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    • #31794
      Tom Foster
      Participant

        Trying to get clear how to establish 'base temp'. From CIBSE Guide A 7th Ed para 2.5:

        “Accumulated temperature differences are calculated as the
        difference between the prevailing external, dry bulb
        temperature and a ‘base temperature’. This is the external
        temperature at which, in theory, no artificial heating (or
        cooling) is required to maintain an acceptable internal
        temperature.”

        Say an uninhabited building, with no solar gain, only subject to conductive/convective fabric losses/gains (via U-value) and to air-change losses/gains, relative to external dry-bulb air temp. No gain from occupants body heat, lighting/cooking/appliances/hot water tank, no solar gains.

        In that case, ignoring thermal lag, stored heat etc, internal air temp will simply follow external air temp. So if I deem the target “acceptable internal temperature” to be say 18C, then 'base temp' – “the external temperature at which … no artificial heating … is required” – must be 18C.

        Correct?

      • #38868
        Tom Foster
        Participant

          Bump! Any comments?

        • #38869
          Tom Foster
          Participant

            Thanks for replying Mark.

            In applying the degree-day method, I am hoping to avoid the complicated guestimating that conventionally seems to be immediately involved, to decide on the appropriate 'base temp' for the building – before looking up in the tables you have to know what 'base temp' applies. Whereas I'd prefer an approach that simply makes 'base temp' equal to desired internal temp.

            Asking the same question on
            http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=10336&page=1
            got mainly scepticism about my suggestion that gains from occupants' body heat, lighting/cooking/appliances/hot water tank, and solar gains could be accurately estimated as the first contribution to reqd heat input, to which fueled input wd then be added to supply the remainder.

            They thought a better approach would be to select from some database of comparable buildings plus do some regression analysis, to arrive at a base temp, say 15.5C instead of 18C, which already assumed those incidental gains, so that only fueled input would remain to be added.

            To me, the latter approach seems far more of a guestimate, whereas I see little difficulty in quantifying the incidental gains.

            Any opinions?

          • #38870
            Tom Foster
            Participant

              Any opinions?

            • #38871
              Mark Siddall
              Participant

                Tom
                It sounds like you should get PHPP and give it a whirl.

                Mark

              • #38872
                Tom Foster
                Participant

                  I do hope AECB still sees ways of approaching questions, other than PHPP as panacea.

                • #38873
                  Tom Foster
                  Participant

                    Not expecting you to do all the answering Mark – but where's everyone else gone?

                    If a simple Q about degree-days can't be asked here, then where?

                    Note I'm not asking how to calculate gains (which yes PHPP wd be great for) – but want to check the validity of a way to establish degree-days 'base temp'
                    Don't tell me PHPP holds the answer to that.

                  • #38874
                    Mark Siddall
                    Participant

                      I've already commented on the basic method. And confimed that you are on the right track.

                      ….erm. There is open software available (auditable i.e. in MS Excel format) that can help you to corroborate that you are using an appropriate calculation methodology. This will help you to ensure that the degree day method that you are employing accords with BS EN standards.

                      Mark

                    • #38875
                      Tom Foster
                      Participant

                        OK, thanks Mark

                      • #38876
                        Tom Foster
                        Participant

                          Thanks – that's v clear.

                          … assuming that gains and losses through the fabric were approximately equa

                          by that you're just confirming, as I said “ignoring thermal lag, stored heat etc”?

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