Shower flow rate help

Discussion in 'Plumbers' Talk' started by rich2766, Oct 12, 2018.

  1. rich2766

    rich2766 New Member

    Hi there,

    I've read many posts and found a lot of help from the forums but I have a problem and I'm not sure how to overcome it...

    First of all I would like to highlight that I am very aware of the difference between pressure and flow rates. It's part of my job, but I'm not in the plumbing trade.

    Okay, I'm remodeling my house and intend to create a new walk in shower. This will be the only shower in the property

    The building of the dedicated walk in shower is going to come with a high price tag and I need the shower to have a high flow rate ... Something in the region of 20 litres per minute.

    My cold water supply runs at 12 lpm and the hot water supply through the taps and current shower runs at 8 lpm.

    Cold water = 12 litres per minute discharge rate
    Hot water = 8 litres per minute discharge rate
    Cold water supply pressure is approximately 3 Bar.

    At present there is a three year old combi boiler providing hot water.

    With the mains cold water supply of 12 lpm a boiler change will result in a flow rate that will match, but not exceed 12 lpm for obvious reasons.

    What method or system of heat generation, storage tank and pump will be required to achieve the flow rate I desire?

    I am happy to leave the current combination boiler in situ to provide hot water and heating for the house and prepared to install a separate dedicated hot water supply purely for the shower.

    The cost of a dedicated hot water supply is not the primary concern. If I can't achieve a flow rate greater than the current cold water flow rate (12 litres per minute) then the building of the new shower cubicle will be pointless and I'll have to go back to the drawing board.

    Any help, recommendations or advice will be greatly appreciated.
  2. kiaora

    kiaora Well-Known Member

    You want 20 l/min
    You got 12 l/min ?

    The maths.....
    How long do you want to stand under the shower?

    10 mins?
    = 200/10
    = 20 l/min
    So you need to have 200 lt of water on demand ?

    A combi can’t do that?

    So a store and a pressurisation unit required,

  3. retiredsparks

    retiredsparks Well-Known Member

    Why exactly do you need 20l/minute ?
    Is 12 not enough ?
    I suspect you will need a bigger cold water supply from outside stop tap.
    Certain shower rose configurations can make the shower spray 'feel' a lot more powerful.
  4. rich2766

    rich2766 New Member

    Hi Peter,

    Thank you for your reply.

    Yes I would expect the average shower to be in the range of 10 minutes.

    And yes, a store of readily available hot water will required.

    The volume of 200 litres is the ball park size I was expecting. Perhaps up to 300 litres so that the entire volume from a 200 litre wasn't exhausted if a shower was to take any longer than 10 minutes.

    But what exactly should I be looking at? Some sort of emersion heater with a hot water feed from the current combi boiler?

    Is there anything on the market that stands alone... ie: cold water supply to a tank which will heat the water (electric heating element?) so that the volume of hot water is available on demand?

    I'm at a loss in regards to what I am going to require to make this work.
  5. CGN

    CGN Well-Known Member

    Try having a bath instead!
  6. rich2766

    rich2766 New Member

    Hi there Retiredsparks,

    I need a flow rate in the region of 20 litres per minute as the current flow rate of (hot water) 8 litres per minute is not adequate.

    I'm not sure how a larger diameter cold water supply to the stop tap will provide me with a readily available flow rate of hot water.

    If I had a cold water supply of 25-30 litres per minute then the solution would be to fit a combination boiler that would be able to heat that supply and provide me with the desired flow rate of circa 20 litres per minute of hot water.

    If such a high supply rate of cold water was available I wouldn't have posted my query, but that isn't the case. I only have a cold water supply rate 12 litres per minute.

