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Rate of heat loss from hot water tank

  • 20-05-2014 11:47am
    #1
    zagmund
    Registered Users, Registered Users 2 Posts: 2,501 ✭✭✭


    I have a 500 litre hot water tank. The water is heated in the Summer primarily by solar, and in Winter by gas. In the shoulder seasons it's heated by a bit of both. Both solar & gas feed heat the same volume of water within the tank.

    Being from Cavan I have been keeping track of the performance over the winter so that I can try to work out the optimal balance between minimising the gas input and having enough water for a shower.

    One thing I noticed recently is that there's a pretty constant rate of loss from the top of the tank overnight. I took some rough measurements and between 00:00 and 06:00 (when there should be no input to or output from the tank as everyone is asleep, the boiler is off and the sun is down) there's a loss of ~2 degrees, plus or minus a bit, presumably depending on the starting temperature of the water, the ambient temperature in the room around the tank, etc . . . This happens consistently, every night.

    The temperature in the middle of the tank stays pretty constant, so it's not like we're losing heat from the entire 500 litres. The bottom of the tank does lose heat, but at a lower rate. I'm sure there's some model which explains this heat transfer in a body of water, but I don't know what it is.

    One thing I've learned from this is that there's little value in Winter in me heating the tank late at night in preparation for the underfloor heating coming on in the mornings as the tank will have lost a reasonable amount of heat by the time the floors need it at 0600. It makes more sense to turn on the boiler at 0530 or 0545 to make sure there's enough in the tank when the floors need it - a sort of just-in-time model.

    Anyway . . . does anyone know whether this rate of loss (see attached image) is normal or reasonable? The tank appears to be well insulated, it's brand new, there's minimal exposed metal where the inputs & outputs flow. I've no reason to believe that the tank is installed incorrectly. The temperatures are measured using probes in pockets which extend into the body of water, so I'm not measuring the outside of the tank.

    Cheers,

    z


Comments

  • #2
    Sleeper12
    Registered Users, Registered Users 2 Posts: 17,155 ✭✭✭✭Sleeper12


    2° is very little to drop over a 6 hour period. I think you are losing sleep over nothing here.
    Putting the gas on a few minutes earlier or later will save you sure a small amount over the year it's really not worth thinking about.
    It sounds like you have a good system. I'm sure you have thermostats on the hot press boiler linked to motorised valves. I'd suggest you let them do their job.
    If iit's a good system you won't save much tinkering with it.


  • #3
    richieburke01
    Closed Accounts Posts: 217 ✭✭richieburke01


    Insulation all pipes with class O
    That's the black type of insulation and no gaps in it
    Especially the hot pipes


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