Overheating vokera maxin 28E

[Joe1970]

We just moved in a new house which have vokera maxin 28E in place for DHW and CH. When we turned on the CH three weeks ago, it was ok for two days. The first time I realise it was doing something wrong was that day my wife told me that it was too hot in the house and asked me if I adjusted the tempature knob of the boiler. I immidiately checked the machine, the tempature was near to 80c and pressure was near to 3 bar. Apparently, it was constantly burning to increase the temprature to that point. But fortunately, it has not yet higher enough to triger the safety cut. And at same time, the hot water was also reaching boiling! so we did not use that night. The second day, it came back to normal again, however, the overheating still happens now and then, and tends to be more and more frequently. Untill now, whenever I turn on the CH or DHW, the burner just won't stop untill safety measures are trigered.

Searching the forum, it seems the faulty of thermistor and has ordered one. So again, any advices on how to check and change the thermistor or any other check could also possibly point to the problem would be appreciated.

 

[Joe1970]

By the way, I should mention that the pressure keep rising when the temperature going mad. It is about 60 degree at 2 bar, and 80 degree near 3 bar. Could there be any single problem pointing to this two related syptoms (tempeature and pressure)?

Thanks in advance.

 

[gas4you]

Expansion vessel could need recharging. Look in FAQ's.

 

[Joe1970]

Thanks Dave. Did you think lack of pressure in EV or faulty EV could cause both temperature and pressure out of control? Hope it is as this will make things simple, though it is not cheap and DIY job.

JOe

 

[Joe1970]

I bought a replacement thermistor and opened the front cover, located the thermistor according to the service manuel. But when I tried to take it out using a span, I found there is water coming out from the contact between the pipe and thermistor. Soon I fit it back then water stops.

I am really confused that is it a wet contact or dry contact for the thermistor? If it is direct wet contact, how could I prevent water leaking when I change the thermistor as there is lots of wires around?

If it is a dry contact, does it mean the pipe sits inside the thermistor is broken making the thermistor out of control?

Joe

 

[chrishutt]

is it a wet contact or dry contact for the thermistor?

Obviously wet!

You have to drain down the boiler. You'd better study the manual before attempting that.