Is my bypass valve broken...?

[radiohead319]

I've looked in the wiki and searched other posts but cannot find an answer. I'm not a heating engineer, but like to think I'm fairly logical and can work things out when/if I understand how things work.

Basically I'm wondering if it is a likely failure mode for an automatic bypass to fail so it is permanently open?

Background - my Worcester Bosch CD40 (modulated?) boiler recently started tripping with a E9 Overtemp fault. After following the fault finding steps in the manual (no use) I "fixed" the problem by turning down the power knob slightly.

But the heating does not seem as effective as it used to be - and I don't think it is the power reduction, because that was quite a small change.

What I have noticed is that the bypass circuit next to the boiler seems to be flowing all the time - even when the house is cold and not all radiators are warm with TRV's open. It has a vernier-type scale on it. I've tried fully closing it (clockwise) but the bypass still seems to flow.

Is it possible for this sort of valve to fail open do you think?

Any other ideas?

Thanks

 

[MrTherm]

Auto by by-pass valves are pressure flow operated and should normally be shut, if correctly set they should 'kick in' and gradually open only when any zone valves or thermostatic rad vales are closed or are closing to allow heated water to return to the boiler temp controls - stats, sensors, thermistors etc' which will let them function correctly.

Ideally select the desired effective pump speed prior to adjustment of the by-pass valve flow rate. If possible put a screwdriver to your ear and touch the valve when the system starts from cold - there should be no flow through the valve with controls set for demand to allow all zone valves to be open.

If the valve will not close then a replacement will be needed.

 

[chantelle]

You lost me at Wiki is that a new boiler model?

 

[Blagard]

Is it possible for this sort of valve to fail open do you think?

It is usually just a spring (adjustable pressure) that keeps it closed. So I reckon if that breaks it will fail open unless it jams the works in the process of breaking.

 

[Charnwood]

You lost me at Wiki is that a new boiler model?

Wiki explained
Plumbing and Heating Wiki

 

[radiohead319]

Thanks for the advice guys.