Worcester 30 CDI - U109 overheating

[flashit99]

Hi everyone,

I guess like most other newbies here I need help/advice, so here goes.

Boiler is about 8 years old & been a bit of hassle for last 12 months. I recently fitted a new PRV to this & suspected from reading up on the matter there maybe another underlying problem & this has proved to be the case.

Visible or audible problem - Rapidly heats upto U9 & pressure gauge jumps from 1 to 3 bar, i suspect from there it dumps through the PRV. The pressure gauge fluctuation is also accompanied with pipe banging sounds within the boiler, this last only a few seconds & then the boiler does start to cool.

Resetting the boiler appears to open a valve or something as the temperature drops as fast as it went up (on most occassions)

The above problem almost always happens when hot water is demanded, the heating does work but I guess the boiler is going the this heating & cooling process so some heat is achieved from the radiators.

From my understanding through reading up the problems could be one or more of the following, pump, expansions vessel, diverter valve prv, heat exchanger, pcb or in otherwords a new boiler.

The diverter valve does appear to have a problem as when the boiler is in cool state it does not always provide hot water, switching tap on & off eventually gives hot water.

Hopefully I have provided enough info for someone to provide tips to help resolve. Thanks in advance

 

[Jamespl]

When you see a U on the display it means the temp is over 100 deg. During this time you won't get any hot water because the primary temp is too high and you'd get scalded. I'd say it's either the primary heat exchanger (heat cell) or plate heat exchanger.

 

[flashit99]

For the benefit of the forum & those who find themselves at this page from searching U109 fault - The pump was faulty.

I actually replaced the boiler due to age & the amount of work it had done, I felt value for money had been achieved.

The new boiler once fitted showed the exact same symptoms out of the box, after several hours messing around thinking the problem MUST now be elsewhere eventually a check on the pump showed it needed assistance to start, once running it was fine.

If I had been sure it was the pump before replacing the boiler I would have repaired it for short term use but it was one of many possible remedies available to me at the time. IF YOU HAVE THIS PROBLEM, UNDO THE SCREW AT THE PUMP, SWITCH ON THE HOT WATER & MAKE SURE THE PUMP SPINS! If it does not either the pump is faulty or the PCB is faulty & not asking the pump to spin. If the pump is spinning I guess you would move onto the plate heat exchanger....but of course check this out.

IF LIKE ME, YOU ARE NOT SURE IF THE PUMP SHOULD WORK ON HOT WATER DEMAND, THE ANSWER IS YES IT SHOULD! I did search this and did not find the answer. Most heating engineers assume you check the basics if trying to fault find, as in my case i had not, so this is not intended as a pop at anyone, it's intended to help others.