Electric heat not energizing on 102-AX

Issue

The 102-AX will not energize its digital outputs to control electric heat strips, but the controller does have a -100% terminal load.

Product Line

TAC Vista

Environment

Xenta 102-AX

Cause

The 102-AX is configured for 1-3 stages of electric heat without a fan, and the heat will not energize. This is because the controller is not seeing enough airflow to safely control the electric heat.

Resolution

Troubleshooting Electric Heat on a Xenta 102-AX

The Xenta 102-AX controller supports up to two stages of auxiliary heat on two digital outputs. This document suggests troubleshooting techniques to help narrow down the cause of improper operation of these digital outputs.

Start with the Universal Outputs

On the Status tab of the 102-AX Plug-in, each of the universal outputs are listed under the “Network Bound Inputs” column.

In the 102-AX Plug-in version 5.1.4, this column has been erroneously omitted. Download TAC Toolpack version 5.1.4 Hotfix 72 from the Buildings Business Extranet to resolve the issue.

The outputs that represent the outputs are Universal Output #2 and #3. The first thing to do is attempt to override the output on. Note: verify that proper airflow is available to prevent overheating.

Figure 1. Network Bound Inputs

If the heating stages energize correctly, then the unit parameters and wiring are all correctly configured. Skip directly to the chapter entitled “Overriding the Output Successfully Energized the Heat.” If the heat did not successfully energize, proceed to the next chapter where three possible problems are addressed.

Overriding the Output Failed to Energize the Heat

There are three main reasons why overriding the output would fail to enable the output.

Unit Parameters

The Unit Parameters tab of the 102-AX Plug-in is where Universal Outputs #2 and #3 are configured as digital outputs. Verify they are set for OUTPUT_DISCRETE and do not have a very long delay on start value.

Wiring

Check the wiring of the heat to the 102-AX. It should be wired as follows:

Point Label Terminal
Universal Output #2 V2 15
Universal Output #3 V3 16
Ground G0 13

Actuator

The last step to troubleshoot an issue with the heat not turning on is to test the electric heat itself. This can be done by jumping out the enable signal to 24Vac power. If the heat still does not turn on, then it is faulty and should be replaced.

Overriding the Output Successfully Energized the Heat

If overriding the output causes the heat to enable, then the configuration of the output and the physical configuration are all correct. The 102-AX controller logic is not calling for heating. There are a number of reasons why this might be occurring.

The Basics

The 102-AX must be in a heating mode to enable heat. It may be displaying either HVAC_HEAT or HVAC_MRNG_WARMUP. The space temperature must be below the effective setpoint to be in heat mode and will generate a negative terminal load down to -100%. The terminal load determines when each stage of heat will be enabled (based upon how many are defined).

Check that the heating/cooling setpoints and heating/cooling bands are set such that no overlap in the two modes occurs. Heating/cooling bands must be greater than 0°.

Hardware Configuration

If the 102-AX is not configured for a sufficient number of stages of heat, it will not enable correctly. If there is no fan, then only one stage of heat needs to be defined. However, if a fan is defined, then there needs to be at least two stages of heat, because the fan is considered the first stage.

Figure 2. Fan Type and Heat Stage

If the auxiliary heating is to be utilized in morning warm up mode (when the duct inlet temperature indicates that the air handling unit is providing central plant heat), then the hardware configuration must state that supplemental heat is allowed during warm up.

Figure 3. Enable Heat on Warm Up

If a supply temperature sensor is wired to Universal Input #1, it could be that it is sending the VAV into morning warm up mode when the heat is enabled, and thus, disabling the heat. If this is the case, there are two options. Universal Input #3 can be used instead and set as an outside air temperature. Alternatively, Universal Input #1 can be configured as “None.”

Figure 4. Input 1 Selection

Setting Input 1 Selection to "None" will not disable the reading of the sensor. UnivIn1_Sense.nvoAuxTemp1 will continue to report the duct supply temperature. Setting it to "None" disables the signal from affecting the internal logic – preventing the 102-AX from entering morning warm up mode.

Note: If the VAV is already in morning warm up mode when the input it set to "None," the box will remain in morning warm up mode. Override the input temperature to something below room temperature prior to changing the input selection.

Application Mode

The SNVT input VAV_Controller.nviApplicMode can allow a supervisory controller to override the current mode of the VAV. Upon receipt of a new application mode, the 102-AX will encounter a brief synchronization period where the terminal load is held to 0%.

If the SNVT is being sent from a data manager, and the period on the output SNVT in Menta is set to 60 seconds, then the 102-AX will zero out the terminal load every 60 seconds, preventing proper operation. Set the SNVT output to a period of 0 seconds, and it will only write a new value on change and allow proper external control of the mode.

Airflow Setpoint

The Warm up Maximum airflow setpoint must be greater than or equal to the Heating Minimum airflow setpoint.  If this is not the case, then the airflow setpoint will default to the unoccupied flow setpoint (typically 0 CFM) any time the VAV is in heat mode, and the heating output will not be utilized.  This can be easily overlooked in applications where Warm up Mode is not being used, but this one parameter must still be set. This is fixed in the Xenta 102-AX firmware v2.18.

Figure 5. Airflow Setpoint

Actual Airflow

In order for the heating outputs to be energized, the airflow must be at least 80% of the minimum heating airflow setpoint. This is hard coded into the controller and cannot be disabled.

The only way to remove the interlock is to set the minimum heating airflow setpoint to 0 CFM; however, in this situation, the VAV will close the damper in an attempt to provide 0 CFM of airflow. The heating will modulate without any airflow at that point.

Figure 6. VAV Status not providing 80% Airflow

Check on the VAV Status tab of the 102-AX plug-in to compare the airflow to the airflow setpoint. If the box cannot provide the necessary 80% of setpoint or there is no airflow available during the commissioning stage, lowering the airflow setpoint can allow checkout to continue.

This 80% airflow requirement is looking at the on-board flow sensor of the Xenta 102-AX. If the flow value is coming from another controller on the network via the nviBoxFlow SNVT, the heat will be disabled. The nviBoxFlow input will override flow values for damper control, but heating outputs will ignore it. In this situation, setting the pressure offset (UCPToffsetPress) to 1” will bypass the limitation. This will be fixed in the Xenta 102-AX firmware v2.18.

Wrap Up

If everything is configured properly physically and in the 102-AX Plug-in, then the box should be enabling and disabling electric stages of heat to maintain the space temperature setpoint.

Disclaimer

The information contained in this document is subject to change without notice. It is also subject to change with versions of TAC Xenta 102-AX. If further assistance is required, or if you would like to add to steps suggested here, please contact Product Support.