Hot water issue - CH working fine

Suddenly got a problem with getting hot water for only about 30 seconds or so before the water becomes tepid for a while and then ultimately cold.

I have replaced PCBs in the past due to buzzing. For the last few months, I've had a little buzz when the system gets active (pump starting to work?) but it wasn't sustained so I didn't replace the PCB. Now that buzz is not happening at all. Could that indicate the PCB has now failed completely?

Is my best bet to replace the XB104 pump drive PCB? Is there anything else I should try first?

Thanks

Might be a scale issue? You could try adjusting the mixer valve although that might also be part of the problem - they are known to fail.

I should have posted a followup message earlier once I had fixed the problem.

It was the mixer valve. I replaced that and the problem was solved.

The valve was heavily scaled up. System was put in in 2000 (when the house was built) and we live in a hard water areas (South Oxfordshire), so not a total surprise. Most of the scale will have built up in the first 10 years because in 2010 we had an Aquatec filtration system installed so we get almost no scale but obviously enough damage was done in the first 10 years. I guess it would have failed a lot sooner without the filtration.

My mixer valve has also failed which is how I ended up finding this topic.  The replacement has arrived and I will install it this weekend.  Is there anything that I need to know in particular?  

The replacement came with new rubber rings but I am not sure if I also need to add some Jointing Compound on the threads?

Many thanks.

I did use a bit of jointing compound but I do think it was a case of "belt and braces". I made sure I used a jointing compound suitable for drinking water.

I'm not an expert plumber but it was pretty easy to do this job. I had a leak at first but that was because I had been over cautious in tightening. Once I tightened it up, it was fine.

I used the rubber washers that came with the replacement.

The old washers had disintegrated, so it required a bit of effort to clean out the residue.

Hope it goes well.

Thanks for the advice - I replaced the mixer this afternoon and all seems to be working. I have hot water again! Although the temperature reduces after a while which I guess is down to scale in the coil - that's an issue for another time. I took some photos of the installation and will post an album to imgur and link here under a separate thread for others to follow.