Differential Temperature Controllers: How They Work, How To Choose & Setup
A differential temperature controller compares two temperatures and switches a pump or fan on when one is warmer than the other by a set amount. It's the standard way to control solar thermal water heating, pool solar heating, heat recovery and air destratification. This guide explains what they do, how our models differ, and how to wire and set one up.
What Does a Differential Temperature Controller Do?
A normal thermostat compares one sensor reading against a fixed setpoint. A differential controller uses two sensors and controls on the difference between them (S1 − S2):
- Sensor 1 (S1) sits at the heat source, such as a solar collector, the top of a warm roof space or a heat recovery coil.
- Sensor 2 (S2) sits where the heat needs to go, such as a hot water cylinder, swimming pool or the occupied floor level.
When S1 is hotter than S2 by the switch-on differential, the output relay energises and runs the circulating pump or fan. When the difference falls to the switch-off differential, the output turns off. The gap between those two values works like hysteresis on a thermostat: it stops the pump cycling on and off as the temperatures settle. See On/Off Control Explained for more on hysteresis.
The pump only runs when there's useful heat to move. You don't waste electricity, and on a cloudy day you don't take heat out of the cylinder.
Most differential controllers also include protection functions:
- Anti-freeze: runs the pump if the collector drops towards freezing.
- Collector overheat: stops the pump if the collector is too hot.
- Tank or pool maximum: stops heating once the store reaches a safe limit.
Typical Applications
- Solar thermal hot water: collector on the roof (S1), cylinder (S2).
- Pool and hot tub solar heating: solar mat or collector (S1), pool water (S2).
- Air destratification: warm air at the roof (S1), floor level (S2). A fan pushes heat back down in warehouses, workshops and greenhouses. See N321-S Success For Air Destratification.
- Heat recovery and transfer: moving heat from a wood burner, boiler or process loop into a buffer tank only when it's worth doing.
- Grain and crop drying: running fans only when outside air is warmer or drier than the store.
How to Choose: Our Differential Controllers Compared
| Model | Key features | Best for | From |
| Ageon A108 | Single pump relay. 110/220Vac. 2 NTC sensors supplied. Reservoir max setpoint, anti-freeze, collector overheat, pump power-up delay, maximum run time, recirculation and alarm limits. | Budget solar and pool systems with a single pump | £35 |
| Novus N321S | Single 16A SPDT relay. 2 NTC 10kΩ sensors supplied on 3m cable, extendable to 200m. 0.6°C accuracy. Anti-freeze, separate collector and tank maximums, start delay and sensor offsets. 100–240V or 12–24V versions. | Professional solar, pool and destratification installs | £52 |
| Novus N321S RS485 | As the N321S, plus RS485 Modbus RTU for remote monitoring from a PLC, SCADA or data logger. | Monitored systems and BMS integration | £64 |
| Novus N322S | Everything in the N321S, plus a second output (3A SPST relay) that switches backup heating when the tank (S2) falls below a setpoint. It can also be set for cooling action. 100–240V or 12–24V versions. | Solar plus backup (immersion or boiler) systems | £58 |
| Enclosed IP65 Differential Controller | A ready-built IP65 enclosure with a differential controller, 2 sensors, 240V supply and 16A output. Mount it on the wall and connect it, with no panel building needed. | Plant rooms, outbuildings, damp or dusty sites | £225 |
Prices are ex VAT and correct at the time of writing; the product page always shows the current price.
Questions to Ask When Choosing
Do you need backup heating? If the cylinder also has an immersion heater or boiler coil that should only run when the sun hasn't done the job, choose the N322S. Its second output handles this automatically. Our blog post Novus N322S: Booster Output (Relay 2) walks through it.
What supply voltage do you have? Off-grid, PV-powered or 24V panel systems need the 12–24V versions of the N321S or N322S. Mains systems can use any model.
Do you need remote monitoring? The N321S RS485 lets you read both temperatures and the pump status over Modbus. See RS485 & Modbus.
