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Question on hot water tank

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  • 29-04-2014 10:15am
    #1
    touchdown77
    Registered Users Posts: 208 ✭✭


    Can anyone help me out on these basic questions;

    I'm looking for a pressurised domestic hot water tank, about 300-400 litres

    -do they come with separate sink and bath elements?

    -can I get a timer that will control the elements separately?
    ie. I want to set the bath element to run each night for a few hours and the sink element to a couple of times during the day.

    thanks !


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  • Options
    #2
    shane 007
    Closed Accounts Posts: 914 ✭✭✭shane 007


    touchdown77 wrote: »
    Can anyone help me out on these basic questions;

    I'm looking for a pressurised domestic hot water tank, about 300-400 litres

    -do they come with separate sink and bath elements?

    -can I get a timer that will control the elements separately?
    ie. I want to set the bath element to run each night for a few hours and the sink element to a couple of times during the day.

    thanks !

    Not combined but some cylinders have 2 immersions, 1 high & 1 low so they could be set up separately.
    You would need to be careful with the current load combined on the cable, as 2 x 3kw immersions will carry 26 amps or 2 x 2.5kw immersions 22 amps.


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    #3
    touchdown77
    Registered Users Posts: 208 ✭✭touchdown77


    OK that's good to know, I presume the overload would only occur if the two elements were on at the same time?

    Would I need two timers or is there a combined one that will control both?


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    #4
    shane 007
    Closed Accounts Posts: 914 ✭✭✭shane 007


    They should be fine to run at the same time, just be ensure the cable & RCD can carry the required load. You will require an electrician for that.

    Regarding timeclock, you will need an immersion timeclock, wired through an immersion switch that will have an on/off switch & a sink/bath switch selector. You could have top as sink & bottom as bath or top as sink & bath as both. Back feed would have to have to dealt with in this scenario.

    If you go for just sink as top & bath as bottom, you will be waiting quite a while as you will be expecting a 2.5kw element to heat 300/400 litres. They tend to burn out a lot.
    You are also heating DHW by the most expensive way, especially for such a high volume. I would consider alternative methods.


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    #5
    touchdown77
    Registered Users Posts: 208 ✭✭touchdown77


    shane 007 wrote: »
    You are also heating DHW by the most expensive way, especially for such a high volume. I would consider alternative methods.


    Yikes, I was basing my assumptions of water heating costs on the article here;

    (I can't post links yet but google "the helpful engineer-water heating" and you'll see the page and article)

    Does the analysis not hold up?

    Regarding time clocks, I was going to use two mechanical types, one for sink, one for bath. Heat the full contents of tank during night (on night rate) and quick 15-30min boost of sink during the evening for top-up.


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    #6
    shane 007
    Closed Accounts Posts: 914 ✭✭✭shane 007


    To heat 300 litres from 10 to 60C with a 2.5kw immersion will take approx 7 hours.
    That's a cost of €3.15 at a rate of €0.18 per kwh or €1.58 at a night rate of €0.09 per kwh.
    I have 250 litre cylinder & use approx 170 litres of hot water per day. It costs me approx €2.50 per week for this volume in winter months.


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    #7
    JohnnieK
    Registered Users Posts: 2,526 ✭✭✭JohnnieK


    OP you should consider a NUOS http://www.ariston.com/uk/NUOS/NUOS_250i_air_source_heat_pump.

    They are fantastic.


  • Options
    #8
    touchdown77
    Registered Users Posts: 208 ✭✭touchdown77


    shane 007 wrote: »
    To heat 300 litres from 10 to 60C with a 2.5kw immersion will take approx 7 hours.
    That's a cost of €3.15 at a rate of €0.18 per kwh or €1.58 at a night rate of €0.09 per kwh.
    I have 250 litre cylinder & use approx 170 litres of hot water per day. It costs me approx €2.50 per week for this volume in winter months.


    I'm working out a 500 litre tank at €2.62 per night (29.06Kwh x €0.09) but what factor does heat retention from the previous night take? (and cycling of thermostat when it reaches 60c)


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    #9
    [Deleted User]
    Posts: 0 ✭ [Deleted User]


    touchdown77 wrote: »
    I'm working out a 500 litre tank at €2.62 per night (29.06Kwh x €0.09) but what factor does heat retention from the previous night take? (and cycling of thermostat when it reaches 60c)

    500litres heated direct is not practical in a domestic environment, usually to achieve any sort of performance they are wired using 3 phase and they can catch fire as they are not respected for the load they can pull.

    I wouldn't go over 300 litres but 200 is more practical.


  • Options
    #10
    touchdown77
    Registered Users Posts: 208 ✭✭touchdown77


    gary71 wrote: »
    500litres heated direct is not practical in a domestic environment, usually to achieve any sort of performance they are wired using 3 phase and they can catch fire as they are not respected for the load they can pull.

    I wouldn't go over 300 litres but 200 is more practical.


    OK, thanks for that advice.

    I had planned on a 500L tank with 6kw and 3kw elements. The 6kw (at bottom of tank) would heat at night rate and 3kw (top of tank) used as a boost during the day. (both on timers)

    Given what you say, maybe a 300L tank is more realistic.


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