Go to Forum Home Building Simulation DHW without circulation

  • This topic has 3 replies, 3 voices, and was last updated 12 years ago by Anonymous.
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    • #31649
      Anonymous

        Hi all, my first proper post so hope its in the right place.

        When modelling DHW for an existing or new build UK property there is quite often no circulation loop. PHPP seems to assume that all the heat in the dead legs is lost (as Internal Heat Gain, but not taken into account for annual heating demand to stop bad design). Which is right, but only if it were left for lots of time between hot water uses (i.e. the pipe totally cools down between uses.)

        Current BR expects some insulation on these deadlegs but there is no input for insulation on dead legs in PHPP. If there was insulation there would be some sort of time delay between uses as to how much heat was lost, and this would probably reduce the predicted DHW demand, especially for poor systems designs that had then been insulated.

        Does it matter?
        Is there an elegant work around, can you change the losses there or will it affect the rest of the model?
        Could the hotwater consumption per person per day be used to create a time factor for heat loss?

        Thanks in advance for any help.

        Will

      • #38481
        Mark Siddall
        Participant

          Will,
          PHPP assumes three draw-offs per person/day. In reality sometimes the draw off may be clustered (morning bathing/showering by occupants) but at other times -most of the time – the pipe will simply cool off between uses. All in all the benefit of insulation, it is could be argued, is limited if the volume of the pipe is minimised (reduced diameter and length.)

          Alan and Nick are the best people to input on your query. In the mean time try reading the AECB water standard:
          http://www.carbonlite.org.uk/carbonlite/waterstandards.php (refer to page 20 of the 2nd volume of the water standards) and this
          https://aecb.net/PDFs/Clarke_Grant_Hot_Water_System_Design_2010_Paper.pdf

          Mark

        • #38482
          Nick Grant
          Participant

            Mark, what do you mean “try reading the water standard”??? 🙂

            As Mark says the dead legs are assumed to cool 3 times per person per day. It's an approximation and close enough to inform design when choosing between circulation and short draw off or longer draw offs and no circulation.

            However what PHPP forgets is the heat loss from circulation when the pump is off. For a non domestic building this volume can be significant and has to be reheated when the pump starts up again.

            The draw off loss is nothing to do with whether water is wasted waiting for hot (the reason to insulate the dead legs unless micro bore). PHPP assumes a modest 25 litres per person stored hot water use and only calculates the losses as a variable.

            The EST report we did and the Dresden Confeence paper (see my website) put some numbers on the different losses and you have the nice graph in colour in your CEPH services book!

            HTH

            Nick

          • #38483
            Anonymous

              Thank you both. I think I'd got there eventually when I saw that 3 – the 'number of tap openings' or occupany coefficient. I thought it might be assuming a constantly heated pipe like the circulation calc. but of course it isn't.

              There is a lot to read – I would be reading more of it if I wasn't still trying to get my head around Less is More. There aren't enough nice graphs in that!

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