DIY Doctor UK DIY Forums

Ideal C28 boiler about 5 years old. I flushed the whole CH system a few days ago an added a flushing chemical for a few weeks to clear out some of the muck in the radiators. Since then a problem has appeared with the combi supplied hot water... When the CH is running the boiler will supply hot water okay but when the CH is off then no hot water. When the hot tap is opened the flow sensor detects this, the 3 way valve diverts to the DHW heat exchanger and the CH pump starts - this then circulates water through the DHW heat exchanger. Within a few seconds the pump stops and the power lamp flashes an error. Have now found that the plunger connected to the CH flow diaphragm extends fully to operate its microswitch when the CH is on but doesn't extend far enough to activate switch when only hot water is being demanded - hence boiler thinks there's no circulation to the DHW heat exchanger. Any ideas why this is happening? Had thought there might be a small amount of air getting trapped in the DHW heat exchanger plates/tubes etc. System is at 1.5 bar and have bled all the air from every place I can.

If the pin isn't connecting with the microswitch then that will be the problem, as the fan will not run etc. Have you tried operating the microswitch with a screwdriver to see if the boiler will flash up - on water only ?

Quite an odd fault if the pin does connect with the heating on, I would strip down the diverter valve and check the diaphragm - it may be stretched or split also clean all ports - something may have become lodged when you flushed the system.

Thanks htg engineer, did wonder if it might be debris stuck in the DHW part of the circuit or a bubble of air in the DHW heat exchanger that was being compressed rather than the pressure operating the diaphragm. Hot water works fine when the microswitch is manually operated with a small screwdriver. Could probably make do for the moment and strip down the assembly when I come to flush the cleaning chemical in a week or so's time.