March 2012

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My Sensor’s Not Working!

Tips on how to troubleshoot a problem

Steven R Calabrese
Steven R. Calabrese
Control Engineering Corp.

Contributing Editor


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The house that I live in is an old bungalow with a hot water heating system, consisting of a gas-fired hot water boiler and three zone pumps, each controlled by a heating-only thermostat. Pretty simple heating system, and overall pretty reliable as well.

Back in the summer of 2010, we were hit with a flood that wiped out our boiler and zone pumps. The old boiler was replaced with a new boiler (half the size!), and the zone pumps were replaced like-for-like. The heating contractor installed a three-zone central pump controller, which accepted the pair of wires from each existing zone thermostat, and in turn engaged the zone pumps through 24-volt pilot relays. All-in-all a pretty good system.

So what does any of this have to do with the title of this column? Well, come winter of 2010-2011, we began to have issues with the zone controller. I would come home from work, and my lovely wife would inform me that it was cold in the living room, so she cranked the thermostat up from 74 to 80. Of course I had to explain to her that, if the thermostat was set at 74 and it was cold in the space, turning it up to 80 would do no good, as there likely was a problem with the system. I just love having to explain this scenario to both my wife and my mother any time there’s a problem. “If the thermostat is set at 74 and it’s 65 in the space, cranking the stat up to 80 ain’t gonna magically bring the temperature back up!!!”.

So back to the issue at hand. I checked out the zone controller, and apparently the relay for the zone in question was pulled in, meaning that the thermostat was doing its job. Upon further inspection, I noticed that the contact inside the transparent plastic relay enclosure was “burnt out”, and not allowing 120 volts to the zone pump. I replaced the relay, problem solved (or was it?).

Come last spring, I sat in a class put on by our Service Manager (shout out to you Craig!), and he spoke about sensor and thermostat issues. I thought that I’d share some of what he had to say here in this column, seeing as it helped me ultimately come to a resolution to my heating system problems (more on this later).

General Troubleshooting – the first rule when it comes to troubleshooting is to have an open mind. Don’t rule out anything, for this is where you can find yourself in a vicious cycle, going through the same few tests over and over because you’ve prematurely ruled out the obvious. Believe me, I’ve been there! The second rule to troubleshooting is to attack it one bit at a time. Don’t take on a troubleshooting endeavor as a huge undertaking. Rather, view it as multiple steps that make up a larger task. Once you’ve gotten into this mindset, it’s easier to fathom the outcome.

Test Equipment – I know this sounds like a given, but make sure your test equipment works! A multimeter is required to check voltages, sensor resistances, and to check for open circuits and short circuits. Again, make sure your tools are in good working order.

Sensor Reliability – sensors don’t just up and die for no reason. Once installed and in place (and proven to be working!), they’re not likely to fail. It typically takes some kind of event to promote failure, whether it be physical damage (sensor crushed by a forklift, or pummeled by a basketball), or electrical damage (sensor subject to an over-voltage condition). So check for signs of physical damage, and check for signs of “operator intervention” that may have resulted in the sensor being, for lack of a better term, “fried”.

Sensor Values – knowing how abnormal computer screen readings correspond to certain problems is a must. For instance, when a sensor is in proper working condition, the values registered at the “front end” operator workstation are real-world temperature values. However, if there’s an open circuit or a short circuit, the value at the front end will typically be something “otherworldly”. For an open circuit (infinite resistance), the reading may be very low, and for a short circuit (no resistance), the reading may be extremely high. Knowing this allows you to pinpoint the issue at hand just that much quicker.

Sensor & Thermostat Troubleshooting – start at the sensor, disconnecting the leads and checking the resistance of the sensor, using a resistance vs. temperature chart to validate the reading. If you’re reading space temperature and the sensor is, for instance, a type III thermistor that has a nominal resistance of 10,000 ohms (10K ohms) at 77 degrees Fahrenheit, then at least you know you’re in the ballpark if your ohmmeter is displaying anything near this ohm value.

If all is good at the sensor, then start checking the “path” (wire and connectors). A voltage drop (i.e. an increased resistance) anywhere along the path would register a lower temperature value. In other words, the resistance of the thermistor and the resistance on the path would combine, resulting in a higher resistance value back at the input, thereby registering an erroneous temperature reading.

If the path checks out and you’re still viewing abnormal temperature readings, time for some insight. Where is the sensor located? Is there a chance that there’s either warm or cool air in the walls? Is the sensor mounted on an outside wall with no insulation? Is the sensor picking up heat from a nearby electric appliance?

For discreet mode thermostats (those that have on-board switches that are either open or closed depending on the mode), check first that the switches are functioning. Induce a call for heat and use your ohmmeter to check for a transition from an open to a closed circuit. Do the same for cooling, and for the fan switch. Then go back to the equipment being controlled by the thermostat, and check to see if there’s voltage being returned on each switchleg. For instance, when transitioning to a heating mode, the contacts at the thermostat that engage on a call for heat will close, allowing voltage from the equipment to travel through the contacts and back to the equipment, to energize the equipment’s form of heat. If there’s no voltage returning on the switchleg, then get out there and start checking out the path!

I’m hopeful that these little tips provide some assistance in your sensor troubleshooting endeavors, as they did for me. However mine turned out to be a very unique issue. Getting back to my story, when I replaced the burned out relay last winter, I considered the problem solved and went on with my life. This winter, the same thing happened, only with a different relay serving a different zone pump, which got me thinking more analytically about the situation. I studied the system for a few days, and I started to notice that the relays “chattered” when they were called into action. Chattering meaning that the coils pulled the contacts in and out very rapidly, causing a chattering sound. This led to the premature burnout of the load-carrying contacts. I attributed the problem to the thermostats, seeing that they were likely over 50 years old, and by no means were they “snap-acting”! I replaced the thermostats, problem solved, this time for good!

Tip of the Month: With an ohmmeter and a type III thermistor, you in essence have yourself a digital thermometer (think about it!). Just make sure you have that resistance vs. temperature chart handy!


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