Active solar water heating systems rely on a pump to move water between the collector and the storage tank, but the pump needs a brain to decide when to run. The most common brains are differential controllers and photovoltaic modules, each using sensors or sunlight itself to trigger circulation only when heat gain is possible. ENERGY STAR describes active systems as using electric pumps, valves and controllers to move water from the collectors to the storage tank. Understanding how these controls work helps you choose, maintain, and troubleshoot your system without guessing.
How Differential Controllers Work
The Florida Solar Energy Center’s 2006 manual (written for Florida conditions) describes a differential controller as the system’s “brains.” It constantly compares signals from two sensors: one placed at the hottest point of the collector and one near the bottom of the storage tank. When the collector sensor measures a temperature that is 5°F to 20°F higher than the tank sensor, the controller switches on the pump. The installer can often choose the turn-on differential from options such as 9°F, 12°F, 15°F or 20°F. The FSEC manual notes that systems with long pipe runs benefit from a higher differential because heat loss along the piping is greater, while short-run systems can work with a smaller differential.
Once the pump starts and warm water moves through the collector loop, the temperature difference between the two sensors gradually shrinks. When the difference falls to about 3°F to 5°F, the controller stops the pump. This off differential is usually set at the factory and cannot be adjusted by the installer. The logic prevents the pump from running when the sun is too weak to add useful heat, avoiding the circulation of water that could actually cool the tank.
Most differential controllers include a manual on/off/auto switch. In the “on” position the pump runs continuously regardless of sensor readings; in “off” it stays stopped; in “auto” the sensor-based logic controls operation. Some models also provide a freeze-protection function that can activate the pump during freezing conditions, a feature the installer must enable manually according to the FSEC manual. A high-temperature limit may also be built in, shutting off the pump when the tank bottom sensor reaches a preset temperature. The manufacturer’s instructions are essential because many functions depend on installer-selected settings.
Sensor Placement and Wiring
Correct sensor placement is critical for a differential controller to read the right information. According to the FSEC installation chapter, the low-temperature sensor is usually attached as close as possible to the bottom of the storage tank. If the tank is not accessible, the sensor can be placed on the collector supply pipe near the tank outlet. The high-temperature sensor may be installed in one of three locations: on the outlet pipe of the solar collector, in the air space between the collector absorber and its transparent cover, or directly against the absorber surface. Whichever location the manufacturer recommends, the sensor must be in firm thermal contact with the pipe or surface and well covered with insulation to avoid being influenced by surrounding air.
The sensors used with differential controllers are thermistors, thermal resistors that change electrical resistance with temperature. The FSEC manual explains that as the sensor temperature increases, its resistance drops. At 77°F, a common 10,000-ohm thermistor should read about 10,000 ohms; at 90°F it drops to roughly 7,333 ohms. A 3,000-ohm sensor reads about 3,000 ohms at 77°F and 2,200 ohms at 90°F. Knowing this relationship lets a technician check a sensor’s health with a volt-ohmmeter rather than guessing.
During installation, sensor wiring should follow several FSEC guidelines. Low-voltage sensor lines must not be run near higher-voltage 110 VAC lines, because induced voltage can interfere with the controller. In attics, wiring should be secured overhead along rafters or taped to pipe insulation so it cannot be damaged by stored objects or foot traffic. Wires should not be stretched taut; thermal expansion and contraction can crack insulation and cause short circuits. All connections should be made weather-tight with silicone-filled wire nuts or waterproof connectors, then sealed and protected from sunlight. A sensor that is mounted on outdoor piping needs to be completely wrapped with insulation.
Photovoltaic Pump Control
Instead of a differential controller and two sensors, some active solar water heaters use a photovoltaic module to run the pump directly. The FSEC manual describes a PV-controlled system where a small solar-electric panel is connected to a DC pump. When sunlight hits the PV module, it generates direct current that powers the pump; when clouds roll in or the sun sets, the module’s output drops and the pump stops. Properly sizing and matching the module to the pump ensures that circulation begins only when enough solar energy is available to collect useful heat, a natural behavior that mimics differential control without any electronic board.
This approach eliminates the controller box, sensor wiring, and the need for household AC power at the pump. The FSEC manual notes that the PV module’s open-circuit voltage on a sunny day should be between 14 and 22 volts, and its short-circuit current under full sun typically falls between 200 and 1,000 milliamps. If a PV-controlled system fails to circulate water, one of the first checks is whether the module still produces voltage and current at its roof-mounted location. Because the pump is DC-powered, it can be checked on the bench with two 6-volt lantern batteries connected in series to see if it spins. The manual cautions that these checks involve live electricity and should only be performed by someone familiar with safe electrical practices.
