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Heating System Thermostat

Heating System Thermostat

This circuit is intended to control a heating system or central heating plan, keeping constant indoor temperature in spite of wide range changes in the outdoor one. Two sensors are needed one placed outdoors, in order to sense the external temperature, the other placed on the water-pipe returning from heating system circuit, short before its input to the boiler. The Relay contact wiring must be connected to the boiler start-stop control input.
Circuit operation:
When Q1 Base to ground voltage is less than half voltage supply (set by R7 & R9), a voltage is generated across R8 and the driver transistors Q2 & Q3 switch-on the Relay. When Q1 Base to ground voltage is more than half voltage supply, caused when one of the n.t.c. Thermistors lowers its value due to an increase in temperature, no voltage appears across R8 and the Relay is off.
C3 allows a clean switching of the Relay. P1 acts as main temperature control.

Notes: R3 is the outdoor sensor, R4 the indoor sensor.
If you are unable to find a 3K3 Thermistor for R3 you can use a 4K7 value instead. The different value can be easily compensated by means of Trimmer R5. R5 allows to set the heating system for outdoor temperatures ranging from about +10°C downwards. The higher R5’s resistance the hotter the heating system and vice versa.
The existing boiler thermostat should be set to its maximum value and not bypassed: it is necessary for safety’s sake.
This circuit can be dispensed with its differential.

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