Shower advice: pair of mixers to suit combi

Discussion in 'Plumbing and Central Heating' started by markocosic, 2 May 2014.

  1. markocosic

    markocosic

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    Hi,


    Large combi boiler. (nominally 15L/min @ 35C dT)


    I'd like to hang two mixer showers off it: running at 9-12L/min when used individually and sharing the boiler output when run simultaneously.

    The combi element is non-negotiable. We aren't getting an unvented system for the number of occasions when both showers need to be in use simultaneously and nor is an electric shower acceptable.


    Are any brands better at dealing with this than others?

    Option A: a thermostatic shower that will reduce the overall flow in response to there being a limited amount of hot water available. Do such beasts exist? (would be easy)

    Option B: a shower that has a button, click-stop or detent so that it is easy to manually set the flow to ~7L/min or 9-12L/min. Do such beasts exist?

    Option C: an overhead/rain shower that'll do 9-12L/min and a hand shower that will run at ~7L/min without feeling too terrible. (it's simple, stupid?)


    Grohtherm F is the type of style we like:

    http://www.grohe.com/uk/4999/bathroom/shower-thermostats/grohtherm-f/

    Built in/concealed valve, overhead shower, plus hand shower. (none of that side spray stuff though!)

    Very much open to less costly ways to achieve a similar look though. We have >100mm depth to play with in the stud wall behind the tiles and above the ceiling.


    Mira Platinum dual also:

    http://www.mirashowers.co.uk/online... showers&resultPageKey=1956297565-0&category=

    Can these internally limit flow? Do they still fall to bits after 5 minutes or do you now get 10 years from them in hard water areas?


    Cheers!
     
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  3. swbjackson

    swbjackson

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    Nothing will do what you want. You've either got to put a couple of eco showers in and have a lower flow rate all the time or provide more hot water.
     
  4. muggles

    muggles

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    Should be easy enough to do. You need to put a flow limiter on the cold water supply to your combi, like this http://www.plumbcenter.co.uk/en/hea...-calflow-plus-flow-15-ltr-per-min-15mm-388224

    That'll give you your 15 litres per minute out of the combi, then you can choose pretty much whichever thermostatic mixer you like, as the hot water flow rate is controlled at source and so it should split reasonably equally between your two showers when they're both turned on
     
  5. markocosic

    markocosic

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    Aha!

    So tee off for toilets/white goods/outside tap/kitchen tap before the flow restrictor, then run the combi feed and the cold feed to showers/taps through that flow restrictor.

    Therefore combined flow must be <15L/minute (hot and cold) and the showers can do as they please. As the cold pressure will drop as fast as the hot pressure you won't get them running cold though.

    What do these things do for pressure? Is there a risk we end up with nothing at the shower?
     
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  7. muggles

    muggles

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    You don't need to run the cold to the showers through the flow restrictor, just the cold feed to the boiler. The cold to the showers should be left unrestricted - you're just restricting the flow through the boiler to ensure you obtain sufficient temperature in the hot supply to run the showers. They aren't pressure reducers, just flow restrictors - you'll end up with 15 litres per minute of hot at one shower, or 7.5 litres per minute of hot at each shower if both are turned on simultaneously. Working pressure will, of course, be reduced over what you'd have with unrestricted flow, but with unrestricted flow you'd have cold showers, which the flow restrictor will prevent
     
  8. markocosic

    markocosic

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    Thanks muggles, I think I follow now.

    Provided hot water temperature stays high enough (restrict flow to/from the boiler) the thermostatic mixer can/will shift the balance from cold to hot so that it maintains the set temperature.

    Without the restrictor the boiler outlet temperature can drop too low for this to happen and you end up with both thermostatic mixers asking for full hot and 25L/minute of lukewarm water coming out.


    15L/minute can't do any harm. If the incoming water is warmer than 10C then the outlet water will be warmer, you'll need less cold, so the showers will run at more than 15L/minute.

    Is it worth dropping down to 12L/minute through the combi and running the hot water hotter? This way it'll still be over 45C even if the incoming cold is REALLY cold, without running the risk of hitting the 60C limit even if incoming rises to 15C?

    http://www.ebay.co.uk/itm/WATER-FLOW-REGULATOR-RESTRICTOR-12-LITRE-PER-MINUTE-CALFLOW-/400368172125

    Or should we try to keep the temperature as low as humanly possible for hard water/scaling reasons?
     
  9. muggles

    muggles

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    I'd keep it to 15L/min, there's no point having a boiler that can produce that much hot water then restricting it, you may as well have bought a lower power boiler. Also a 12L/min restrictor would knock you down from 7.5L/min per shower with both showers running to 6L/min, which is well into electric shower performance territory
     
  10. markocosic

    markocosic

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    It wouldn't work that way though. It'll flow less water but at a higher temperature - still using the full boiler output - with the shower making up the difference by blending in cold.

    It would avoid the showers running cold when incoming mains is <10C. Easy enough to experiment with after the event anyhow - thanks for the pointer!
     
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