Making sense of thermistor sensors

By Bristol Stickney, technical director,
Cedar Mountain Solar Systems, Santa Fe, N.M.

A thermistor sensor is an electrical resistor that changes resistance with temperature. In the most common types, the resistance (measured in Ohms) becomes lower at higher temperatures. New condensing modulating boilers now come with thermistors to control the outdoor reset function and to monitor the domestic hot water (DHW) tank temperature. Solar hot water systems have long been controlled by the differential thermostat, which requires at least two matching sensors. Other temperature control equipment in a boiler room may have their own thermistor sensors, such as motorized thermal mixing valves and energy management and temperature control devices.

Not all sensors are alike, and it is important to be able to tell them apart and install only the correct sensor on any given controller. Here is a brief discussion of some of the important issues when installing, repairing and maintaining them in solar heating applications.

Temperature ratings

In solar combisystems, the need for thermistor sensors can multiply rapidly. The hot outlet at the solar collectors must be monitored and compared to any heating load that might need the solar heat. The top and bottom of the DHW tank, the top and bottom of any supplementary heat storage water tanks, heat supply and heat return pipes, room temperatures, floor temperatures, pool and spa temperatures may each require a thermistor sensor. They are packaged in various shapes to match their intended end-use, as seen in Figure 46-1. Some are easier to clamp to the surface of a pipe, others fit easily into a narrow sensor well.

Temperature control manufacturers use sensors with various high temperature ratings; higher temperature-resistant materials have a higher cost. Some of the common temperature ranges available and their intended uses are as follows:

Air sensors may have a normal operating range of -60 to 140 F.

Water tank sensors might be supplied in the range of -60 to 221 F.

A common solar collector “hot” sensor might have a range of -60 to 255F.

The most robust solar “hot” sensors have a range of -40 to 356 F.

The proper temperature range must be chosen especially for the hottest temperature locations. A common cause of sensor failure on the solar hot outlet is the use of a sensor with too low a temperature rating. The temperature limit is not usually printed on the sensor itself, so the installer may have to look it up in the manufacturer’s specifications to be sure.
Resistance type (10K and 1K)

There are common sensors that look similar but might have completely different characteristics. The most widely used types in the solar heating industry are called “10K” and “1K” sensors. These types of sensors are not interchangeable, since each temperature control device is usually designed to be compatible with only one type. For example, 10K sensors are used by Eagle, Delta T, Goldline, Azel, Tekmar and SolarLogic solar temperature controllers. 1K sensors are used with Resol solar heat controls, including Caleffi and Steca.

Since individual thermistors are often unmarked, an Ohm meter is often the only way to identify them. Use the resistance table that comes with your sensors to compare your meter readings.

When using multiple sensors attached to a single control device, they must all be matched. This is especially important when making repairs and modifications, when older sensors already exist. Installing the wrong sensor is an easy mistake to make, since sensors can look the same even though they might have very different temperature and resistance ranges.

The Beta curve

When 10K sensors are obtained from various suppliers, another question comes into play. The performance of the sensor over a wide temperature range can vary from one brand of 10K thermistor to another. The name 10K comes from the fact that this type of sensor responds with 10,000 Ohms at a temperature of 25 C (77 F). But the response of different resister materials over a wide temperature range may vary. This variation can be defined mathematically, graphed as a curve and assigned a numerical value called Beta. For example, the 10K thermistors supplied by Tekmar are listed at a Beta value of 3,892. If we wanted to replace an old Tekmar sensor with a different brand, we would ideally need to find one with a similar Beta value.

Figure 46-2 shows this Tekmar sensor graphed along with two other sensors to show the difference between three 10K sensors with different Beta values. In this example, the greatest possible error occurs at the coldest temperatures, with less variation when warmer. To avoid this possibility for temperature control error, the easiest solution is to always buy replacement sensors from the original control device manufacturer.

Sensor wiring connections

Each thermistor requires only two wires to connect it to a typical solar control device. There is no polarity, so either of the two signal wires may be connected to either of the sensor wires. In spite of this wiring simplicity, sensor problems are caused more often by the wiring than by the sensors themselves. The wire connection is the most common point of failure. If wires are joined using wire nuts (marrettes) that are too big for the job, the joint will fall apart. If wire nuts or crimp caps are left exposed to moisture or the outdoor elements, the connection will corrode and produce a false high resistance reading. It is also very easy to forget to tighten a screw terminal when joining many wires into a control box.
Tip: After connecting wires with a wire nut, crimp lug or screw terminal, try to pull it apart with a gentle tug.

Any copper wire has an electrical resistance, which is added to the resistance of the sensor itself. A very long small-gauge wire can alter the apparent temperature that the sensor is reading. This effect is more prominent in lower value thermistors than in higher ones. So, when dealing with long runs of existing small-gauge wire, 10K sensors may be preferable to 1K. Follow the equipment manufacturer’s recommendation for wire size and maximum length. For example, a note with one of the Tekmar solar controllers mentions that 18-gauge wire up to 1,000 feet long will provide a plus or minus ½ degree F accuracy for their standard 10K sensor.

Signal interference

The wire cable itself can affect the performance of a thermistor sensor. Long lengths of unshielded straight wire (commonly used for room thermostats) can act like an antenna. Interference from nearby power equipment can cause random "signals" to be induced in unprotected wiring such as this. This can cause the temperature reading to become unstable and jump around rapidly instead of settling on the actual temperature at the sensor.

To prevent these unwanted signals, use metal conduit, shielded wire and twisted pairs whenever you can. Connect the metal shield to the “Common” terminal in the control box (the low voltage or sensor common). It is interesting to note that sensors with higher resistance (10K) are more sensitive to this kind of interference. Sensors that operate with a lower resistance range (1K) are less likely to be affected. So, when choosing temperature controls for an unshielded and noisy electrical environment, equipment compatible with lower sensor values might be preferable.

Final notes

These articles are targeted toward residential and small commercial buildings smaller than 10,000 square feet. The focus is on pressurized glycol/hydronic systems, since these systems can be applied in a wide variety of building geometries and orientations with few limitations. Brand names, organizations, suppliers and manufacturers are mentioned only to provide examples for illustration and discussion and do not constitute recommendation or endorsement.

Thanks to Fred Milder at SolarLogic for information on Beta curves.

Bristol Stickney has been designing, manufacturing, repairing and installing solar hydronic heating systems for more than 30 years. He holds a Bachelor of Science in Mechanical Engineering and is a licensed mechanical contractor in New Mexico. He is the chief technical officer for SolarLogic LLC in Santa Fe, N.M., where he is involved in development of solar heating control systems and design tools for solar heating professionals. Visit www.solarlogicllc.com for more information.