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Thread: Adjusting an ABV and measuring flow rates through the boiler

  1. #1
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    Default 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.

  2. #2
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    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

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

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    Quote 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.

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    Quote 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.

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

  7. #7
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    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:

    https://www.youtube.com/watch?v=KfJ5SrBnoS4

  8. #8
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    Quote 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; 8th August 2017 at 10:31 AM. Reason: Opening quote added for context

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

    Nice, though!

    P.

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

    http://www.anchorpumps.com/grundfos-...oaAt5IEALw_wcB

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