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Shower / boiler flow and fill rate - help needed

Discussion in 'Plumbing and Central Heating' started by lsd6294, 7 Oct 2010.

  1. lsd6294


    23 Sep 2008
    Thanks Received:
    United Kingdom
    Hi I want to fit a digital shower pumped type as i currently have a standard gravity fed boiler setup.

    My question is based on my flow rate calcs the hot water tank will empty in about 11-12 mins with the shower on maximum (16l/min), this may be a stupid question but on average how long will the boiler take to refill. to allow for a second shower.

    I presume this is a pretty standard setup, and realise that the shower pressure can be turned down to conserve the water in the tank. but would like to hear from anyone who has a similar setup and how it works for you.
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  3. ajrobb


    23 Jan 2010
    Thanks Received:
    United Kingdom
    I think you forgot to calculate the cold water in the shower. The hot water cylinder has to be at least 60°C by law to kill legionella bacteria. Shower temperatures are typically 38°C and cold water temperature from a cold tank might be as low as 5°C. The hot water has to be cooled by 22°C and the cold heated up by 33°C so, for every 33 litres of hot water you will also use 22 litres of cold water (or 3:2), so you get 5 litres of shower from 3 litres of hot water and that is the worst case in the winter. In other words, your 12 minutes becomes 20 minutes (with the boiler off).

    How long the hot water cylinder takes to reheat depends on the heating coil, the boiler set temperature and boiler size. It could be 30 minutes to an hour or even longer to heat a tank from cold. This can start soon after the shower starts so your 20 minute shower could extend to roughly 40 minutes.

    You also have to refill the cold tank. These are typically 25 or 50 gallons (roughly 100 or 200 litres). If your water main can provide the full 16 litres a minute then you don't have to worry about it. However, it might have to provide no more than 9 litres a minute. In which case, for every 9 minutes of shower, it will take 16 minutes to refill the cold tank. Or, in other words, the tank level would drop by 7 litres a minute and a 100 litre cold tank could run dry in less than 15 minutes.

    There are checks you can make yourself (how you square these with your family is up to you ;) ):
    • turn off the hot water heating and let the cylinder go cold (optional)
      at the beginning of the test, turn off the boiler and turn off the water (at the stopcock)
      drain the cold tank by running the hot tap(s) only (to get the cylinder really cold)
      if you don't know the size of the cold tank, you can drain the cold tank into buckets to measure it
      time how long it takes the cold tank to refill when you turn the water back on
      turn the central heating off and time how long it takes your boiler to heat the hot water (usually until the boiler and pump go off)

    If you can't adjust the pressure from your pump, you could fit a flow restrictor to your shower head to conserve water.
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