Boiler control

  
The purpose of this circuit is to control the water temperature in a hot-water heating system. What it does is to lower the boiler temperature as the outside air temperature increases. The op amp is used as a comparator. Thermistor TH2 and R2 form a voltage divider that supplies a reference voltage to the op-amp`s inverting input. Thermistor TH2 is placed outdoors, and the values of TH2 and R2 should be chosen so that when the outside temperature is 25 °F, the resistance of the thermistor and resistor are equal. Resistor Rl and thermistor TH1 make up a voltage divider that supplies a voltage to the op amp`s noninverting input
Boiler control - schematic

Thermistor THl is placed inside the boiler and the values of THl and Rl should be chosen so that when the boiler"s temperature is 160 °F, their resistances are equal. The output of the op amp controls Ql, which is configured as a transistor switch. When the logic output of the op amp is high, Ql is turned on, energizing relay RYl. The relay"s contacts should be wired so that the boiler"s heat supply is turned off (relay energized).




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