The pump has To some central-heating systems r even three speeds. At the lowest speed, not much hot water is pumped around the circuit and this may result in the boiler over-heating. The switch circuit proposed here prevents that happening.
The electrics of the pump are as shown in the diagram. The main winding is normally 175 Q and the auxiliary windings 135 Q: these values may, however, be different in certain pumps and this should, of course, be checked. The capacitor in series with the windings provides the necessary phase shift that enables the motor of the pump to rotate. In positions a and b, the impedance is increased and this results in a weaker field so that the motor runs more slowly and the pump displaces less water.
A simple circuit enables the 7aato.matic switching between a and b or between b and c. Its 24-V input is parallel with the drive of the gas valve of the main burner. When this valve closes. the speed of the pump increases and the boiler can not overheat.
If the installation uses a 240 V drive for the gas valve, a small transformer may be used to obtain the 24 V. On a safety note. bear in mind that the 24 V supply, which reaches the thermostats in various rooms, is insulated from the mains only by the relay. Therefore, this relay should be a heavy-duty type that provides adequate insulation.