Adjusting an ABV and measuring flow rates through the boiler

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  • bruce_miranda
    Automated Home Legend
    • Jul 2014
    • 2307

    Adjusting an ABV and measuring flow rates through the boiler

    With so much talk about OT and boiler condensing in other threads, I shifted my attention to the little spoken about 'guy', the Automatic Bypass Valve.

    In today's world of self modulating pumps, TRVs and zones, how does one set up the ABV?

    This is likely to become a topic as debated as "is balancing really relevant with TRVs and zones".

    Most boilers will specify a minimum flow rate. So how does one actually measure the flow rate running through the boiler.
  • the crooner
    Automated Home Jr Member
    • Jun 2017
    • 33

    #2
    Another interesting point.

    The Baxi HE boiler my sons has says no ABV required and no heatsink radiator required as long as th boiler controls the pump.

    But the boiler has no live feed, just switched from the zone valves as shown in yjr diagram in the manual.

    Can it really cope with no bypass at all?

    Tony

    Comment

    • the crooner
      Automated Home Jr Member
      • Jun 2017
      • 33

      #3
      And why did British Gas install an ABV if none is required?

      Comment

      • bruce_miranda
        Automated Home Legend
        • Jul 2014
        • 2307

        #4
        Originally posted by the crooner View Post
        And why did British Gas install an ABV if none is required?
        Probably best, they did. You can always turn the ABV down so that it never operates. But you cannot magic one in, if you need it.

        Comment

        • DBMandrake
          Automated Home Legend
          • Sep 2014
          • 2361

          #5
          Originally posted by bruce_miranda View Post
          Probably best, they did. You can always turn the ABV down so that it never operates. But you cannot magic one in, if you need it.
          Well, you can't completely disable them - most only let you set them as high as 0.6 bars differential pressure. You're unlikely to see this pressure if there is another relief valve somewhere though.

          Comment

          • bruce_miranda
            Automated Home Legend
            • Jul 2014
            • 2307

            #6
            How does one measure flow rate through the boiler?

            Comment

            • HenGus
              Automated Home Legend
              • May 2014
              • 1001

              #7
              The technical data for my boiler shows the water displacement that the boiler pump can deliver for a 'particular resistance' of 910L/hr at 20C (max resistance 250mbars) with the boiler adjusting water flow until a temperature difference between flow and return water is reached that is acceptable for the control system is achieved.

              So what does all mean when setting the Honeywell ABV 145? The graph 'appears' to indicate that for a minimum flow of 910L/hr at 250 mbars, the valve should be set at about 3.

              Then again you could just do what this guy suggests from minute 7.00 onwards:

              Comment

              • Edinburgh2000
                Automated Home Guru
                • Dec 2016
                • 134

                #8
                Originally posted by bruce_miranda View Post
                How does one measure flow rate through the boiler?
                I use two methods.

                1) Have a modern pump that displays the flow rate. Here is a picture of my pump, showing the head and flow rate. (1 m3 = 1000 litre)

                Pump.jpg

                However, I do not trust the readings from the pump, especially at lower power inputs, so I also use a more direct calculation method:

                2) Derive the flow rate from the power input and the temperature difference

                The flow rate is given by the following equation, in which we assume that 1 litre of water has a mass of 1 kg and that the specific heat of water is 4.2 kJ/kgK.

                m (litre/s) = Power (kW) / (4.2 * (T2 - T1))

                where T2 is the boiler outlet temperature (oC) and T1 is the inlet temperature. (To use this equation you do need to have thermometers on your boiler inlet and outlet pipes.)


                For me, I can use this equation in the first hour of heating on a winter's morning, when I know that my boiler is running at full power while the outlet temperature is still below the set point. I also have a Loop energy monitor which confirms to me from the gas consumption rate that the boiler is running at full power. My boiler is rated at 65kW and I can measure the temperature difference across the boiler as 11oC, giving me a flow rate of (65 / (4.2 * 11)) = 1.4 litre/s = 5000 litre / hour. My boiler specifies a maximum flow rate of 5000 litre / hour and I have programmed my pump with that limit. For this high power scenario, the two methods correlate well for me.
                Last edited by Edinburgh2000; 8 August 2017, 10:31 AM. Reason: Opening quote added for context

                Comment

                • paulockenden
                  Automated Home Legend
                  • Apr 2015
                  • 1719

                  #9
                  Originally posted by Edinburgh2000 View Post
                  Here is a picture of my pump
                  A Magna 3 costs more than many boilers!

                  Nice, though!

                  P.

                  Comment

                  • Edinburgh2000
                    Automated Home Guru
                    • Dec 2016
                    • 134

                    #10
                    Originally posted by paulockenden View Post
                    A Magna 3 costs more than many boilers!
                    £359 + VAT here:

                    Comment

                    • The EVOHOME Shop
                      Site Sponsor
                      • Dec 2014
                      • 483

                      #11
                      Originally posted by HenGus View Post
                      Then again you could just do what this guy suggests from minute 7.00 onwards:

                      https://www.youtube.com/watch?v=KfJ5SrBnoS4
                      LOL, well that is the worst ABV I think I have seen installed for a while! Think he needs a heating engineer out, as he seems to have left himself a leak on the pump valve (6:59).

                      I personally also use mass flow rate calculation to work heating flow rates out...

                      So the flow rate required in kg/second = H/TD*SH (H = heat required in Watts, TD = temperature drop of the system and SH = Specific heat capacity of the fluid (Water = 4187 joules)).

                      Pub Question... If a boiler requires a minimum flow rate 1032 litres per hour with a 20 degrees C differential, what is its heat output in kW?

                      Comment

                      • Dan_Robinson
                        Automated Home Ninja
                        • Jun 2012
                        • 347

                        #12
                        Originally posted by The EVOHOME Shop View Post
                        LOL, well that is the worst ABV I think I have seen installed for a while! Think he needs a heating engineer out, as he seems to have left himself a leak on the pump valve (6:59).
                        Could be worse... Could be that plumberparts doughnut.
                        Kind Regards - Dan Robinson (Jennings Heating Ltd)

                        Comment

                        • The EVOHOME Shop
                          Site Sponsor
                          • Dec 2014
                          • 483

                          #13
                          Originally posted by Dan_Robinson View Post
                          Could be worse... Could be that plumberparts doughnut.
                          I was thinking of you and your shiny pipes when I watch that...

                          Output in kW please...

                          Comment

                          • Dan_Robinson
                            Automated Home Ninja
                            • Jun 2012
                            • 347

                            #14
                            24.7kw. Plus some characters to make minimal post requests. :P
                            Kind Regards - Dan Robinson (Jennings Heating Ltd)

                            Comment

                            • The EVOHOME Shop
                              Site Sponsor
                              • Dec 2014
                              • 483

                              #15
                              Originally posted by Dan_Robinson View Post
                              24.7kw
                              Correct, 24kW would have been sufficient though... Bet you used your excel spreadshhet for that didn't you...

                              Comment

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