I was wondering if you could help. I have recently (about 18 months ago) had a Worcester Greenstar 24i installed, had nothing but trouble with it but luckily we have a 7 year warranty. The problem we have is when you put a hot water tap on the water runs cold, boiler tries kicking, water warms up a little, then runs cold. boiler then makes some clunking noises - enough to make me jump the first time - then sounds as though it is filling up.
CH works fine no problem, after the water problem I turn off CH, turn it back on and the hot water appears to work fine. This has happened 3 times in the last 7 days. Each time the CH and the hot water has worked fine before and after. Water pressure is fine throughout - about 1.5.
I've tried turning the taps on slowly till it warms up then turn it up faster to fill bath/basin etc... And the problem does not appear to occur. When my missus or the bairn are in though they put the water on full blast, run cold water at same time, put 3 taps on at once, run shower and washing machine etc... run the hot water tap to rinse hands and turn it off before the water has a chance to warm up etc.
Before I call worcester out for the 5th time in 18 months I want to understand if this is the boiler or the system. During installation the system appears to have only been cold water flushed, after 12 months the boiler died due to all the build up of gunk. Worcester replaced all the parts and we had to get a chemical flush from installers at a cost of £350!
On boxing day the pump seized so Worcester replaced it, the Engineer checked magnaclean and said it was clean no problem with system. 2 days later same thing happened and without checking magnaclean another worcester engineer advised on a cold water flush. I got a different CH engineer out who drained the system and said it is clean - no crap in it and does not need a flush. He said the cold water flush bypasses the boiler anyway so there is no point as the system is now clean.
Last week we started hitting the hot water issues.
Sounds like you've been having a number of issues. Before you do call Worcester to book another, is your boiler displaying a fault code? When did you service your boiler last? the following website http://www.mrboilerrepairs.co.uk will provide you copies of your boiler manual and details of faults codes if you need it.
It could be a number of things as you probably already know.
If there was an issue with the initial cleanliness of the system and flushing techniques used, I would still get Worcester out as you have the warranty, as them to check and if possible replace the Plate Heatexchanger if it hasn't already been changed.
There's 2 sides and 4 ports to the plate heatex. There a flow and return which is for the water from the heating side of the system and then there's the other two ports which is the cold water inlet and hot water outlet which goes to your hot taps.
As you can appreciate to heat water from cold to hot takes some doing, so the waterways are small to extract as much heat as possible - so it doesn't take much to block them either, especially if the system hasn't been flushed or maintained properly.