i have a vaillant boiler when i turn the hot tap on the boiler only fires up when tap is fully open when you slow the flow of water down to get hotter water the boiler switches off
I had the same problem and it turned out to be a tear in the diaphragm.
Boiler Trouble, January 2010
THE BOILER HAS A MINIMUM & MAX FLOW RATE FOR HOT WATER. MEASURED IN LITRE PER MIN.
CHECK YOUR MANUAL FOR FLOW RATES OF YOUR MODEL.THERMISTORS & WATER PRESSURE DIFFERENTIAL VALVE DIAPHRAGM ALSO PLAY A PART IN FLOW RATE & TEMP SENSING.
ALAN, December 2009
The boiler needs a constant flow rate to make the boiler know it needs to fire up. By slowing the water down the boiler thinks no hot water is required and so shuts down.
