Solar Water Heating System Troubleshooting and Repair: Part 2

Controls, Sensors and Tanks
Intermediate

Inside this Article

A solar technician
A solar technician adjusts the settings on a solar controller.
Solar Water Controllers
A basic 10K controller (left) has been in service for more than 30 years. LED lights show when it is powered up and when the pump(s) should be operating. A modern 1K controller (right) has digital readouts and a graphic interface.
Testing a 10K sensor
Testing a 10K sensor to see if it will operate the controller.
A variety of 10K sensors
A variety of 10K sensors (left to right): A basic strap-on sensor with a hole for mounting on a lug; two sizes of immersion sensors; and a plastic swimming-pool control sensor. Note the O-ring on the pool sensor—it is designed to be placed in a hole drilled in the PVC piping and serve as an immersion sensor when clamped to the pipe.
Multimeter and sensors
The top sensor is a 10K•ohm sensor; the bottom two are 1 K•ohm sensors. The multimeter is set at K•ohms and reads 1.095 K•ohms when connected to the middle 1 K sensor—close to the reading for 68°F (see table at upper left).
Rheem Marathon tank
This Rheem Marathon tank has a seamless, blow-molded, polybutylene liner and has a limited lifetime tank warranty and a six-year parts warranty.
A cutaway view of a Stiebel Eltron glass-lined tank
A cutaway view of a Stiebel Eltron glass-lined tank with a submerged, glass-lined heat exchanger.
Sacrificial anode rods
The sacrificial anode rod (left) has served its function and is corroded. The new anode rod (right) will replace it. This type of anode is located under the water heater’s hot water outlet—evidenced by the pipe fitting on top.
A typical SWH expansion tank
A typical SWH expansion tank is properly installed with its bladder end up.
A solar technician
Solar Water Controllers
Testing a 10K sensor
A variety of 10K sensors
Multimeter and sensors
Rheem Marathon tank
A cutaway view of a Stiebel Eltron glass-lined tank
Sacrificial anode rods
A typical SWH expansion tank

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.

Controls & Sensors

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).

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