Troubleshooting Modern DDC Systems

May 18, 2009

The vast majority of commercial buildings today include direct digital control (DDC) systems. These systems are also known by other names, such as facility management systems (FMS) and building automation systems (BAS), and they’re typically maintained by an HVAC controls technician.

This article describes several common troubleshooting and diagnostic procedures for DDC system wiring.

Building operations use these systems for HVAC system control, fault alarming, energy monitoring, and many other critical functions. Operating these systems properly depends on following accurate installation and wiring guidelines for the various devices.

For troubleshooting, use a high-quality digital multimeter (DMM). Avoid using inferior and older analog meters. This is a case where inaccurate or misleading readings can substantially affect your outcomes. Common problem areas include incoming power, ground loops, communications such as RS485, inputs (analog and digital), and outputs (analog and digital).


Testing for transformer isolation to ground. This test checks whether the transformer power supply is properly isolated from ground. If it is not, the controller communications may be disrupted. You can test the transformer secondary with a good digital ohmmeter to ground. The resistance between the secondary and ground should be above the manufacturer-specified level.

Testing for RS485 communications. Many DDC systems use an RS485 bus. This communication bus is a daisy chain communications line. Essentially, it consists of three wires that carry three signals: RS485+, RS485-, and REF. The RS485+ and RS485- lines carry the actual data signals. The Com line provides a common reference so that each connected device is capable of electrically receiving and transmitting data by creating a common voltage reference among all the devices connected together by the RS485 bus.

The system needs three wires. It is important that the RS485+ and RS485- lines are twisted together, because that allows most induced noise (common mode noise) from external sources to affect both lines equally, thereby canceling it out. In most installations, the RS485 bus works fine with unshielded cable. However, in noisy environments (such as near gas ignition devices and arc welders), shielded twisted wire must be used. Otherwise, the noise disrupts RS485 communications and the field controllers.

Optoisolation is another important feature of the RS485 bus. Isolation prevents interruption of all bus communication if any of the bus controllers become grounded. This procedure tests for proper levels of RS485 voltage on three wires (RS485+, RS485-, and Com). Fortunately, RS485 voltage levels are standardized for many manufacturers.

Use a digital multimeter for these tests. If these voltage levels are not present, you have a problem with the communication trunk wire or one of the controllers. Test halfway across the length of the trunk for the proper voltages, then split in half again and again until you find the problem controller.

Testing for communication trunk isolation. Testing communication trunk isolation from ground is very similar to testing power supply. Test the RS485 communications wire to ground with a quality digital ohmmeter. The resistance must be above a specific level or the communications trunk is not isolated properly. If it isn’t, one of the inputs, outputs, or even the power supply may be improperly grounded.

Testing for analog input resistance values. You can easily check thermistors and other analog inputs by testing their resistance and matching that to a manufacturer-supplied chart. Use a quality thermometer to determine if the sensor readings are on the proper spot on the chart. Thermistor-type sensors can be either PTC or NTC. A PTC sensor will increase its resistance value as the temperature increases, and vice versa. An NTC sensor resistance value will decrease on a temperature increase, and vice versa.

The coefficient of a thermistor-type sensor is the amount of resistance change per °F change. For instance, a thermistor-type sensor may have a coefficient of 2.2 Ohms (O) per °F. This means that this sensor resistance value will change 2.2 O every time the temperature changes 1°F. The base or nominal value is given as well. A common number that is used is 1,000 O at 70°. This means that this sensor will have a base reading of 1,000 O at 70°.

If its coefficient is 2.2 O per °F, and if it’s a PTC sensor, then the resistance value will increase by 2.2 O for every 1° temperature change at the sensor. Using an accurate digital multimeter and thermometer, it’s easy to find out if the sensor is reading properly. Since 2.2 O per °F is commonly used, it’s easy to understand why the resistance due to long wire runs can affect the reading of the sensor.


To confirm that the element is functioning correctly:

1. Measure the temperature at the sensor using an accurate thermometer.

2. Determine the element resistance at ambient temperatures by applying the appropriate compensation (for models with nominal resistance values less than 1,000 O).

3. Measure the resistance of the sensor using an ohmmeter and compare actual and expected values.

4. Replace the sensor if the measurement indicates (a) open circuit (infinite resistance), (b) short circuit (zero resistance), or (c) out of tolerance indicated for the sensor.

It’s easy to check the electrical continuity on digital input and output wiring. Disconnect the wires from the controller and digital device, connect your meter, and read the resistance. Normally open and normally closed contacts must be set up properly. If 24 vac should be present at a DO, the voltage can be easily checked with a digital multimeter.

Publication date: 05/18/2009