Active solar water heating systems are easy to spot because they include a circulating pump, a controller, and a pair of sensors that work together to move water or heat-transfer fluid through the collectors. According to the Florida Solar Energy Center’s 2006 Solar Water and Pool Heating Manual, a Florida-focused guide, active systems fall into two main categories: direct, where the household water flows through the collectors, and indirect, where a separate fluid circulates in a closed loop and transfers its heat via a heat exchanger. The manual’s installation and troubleshooting sections offer detailed, practical steps for setting up and checking these components, and while the numbers and freeze-strategy advice reflect Florida’s climate, the underlying principles apply to any active solar water heater.
How Active Systems Differ from Passive Systems
The FSEC manual divides solar water heating into two broad categories: active systems, which use a pump to control water flow, and passive systems, which rely on natural convection or gravity and have no pump. Both families can be direct or indirect. An active direct system sends potable water from a storage tank through the collectors and back, while an active indirect system circulates a freeze-resistant fluid through a heat exchanger. Passive setups, by contrast, use no controller or pump; instead they depend on thermosiphoning or integrated collector‑storage designs. Because active systems introduce electrical components, installing one connected to a home’s pressurized drinking‑water supply must follow local plumbing and building codes and usually requires a permit and a qualified, licensed installer.
The Circulating Pump: Heart of an Active System
ENERGY STAR describes active, or forced-circulation, systems as using electric pumps, valves and controllers to move water from the collectors to the storage tank, and notes they are common in the U.S. Every active system contains a pump to move fluid between the storage tank and the collectors. The FSEC manual explains that many solar water heating applications use inexpensive, low‑horsepower pumps that can circulate enough liquid for efficient operation. The most common type is a “wet‑rotor” pump, where the rotor is surrounded by liquid that lubricates it during operation, eliminating the need for manual lubrication. Some pump motors, however, still require periodic oiling, and installers should show the homeowner how and when to do this.
Material choice is critical. For active direct systems that handle potable water, the manual says the pump must have a bronze or stainless‑steel housing and impeller. Using a cast‑iron pump in a direct system will cause rust because the drinking water comes into direct contact with the pump. Cast‑iron pumps can be used in certain indirect systems that circulate glycol or other heat‑transfer fluids. The manual’s pump applications table lists that a direct active system calls for a bronze or stainless‑steel low‑head pump; a direct drain‑down system needs bronze or stainless‑steel with low‑to‑medium head; and glycol‑based indirect systems can use cast‑iron pumps.
Drain‑back systems deserve special attention. In a drain‑back design, the fluid drains into a reservoir inside the building every time the pump stops, so the pump must be capable of lifting the water from the storage level to the highest point in the collector loop, a higher‑head pump than a pressurized system requires. The FSEC manual notes that most solar pumps are not self‑priming, so the impeller must be located below the liquid level, and the pump may be mounted in any orientation as long as air is not trapped in the pump cavity. Pump rotation must push fluid in the correct direction, and all pumps and motors must be UL‑listed.
Proper pipe sizing helps the pump do its job. The manual’s installation section recommends ¾‑inch pipe for forced‑circulation systems where the total run exceeds 100 feet, or where the flow requirement is above one gallon per minute and the pump is less than 1/20 hp. Isolation valves on both sides of the solar loop, a motorized check valve or a high‑temperature vertical spring‑check valve, and drain valves that allow service and back‑flushing are all part of a well‑installed pump circuit. To find out how the pump fits into the bigger picture of system components, see our Pumps and Controllers section and the solar water heater component overview.
Differential Controllers: The System’s Brains
The FSEC manual calls the controller the “brains” of an active system. A differential controller continuously compares the temperatures read by two sensors, one at the hottest point of the collector and one near the bottom of the storage tank. When the collector sensor registers a temperature that is 5 to 20 °F higher than the tank sensor, the controller starts the circulating pump. Many controllers let the installer choose the exact on‑differential, for example, 9, 12, 15 or 20 °F, depending on the length of the pipe runs. Longer pipe runs lose more heat, so the manual suggests using a higher differential; shorter runs can use a smaller gap. The off‑differential, typically around 3 to 5 °F, is usually set at the factory and cannot be adjusted by the installer.
