The Novus N321S is a differential temperature controller purpose-built for solar water heating systems. It compares the temperature of two NTC thermistor sensors—one on the solar collector and one in the storage tank—and activates a circulation pump when the collector temperature exceeds the tank temperature by a configurable threshold (dON, default 8 °C). When the temperature difference drops to the lower threshold (dOF, default 2 °C), the pump stops, preventing reverse thermosiphoning that would cool the tank at night. The SPDT relay output is rated for 16 A resistive load (1 HP at 250 Vac), sufficient to directly drive a circulation pump without an intermediate contactor.

Differential Control Logic

• Pump ON Threshold (dON): When the temperature difference between the solar collector (T1) and the storage tank (T2) exceeds the dON setpoint, the SPDT relay closes and the pump energises. The default dON value is 8 °C—a value that prevents the pump from short‑cycling during intermittent cloud cover while ensuring that useful heat transfer begins promptly once a sufficient temperature gradient exists.
• Pump OFF Threshold (dOF): When the difference drops below dOF (default 2 °C), the relay opens and the pump stops. Maintaining a positive dOF prevents the pump from running when the collector is barely warmer than the tank—a condition where energy consumed by the pump would exceed the thermal energy transferred.
• Field‑Adjustable Setpoints: Both dON and dOF can be adjusted directly on the front panel by pressing and holding the SET key for two seconds. No programming software or PC connection is required, allowing the installer to adapt the controller to different system sizes and climate conditions on site.
• Default Values Are Pre‑Programmed: The controller ships with dON = 8 °C and dOF = 2 °C—values that work for the majority of residential solar thermal installations. For swimming pool heating systems, dON may be set lower (e.g., 4 °C) to maximise heat transfer from the collector to the pool, while for large storage tanks a wider dON (e.g., 10 °C) may be used to reduce pump cycling.

Freeze, Overheat, and Sensor Fault Protections

• Freeze Protection: When the collector temperature (T1) drops to 3 °C, the controller automatically activates the pump for a brief period—circulating warmer water from the storage tank back through the collector and exposed roof piping. If the collector temperature continues to drop and reaches -2 °C, the pump is shut off and the display flashes "LO", alerting the user that freeze protection has been exhausted. This prevents both pipe burst damage and the energy loss that would result from continuous pump operation in deep freeze conditions.
• Overheat Protection: When the storage tank temperature reaches the user‑configurable high‑limit setpoint (HI), the pump is stopped even if a positive temperature differential exists between collector and tank. This prevents scalding temperatures in the storage tank and protects the tank lining and connected plumbing from thermal damage. The pump resumes normal differential‑based operation once the tank temperature drops below the HI setpoint.
• Sensor Fault Shutdown: If either NTC probe develops an open circuit, a short circuit, or a connection fault, the controller immediately de‑energises the relay output and displays an error code: E1 for a T1 (collector) sensor fault, E2 for a T2 (tank) sensor fault. This prevents the pump from running continuously in the absence of valid temperature data—a condition that would, in a solar thermal system, transfer stored heat from the tank back to the collector and dissipate it to the ambient environment overnight. The error clears automatically when the sensor connection is restored.
• Factory‑Calibrated Sensors: The two NTC thermistors are factory‑matched with an interchangeability error of just 0.75 °C. This means a replacement sensor can be installed without recalibration—the controller's differential measurement remains within specification even after a sensor swap, which reduces service time and eliminates the need for specialised calibration equipment on site.

Installation, Power, and Applications

• Universal Power Supply: The N321S operates from any AC or DC supply voltage between 100 V and 240 V (±10%), at 50 or 60 Hz. An optional low‑voltage version accepts 12–30 Vdc/ac. This means a single model can be installed in a 120 V North American system, a 230 V European system, or a 24 V battery‑backed system—reducing the number of controller variants that installers must carry.
• Standard Panel Cut‑Out: The 70 × 29 mm panel cut‑out matches the industry‑standard format shared by many HVAC and solar controllers. The controller is inserted from the front and secured by snap‑fit tabs—no screws are required for panel mounting. The IP65‑rated front panel allows installation in outdoor enclosures exposed to rain and dust, while the IP42‑rated polycarbonate housing (UL94‑V2) provides mechanical protection for the electronics behind the panel.
• Typical Applications: Domestic solar water heating systems (flat‑plate and evacuated‑tube collectors), swimming pool solar heating, hydronic floor heating with solar pre‑heat, and any liquid‑based thermal system where circulation is controlled by the temperature difference between a heat source and a thermal store.