Startup Checks for Controls and Sensors
After a solar water heater is pressure-checked and flushed according to the FSEC startup procedures, the controls and sensors must be verified before the system is left to operate on its own. The FSEC manual describes several checks that a solar installer normally performs, and a homeowner should understand what they look for, even if they hire a professional.
For differential controllers, a basic checkout can be done without specialty tools. The installer disconnects all sensor inputs, then shorts the collector sensor terminals together. This simulates a very hot collector, and the controller should turn on the pump. Removing the short simulates the collector cooling down, and the pump should stop. For controllers that use positive-temperature-coefficient resistance temperature devices (RTDs), shorting the tank sensor terminals is the action that should start the pump. Some controllers have a built-in time delay to prevent rapid cycling, so the pump may not respond instantly.
Sensor resistances are also measured with a multimeter and compared to the manufacturer’s temperature-resistance chart, like the ones printed in the FSEC manual. If a sensor shows an open or short circuit, the wiring between the controller and sensor is the first place to look, broken connections are a common cause of an open reading, while a pinched wire can create a short. Freeze sensors and upper-limit sensors, which are temperature-actuated switches, can be checked by placing them in ice water and confirming the switch changes position above 32°F, typically between 38°F and 42°F.
On the PV side, the FSEC manual recommends measuring the module’s voltage and current output under sunny conditions before connecting the pump. If the module’s numbers are within range but the pump still does not run, the wiring between the module and pump or the pump itself may be defective. All electrical checks should follow the controller manufacturer’s instructions and local electrical codes. Any work on household AC wiring must be done by a licensed electrician, and even low-voltage sensor circuits deserve attention to proper grounding and polarity where required.
Common Faults with Controls and Sensors
When a solar water heater stops performing, the controls and sensors are often the cause. The FSEC problem-assessment chapter lists several typical faults that a knowledgeable homeowner or a service technician can look for. The table below summarizes the main players and what can go wrong, based on that manual.
| Component | Role in the System | Common Fault (per FSEC Manual) |
|---|---|---|
| Differential controller | Compares sensor temperatures and switches the pump on/off at preset differentials | Fails to turn on or off; incorrect differential setting; internal failure; loose or broken connections |
| Sensor (thermistor or RTD) | Measures temperature at collector and tank; its resistance changes with temperature | Open circuit (broken wire), short circuit (pinched wire), resistance drift making readings inaccurate, or poor thermal contact to pipe |
| Photovoltaic module | Converts sunlight to DC electricity that powers the pump directly | No voltage or current output; degraded wiring; cracked panel; pump not matched to module output |
Sensor problems often reveal themselves as a controller that never starts the pump, or a pump that runs continuously even when the collector is cold. A quick inspection of the wiring for breaks, shorts, or corrosion can isolate the fault. If the controller does not respond as expected during a simple shorting check, the FSEC manual describes a more detailed controller check with a dedicated controller check tool that contains fixed or variable resistances and shows the exact turn‑on and turn‑off temperatures. The manual says it is generally calibrated for a specific manufacturer’s line of controllers.
Other system components that interact with controls can also cause trouble. A check valve that fails to close allows reverse thermosiphoning at night, making the differential controller look as if it is working while the tank loses heat. A freeze sensor that does not close on temperature fall can leave the collector unprotected. For this reason, the FSEC manual recommends at least two methods of freeze protection and advises connecting the collector and freeze sensors in parallel or series according to the controller’s design.
Frequently asked questions
What is the difference between a differential controller and a PV controller?
The Florida Solar Energy Center’s manual explains that a differential controller uses two wired sensors and electronic logic to decide when to run an AC pump, while a photovoltaic controller uses a PV module that generates DC electricity directly for a DC pump whenever sunlight is sufficient. The differential controller allows adjustable turn-on and turn-off settings; the PV controller’s behavior is determined by the electrical match between the module and the pump.
How can I tell if my differential controller is working properly?
According to the FSEC startup chapter, a basic check involves shorting the collector sensor terminals at the controller to simulate a hot collector: the pump should start. Removing the short should stop it. If this does not happen, the controller, sensor wiring, or sensors themselves need further inspection.
Do I need special sensors for a drainback or indirect system?
The sensors themselves are the same thermistor type, but drainback systems require careful pump sizing and piping slopes, not different sensors. For indirect systems using glycol or oil, the FSEC manual notes that manual air vents should be used to purge air, and the sensor wiring must still be sealed against moisture. Always follow the controller manufacturer’s specifications for sensor type and mounting.
For more detail on choosing, installing, and troubleshooting these parts, visit our Controls and Sensors section, the parent DIY System Components page, and the pillar article DIY Solar Water Heater Components: What Each Part Must Handle. You may also find helpful guidance in our articles on scalding prevention, pressure and temperature relief, and troubleshooting temperature problems.
