Got a Worcester Bosch Combi boiler, it was fitted about 7 months ago and has been working fine. I had a home boost kit installed as my water pressure was so low. I opened up the filling loop to slightly increase the water pressure in the boiler to just over 1 bar. Filling loop is now closed. Now every time the heating comes on it starts to heat up, the next suddenly dumps a load of water out of the overflow. The pressure gauge on the boiler stays exactly in the same place throughout all of this, doesn’t go up when it starts heating up, doesn’t go down when water is released...
It also heats up to about 45 degrees before shutting off, then comes back on and heats up again and then suddenly gurgles and then it dumps the water.
It sounds as though the water is not circulating and building up steam inside the boiler causing it to overheat, switching off the burner then purging the excess pressure. When it cools the cycle starts over again. If your pressure gauge is perfectly still through all this then either there is a fault in the gauge or electronics somewhere, if it's a Worcester it probably has an analogue one, or the boost kit is stabilising the pressure around the sensor. It could be anything from a blockage or even the boiler pump. The most obvious thing to check first is that any gate valves on the system are fully open. Always amazed how many are not open properly or have been inadvertently turned off.
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