Worcester Bosch Greenstar 24i, hot water not very hot or even cold sometimes

[talktalk]

This was installed about four and a half years ago, and apart from a leaking plastic hydraulic block which had to be replaced last year it has been fine. CH is fine, and the shower water is hot. But for the last few months the bath water is only warm and the hot water supply in the kitchen is sometimes hot, sometimes warm and sometimes cold. I had the Worcester Bosch engineer out last week, he replaced what he called the plate (I think it was the heat exchanger) which he said was full of grit. There is no improvement to the hot water supply. He blamed the warm bath water problem on the boiler not being powerful enough (only 24kw) except for a shower. He also blamed the weather - the cold water supply temperature is low and the boiler can only heat the water by 30 degrees. But the hot water supply problem only started a few months ago - last winter was just as cold if not colder and we had hot bath water! The condensate pipe froze last December, that's how cold it got! Has anyone got any ideas that I can pass onto the next engineer? It is installed in the loft of a ground-floor bathroom but so far no one has blamed that.

 

[talktalk]

Update: I reduced the cold feed so the valve is only open a quarter of a turn and that seems to have fixed the problem. Hot water is coming out at 45 degrees C with the hot taps fully open and the flow rate is like it was a few months back before this problem developed. Thanks for everyone's help and info.

 

[SILKCUTZ]

Our plumber explained this to us really well. This is a good boiler but not the largest. It heats water by firing on cold water being pushed through it. In summer it's fine. The temp of the water going through it is higher so raising up the temp is easy. In winter the cold is very cold. Raising it Up 30% still doesn't make it warm enough. The answer is to not turn the taps on to full but back a few notches. Reducing the flow allows it to heat a smaller amount of water to a higher temp.

Watch out though to small a flow will not kick the hot water in. The flow has to be powerful enough to turn the hot water on.