These controllers normally have an on/off/auto switch. In “auto” mode, the sensors and controller work together to run the pump only when useful solar heat is available. In the “on” position, the pump runs continuously regardless of what the sensors say, which is handy during service checks. An off position disables the pump entirely. Many differential controllers also incorporate a freeze‑control feature that circulates warm tank water through the collectors during near‑freezing conditions; this function must be activated by the installer according to the manufacturer’s instructions. Some models add a high‑temperature limit that turns the pump off once the tank sensor reaches a preset temperature, although the manual warns that the top of the tank can be hotter than the sensor location at the bottom.
The installation guidelines say to mount the control box indoors, protected from weather and tampering, often connected to the pump with pre‑formed conduit and a wall plug. During startup, a simple checkout can verify the controller: shorting the collector sensor terminals together (with the controller in automatic mode) simulates a very hot collector and should turn on the pump. Removing the short should shut it off, though some controllers have a built‑in time delay to prevent rapid cycling. For RTD‑type sensors, the tank sensor input is shorted instead. If the controller does not respond as expected, the sensors or wiring might be faulty, or the control itself may need attention. More information on selecting monitoring equipment is available in our guide to controls and monitoring.
Sensors Keep the Controller Informed
The differential controller’s decisions depend entirely on accurate temperature readings. The sensors used are thermal resistors, or thermistors, that change their electrical resistance as the temperature changes. The FSEC manual describes the relationship: when the sensor’s temperature goes up, its resistance goes down. At 77 °F, a typical 3,000‑ohm sensor reads about 3,000 ohms and a 10,000‑ohm sensor reads about 10,000 ohms. At 90 °F, those numbers drop to roughly 2,200 and 7,333 ohms respectively.
Placement matters. The tank sensor should be attached as close to the bottom of the storage tank as possible; if the tank is inaccessible, it can be mounted on the collector supply pipe near the tank outlet, but must be well insulated from the surrounding air. The collector sensor can be fixed to the collector outlet pipe, in the air space between the absorber and the glazing, or directly against the absorber surface, always following the manufacturer’s instructions. The manual’s startup procedures recommend verifying sensor calibration with a digital ohmmeter and checking freeze sensors by immersing them in an ice‑water bath. A freeze sensor should activate (open or close its switch) above 32 °F, typically between 38 and 42 °F.
Wiring details matter too. Sensor connections should be made with silicone‑filled wire nuts or waterproof telephone‑style connectors, sealed against moisture and sunlight. Teflon‑insulated wire is recommended close to the collector, and low‑voltage sensor lines must never run alongside 110 V AC wiring, because induced voltage can confuse the controller. The manual also advises not to stretch sensor wires taut, as thermal expansion and contraction can damage the insulation and cause short circuits. A common wiring fault is a broken connection that shows up as an “open” resistance reading, while a pinched wire touching a metal surface can create a short to ground.
Photovoltaic (PV) Control: An Alternative to Wired Controllers
Some active direct systems forgo a wired differential controller entirely and instead use a photovoltaic module to power a DC pump. The FSEC manual explains that in a PV‑controlled system, the module converts sunlight directly into DC electricity that drives a small pump. Because the pump’s speed depends on available sunlight, the system inherently starts circulating when the sun is strong enough to heat the water and slows or stops under overcast skies. No separate temperature sensors or controller box are needed for the basic function, although freezing climates still require a freeze‑protection strategy.
The installation section advises covering the PV module before connecting the DC pump to avoid an electrical spike. To check a PV controller on a sunny day, the manual’s troubleshooting procedure recommends disconnecting the pump and measuring the module’s open‑circuit voltage with a volt‑ohmmeter. In almost any amount of sunlight the voltage should be between 14 and 22 volts. The short‑circuit current, measured with a meter that can handle milliamps, should fall between 200 and 1,000 milliamps depending on the module’s voltage rating. If the pump doesn’t run even when those readings are good, connecting two 6‑volt lantern batteries in series (12 V) directly to the pump can tell whether the pump is defective. PV‑controlled systems use a DC pump. For a broader look at how PV pumps work, see our article on solar water pumps.
