The kind of thermostat you’re probably most familiar with—an ordinary room thermostat—works great atº keeping our homes and offices comfortable. Set the thermostat and it will keep the temperature hovering around the setting. Conventional water heaters have a similar thermostat that keeps the temperature of the water where the thermostat is set. But solar water heaters need more sophisticated controls to utilize solar energy most efficiently.
To get all the solar energy we can when it’s available and needed, we have to measure the sun’s energy and simultaneously measure the water temperature in the water heater or solar storage tank. When the solar-heated water (or, in some systems, the antifreeze solution) in the solar collector will add heat to the tank, we want a pump to turn on and move the warmer water from the collector to the tank. Measuring two temperatures at once and cueing a pump based on the temperature difference is a job for a differential thermostat—a.k.a. differential controller.
Differential thermostats have two sensors that are constantly monitored by the electronics in the controller, which turns equipment on and off based on the temperature differential. When the controller determines that one sensor is warmer than the other sensor, it energizes a relay (an electromagnetic switch) that can activate an electric device, usually a circulator pump. When not enough difference in temperature exists, the differential thermostat turns off the pump or other device.
In a solar hot water (SHW) system, one sensor (known as the “hot” or “collector” sensor) is placed on the piping, very close to the collector’s outlet, and the other sensor (the “cold” or “storage” sensor) is placed near the bottom of the storage tank where the water is coolest. Sun shining on the collectors causes the outlet temperature to increase quickly. The sensor’s resistance will change (see discussion of sensors below) and the controller’s electronic circuits will activate the relay, turning on the pump. When solar energy eventually lessens on the collectors, or the storage tank water becomes hot enough, a large “differential” will no longer exist between the two sensors, and the thermostat will shut off the pump.
The sensors that measure temperatures for a differential thermostat are thermistors, a type of resistor with resistance that varies with temperature. The industry standard in the United States is a 10,000-ohm inverse thermistor, which measures 10K ohms at 77°F. As the temperature rises, the resistance drops, hence the “inverse” designation.
You can’t mix sensors of one value with a controller designed for a different sensor, and beware—not all 10K sensors are the same. Each thermistor has a “curve” of resistance at changing temperatures. To ensure two sensors are equal, you need to compare the curves and verify they are the same. If resistance curves are too different, they can cause the control to have imprecise on and off differentials. Some thermostats require different sensors—make sure the sensors used are compatible with the chosen controller.
Care should be taken in how and where the sensors are placed. The collector output sensor should be placed within 1 inch of the collector, clamped to the hot pipe going to the storage tank. Clamp the flat part of the sensor to the collector tube with a hose clamp and insulate it well with closed-cell, high-temperature insulation. If the sensor has no flat section, don’t clamp it so tightly that it will crush the sensor housing—thermistors are much smaller than their housing and are delicate. Manufacturers typically house the sensors in metal cylinders, usually copper, and encapsulate the thermistors to protect them from water and weather.
The tank (or storage) sensor should be placed inside the bottom access cover on the tank—behind the insulation and making direct contact with the tank. It may also be clamped to the tank drain valve if it can be insulated. Some solar tanks have a threaded lug at their bottom where a sensor can be bolted. Incorrect sensor placement will create an inefficient system, causing a pump to run when it shouldn’t or not run when it should. Individual controller manufacturers may have specific instructions for sensor placement.
Parasitic loss occurs when energy is consumed or lost in order for a system to make more energy. A good example of a parasitic loss is the amount of energy that bearing friction consumes in the common internal combustion engine. In grid-powered SHW systems, the pumps and controllers take a certain amount of electrical energy to operate, and there are some heat losses through the piping and other points in the system. If the parasitic losses are greater than the solar energy being put into the SHW system, a net loss of energy results.
In PV-direct DC systems, the pump energy losses are inconsequential because the energy from the PV module is there regardless, and at no cost. But in AC systems, a wider temperature differential is used to ensure that the parasitic losses are not greater than the solar energy gained. Many manufacturers set their AC controllers with a 4°F or 6°F turn-off differential. With any smaller differential, there is no net energy gain because of parasitic pump consumption and system heat loss. Any greater differential would waste valuable solar energy by turning the pump off too soon. Since there is no utility-generated parasitic power consumed with a PV-powered DC differential control, the turn-off differential is lower—sometimes zero is appropriate if the pipe losses are negligible.
The pump turn-on differential is usually adjustable in both AC and DC controllers. The turn-on differential is set higher than the turn-off to make sure that the solar input can be sufficiently maintained to add heat to the tank and to avoid excessive on–off pump cycling. With minimal sun, the collector temperature will increase somewhat. But the possibility is that when the pump goes on, the temperature won’t be maintained when the barely warm liquid from the bottom of the collector reaches the sensor at the top. A higher turn-on differential makes that possibility less likely, and decreases the likelihood that the system will cycle on–off repeatedly during times of reduced solar input, like during the morning and evening.
A low turn-on differential setting (8°F to 10°F) is most efficient for direct forced-circulation systems that don’t have a heat exchanger. A somewhat higher differential setting (14°F to 18°F) is recommended for indirect forced-circulation systems (antifreeze and drainback) with heat exchangers. This is because these systems have higher pipe and system heat losses, making it necessary to assure enough heat input to overcome those losses before turning the pump on.
A basic differential controller needs only an input power source, an output, and two sensors. A few such basic units were made in the past—they didn’t even have a single LED to indicate if the unit was on. In the past, thousands of these basic thermostats had been produced—many are still working after twenty years or more of service.
The controllers listed in the guide have many more features than this basic design. LED lights are useful to indicate the control and system status, but digital displays are now common. Another popular feature is multiple differentials—the ability to control two or more pumps at different times is used in systems supplying both domestic hot water and space heating.
Our table of differential thermostats includes all of the U.S. manufacturers and six overseas products that are available in North America. Control freaks and data junkies are sure to be happy with recent innovations in some controllers. Lots of goodies are available—digital displays, remote displays, computer interfaces, and data logging capabilities. Even if you’re not a number nerd, you’ll find many of the new features handy.
The wide selection of differential thermostats is a welcome sign of a growing, mature industry. Reliable controllers with lots of options can make design and installation easier and more attractive. The information provided by the new crop of controllers will be welcome by many system owners. Some of the controllers on our list have data logging capabilities, such as recording sensor temperatures over a period of time. Add a flow meter to the temperature data and you can compute Btu for tracking total output.
A good differential thermostat puts you in control of your hot water system and collects as much of the sun’s energy as possible. For most systems, a basic differential control is all that is needed. The addition of a digital display is helpful if you want a good understanding of system operation and the data is meaningful to you. A digital display is helpful in troubleshooting problems if the system is inoperable or losing efficiency—it’s hard to have too much data in these cases. Two or more differentials in one controller are useful for collectors doing more than one job.
Contributing editor Chuck Marken is a New Mexico-licensed plumber, electrician, and heating and air conditioning contractor. He has been installing and servicing solar thermal systems since 1979. Chuck is a part-time instructor for Solar Energy International and the University of New Mexico.
SHW Differential Controller Manufacturers:
ArtTec • www.arttecsolar.com
Caleffi • www.caleffi.us
Heliodyne • www.heliodyne.com
Heliotrope Pool • www.heliotropethermal.com
IMC Instruments • www.imcinstruments.com
Pentair Water Pool & Spa • www.pentairpool.com
Steca • www.steca-solar.com
Sun Spot Solar • www.sssolar.com
SunEarth • www.sunearthinc.com
Tekmar • www.tekmarcontrols.com
Thermomax • www.solarthermal.com
Viessmann • www.viessmann-us.com