    If everyone out there see's re-piping the current cold water supply from the domestic suppliers feed to my property as being the only solution then I'll peruse that avenue, but I thought some system of tank and heat source would provide a better solution.
  7. rich2766

    rich2766 New Member

    The bath is the easy one. Just open the taps and wait a little longer for it to fill.
  8. The Teach

    The Teach Well-Known Member

    rich2766,tbh consider getting in a proper plumbing company to specify and install a system to your requirements.

    currently we are fitting a walk in shower with 6 body jets and an overhead rain effort shower head. it can be done but youl need to splash some cash
    rich2766 likes this.
  9. kiaora

    kiaora Well-Known Member

    Looks like you really want a top specs system, give this company a call, we have fitted them in various places, and considering the performance you want, they’re not too expensive,
    The rep will sort you out,
    rich2766 likes this.
  10. rich2766

    rich2766 New Member

    I'm gobsmacked that nobody has any idea how to achieve a flow rate of hot water higher than the cold water supply.

    I may well be ignorant in the field but this isn't my profession.

    Does anyone remember immersion heaters? Does such a system not exist today?

    So other than employing the services of a specialist company there is no way that I provide hot water to a shower that's at a flow greater than the cold water supply?
  11. Allsorts

    Allsorts Well-Known Member

    Hi Rich.

    3 bar pressure is 'ok' although not startling. It might, however, be enough for an unpumped one of these:

    It's essentially a giant expansion vessel for mains water. It fills from mains pressure (ideal for situations where pressure is good but incoming flow rate is poor) and this compresses the inbuilt diaphragm. Once filled, it'll then deliver it's stored quant of water at the stored mains pressure - around 3 bar in your case - and at well over 20lpm.

    When you then consider that a shower from a combi is achieved by a mix of hot and cold water - I'm guessing roughly 70:30 or so? - then a decent combi (38kW or so) will provide over 15lpm of ouchy hot water which will need some mixed cold to calm it down = as near 20lpm as makes no difference.

    20lpm is a hell of a shower :eek: :)

    Should you find that the incoming mains isn't as good as you thought - ie pressure not a consistent 3+ bar - then consider the model with a built in pump which pressurises the cylinder to full capacity.

    I haven't had user experience of this actual model, but did speak to the guy - Mr Challis himself - when I was trying to sort my bro's very poor mains supply, and was impressed. Bro ended up getting a Grundfos equivalent instead, tho', but only because his chosen plumber wouldn't use anything else. Bro's water pressure is now utterly nuts - I had to ask Grundfos how to turn the bludy thing down... (answer - fit a PRV on the outlet!)
  12. Heat

    Heat Well-Known Member

    A larger mains pipe going to your house should increase the flow rates.
    (On that point, - you must take flow rate from an open unrestricted mains pipe to find maximum flow rate).
    An unvented cylinder - best heated indirectly from your gas boiler (boiler can be heat only, or a combi because a combi can still be also used to heat a cylinder if rads are zoned.
    There is another way to boost the flow rates, but you might be best going to a dedicated plumbing forum and asking for advice directly.
    Allsorts just beat me to it! :)
  13. Heat

    Heat Well-Known Member

    What sort of immersion heaters?
    Where do you get the flow boost from, that you require?
  14. jonathanc

    jonathanc Active Member

    It’s not that hard to spec out the kind of system you need. Just thinking through what amount of hot and cold storage you need and pumping capacity. However given that you are “gobsmacked” that in a few hours of being on this forum you haven’t been given the answer you need for free by other generous contributors, the best suggestion is to pay someone or look elsewhere. That way you can be less “gobsmacked”
    WillyEckerslike likes this.
  15. Tyro

    Tyro Member

    Not a plumber, but if you want more flow (of anything) than the existing supply can manage, there are in principle only two options:

    1. Increase the flow rate of the incoming supply, either through a larger supply pipe or fitting a booster pump on your rising main. Talk to your water company about what is technically feasible and/or permissible under regulations

    2. Use an accumulator of some sort, in practice a large cold water cistern feeding a hot water storage cylinder. Depending on the head of water available from that cistern, you may need a booster pump to feed your shower at the desired pressure and flow rate.

    The cheapest and simplest system is a cold water cistern feeding a vented hot water cylinder, said cylinder being heated either by your boiler or an immersion heater, or possibly both. Unvented systems are more expensive and complex.

