designer.

    I'm no expert but I suspect this HW output is only available under the latest 'HW priority' or PDHW installations - where the boiler runs at high/max temp, with no condensing, to reheat the cylinder while the CH circuits are closed off (even if any CH room stats etc are calling for heat). 
    Once DHW is satisfied the boiler then runs in an economic low-circulating water temperature (condensing) mode to heat the radiators etc,..

    That will only happen if plumbed up and wiring / controls are suitable for that mode of operation.

    https://theintergasshop.co.uk/content/189-why-hot-water-priority-pdhw-is-the-reason-s-and-y-plan-should-be-banned probably explains it better than I have done.


  • ThisIsWeird
    ThisIsWeird Posts: 4,458
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    Very interesting, Rodders.
  • kittennose
    kittennose Posts: 142
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    Hi Kitten.
    My advice - whoa!
    I don't mean that in a bad way - that boiler could be perfectly fine, and Vaillant are generally considered decent - but to listen to the advice given above.
    For a start, if Vaillant cannot give you clear answers aimed at a laypeep, then imo they barely deserve your custom.
    Next, why Heat-Only? Why not 'system' - ie do away with that nasty wee tank in the loft that will give you bovver over time? Ok, you don't have to, but have you considered it?
    And then consider alternatives like wot FreeBear has pointed to.
    As for your actual Q - no idea :neutral:



    We don't have the flow/pressure for unvented unless we spend £££ on accumulators.
  • kittennose
    kittennose Posts: 142
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    edited 5 January at 6:18PM
    FreeBear said:
    kittennose said: Our new boiler is a Vaillant ecoFIT Pure 418 which obviously has a max output of 18kw (actually looking at the spec it's 19.1)

    However it also says for domestic Hot Water it is 25.2KW which appears to be perfect for us.
    Is this "new" as in being ordered, or have now installed ?

    The specs on the 418 says a maximum heat output for CH is 19.1KW (with a 50°C/30°C flow/return) - That is way more than most houses really need. The minimum output being 5.2KW - A very poor modulation range and you may well find the boiler short cycling quite a bit...
    Putting out 25KW to heat the DHW tank is perfectly doable - It just means running the burner at 130% for the short time that that much heat is needed (the boiler will modulate down to 5KW if needs be). When I use my combi boiler for DHW, it initially peaks at 127%, and then drops to 105-110%.

    If it is not too late to change your mind about boilers, I'd suggest either Viessmann or Worcester Bosch - Both come with 10+ year warranties and a much better modulation range (typically 10:1). Viessmann also use stainless steel for the main heat exchanger and is OpenTherm compatible.

    Ordered (actually it's here but I assume still time to swap if I suck up a return shipping fee...)

    We could get a Worcester Bosch Greenstar 18Ri but I couldn't find any info on modulation etc. Website is terrible. And everyone keeps saying how they are the worst manufacturer (I always thought they were known to be decent).

    I believe we need no more than 8kw for heating on a very cold day, but we need to balance with our hot water needs. We will be having a new cylinder which has a max input of 25kw. We need decent reheat times as we are a family of 4.

    We will also be having a new hot water storage in the loft, to help with the issues an old one brings.

    Or....we go back to the original plan of a combi, but our flow and pressure may not be enough at 14l per min / 2.0 bar. Seems to be 50/50 who you ask!

    To be honest it's driving me a little crazy now.
  • FreeBear
    FreeBear Posts: 14,274
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    kittennose said: We could get a Worcester Bosch Greenstar 18Ri but I couldn't find any info on modulation etc. Website is terrible. And everyone keeps saying how they are the worst manufacturer (I always thought they were known to be decent).

    Or....we go back to the original plan of a combi, but our flow and pressure may not be enough at 14l per min / 2.0 bar. Seems to be 50/50 who you ask!
    Greenstar 18R1 has a rated heat output of 19.28KW. All I can find on minimum is the rated heat input which is not the same, although it can be used as an indicator.. 5.54KW to 18.48KW is what is listed in the installation manual - Like the Vaillant, not brilliant. WB do have a very good reputation for not just reliability, but also for warranty claims in the event of a breakdown. Just about all the local gas engineers that I contacted were pushing WB over any other make.

    With a flow rate of 14l/min and a pressure of 2 bar, you are way ahead of me (10l/min @ 1.5 bar), so a combi would work for you depending on the number of bathrooms/showers. As I only have a single bath (and an electric shower), the flow rate & pressure is adequate, and I went for a combi.
    Her courage will change the world.

    Treasure the moments that you have. Savour them for as long as you can for they will never come back again.
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