A few months ago DHW temperature started to fluctuate mildly between hot and warm. This problem got worse so after some research I replaced the plate HE but the problem has not been resolved, however I note that when the DHW runs hot it is much hotter than before. The push rod operated by the domestic diaphragm operates immediately hot water is called for and operates the flow micro switch, so I am assuming that the diaphragm is OK. The micro switch remains switched all the time the hot tap is open. The burner can take as long as a minute to fire up and never sooner than 25 seconds. All the time DHW is being called for the burner alternates between lit and not lit, the fluctuations range from 20 to 40 seconds (approx) and this condition is the same whether the heating side of the installation is active or not. The hot water tap used to test is within 3 metres pipe run of the boiler. Anyone got any ideas please?

Jack H, November 2019

Well I have not resolved this and I am getting a lot of grief from family members. I have thrown enough money at the problem without a solution so I have requested quotes for a boiler replacement. Going for a Worcester Bosch.

Jack H, December 2019

It's one of those hit and miss situations. I was going to say maybe spark electrode next. But you have already changed those. So would be the. PCB next. Maybe a sticking relay or just worn components. Looking on eBay they don't seem to be to expensive. So worth trying this as there's not many more area it could be.

k, November 2019

Domestic micro switch and temperature sensor (thermistor) replaced with new items. No change, could it be the PCB ?

Jack H, November 2019

Part 13 Domestic micro switch and part 20 Temperature sensor (thermistor) . Is the parts I am talking about . Those are shown on the 1 at page of the installation and installers manual. But you can always call the vokera helpline. That's what most engineers do when it's a tricky issue. Calm the manufacturer they know there boiler the best and are usually helpful.

k, November 2019

I have replaced the safety and fan overrun thermostats and the spark and sense electrodes and taken the opportunity to clean and service the burner and main heat exchanger. No change burner still takes about 30 secs to fire up on demand and alternates between lit and not lit all the time DHW is in use.

Jack H, November 2019

Try replacing the hot water temp sensor and if fails the micro switch that activated by the pin.
There not expensive parts. But with out being there it will be trial and error.

k, November 2019