How to set auto bypass valve / is it necessary?

Discussion in 'Plumbers' Talk' started by Rob91, Feb 11, 2021.

  1. Rob91

    Rob91 New Member

    Good afternoon all,

    I have an Ideal Classic FF360 boiler (and an unvented HW cylinder). Boiler is 18 years (ish) old and cylinder was replaced last year, with a bypass also being added. Initially, the bypass had a manual gate valve installed however an auto bypass valve was subsequently fitted before the gate valve (presumably making the gate valve redundant, but it still there).

    The boiler constantly cycles and my theory is it because the bypass is always allowing water through. The bypass valve and pipework after it is immediately hot when CH is running, and maintains a similar temp to the main CH pipes. My understanding is the the auto bypass should only allow water through once all the TRVs around the house have closed, but it seems to be letting water through all of the time thus it heads straight back to boiler whilst hot, making the boiler cycle.

    Two questions:

    1. How do I set the auto bypass? It is a plumb centre branded one with a dial from 0.1 to 0.6. It is currently on 0.2 which must be how the installer left it. Reading the manual for the valve, I need the minimum flow rate of my boiler (I cannot find this in the manual, only reference is to a flow rate table on page 7 but it does not say "minimum" - figure is 22.8L/min) and the "pump head" of the pump at minimum flow rate. The pump I have is a Grundfos UPS3. If I plot the 22.8L (1,368L/hr) on the pump curve (at speed 2) I get a head of around 4.5m. If I then plot this on the auto bypass valve's chart it puts me off the bottom below the 0.1 setting, so something must be wrong. Any ideas?

    2. Can I not just close the gate valve therefore shutting off the bypass and ignore the auto bypass valve? All radiators in the home have TRVs except for the radiator in the hallway (with the stat) and then there are 3 towel rails without TRVs. Will these provide sufficient circulation without needing the bypass?

    Thank you for your help. Links to the boiler manual and pump manual are below.

    Boiler manual:
    Pump manual:
  2. kiaora

    kiaora Screwfix Select

    Close the gate vale and see what happens ?

    good luck
  3. Rob91

    Rob91 New Member

    Hi Peter,

    Thanks for the reply and i’ll give this a go tomorrow AM when heating comes on from cold. But is there not a risk kf damaging the pump if there is pump overrun and/or if both motorised valves for HW and CH are closed?

  4. sam spade

    sam spade Active Member

    Your calculations are correct; the ABV needs to be set to approx 0.1. I would set it to just above the minimum and monitor the results.

    The flow rate given in the boiler manual assumes that the temperature differential is always 11C. This rarely happens, particularly with fixed speed pumps, so the actual flow rate could be higher or lower. You will have to expriment with the ABV setting.
    Dave does Gas likes this.
  5. Rob91

    Rob91 New Member

    Thanks Sam. I’ve turned it to 0.1 but doesn’t seem to have made any difference (bypass pipework is still really hot after an hour or so, I assume plenty of water therefore continuing to pass that way). A stupid question for you - does turning it down (to 0.1) reduce the water flow or turning it up (to 0.6) reduce the flow? I assumed increasing the figure since this would mean a higher pressure is needed before the valve opens? And an even stupider question...if the whole system is pressurised at 1 bat (when cold), is this not already a high enough pressure to open the ABV?

    Thanks again for your time
  6. sam spade

    sam spade Active Member

    To answer your second question, first.The static pressure is irrelevant; it's the difference in pressure across the ABV which determines when it opens.

    As for your first question, if the pipe on the exit side of the ABV is hot you need to turn the ABV to a higher setting. It's just a case of trial and error. There should be no flow through the ABV when all radiator TRVs are fully open; and the ABV should start opening as one TRV shuts off.
  7. Rob91

    Rob91 New Member

    Thanks Sam. I will gradually increase from 0.1 to 0.6 and see how I get on (will do in AM when heating first comes on to ensure all TRVs open)

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