Intermittent dhw problem

[Mechanic1]

Help!!
I have a baxi 105e boiler which has served well for many years but recently it has been playing up. When dhw is called for by the shower it fires up ok and gets up to temp, but after a few minutes the boiler cuts out until the temp drops to 60 degrees then fires up again, causing a cold and hot shower. It makes no difference if the heating is on or not, also it's the same via the hot tap. Heating working fine.
I've changed the dhw thermostat and the thermostatic cut out but still the same. I have also noticed that the boiler pressure is creeping up even though the filler loop in not connected. I am wondering if the diverter valve may be sticking slightly. Would this cause the symptoms that we have? I have ordered a repair kit to overhaul the diverter valve as a matter of course, bearing in mind the boilers age..
Any thoughts would be appreciated.
Thanks

 

[45yearsagasman]

You could also have a blocked plate heat exchanger with those symptoms.

 

[bazdaman]

Poss gas related or blocked plate can you take a temp reading accross plate?

 

[Mechanic1]

Many thanks for the advice.
Would you recommend overhaul of the diverter valve anyway, and can the diverter valve cause the problem we are having. It's not a major issue at the moment but a bit of a shock to the system when the temp in the shower drops, albeit only for 30 seconds .

 

[bazdaman]

When system is cold run hot tap and feel firt pipe on left (ch flow) if it starts to get hot with hw running then overhaul diverter valve, if not then save the new diverter for future use.

 

[Mechanic1]

Thanks. Will try that tomorrow when heating is cold.

 

[Mechanic1]

I've tried the feel test on the ch feed with the dhw running. It didn't get hot. I also turned the dhw temp regulator low and the burner did not cut out unless the temp was raised above 60c. As soon as the temp was increased it started to cut out and restart going down to 60c before firing up again. Do you think this could be the plate heat exchanger being partially blocked as was mentioned previous?
Another question, do I need to remove diverter valve assembly to get to heat exchanger, in which case I will overhaul at the same time.....

 

[45yearsagasman]

That sounds like the plate partially blocked...The plate can be removed without removing the diverter.
Descaling it is possible, but new ones are reasonably priced. Don't get any water on the electrics!
From memory there are only 2 screws holding it in place. Renew the 4 seals when refitting..

 

[Mechanic1]

Thanks for that. I've ordered the seals and will give it a bash. Thanks again.

 

[Mechanic1]

Well bought the diverter valve overhaul kit. Stripped it down and rebuilt it. Removed the plate heat exchanger and gave it a good clean out with brick acid ( full of crud) all rebuilt and system pressurised.
So far so good !!
Now it won't fire up. I have a warning light which say pump fault or low pressure. The gauge pressure reading is ok. The diverter valve appears to be working ok.
When in hot water mode nothing happens, when in heating mode the led green light comes on for the heating, the pump runs, but nothing else. No fan and no ignition.
Can anyone shed any light on a potential cause??

 

[bazdaman]

Did u put the correct microswitch back on diverter valve?(red wires) Check operation of pump flow microswitch, is it making?

 

[Mechanic1]

Forgot to mention that the pcb was kept dry. It was removed to protect it. Have even tried a second pcb to see if that made a difference, it didn't.

 

[Agile]

Would it really have been too expensive to have called a professional to do it all for you and leave it working?

Tony

 

[Mechanic1]

You may be on to something. There are green and red. Thought they were red and green on each of the microswitches. Are you saying the Reds should be on the deverter valve and green on the dhw microswitch?
I have a house converted from two but kept two separate systems, the other is also a baxi so could check that and compare.

 

[Mechanic1]

No Tony it probably wouldn't but what's life without a challenge?