Vaillant VCW GB 242 EH, CH temp goes down once DHW is drawn

[don1955]

I have read a lot of your usefull advice on heating and specially on Vaillant boilers and was hoping that you may be able help me.

When CH is switched on boiler fires and run for 25 mts to get the temperature to 80. On reaching 80 flame goes off and comes back when the teperature has fallen to about 50. This will go on without a problem if no DHW is drawn. Fine.

Once DHW is drawn & taps closed ; on returning to CH the flame goes off on reaching a temperaturte of about 70 and does not come back untill the temperature has dropped to 40 or below and will continue to work between 40 and 70 degrees

How can I rectify this.

The system would not fire at all after the cold spell, not having been used for about ten days. Rads were taken out and flushed, diverter valve assembly cleaned and boiler flushed. Pump flushed and impeller cleaned and fitted. gas sound. The DHW flow rate incresed but heigher temperature can be achieved by contrlling the flow at the taps.
(however still top of rads feel little warmer than the bottom)

Many Thanks
Don

 

[Agile]

The temperature is sensed by the NTC sensor.

The decision to turn the gas back on is taken by the PCB and it seems that the hysterisis is too great and presumably its either a badly scaled up sensor or the PCB at fault.

Clean the sensor and if theses are dry pocket as I recall then add heat transfer paste and see if that makes any difference.

First I would confirm that the o/h stat is not operating at too low a temperature.

Tony

 

[ChrisR]

NTC is dry pocket. HW overheat stat screws on (blue). Your heat exchanger (orange) may be the original black one.

 

[pannierstan]

Is this really a fault or are you spending too much time looking at your boiler!!
What is the actual problem related to these temperature readings! ie are you cold or what?

Stan

 

[kevplumb]

howay stan who trusts the temp gauge on a 242

 

[don1955]

Thank you all . I shall attend to the suggestions and come back.

Stan,
the problem is when some body has had a shower the radiators run less warmer and the house gets cold. When the system has been switched off overnight and when it comes on in the morning CH works fine until hot water is drawn. Once this happens it does not improve during the rest of the day.

Kev,
temperature readings were used to show by how much of a change in temperature occurs at the boiler. I do not trust the readings either but the house get cold.
thanks

Don