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Confused... Central heating / combi question

Discussion in 'Plumbing and Central Heating' started by DaveApp, 24 Mar 2017.

  1. DaveApp

    DaveApp

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    Hello.

    I live in a 1930's mid terraced 3 bedroom house and about to pull the trigger on a loft extension and ground floor refurb. We need as much space as possible (can't afford to move) so as a result we will have to remove the cold water tanks from the loft.

    The loft extension will mean that we end up with 2 bathrooms - the en-suite in the loft and the family bathroom to the first floor. Since our current central heating system is working off of a single pipe setup, we're going to pretty much start again and install new boiler and feed and return pipework.

    So here's the problem. I want to install a combi boiler to supply the house with it's hot water, but having read a little I'm starting to think that it might not be quite up for the job. Being realistic, It would only be rare that both shower and bath/shower would be used at the same time, but my kids are getting older so I can't really determine what the demand will look like in a few years time.

    Ideally, I'd like a combi to be installed in to the kitchen and be hidden behind a cupboard, but then maybe I'm after too much.

    Although I haven't planned for it, I suspect I could fit a megaflow in the loft somewhere, but again, that's going to eat up space which we really don't want to do unless we have to.

    So the question is... Given a clean slate, what options do you think I could consider here?

    Thanks in advance,
    Dave.
     
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  3. Agile

    Agile

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    You can consider any option you like.

    But no combi is going to run two proper showers.

    A combi and an unvented both depend on the incoming dynamic flow rate.

    You really need a minimum of 22 litres/minute @ 1.0 bar to get the full benefit from an unvented and be able to run two reasonable showers.

    Tony
     
    Last edited: 25 Mar 2017
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  4. ianmcd

    ianmcd

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    Absolute *******s , many decent combis around these days that can comfortably cope with two showers at the same time if installed properly, all depends on the incoming water FLOW rate not pressure
     
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  5. You can run 2 showers off a decent suitable combi but planning and a lot of considerations have to be met,otherwise it wont work as well as you would like.

    First consideration is the incoming water supply pipe,your plumber/heating engineer will be able to test these and advise.

    Good luck with your project.
     
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  6. bernardgreen

    bernardgreen

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    and how do they perform when modulated down to provide just the background heating load for the house. What efficiency do they achieve then ? What is the actual efficiency when operating at minimum heat load and not the calculated SEDBUK efficiency rating.
     
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  8. maybe look at a Combie with an added hot water store, they stick out quite far off the wall so wouldn't fit into a cupboard, its a Valliant 938 we usually fit​
     
    Last edited by a moderator: 25 Mar 2017
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  9. DP

    DP

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    Another option is horizontal UV cylinder for washing, 24kw combi in kitchen for heating and HW there. Best of both world with cylinder being your belt and braces when combi stops working

    Do consider ( sleeping area and living area) zones as well for fuel efficiency
     
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  10. Agile

    Agile

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    That depends on what flow rate you want from your shower. Most people who like showers expect at least 10 litres per minute. Where I stayed last Thursday the shower was about 15 litres per minute and very pleasant!

    You don't get a good shower without a residual dynamic pressure in the supply pipes to ensure the shower water is ejected at a good velocity to give that nice tingly feeling.

    You only get a good flow when the supply has a good dynamic pressure.

    Tony
     
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  11. DaveApp

    DaveApp

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    Thanks everyone for the responses, it really helpful to hear different takes on it.

    I've just measured our pressure to be pretty much bang on 3 bar and the flow rate to be approximately 15 l/min. From what I've read online others have said that to have a two showers running from a single combi then you need to have a flow rate of at least 16 l/min. And as bernardgreen said, if the boiler has other demands at the same time then the experience is only going to get worse. I kind of feel like we're at the low end of what is required.

    The other issue is that if we need a high end combi, then it's unlikely to fit within a kitchen cupboard (if only we had a utility room!) which I think the other half won't be too thrilled with.

    So it's looking more like the UV cylinder could be the way forward if we can tuck it away in the loft somewhere and a far smaller combi for the heating and kitchen sink.

    I have one last question..

    Out of interest, I wonder if you could explain how I might work out my dynamic pressure?

    Thanks again for everyone's input.

    Dave.
     
  12. Agile

    Agile

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    An open pipe flow rate is just that.

    To operate a shower you need to have a residual pressure in the supply pipe of about 1.0 bar. In your case that might only be possible at a flow rate of about 10 li/min which is adequate for one fair shower.

    I think you can read in the FAQ about how you can measure the dynamic flow rate.

    Simply you put a pressure gauge on your supply perhaps at an outside tap or a washing machine connection and see what flow you can take from taps whilst still leaving say 1.0 bar on the pressure gauge.

    A static pressure of 3 bar is good and would give a good flow rate if your supply pipe could supply it. You may need to upgrade to 32 mm from the street connection.

    Tony
     
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