Avoidable errors with sensors

The energy management control systems we use most commonly today depend heavily upon thermistor sensors and other types of sensors connected to a central control system usually by low voltage wiring.  In our new standard primary loop solar/ hydronic combisystems, well over a dozen sensors are typically connected to a central control system where all the control decisions are made in computer software.  

Any problem with a sensor, a wire or any terminal connection can seriously undermine the control decisions being made. A false or inaccurate temperature reading can cause and intelligent controller to do dumb things.  

Following are a few examples taken from recent installations in the field. These are mistakes that are easy to make and seem to happen far too often. They are easy to avoid if you know what to look for.

Heat storage water tank temperature sensors

Our most typical standard control configuration for heat storage water tanks always includes dual temperature sensors on each tank, one near the top and one near the bottom. Our heat storage tanks usually take advantage of the natural heat stratification in the water, where the hottest water rises to the top and the cooler water sinks to the bottom. The top sensor is used to make the control decision of whether to take heat out of the tank. The controller asks the question, “Is the top of the tank hotter than the heat load that needs the heat?”

 The bottom sensor is used to make the decision whether to put heat into the tank, to provide pre-heating to the coolest water in the tank. The most common mistake is for the a novice installer to choose a pipe, drain or other fitting that is attached to the tank, and clamp the temperature sensor to that for convenience. The pipe temperature will rarely provide an accurate reading of the water temperature in the tank. In some cases, we have seen uninsulated sensors attached to tank-connected pipes that literally provide only the room air temperature in the mechanical room, which is about as far from the internal water tank temperature as you can get. When this happens, a hot tank will be seen by the controller as being cool, and the control decision to put heat into the tank may actually result in heat being pumped out of the tank for no useful purpose.

To avoid this kind of mistake, always put the temperature sensor as close to the fluid of interest as possible. Many heat storage water tanks have temperature wells built into them. A temperature well can allow the sensor to slide inside the tank. A vertical well can sometimes allow two sensors in the same tube, and the low sensor can be inserted below the top sensor with enough distance between them to give a reasonable high and low reading. In many situations, where there is no easy way to put a sensor in contact with the water of interest in the tank, our installers have often cut away the thermal insulation on the tank (e.g. with a 2-inch hole saw) to expose a small patch of the bare metal tank. A thermistor can then be inserted between the insulation and the metal tank surface and an insulation patch is then put over the cut.  

Hot sensor on the solar heat collector

In most of our installations, each array of solar heat collectors must be monitored at the heat output end of the array. When a bank of collectors gets hot enough to be useful, the control system needs to know it and  have an accurate solar hot temperature. In some cases, we have noticed that if there is a plumbing fitting near the outlet, like a union, tee or air vent, the inexperienced installer might choose to avoid the fittings and clamp the thermistor a little too far away from the heat source inside the collector. This can result in the control system waiting much longer for the outlet pipe to heat up and turning on the solar heat collectors much later than necessary. As a result, the solar collectors run hotter, and some useful heat is ignored by the controller each day, so less heat is collected and the panels operate at a lower thermal efficiency because they are hotter than necessary.

The best place for a hot sensor on a solar collector is in a factory-provided temperature well, which can sometimes be ordered with new panels from a collector manufacturer.  However, many solar heat collectors have no temperature wells built-in, and so the best place to clamp a thermistor is on the hot pipe (union or coupling) at the hot outlet of the panel, as close to the collector frame as possible.  A metal clamp should always be used (e.g. automotive style hose clamp) since plastic wire ties can actually melt or become brittle and break at normal solar temperatures. Certain kinds of sensors can be crushed by over tightening a metal clamp, so use moderation to get a snug fit. Of course, the thermistor must be covered with pipe insulation that is permanent, weather proof and invulnerable to both high temperatures and sunlight.

Sensor wiring

Sensor wiring is typically very small gauge wire, operating at low voltages, and is vulnerable to damage and installation errors. Figure 89-1 shows a snapshot of data from a pair of solar heat sensors where one of them is malfunctioning with an intermittent bad connection. The most common reoccurring errors we see in the field involve heat, moisture and connection issues.  

The heat from a solar pipe can be hot enough to melt plastic. When sensor wiring comes in contact with the bare hot metal of a solar heated pipe, the plastic electrical insulation can melt, causing a short circuit between the sensor wires. Never run electrical sensor wiring along a bare metal hot pipe. Sensor wiring should always be separated from hot plumbing with permanent thermal insulation between the wiring and the plumbing. To be safe, always choose high-temperatureresistant wiring as well.

Wiring that runs outdoors, exposed to rain or snow, or that is mounted near other sources of moisture must be waterproof.  Some wiring, even with plastic electrical insulation, can absorb water which can cause a partial short between the electrical conductors and produce a false sensor reading. If the wire can get wet, it must be waterproof. If it runs outdoors, it must be weatherproof and be installed to resist sun, wind, rain, ice and snow.

Terminal connections can also cause unexpected problems. It is common for a sensor to be connected to its wires with small wire nuts or crimp connections. These connectors can actually collect water and cause the wire connections to corrode if not protected properly. Corrosion will cause the resistance of the circuit to change and then give a false reading. Always mount your connectors in a position that sheds moisture and then wrap them so they are weatherproof.

Finally, the most common connection problem is no connection. Sensor wire can be easily broken by a nail or sheet rock screw. It is also very common for a wire to break at the moment when a wire nut or crimp connector is being installed. This can also happen when installing a wire into a screw terminal. Also, we have seen cases where a poorly stripped wire, with some plastic electrical insulation remaining in the wrong place, can prevent a screw terminal from making contact with the signal wire. It pays to be methodical, so tug lightly on each connection to make sure it is firmly attached and then verify that proper continuity or sensor signal exists as soon as possible after it is installed.

Final notes

These articles are targeted toward residential and small commercial buildings smaller than ten thousand 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 in these articles only to provide examples for illustration and discussion and do not constitute any recommendation or endorsement. 


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. 

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