Vaillant turboMAX F23 - confused!

10 Jun 2010
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United Kingdom
Heating has stopped working. BG engineer visits and diagnoses the problem as being a blockage in the radiator network somewhere. We have an old property with an old one-pipe system, so although BG cover the boiler, they don't cover the rads.

What confuses me is this:- when I run the kitchen hot tap, I get some hot water out (a bit tepid), and then after a short while the boiler is reporting F23; here's the sequence on the boiler from when the tap is first turned on:-

  • S10 - Hot water demand (hot water operation)
    S13 - Ignition sequence (hot water operation)
    S14 - Burner ignited (hot water operation)
    ~~ the above then repeats a few times over several minutes ~~
    S53 - Delay mode due to lack of water in system (~2,5 mins.)
    ~~ then back to S10,13,14 again for a few minutes ~~
    F23 - Dry fire protection (difference between flow and return NTC's too large) - No water in appliance, pump cable defective, pump blocked or defective, pump output too low

I know that the boiler has a valve that diverts the hot water from the primary heat exchanger through to the secondary heat exchanger to heat the water that comes out the taps. So I would think that the rads could be in any state - this valve has closed them off so what does it matter.

So why is this caused by a blockage in the rads? The central heating is not on - there is only DHW demand. Cold boiler pressure is 1.2 bar
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Your boiler has a diverter valve which can get jammed so until that is checked its not clear exactly what your fault may be.

There are many different one pipe systems. The worst just have a 15mm pipe with the rads in series. They usually don't allow enough flow to make your boiler happy.

Depending on the version of your boiler you might be able to see both the flow and return temperatures using the d40 and d41 values. Earlier ones only showed d40.

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Its a very easy fault to diagnose.

I have written up the way to diagnose them in the FAQ on this site!


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