Second only to valve problems, differential controllers and their sensors have the largest failure rates of all solar water heating (SWH) components. Voltage spikes from nearby lightning strikes are blamed for many of the failures, but this is difficult to verify. Differential controls monitor two sensors: one placed at the collector outlet and one placed at the storage tank. At often-adjustable set points, the controllers turn on pumps, blowers, or motorized valves. Of the three control components, my experience is that the differential controls fail most often, with sensors next.
U.S.-made differential controls use “10K” sensors, an industry alias for a thermistor, which vary their resistance as temperature changes. Thermistors used in the solar industry have an inverse function: the resistance decreases as the temperature rises. Ten-K sensors have an electrical resistance of 10 K•ohms (10,000 ohms) at 77°F.
Newer to the U.S. market are European controls that use sensors known as resistance thermal devices (aka resistance temperature detector; RTD). Most use a model PT1000 sensor. RTD sensors used in solar controls are proportional devices where the resistance increases with temperature. They have a resistance of 1,000 ohms at 32°F. European controls have captured some of the market share in the United States in the last few years because they incorporate features that are popular with many consumers: digital displays; multiple inputs and outputs; and other functions, such as a vacation mode to help prevent overheating. Ten-K sensors will not function correctly in European controls and vice versa.
Some controls have built-in diagnostics to detect sensor malfunctions and report them on the display. A display of sensor temperatures is also valuable for evaluating operation and flow problems. Controls that don’t have digital diagnostics require a multimeter to discover which control component has a problem.
Troubleshooting. A control failure is evidenced most often by equipment (pumps, etc.) not turning on when expected. For example, a pump is expected to be energized when the collectors are exposed to bright sunshine and the tank contains cold water. A more unusual circumstance that indicates a control failure is equipment running when it is expected to be off. An example is a collector loop pump running at night.
At room temperature (72°F), a 10K sensor should give a reading of a little more than 10,000 ohms on a multimeter’s resistance setting; a PT1000 will read about 1,100 ohms. A heat source is needed for testing sensors. Wrapping your hand around the sensor will work and give a slow rise in temperature. Solar technicians sometimes use a propane lighter or torch for quicker temperature changes. An ice cube can also be applied to the sensor to test that the sensor isn’t stuck at a temperature. Increasing temperature will cause the resistance to drop with a 10K sensor and rise with a PT1000. A sensor that gives a reading of either zero or infinite resistance on the meter needs to be replaced.
A temperature difference of about 16°F between the collector and storage sensors usually triggers the SWH pump, blower, and/or valve to activate the system (the “on” differential). Many controls have field-adjustable differentials (although, in some cases, only the “on” setting is adjustable). Some controls have no differential adjustment.
A control can be tested with two good sensors connected to the sensor terminals. If both sensors start at room temperature, the ON differential can be reached using body temperature to heat the collector sensor. An ice cube on the storage sensor will accomplish the same thing. If the control doesn’t turn on when the differential is reached, the control is probably defective. Most controls have an ON-AUTO-OFF switch and it must be in the AUTO position for the control to function on sensor differentials.
Most solar service technicians just replace defective differential controls—they don’t repair them, since a strong electronics background and difficult-to-source parts are required. If you have an older SWH system that uses a discontinued controller, you can send it away for repair by Conifer Solar Consulting (conifer-solar-consulting.com).