vokera lina 28 hot then cold

[blake floyds]

Can any one help

The hot walter runs hot then cold about 30 seconds

I have replaced thermistors
overhauled the diverter valve
replace over heat stat
checked gas perishers
Cleaned out secondary heat exchanger
represhrised expansion tank
The water preshure is high
checked the fan
checked the pump

The only way to keep it running hot is to restrick the flow so it is very slow
could this be the heat exchanger , but i did clean it with caustic soda but did not get much out .

is there any thing else to clean it with
I have checked other threads but can not work this out
wolud be grate if some one can help

 

[paulrushton]

I had a Vokera which did this and I found it to be the DHW thermistor. It wasn't sending a true reading to the PCB and the water would constantly heat up and up until the safety thermistor cut in and killed the heating. The water would cool again and then the boiler would fire up etc and do this over and over.

A new thermistor is about 10 quid and is an easy fit, make sure you get the correct one though as the threads can differ and you dont want to strip it. You can test them by using a multimeter set on resistance. If it gives a reading put it into hot water (the end without the connections) and see if the resistance changes. If the resistance doesn't change or the reading is infinity then its knackered.

Hope it helps.

 

[gas4you]

Could be the dv, does the ch flow get hot when heating off and hot taps run?

There is a kit to replace/service this.

 

[blake floyds]

Could be the dv, does the ch flow get hot when heating off and hot taps run?

There is a kit to replace/service this.

I have all ready fitted this kit no diffrence

 

[blake floyds]

I had a Vokera which did this and I found it to be the DHW thermistor. It wasn't sending a true reading to the PCB and the water would constantly heat up and up until the safety thermistor cut in and killed the heating. The water would cool again and then the boiler would fire up etc and do this over and over.

A new thermistor is about 10 quid and is an easy fit, make sure you get the correct one though as the threads can differ and you dont want to strip it. You can test them by using a multimeter set on resistance. If it gives a reading put it into hot water (the end without the connections) and see if the resistance changes. If the resistance doesn't change or the reading is infinity then its knackered.

Hope it helps.

I have changed both thermistors still the same

 

[scatmanjohn]

Plate heat exchanger...fit a new one.

 

[superdupergasman]

Plate heat exchanger...fit a new one.

He could be correct...check by testing how hot the flow from main H/E gets, if its up to 80 degrees rapidly and then the boiler cuts off I would say its the Domestic H/E.

 

[scatmanjohn]

easier still..turn off the hot tap just as the boiler cuts off, observe the temperature readout on the boiler. It will change to roughly 90 deg.

If it does, change the heat ex.

 

[blake floyds]

thank for the reply
i will try that

 

[superdupergasman]

easier still..turn off the hot tap just as the boiler cuts off, observe the temperature readout on the boiler. It will change to roughly 90 deg.

If it does, change the heat ex.

Use the little black filling loop lever on the right to do this....when it cuts off turn anti clockwise to the horizontal position, cutting off supply to taps...then you will see display change from DHW temp to boiler temp.

 

[blake floyds]

when I shut the tap off when the boiler went off it displayed 96

Is this the heat ex, if so I take it that it is the dhw heat ex not the main one

 

[scatmanjohn]

Yes, dhw heat ex required.

 

[blake floyds]

can you not clean them out
only I was told there £98+ VAT

 

[Terrywookfit]

I have never replaced one due to blockage!

I usually fill with FX 2 and leave to ferment, then blast with a power washer!

 

[Agile]

Can any one help

The hot walter runs hot then cold about 30 seconds

checked gas perishers

What is a gas perisher???

How do you check it?

Tony