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Yes that is certainly a potential reason. I will do as you suggest and finally enjoy a well deserved bath this evening as last two weeks were a bit tough (immersion heater stopped working too).

Just for anyone coming across this topic in the future after manually pulling the spindle to CH and HW few times (leaving it in each setting so I can sense the A and B could warm up on their own) and after doing the tests that @Harry Bloomfield suggested I just turned the power off, screwed the...

Ok thanks @Madrab. So all in all there is nothing wrong with the system apart from the boiler going into DRY FIRE. Could the incorrect/stuck 3 port valve cause the flow/return reversing or no flow issues at the boiler or is it an air lock that could do that? But then still the boiler works fine...

The boiler goes into DRY FIRE status mode and this is the description from the sticker: No Water flow or Flow/Return reversed.

Would you replace the actuator head in this instance or the valve or look somewhere else? The boiler seems to be going into DRY FIRE when it remains in the stand by mode (not burning) - not sure that is normal. Maybe the fault is there?

@Harry Bloomfield I have just tested the above and observed actuator moving. I have actually finally also tested the B side of the valve by manually turning the spindle to HW and turning up the cylinder thermostat from min to 55C and only the B side got very hot so the valve is ok I think. I...

@Harry Bloomfield how do I check for sign of movement on the actuator? Are you referring to the movement of the latch there? I do not observe any movement from AUTO position and also it is impossible to latch it into manual setting as it drops out of the slot. Btw I have taken the actuator out...

@Madrab The cylinder stat clicks when I turn it to min and back up to 55deg.

So two showers later boiler still goes into Dry Fire and with the CH OFF and HW ON on the programmer the flow and return coil pipes are becoming only luke warm again. It seems that the issue persists.

This is the setting (attached) I have got at the moment which got my hot water from taps. This has been like that since yesterday. I know that CH is still on and I now disabled it to OFF position. I have to wait until the system is cold to retest now I suppose.

Ok this is very strange now. After I turned the cylinder thermostat down a bit to 55C yesterday evening and cranked up the boiler temperature gauge up a bit there is hot water today and I can feel the feed and return of the coil getting warm finally. I still think there is an issue with the...

@Harry Bloomfield Thanks for your comment. If it turned out I need to change the whole lot would it not be possible to use the rad valve change kit to create a vacuum which would avoid draining the system down?

Thanks @HERTS P&D. Just before I crack on is there any possibility it is not that part that's causing the issue? I know that all the signs are pointing towards it but want to make sure I check everything before I change the valve.

Thanks @HERTS P&D. Am I able to change it myself (I am fairly handy DIYer) or shall I wait for an engineer (from which all are busy) until they become available next week?

@CBW Would you suggest doing another check anywhere? Is it possible to connect the system to the mains or drain and refill the system to rule out any blockages?

Also what I do not understand is that when the motorised valve fails it is usually CH that does not work with HW working but my case is the other way round which baffles me.

One other thing I noticed on the motorised valve is that I cannot make the lever latch into a notch in the casing to keep it open. Not that it changes anything but maybe someone knows if it is relevant here. Maybe that indicates the valve is gone?

Hmm but it would not get same level hot I suppose - it wouldn't make sense.

Could it be the motorised valve is stuck in the CH position and the B pipe gets warm from heat transfer from A?

Also the only time pipe B gets warm is when rads are on and then B and A gets same level warm.