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Basic Control System Troubleshooting
More tips from Craig
month I discussed some general troubleshooting tips pertaining to
thermostats and temperature sensors, much of which I’d garnered from
practical experience, and some of which I learned in a class put on
last spring by Craig, my company’s Service Manager. This month I’d like
to share more practical tips and tidbits that I got out of Craig’s
How is it that something suddenly doesn’t work? What caused the “sudden” failure? Was there something that preceded the failure, that could have been a direct or even an indirect cause, for instance, a storm? Was there some sequence of events that could have led to the failure? Was there an issue prior to the failure, of which the failure could have been attributed to? Was there a recent “fix” to a separate issue that could have created another problem?
Perhaps the first place to start is with the equipment wiring diagram, whether it be a factory-furnished wiring diagram for a packaged piece of equipment, or a control drawing generated by a temperature controls contractor. It is important to note that there is a difference, especially when it comes to packaged equipment, between a point-to-point wiring diagram and a logic or ladder diagram. The former will show wire numbers, colors, terminal designations, etc. to depict the physical wiring of the unit’s controls, whereas the latter (ladder!) will show in schematic form, the logical representation, using source/path/load type symbols. Both are essential in understanding how a piece of equipment is wired and how it’s supposed to work, the ladder diagram more so for understanding the logic, and the point-to-point for understanding how the unit is physically wired.
Next to consider are environmental issues. Are there any signs of moisture? Is there a leak that can allow water to enter the control cabinet? What about condensation and humidity? Is the control cabinet located in a dusty area? All of these are to be considered and ruled out. If there is a problem due to environmental factors, best to address the cause of the problem at the same time you’re fixing the problem, otherwise you’ll be back again to deal with the same issue.
Tools of the trade: do you have the proper toolset? Fundamental is the possession of a voltmeter / ammeter. Do you know how to use it? How to check voltage? How to check current draw? How to check resistance? It’s surprising how few know how to properly utilize a multimeter, and to its full potential. Get someone who’s been around for awhile to give you crash course on using a meter.
Source/Path/Load: as discussed above when I was describing the ladder diagram, all electrical circuits consist of a source of power, a path of which the power travels, and a load that receives the power and performs some function. The source of an electrical control circuit is typically a transformer. Feel the transformer (don’t feel the terminals!). Is it warm to the touch? Is it hot? Cool? Check the primary voltage (with your voltmeter, remembering that the primary voltage is/can be lethal). Check the secondary voltage. If the transformer is warm to the touch, and primary and secondary voltages check out, then move on to the path. Is the wiring okay? Use your multimeter to “ring out” the leads. What about the load? Is the end-device getting power but not functioning?
What causes a piece of equipment to be shut down? A packaged rooftop unit or built-up air handler, for instance. If it’s DDC-controlled, start at the controller. Does it have power? Is the power for the digital controller derived from the rooftop unit’s control transformer? If so, is the unit online, meaning does the unit itself have power? If not, what is the issue? Is the circuit breaker tripped? Is the electrical disconnect thrown?
With regard to an air handler, are any of the safety devices tripped? If so, ask yourself “why” before resetting the device. If the fan starter overloads are tripped, what could have caused this? With freezestats, the biggest cause of “nuisance trips”, from our experience, stems from improper installation of the capillary. The cap needs to be installed horizontally across the coil, not vertically. With high and low limit duct static pressure switches, again the biggest issue we find with these is improper installation. The bellows of the device needs to be in the vertical plane for proper and effective operation. Any position straying from vertical can be the source of ongoing issues.
In the control panel, check to make sure that the plug-in relays are firmly seated in their bases. Vibration can work the relays loose, and this can be remedied with some cheap clips that hold the relays in place. Check to see that relays pull in when commanded to. If the relay has a light on the top of it to indicate that it’s on, then use that to determine if the coil has gone bad. If the light is on and the relay is still not passing current through its contacts, the contacts themselves have likely been fried. Check the current rating of the relay contacts, and then check to be sure that you’re not exceeding that rating.
If a digital controller is installed in the control panel, check to see that it has power. Now, is the controller communicating? Use the transmit/receive LEDS on board the controller to verify. Also check the input/output indicator LEDs (if there are any), and use the on-board override switches (again if there are any) to force digital outputs to an energized state. Just understand what you’re doing and use caution whenever manually forcing anything to an “on” mode.
Well, that’s it for this column. I’ve omitted some other items and issues, such as damper actuator troubleshooting and communication cabling wiring/shielding issues, thinking that I’ll tackle these topics with another future column, and perhaps start a “back to basics” series to span the summer months.
Tip of the Month: A few tricks of the trade, if you will…to check a solenoid valve, knowing that it is an electromagnet in its energized state, you can tap the head with a screwdriver to check for magnetism. If the head sticks, it’s working. To simulate a call for heat from a space sensor, give it the “breath test”. And if you run across a situation in which you’re overamping one set of contacts of a double-pole relay, and the other set is not being used, try splitting the load so that the current travels through both sets of contacts. You’ve effectively reduced your current through the original set of contacts by half, allowing the other set of contacts to pick up the other half. Problem solved!
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