• Thermostat – a device that automatically controls cooling and heating equipment in such a way as to maintain a temperature at a constant level or within a specified range, generally using a thermometer capable of triggering electrical switches that activate or deactivate the equipment
• BC337 – a small signal NPN Silicon AF medium power transistor used for general purpose switching and amplifying applications with features such as TO-18 manufactured package, suited for AF driver stages and low power output stages, and divided into three group types
• BC547 – NPN small signal transistors designed for general purpose switching and amplification due to its low voltage, low current and three different gain selections
• BC557 – PNP general purpose transistors used for amplification and switching due to its low current and low voltage

The circuit assembly consists of two sensors where one is located in the outdoor so that the external temperature can be measured. The other sensor is located in the water pipe coming from the heating system circuit which is short before its input to the boiler. The start-stop input control of the boiler should be connected to the SPDT relay contact wiring with 220 V at 2 A switch rating.

A voltage is produced across R8 when R7 & R8 sets the supply voltage to less than half on Q1 base to ground voltage. At this stage, the relay will be switched ON by Q2 & Q3. When one of the NTC thermistors sets more than half the supply voltage on Q1 base to ground voltage, no voltage will appear across R8 sin the thermistor is lowering its value due to an increase in temperature and simultaneously turning OFF the relay. The control of temperature is adjusted by the linear potentiometer P1 while a smooth switching of the relay is managed by capacitor C3.

From the diagram, the indoor sensor is the NTC thermistor R4 while the outdoor sensor is R3. The trimmer R5 is used to compensate any changes in the value of 3K3 thermistor for R3 in case 4K7 is used. For the outdoor temperature ranging at approximately +10°C downwards, the heating system will be set by R5 since the higher the resistance of R5, the hotter the heating system.

A maximum value should be set for the existing boiler thermostat and should not be bypassed to ensure safety. Due to its differential feature, the circuit can be distributed and be converted into a regular precision thermostat by removing R3 which is only applicable for outdoors.

The heating system thermostats are being used as floor heaters to control the underfloor heating. It also controls a room’s ambient temperature with one floor sensor located in the control module and connected to the power base. The room temperature is detected by the sensor and the heating system will be closed when the room temperature reaches the set-point.

The thermostats are suitable for greenhouses, warehouses, cowsheds, and industrial plants. Also, the relay stations and switchgear cabinets can be monitored by using thermostats. Some designs come with time-clock functions and other features such as, keylock facility, adjustable frost setting, remote control facility, slim line design, optimum start, large display, and user friendly.