How long are the sensor runs? The Novus controllers' NTC sensors come on 3m cable and can be extended up to 200m. That suits a roof collector with the controller in a ground-floor plant room.
How big is the pump or fan? The Novus relays are rated 16A resistive. For larger motors, three-phase loads or anything inductive near the limit, switch a contactor coil rather than the motor directly.
Do you want it pre-built? The enclosed IP65 unit comes ready to mount. The panel-mount models fit a standard 70 × 29mm cutout (Novus) and suit an existing control panel or a small enclosure.
How to Set Up a Differential Controller
1. Position the Sensors
- S1, collector or heat source: put it at the hottest point, normally the collector outlet (flow) pipe or the collector's sensor pocket. Use heat-conducting paste, insulate over the sensor, and keep it away from direct sun on the probe itself.
- S2, tank or pool: for a cylinder, place it in the sensor pocket in the lower third, level with the solar coil, so it reads the water the solar circuit actually heats. For a pool, place it on the return pipe from the pool before the solar mat, where the water is representative of the pool.
- Run sensor cables separately from mains and pump cables, preferably in their own earthed conduit, to avoid interference.
The controllers use NTC thermistor sensors. Faulty or damaged sensors can be replaced with our NTC temperature sensor (5 × 30mm, 3m cable). See Temperature Sensors Explained for more on NTC sensors.
2. Wiring
Safety first: isolate the supply before working. Mains wiring should be carried out by a competent electrician to BS 7671, and the controller should be supplied from a fused spur or a suitably rated circuit breaker.
All the controllers follow the same basic layout:
- Supply: Live and Neutral to the power terminals. Check the voltage version before connecting: 100–240V, 12–24V, or the A108's 110V/220V selection.
- Sensors: S1 (collector) and S2 (tank or pool) to their sensor terminals. NTC sensors are not polarity sensitive.
- Pump output: take a Live feed to the relay common (COM). Connect the normally open (NO) terminal to the pump Live, and the pump Neutral and Earth back to the supply.
- Inductive loads: fit an RC snubber (47Ω + 100nF in series, as Novus recommends) across contactor and solenoid coils to protect the relay contacts.
Novus N321S / N322S: terminal numbering is shown on the label on the controller's case and in the manual linked below. Use that diagram for the exact supply, sensor and relay terminals. On the N322S, Output 2 is rated 3A at 250Vac. It is suitable for switching a boiler demand or a contactor coil, not an immersion heater directly. A 3kW immersion draws about 13A and needs a contactor.
Ageon A108 (from the manufacturer's instructions):
| Connection | Terminals |
| Supply 110V | 6 and 7, with a link from 7 to 5 |
| Supply 220/230V | 6 and 8, with a link from 8 to 5 |
| Pump relay | 4 and 6 |
| Collector sensor (T1) | 12 and 13 |
| Reservoir sensor (T2) | 11 and 13 |
3. Basic Parameters: Novus N321S / N322S
Novus controllers use three programming levels. Settings save as you step to the next parameter. After 20 seconds without a key press, the controller saves and returns to the temperature display.
- Level 1, differentials: hold the P key for about 2 seconds until D0n appears.
- Level 2, configuration: keep holding P until Unt appears, then release and step through.
- Use the up and down keys to change a value.
- The factory password, if protection is on, is 111.