    For a ten minute shower at 20 litres per minute on a mains supply of 12 litres per minute, you would need at a minimum a cold water tank with a working capacity of (20-12) x 10 or 80 litres. This would be completely drained at the end of your shower. In practice, you would install a tank of perhaps 200 litres capacity as high up in your house as possible. Note that this means adding a load of around a quarter of a ton to your roof structure. If your house was not designed for this, you should take advice on whether it is feasible.

    You would also need to install a large hot water cylinder. These can be made to order within reason. You would need to fit an electric immersion heater and/or have it heated by your boiler. The cylinder does not need to hold 200 litres of hot water since the shower will be a mix of hot and cold, but for your proposal would need to be one of the largest sizes readily available.

    Electric immersion heaters for domestic use are normally 3kW in rating. Assuming your cold water temperature in the cistern is say 10 degrees and you want to heat it to 60 degrees, the temperature increase is 50 degrees. Assuming your shower water mix is 70% hot water and 30% cold, you'd need to heat 200 x 0.7 or 140 litres of water. The specific heat capacity of water is approximately 4.19 J/g.K which means that to heat 140 litres (i.e. 140kg, close enough) by 50 degrees you would need 4.19 x 140 x 1000 x 50 or 29,300,000 joules of energy assuming 100% heater efficiency and no heat losses from the cylinder. Since 1kWh is 3.6MJ, this means 8.15kWh of energy needed. A 3kW heater would need 2.7 hours to heat this, so to do it faster you'd need multiple heaters. A gas boiler will heat the water quicker.

    Adding the additional pipework will increase the resistance to flow and thus pressure drop, in the same way as lengthening an electrical circuit will increase voltage drop. You can minimise this by careful design of the pipework, for example using large bore pipes, forming bends rather than using soldered or compression elbows, locating tanks and cylinders appropriately, and so on. A professional plumber would advise you on this.

    But you may also want to go back to the beginning and work out why you need a ten minute shower at 20 litres per minute. The advantage of a shower is that you use significantly less hot water than a bath and therefore economise on heating costs. Two hundred litres is about the volume of water for two baths, so your proposed arrangement is much less efficient than just having a bath.
  16. Pollowick

    Pollowick Well-Known Member

    For a 20 lpm shower you do not need 20 lpm of hot water. A combi boiler will run at say 65 degrees and the incoming cold may be at 5 degrees. A shower is usually 35 to 38 degrees - round that to 40 degrees.

    So, 40 deg x 20 litres = 65 deg * (20-a) + 5 deg * a where "a" is the cold flow rate.

    40 * 20 = 65*20 - 65*a + 5*a

    800 = 1300 -60a

    60a = 1300-800 = 500

    a = 500/60 = 8.3

    So you will need about 12.7 lpm from the boiler.
  17. rogerk101

    rogerk101 Well-Known Member

    I'm surprised that no one has commented yet on the sheer ostentatious greed of anyone who thinks he/she is so far above us mere mortals who would gladly take a 10 minute shower at a flow rate of 12l/min that he/she needs a 20l/min shower flow rate.
    What makes him/her so special that they think they're worth it?
    Perhaps there's more to the OP's username than we might have thought.
    A sense of entitlement is an understatement here!
  18. Allsorts

    Allsorts Well-Known Member

    It's ok, Roge; Rich has already PM'd me about that in case I thought he was being greedy. He isn't. The way he plans to use the shower means it works out at only 5 litres per minute per person.

    No, you are not invited.
    Tyro likes this.
  19. The Teach

    The Teach Well-Known Member

    The job we did #8 involved fitting an unvented cylinder with balanced hot & cold to supply one wet room with multiple shower outlets. The existing combination boiler retained for other hot water draw offs & heating,supply and fit cost for the unvented cylinder,fitting shower mixer/diverter 8 body jets just shy of £3 Grand. not including building the wet room or drainage :).

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