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My Ferroli sigma 20/40 boiler overheats when you switch it on. Turning the temperature control knob on the boiler makes no difference, the radiators get really hot and then the pipes rattle as the overflow kicks in and flows into the expansion tank. Often this overheating takes about 30 mins...

My Ferroli sigma 20/40 boiler also overheats when you switch it on. Turning the temperature control knob on the boiler makes no difference. I suspect it is a simple problem (boiler thermostat or mother board). How much should this problem cost to repair? I have seen that new thermostats on...