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High end Combi vs. Unvented for 3-bed semi-D & 1 bath.

Discussion in 'Plumbing and Central Heating' started by EMFTeo, 18 Jan 2012.

  1. EMFTeo

    EMFTeo

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    Hi there,

    We've just moved into a new 3 bed semi-Detached with a 25 yo++ LPG Potterton boiler and would like it replaced with a modern efficient LPG boiler.

    The aim is to also free up the airing cupboard where the water cylinder is stored and to remove the storage tanks from the loft.

    There are only 2 adults with 1 bathroom and we're planning a little one sometime later this year :)

    We would like to have a power shower installed in the bathroom (where the airing cupboard is). We rarely take baths, perhaps once a month?

    We've had a plumper measure our mains pressure:

    5Bar pressure and 24 litres/minute flow rate from outside tap at back of property

    One plumper is saying a Vaillant Ecotec Plus 837 37kw Combi Boiler will do the job quite well as we have more than sufficient mains pressure to cope.. but other plumbers are saying to go for the more expensive Vaillant EcoTecPlus 618 LPG system boiler & Unistor 155 indirect unvented hot water cylinder solution.

    The unvented solution works out almost double the price of the combo solution (with a water softener).

    Can anyone out there share their experience ? i.e. whether the combo will do the job well? or to go all mad and pay for the unvented solution?

    Thanks
     
  2. stem

    stem

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    If you want a 'power shower' you will need to keep your hot water cylinder. A power shower uses a motor driven pump to boost the flow of stored water to your shower. This produces a steady flow of water that is unaffected by other water demands ie flushing a WC, running a tap.


    Combi's can provide a good hot water directly to a shower as they heat cold mains water directly. The output you get will depend on the capacity of the boiler, your water pressure, (which will change if any water hot or cold is used elsewhere in the house) and the temperature of the incomming cold water.
     
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  3. EMFTeo

    EMFTeo

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    Thanks for that information, can you comment based on the plumper's combi-boiler recommendation and the flow-rate measurement would suffice ?
     
  4. Agile

    Agile

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    I am not impressed by your plumper!

    He has measured your static pressure and open pipe flow rate but not the dynamic flow rate which is whats relevant for decision making.

    Based on what he has measured its very good and will be fine for an unvented cylinder and thats what I would strongly recommend.

    Some areas are very hard water like Reading and Bracknel and a water softener would be a good idea. To save running costs I would probably only fit one on the hot supply but I would be very careful about the flow rate they can give as many are very restrictive.

    Tony
     
  5. D_Hailsham

    D_Hailsham

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    Are you sure that the 837 is available in the LPG configuration? Vaillant only list the 831 for LPG (the 618 is NG or LPG).

    The plumbers have only taken the HW requirement into consideration and have completely ignored the heating requirement.

    The CH output of the boilers are:

    837: 12- 28kW,
    618: 6.7-18kW
    831: 8.7-24kW

    The heating required for a typical 3 bed semi will be under 12kW in the coldest weather, so the 837 will be running in the inefficient on/off mode all the time.

    If you want to know what your heating requirement is, use the Whole House Boiler Size Calculator. Set the Domestic Hot Water Allowance to zero as you have a combi boiler.

    For maximum efficiency the minimum CH output of the boiler should be as far below the calculated requirement as possible.
     
  6. EMFTeo

    EMFTeo

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    Hi there,

    Which would be the better combi-boiler pls? I will provide this feedback to the plumber.

    Thanks

    Eric
     
  7. D_Hailsham

    D_Hailsham

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    I need the results from using the Whole House Boiler Size Calculator. (Set the Domestic Hot Water Allowance to zero as you have a combi boiler.)
     
  8. EMFTeo

    EMFTeo

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    Based on that,

    The required boiler output is 12.65 kW

    We are planning to have our walls cavity filled.
     
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  10. muggles

    muggles

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    If your whole house requires 12.65kw then the 831 is the one to go for, you'll get less hot water performance but it's more suited to your heating system. If you're planning to improve your insulation then your heating requirement will drop further, maybe to 11kw or so depending on how good the insulation is now compared to how good it will be when you're done. The 837 would just cycle all the time, which won't do it any good and will make it inefficient as well.

    You'd be much better off having an unvented cylinder and system boiler.
     
  11. EMFTeo

    EMFTeo

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    Can you give a pricing estimate for an unvented cylinder solution?

    Thanks
     
  12. dcawkwell

    dcawkwell

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    Hi end combi like Atag A325 ECX with 95% efficiency for hot water.
    Will give you all you need for a 3 bed semi and give a good shower.

    Even something like the big Ravenheat 42kW boiler can give
    17 litres a minute and is bargain basement money.
     
  13. EMFTeo

    EMFTeo

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    Can anyone then recommend a reputable LPG Gas safe certified engineer willing to travel to Thames Valley region?

    Thanks
     
  14. muggles

    muggles

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    The best person to give you an estimate for this is the person looking at the job, not a bloke sat at a PC a couple of hundred miles away. Ask your neighbours who they recommend
     
  15. Agile

    Agile

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    There are plenty of LPG registered installers in Berkshire.

    I would recommend an 18 kW ( or lower power ) heat only and an unvented cylinder.

    More efficient on fuel consumption which is important on expensive LPG and a better flow rate for hot water as well.

    Vaillant unvented cylinder are more expensive than cheaper to buy makes like Santon.

    Tony
     
  16. EMFTeo

    EMFTeo

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    Just had a chat with the heating engineer, he said the 837 is 'future proof' by means of being able to switch to natural gas (we are still hoping the area will go to natural gas) and also have better output for hot water.

    Secondly, with regards to the central heating output range it can be configured via the computer panel to work on the minimum 'condensing' mode, which will not burn too much LPG...

    With regards to the unvented solution.. it's just way out of our budget.. it's an additional £1K++ and there's also the extra cost of maintenance?
     
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