After 3 years of faultless operation, my Baxi Combi Instant 105E boiler is now struggling to produce domestic hot water.   Its OK if I turn the tap on full but even at a flow rate of 6 litres a minute it doesn't heat up. The book says the minimum flow rate for operatioion is 2.5 l/m.  There are no flashing fault lights and everything else is OK.  Input pressure at the stop tap is 4 bar,  central heating seems fine.  I rang Baxi and they quoted £259 to fix it  (repeat £259).  Is there a  DIY solution? ???

That's right.  As I said, I can turn the tap on a fair way (giving a flow of 6 litres a minute) before the boiler fires up.

The  "differential valve - DHW" seems to be the bit that detects that the tap is on and thus triggers the burner, pump etc.  If this is faulty then I may have found the solution.   I'm still hoping that someone may respond who's seen the same problem before and can verify that a replacement valve fixed their problem.

The local  Corgi plumber has had a look at the boiler and is willing to get and fit a new valve but he doesn't sound totally confident that it'll improve matters. Also, he didn't seem convinced that a tap turned "half on" should trigger the boiler to fire up.

Did some googling and found this...

http://www.baxi.co.uk/products/literature/Baxi-Combi-80e-105e-Manual.pdf

On page 6 bullet point 7 implies a flow rate of 2.5 l/min should be enough to fire the boiler.

Also this for another model in the range...

http://www.potterton.co.uk/products/literature/performa24iHeInstallation.pdf

....has a fault finding flow chart.  On page 43 it suggests the boiler should also switch on when the flow rate is more than 2.5 l/min.

It says that if it doesn't switch on and the flow rate is ok (eg more than 2.5 l/min) you should check...

"Is mains water filter and differential assembly clean?"

If you've got a mains filter on your model give it a check.

If the filter is Ok it suggests you check if the "DHW flow valve diaphragm is damaged.?" I guess that's equivalent to the Differential Valve on yours so it looks this is the right track acording to the book.

If it's not that then I can't think of anything else to try.

Bought a diaphram fot the DHW diff  this morning - expensive bit of rubber  (over £9 with VAT) but £250 cheaper than the baxi call out fee.  Instructions in the manual looked straight forward so I did what it said - took out the old diaphram (didn't look damaged, just a bit old) and fitted the new one.   I also cleaned a bit of scale from inside the diff assemby.  And....

PROBLEM SOLVED!!!!!!!!!

Thanks for your help.

[quote author=billythekid  link=1135079327/0#9 date=1136397129]wtg banjo, and wtg CW for some good advice..
btk [/quote]

Blush.

I reckon CWatters will be "Member of the month" for that!