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? Thanks!
How are you checking for 'power'? Are you using a neon screwdriver or multimeter? Have you checked the HW thermostat by adjusting the setting? Keep a note of the position it is currently set to.
I used a multimeter to get 240/0 volt of the orange wire if the HW was on/off, I can switch on/off HW by turning the HW thermostat.
If you are getting 240V on the orange wire from the HW valve when it opens, then the microswitch is working.
It actually sounds like the zone valve works fine. Is there maybe some restriction on the HW circuit. It could be that the boiler is getting up to temperature due to the water not getting away quick enough. This would result in the boiler cycling on and off.
I wonder if there is trapped air in HW circulation system, once the air is circulated to boiler, the boiler is cut off for overheating. I changed my pump three weeks ago for pump leaking, after that, the boiler was making big bang noise for trapped air in heating system. After the trapped air was released, CH works normally. I don't know if there is separate air bleeding for HW system.
If the fan is running slow due to tight bearings, it will not supply sufficient air to support combustion. This may also be happening on CH but you don't notice it as the boiler may reset the flame failure and try again over and over again. On most boilers their is a small window through which you can see the flame, set it to run on hot water and watch the flame. It should be blue, but if the combustion air is restricted by the fan running slowly, the flame will turn yellow and then go out.