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Underfloor Heating Controller Schematic Circuit Diagram

Optimizing Central Heating Systems with Underfloor Heating

In central heating systems incorporating underfloor heating, the additional pump responsible for circulating water through the underfloor pipes typically operates continuously. This occurs because the central heating controller lacks a distinct control circuit and output for the underfloor heating pump. The purpose of this circuit is to enable independent control of the underfloor heating pump, either directly or through the switch integrated into the living room thermostat. The design of this circuit emphasizes flexibility, offering the ability to connect it in four different configurations.

Underfloor Heating Controller Schematic Circuit Diagram

  1. The temperature sensor 1 is connected to the inlet pipe of the underfloor heating, a Temperature sensor 2 is shorted. The pump is turned on when the inlet pipe becomes warm enough. When the temperature of the inlet pipe drops below the trigger temperature the pump will continue to run for 20 minutes.
  2. The temperature sensor 1 is connected to the inlet pipe of the underfloor heating, Temperature sensor 2 is connected to the outlet pipe. This works in a similar way to that in the previous configuration, but also: as long as the inlet pipe is warm the pump will be stopped (temporarily) when the outlet pipe rises above the trigger temperature.
  3. Switch input connected to the living room thermostat. As long as the switch (connected to the same input as for Temperature sensor 1) is closed, the pump will run. When the switch opens the pump stops after 20 minutes.
  4. Switch input connected to the living room thermostat. Temperature sensor 2 is connected to the outlet pipe of the underfloor heating. This works in a similar way to that in the previous configuration, but also: as long as the inlet pipe is warm the pump will be stopped (temporarily) when the outlet pipe rises above the trigger temperature.

Temperature Sensor 2: Preventing Overheating in Underfloor Heating

Temperature sensor 2 offers a valuable safeguard against overheating in underfloor heating systems. To activate this feature, adjust the trigger temperature to approximately 50 degrees and connect the sensor to the inlet pipe of the pump. The core of the circuit centers around an ATtiny25 microcontroller, equipped with two ADC inputs responsible for monitoring the voltage across both PTCs. The software compares the voltage across the first temperature sensor with a predefined trigger value or zero. When the trigger value is surpassed or the value is zero due to an external switch, the Motor power pin (pin 5) is raised to activate the pump through an opto-triac. Simultaneously, another output (pin 6) is brought low, enabling the connection of external components like an indicator lamp. To prevent a continuous current flow through the PTCs and temperature sensors, the PTCs are grounded through a software-controlled FET only during the measurement process.

Microcontroller Configuration and Interrupt Handling

A configuration fuse within the microcontroller is set to ensure the internal clock runs at 128 kHz, sufficient for program operation. This frequency is divided by 1024 in the timer1 prescaler, with timer1 generating an interrupt approximately once per second. During the interrupt routine, the pump’s operating state is determined. If Temperature Sensor 1 exceeds the trigger value or equals zero (due to the switch input), the pump timer is initiated for 20 minutes. This extended time ensures the pump remains active for an additional 20 minutes even after the temperature drops below the trigger level. Should the second temperature sensor exceed the trigger level, the pump is immediately stopped.

Measurement Process and Pump Control

The interrupt routine initiates a measurement by first enabling the FET to connect the PTCs to ground. Subsequently, an ADC routine is executed to read in the values from the temperature sensors, alternating between the two sensors, resulting in a 2-second measurement interval for each sensor. The circuit ensures the pump operates for at least 5 minutes within an 18-hour period, facilitated by a Summer-timer. This timer keeps track of when the pump was last turned on. If the Summer-timer remains unchanged for 18 hours (equivalent to 65,536 seconds), the pump timer is set to 5 minutes, maintaining the pump’s activity. For U.S. readers, it is essential to replace Tr1, M1, and F1 with components suitable for operation at 115 VAC and 60 Hz.

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