boiler firing fails and burner flashing in HW

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My Ideal icos boiler develops to a weird problem. It works fine with heating, but if hot water is required, the boiler has 'c' stayed on and burner led begins flashing, then gone, it starts work. It seems everything normal. After 2/3 minutes, boiler firing stops suddenly and led flashing comes back, this process always repeats. I checked the zone valve of the Drayton actuator (ZA5), it opens (at B) once ignition starts, the orange wire has power (means microswitch opened), the pump works, it is a new pump. During the repeated processes, the zone valve, the orange wire power and the pump are always on, I guess HW is heated by CH when HW zone valve is in open state. If I turn down temp of HW thermostat, I can hear a click, then the zone valve, the orange wire power and the pump are switched off automatically. To me, it seems the zone valve is okay, but somebody says the microswitch could be faulty. I am not sure if the microswitch or the HW thermostat is fault, any idea?
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CBW

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What voltage are you getting on the orange wire? You’re definitely checking the correct zone valve?
 
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240V, It is drayton 2 port zone valve for HW. CH controls Sunvic 2 port valve.
 
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My Ideal icos boiler develops to a weird problem. It works fine with heating, but if hot water is required, the boiler has 'c' stayed on and burner led begins flashing, then gone, it starts work. It seems everything normal. After 2/3 minutes, boiler firing stops suddenly and led flashing comes back, this process always repeats...!
 
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My Ideal icos boiler develops to a weird problem. It works fine with heating, but if hot water is required, the boiler has 'c' stayed on and burner led begins flashing, then gone, it starts work. It seems everything normal. After 2/3 minutes, boiler firing stops suddenly and led flashing comes back, this process always repeats...!

That seems like a quite normal boiler operation to me. Are you suggesting that it has acted differently previously? Hot water generation can be a low boiler load, so the boiler first modulates to low fire (about 5kW) as the boiler water temperature rises. As the DHW temp rises so the load on the boiler reduces even more, and the boiler thermostat turns the flame off (flashing led) to prevent overheat.

Try turning the boiler temperature knob to about 80% of maximum, and the cylinder temperature to between 55 and 60C.
Does that solve your 'problem'?
 
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Boiler is fine you have a circulation issue on hw only.
 
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My HW thermostat was set to 60. It doesn’t behave like CH, once a programmed temp is reached, the burner led is off, the two port valve and the pump are turned off, after some time, c becomes 0.
 
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My HW thermostat was set to 60. It doesn’t behave like CH, once a programmed temp is reached, the burner led is off, the two port valve and the pump are turned off, after some time, c becomes 0.
The boiler thermostat is off as reached temp but there is still a demand from external controls. It’s normal operation
 
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