Baxi 105e - DH Only on High Demand. Replaced Diaphragm - Still having issues

Discussion in 'Plumbing and Central Heating' started by Eddamoo, 23 Nov 2020.

  1. Eddamoo

    Eddamoo

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    Hi All,

    Moved house ~6mths ago and inherited this boiler which is ~11y old. Its always been very lazy to turn on for hot water demand. The Kitchen Tap with an aerator needed to be on full, and the shower needed to be run high and hot for it to stay on.

    A few weeks ago i repaired the common leak around the pin which contacts the DH demand micro-switch. I hoped to see a split diaphragm to explain the demand issue, but it was fine. I replaced the diaphragm and bolt (the one with the hole in).

    The leaking has stopped, but the DH demand/switch-on issue is even worse. Sometimes the boiler wont even switch on with 1 tap on fully - you need a 2nd tap to increase the flow. The pin moves very slowly.

    I've since taken the chamber apart again to check for free movement and re-lubricate. Everything moves freely. I've now ordered another kit just in case.

    Since watching some excellent Youtube videos by 'Dr Pipe', i can see the back of the housing has another pin which he places a tiny o-ring on after installing the diaphragm and disk/pin. As i didn't remove the housing fully to perform the change, i can only assume this O-ring fell off when removing the plastic disc. None of the guides i watched prior mentioned this (including Baxi themselves) so that's alarming.

    Does anyone have any ideas for me please? Some things i'm thinking as a complete amateur:
    • Perhaps the small pressure-differential copper pipes are fouled which lead to the chamber interrupting the flow? We aren't in a hard water area, but the back of the housing did have some buildup.
    • Something to do with the CH pressure, but it should be fine at ~1.4Bar cold?
    Many thanks :) Stay safe
     
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  3. picasso

    picasso

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    If you do a search on here for hydraulic Venturi you will find the info your looking for.
     
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  4. Eddamoo

    Eddamoo

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    Wow thanks picasso, super quick reply! I was so busy searching for 'diaphragm' that i must've missed this! The posts do indeed look to be what im looking for.

    Ill get one ordered and report back asap :D
     
  5. ianmcd

    ianmcd

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    yep hydraulic venturi will have broken up
     
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  6. Eddamoo

    Eddamoo

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    Thankyou Picasso and Ianmcd - you were both absolutely correct! The Venturi had indeed broken up!

    Nice and easy fix - just a 24mm spanner to remove and some grips to remove the 1-way check valve from the assembly. I would've struggled to get an adjustable wrench in there but thankfully had a 24mm in my garage thanks to my spanner addiction. The pin now pops out with gusto, and with 1/4 the flow.

    Took my time as access was cramped and the longest part was trying to fish out the loose bits of the old one. Cleaned out the 2 inlet gauze filters while i was at it. Took a leisurely 45mins. Well chuffed!
     
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  7. ianmcd

    ianmcd

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    yep can be a bit of a pain in the butt getting the debris out, a very common fault but really good boilers, well worth keeping
     
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  8. picasso

    picasso

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    Thanks we do appreciate the feedback.
     
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  9. DIYnot Local

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