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Vaillant ecoTEC Plus 938 - Low Flow Rate

Discussion in 'Plumbing and Central Heating' started by gmax333, 16 May 2020.

  1. gmax333


    16 May 2020
    Thanks Received:
    United Kingdom
    Hi guys. Hope you are able to help me, and thank you in advance for anyone who can contribute.

    I've got a Vaillant 938 and notice that the hot water output isn't near the claimed flow. I've tested the pressure at the stop cock and it's around 3 bar. I've tested the cold flow rate from the outside tap and it's around 21L per minute.

    With one bath tap turned on max (Aqualisa Shower/Bath mixer) with JG Speedfit 15mm flexi's i'm getting 11.0L on D.36. If I open 3-4 other outlets I can get D.36 to report a maximum of 13.1 - 13.2L.

    The manual states 15.9L output (however I thought this boiler could output up to 20L as that's why I bought it). So if the boiler is only capable of 15.9L - how can I acheive this, and where does the 20L figure come from?

    For a little bit of detail about the pipe work as I think i made it far to complicated, however it's:

    -Hot = 15mm DHW outlet is converted straight in to 22mm for a distance of 1.5m and run in to the ceiling. From here it's reduced to 3x15mm pipes. 1 pipe goes straight to the bath, the other pipe goes straight to the shower and the third pipe goes to all other outlets including basin, sink and outside hot tap.
    - The stop cock was replaced with a 25mm > 22mm full bore stop cock
    - 22mm direct feed is first tee off the stop cock going to the boiler (stopping off at the cold for the bath and shower only on it's way).
    - 15mm pipe is used as second tee above to feed all other outlets including toilets, basins, sinks etc

    The 21L flow rate was tested from the second tee'd 15mm pipe.

    Cheers guys
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