Recognizing Pump and Controller Problems
The FSEC manual’s problem‑assessment module provides a handy list of signs that suggest a pump or controller may not be working correctly. On a sunny day, the pump should run most of the time, though passing clouds can cause an on‑off cycle with a differential or PV controller. On a heavily overcast day or at night, the pump should stay off. If the pump doesn’t turn on when you expect it to, first make sure the controller switch is set to “auto” or, if you are doing a service check, manually switch it to “on” to see if the pump itself has power. A slight hum, visible fan movement, or a gentle vibration usually means the pump is running, but the manual notes that pump motors can become very hot after a long stretch of operation, so be careful when touching the motor housing.
To check whether fluid is actually moving, the manual suggests peeling back a short section of pipe insulation a few feet from the tank and feeling both the collector supply line and the return line. On a sunny day with the pump running, the return line should be noticeably hotter than the supply line, a temperature difference of about 5 to 15 °F indicates flow. If you can’t feel any difference, turn the pump off for a few minutes, then restart it. A surge of hot fluid should pass through the return line within seconds. If that surge never appears, there may be a blockage in the collector, the pump, or the pipes.
Controller problems often show up as a pump that never runs or runs continuously when it shouldn’t. The manual’s checkout procedure for differential controllers involves shorting the sensor inputs to trick the control into turning on. For a thermistor‑based control, bridging the collector sensor terminals should start the pump; for resistance‑temperature devices (RTDs), the tank sensor terminals are the key. A control that ignores these inputs may need closer inspection or replacement. Indirect systems add another dimension: the heat‑transfer fluid should be inspected periodically for rust, scale, sludge, or gummy materials that signal degradation, leaks, or seal damage. If you notice any of these signs, the system likely needs service soon. For more troubleshooting tips, visit solar water heater troubleshooting.
Common Components and Warning Signs
Below is a quick reference table linking each major control component to its role and a warning sign that may indicate a problem.
| Component | Role | Warning sign |
|---|---|---|
| Circulating pump | Moves water or heat‑transfer fluid from the tank to the collectors and back. | No hum or fan movement on a sunny day; supply and return pipes feel the same temperature. |
| Differential controller | Compares collector and tank sensor readings and switches the pump on or off at preset temperature gaps. | Pump runs continuously during the night or never activates even when the collector is hot. |
| Temperature sensors (thermistors) | Feed resistance‑based temperature signals to the controller. | Controller behaves erratically; resistance readings show open circuit (broken wire) or short to ground. |
| PV module and DC pump | Generate pump power directly from sunlight, eliminating a wired controller. | Pump does not spin on a sunny day; measured open‑circuit voltage outside the 14 to 22 V range. |
Frequently asked questions
How can I tell if my active system’s pump is actually running?
Listen for a subtle hum and look for fan movement or gentle vibration on the pump motor. If in doubt, peel back a short piece of pipe insulation and feel the return line; on a sunny day it should be noticeably warmer than the supply line. The FSEC manual says a temperature difference of 5 to 15 °F normally confirms that fluid is circulating.
What does the “differential” setting on my controller do?
The differential setting defines how much hotter the collector sensor must be than the tank sensor before the pump turns on. According to the FSEC manual, this can be set by the installer to values like 9, 12, 15 or 20 °F. A higher setting reduces the number of pump starts but may harvest less heat on days with partial sun; a lower setting captures more small gains but can cause more frequent cycling, especially with long pipe runs.
Can I add a PV module to an existing AC‑pump system to save electricity?
The FSEC manual describes photovoltaic‑controlled systems as a dedicated design where a DC pump is directly matched to a PV module. An existing AC pump cannot simply be swapped over because the pump motor and control wiring are fundamentally different. Converting a system would require a new DC pump, potentially new plumbing connections, and careful matching of the PV panel to the pump’s electrical characteristics.
For additional guidance on selecting, upgrading, or maintaining the controls that run your solar water heater, visit our Pumps and Controllers category page and explore related resources on the System Components hub.
