Shower spec.

Discussion in 'Plumbers' Talk' started by johnfr, Jan 11, 2020.

  1. johnfr

    johnfr New Member

    Can anyone explain the 'flow and pressure' spec of showers? Example: 2 litres per second at 0.7 bar. Fine but AT WHAT TEMPERATURE?! I'm looking at several 9.5kw showers, specifically multi fit unit, however they all have different specs, some higher pressures, some higher flow, some both, but how is this when the power is the same? The Mira multi fit is 2 l/s at 0.7 bar which dont sound great compared with some other 9.5kw units
  2. Jimbo

    Jimbo Screwfix Select

    Temperature and flow rate depend on the incoming supply temperature. Water needs 4.2kJ per litre per *C it's heating (=4.2kW per litre per *C per second). To achieve 37*C flow from say 10*C inlet at 11 litres per minute would need:

    (37-10) * 11 litres * 4.2kJ / 60 seconds = 20.79 kW.

    Re-arranging to get flow,

    litres per minute = 60 second * 9.5kW / ( (37-10) * 4.2kJ ) = 4.7 litres per minute.

    When inlet temperature is 6*C this would reduce to 4.4 litres per minutes
    When inlet temperature is 15*C this would increase to 6.2 litres per minutes

    Hope that helps.
  3. terrymac

    terrymac Screwfix Select

    No electric shower delivers 2 litres in a second !! Expect you mean per minute.
    The ability to heat the water is determined by the power of its heating elements ,so any 9.5 KW shower has the same ability , irrespective of the brand. All instantaneous electric showers take in cold water , usually from the mains ,and raise the temperature X degrees for a given flow rate. The faster the water flows through the shower ,the lower the temperature delivered at the shower head ,and vice versa. To achieve the temperature desired for showering ,the flow has to be reduced by the user ,and typically would be around 6 litres per minute. This will vary summer / winter because the water temperature coming into the property will be higher in summer than in winter ,and the showers ability to raise temperature is still X.
  4. johnfr

    johnfr New Member

    Correct! 2 l/m! Working in water supply I'm used to l/s!

    I understand fully about flow and temp rise based on power and inlet temp, but these different showers are all quoting different specs in flow and pressure
  5. terrymac

    terrymac Screwfix Select

    All that's telling you is how many litres can pass through the shower at a given pressure ,it's got nowt to do temperature. The simple fact is all 9.5 KW showers have exactly the same power. So buy whichever you like , there is no discernible difference in their performance .
  6. johnfr

    johnfr New Member

    Absolutely fantastic chaps! Everything I needed to know!

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