| Code | What it does | Typical starting point |
| D0n | Differential to switch the pump on (S1 − S2) | 6–8°C for solar hot water |
| D0f | Differential to switch the pump off (must be lower than D0n) | 2–4°C |
| Sp1 | N322S only: tank temperature below which Output 2 (backup) turns on | Set to the minimum acceptable hot water temperature |
| Unt | Display units, °C or °F | 0 (°C) |
| ind | What the display shows: 0 = S1, 1 = S2, 2 = S1 − S2, 3 = cycles through all three | 3 while commissioning |
| I(e | Anti-freeze: the pump runs if the collector (S1) falls below this value | A few degrees above 0°C in water-filled systems. Not needed with antifreeze-filled (glycol) circuits. |
| kt1 | Collector maximum: the pump stops if S1 exceeds this value | Per collector and system manufacturer |
| Kt2 | Tank or pool maximum: the pump stops if S2 exceeds this value | About 60–65°C for a cylinder, or your chosen pool temperature |
| A(1 | N322S only: Output 2 action: 0 = heating, 1 = cooling | 0 for backup heating |
| kys / ky1 / ky2 / b1y | Hysteresis for the anti-freeze, collector max, tank max and backup functions | 2–5°C |
| dly | Delay before control starts after power-up (0–250 s) | Useful to stagger restarts |
| 0f1 / 0f2 | Sensor offsets (±10°C) to correct a reading against a reference thermometer | 0 unless checked |
| spl / spk | Lower and upper limits for the adjustable setpoints | Leave wide unless locking down |
| Adr | RS485 versions only: Modbus address (1–247) | Unique per device |
4. Basic Parameters: Ageon A108
- To change the reservoir setpoint (SP), hold Set and adjust with the up and down keys.
- To reach the other parameters, hold Up + Down together for 4 seconds until Cd appears. Then hold Set and enter 28 for basic parameters or 38 for the full list.
| Code | What it does | Factory default |
| SP | Reservoir maximum temperature: heating stops when the tank or pool reaches this value | 40°C |
| r0 | Differential to switch the pump on (T1 − T2) | 4.0°C |
| r1 | Differential to switch the pump off (lower than r0) | 2.0°C |
| td | Main display: t1, t2 or the difference | t2 |
| r9 | Pump delay after power-up (0–20 min) | 0 |
| P0 | Collector anti-freeze: the pump runs below this value (or oFF) | 2.0°C |
| P1 | Collector overheat: the pump stays off above this value | 90°C |
| P9 | Maximum pump run time on a differential start | 300 min |
| C1 / C2 | Collector and reservoir sensor offsets | 0.0 |
For hot water, raise SP from the 40°C default to suit the cylinder, typically 55–65°C. We also offer a pre-programming service for the A108. Ask when ordering.
5. Commissioning Checks
- Set the display to cycle through S1, S2 and the difference, and check that both sensors read sensibly against a reference thermometer. Correct any error with the offset parameters.
- Warm S1 by hand or with a hot cloth and confirm that the pump starts once the difference passes the switch-on value.
- Check that the pump runs in the right direction and that air is purged from the circuit.
- If a sensor shows an error (for example E1 or E2 on the A108), check for a broken or shorted sensor cable. The output switches off on a sensor fault.
For general faults, see our Faultfinding guide.
Manuals & Downloads
Novus N321S Datasheet Download
Novus N321S / N322S Modbus Register Table Download
Ageon A108 Instructions Download
Frequently Asked Questions
What's the difference between a differential controller and a thermostat?
A thermostat switches when one temperature crosses a fixed setpoint. A differential controller switches when the difference between two sensors crosses a set value, so the pump only runs when there's heat available to move.
What differential settings should I use for solar hot water?
A common starting point is on at 6–8°C and off at 2–4°C. If the pump cycles on and off in weak sun, widen the gap. If the system is slow to start, reduce the switch-on value slightly.
Can I use it for a swimming pool?
Yes. Put S1 on the solar mat or collector and S2 on the pool return pipe, and set the tank maximum (Kt2 on Novus, SP on the A108) to your target pool temperature.
Can it control an immersion heater as backup?
The N322S has a second output for exactly this. Because Output 2 is rated 3A, switch the immersion through a suitably rated contactor.
Can I extend the sensor cables?
Yes. Novus specifies up to 200m. Use a twin cable of at least 0.5mm² and keep it away from mains cables.
Can I run it from 12V or 24V?
Yes. The N321S and N322S are available in 12–24V versions for off-grid and low-voltage systems.
Need Help Choosing?
Tell us about your system and we'll recommend the right controller and settings. Contact us or browse more guides in the Knowledge Base.