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how long to heat 200L cylinder

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  • 06-04-2014 3:45pm
    #1
    cargen
    Registered Users Posts: 270 ✭✭


    Hi,

    in your opinion how long should take to heat a 200L cylinder with a system boiler?

    thanks


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


    It would depend on a number of factors such as the fuel being used, the kw output of the boiler, the efficicency of the boiler, the amount of pipework between the cylinder & the boiler, incoming water temperature & the desired water temperature you are trying to achieve, etc.

    Basically, as a guide, a 26kw standard efficiency oil boiler set at 23kw output running at 80% efficiency with 10% system losses should heat a 200 litre cylinder from 10C to 60C in approx 43 minutes. Fluctuations would occur on this time depending on the ability to the coil to transfer the heat at the calculated rate.


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    #3
    cargen
    Registered Users Posts: 270 ✭✭cargen


    shane 007 wrote: »
    It would depend on a number of factors such as the fuel being used, the kw output of the boiler, the efficicency of the boiler, the amount of pipework between the cylinder & the boiler, incoming water temperature & the desired water temperature you are trying to achieve, etc.

    Basically, as a guide, a 26kw standard efficiency oil boiler set at 23kw output running at 80% efficiency with 10% system losses should heat a 200 litre cylinder from 10C to 60C in approx 43 minutes. Fluctuations would occur on this time depending on the ability to the coil to transfer the heat at the calculated rate.

    thanks for the answer.

    it is a 28kw gas system at 98% efficiency.
    cylinder is just beside and it is taking ages to heat.


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


    cargen wrote: »
    thanks for the answer.

    it is a 28kw gas system at 98% efficiency.
    cylinder is just beside and it is taking ages to heat.
    There is no gas boiler on the market that outputs at 98% efficiency. That is combustion efficiency, not boiler efficiency. Nearly every gas boiler on the market's efficiency is 90.0 - 90.5%.
    Also the 28kw maybe gross output or net output. This would also have an effect of approx 11% the results.

    You must also consider are there any radiators on the circuit while you are trying the heat the hot water as this volume of water would dramatically increase the time.

    If you are able to isolate & only heat the cylinder, then giving that your cylinder can transfer that amount of heat at a given time (which it won't) the time would be approx 31 mins if your boiler output is net or 35 mins if gross.
    This does not account for the volme of water withinin the boiler or within the flow & return pipework between the boiler & the cylinder.


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    #5
    shane 007
    Closed Accounts Posts: 914 ✭✭✭shane 007


    cargen wrote: »
    cylinder is just beside and it is taking ages to heat.

    Probably nothing to do with the boiler but as I mentioned before the ability of the coil of the cylinder to transfer the heat.
    Most likely if your cylinder is old & you are in a hard water area, the coil could be affected by limescale. This can put hours on heat transfer times.

    I had a case a while back where the customer put on their oil boiler at 3pm to have luke warm water at 6pm. The cylinder was full to the brim with lime. That was an extreme case & they were on mains water fed from a council water tower up the road.


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    #6
    makfli
    Registered Users Posts: 157 ✭✭makfli


    Just out of interest. How is the calculation done?


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    #7
    shane 007
    Closed Accounts Posts: 914 ✭✭✭shane 007


    makfli wrote: »
    Just out of interest. How is the calculation done?

    Time (seconds) = (volume x specific heat capacity of water x ΔT) / KW

    So

    Time (seconds) = (volume of water being heated x 4.186 x 50 (60-10)) / Net heat output from boiler to include efficiency losses, combustion losses, pipework losses, etc.


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    #8
    shane 007
    Closed Accounts Posts: 914 ✭✭✭shane 007


    I didn't but to be more accurate you could add the boiler water volume & pipework water volume to the cylinder volume.
    You could guesstimate or calculate the pipework lengths with pipework diameter. For this use L x Pi x r². Pipe length x (3.186) x (pipe radius x pipe radius).

    3/4" pipe would have approx 0.32 litres per meter or 0.64 litres for every meter of flow & return pipework.

    Gas boilers will have extremely low water contents, probably 1 - 2 litres if even, whereas an oil boiler would have in excess of 20 litres.


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    #9
    antoinolachtnai
    Registered Users Posts: 9,774 ✭✭✭antoinolachtnai


    I would question the use of the boiler capacity as the heat input.

    The main issue here is going to be the capacity of the coil in the tank. My tank seems to only have a 5 or 6 KW coil. This means that no matter how inefficient or efficient my boiler is, it can only heat the water at a maximum of 6 KW.

    You can get cylinders with larger coils which will heat the water faster. On the continent, they seem to generally have larger coils.

    You can tell whether coil output is the issue by listening to see if your boiler is modulating or cutting out altogether during the heating time. If it is is, it means that it is constrained by the capacity of coil.

    There are lots of 'interesting' ideas out there about how to plumb a hot tank in Ireland. Basically, the issue is that in general in Ireland, people like the house to heat fast, at the same time as the hot water. So the whole thing is sometimes set up to give the hot water tank very low priority. One way of doing this that I have heard of is to plumb the hot water coil with 1/2 inch pipe rather than the 3/4 inch you would expect. Obviously, this will greatly slow down the rate at which the tank heats.


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    #10
    JohnnieK
    Registered Users Posts: 2,526 ✭✭✭JohnnieK


    If it's taking ages and it's right beside the boiler it sounds like there is a balancing valve closed right down or maybe a zone valve not opening correctly if zoned.

    It also depends on the output temperature the boiler is set at.


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