I have a Myson Marathon 700B boiler. It operates with two pumps serving rads and hot water tank. When set on hot water from time to time there is a loud whooshing noise above the hot water tank as though the boiler is "letting off steam". I have replaced the tank thermostat but that doesn't seem to have cured it. Could it be the bolier thermostat that's faulty or could there be a blockage somewhere. The boiler thermostat does click when turned but this problem is occurring when it is on a low setting.
Thanks.
Thanks.
