Miles Ryan, P.E., writes a monthly column in Engineered Systems Magazine on Building Commissioning. Read December’s column below:

For years, the conventional VAV control sequence included the reheat valve controlling directly to space temperature. More and more often, discharge air temperature (DAT) sensors are now being installed, and the reheat valve is specified to control the DAT to a setpoint (see Figure 1). That DAT setpoint is then reset to maintain space temperature at its space heating setpoint. Proper implementation of this strategy requires DAT readings that are representative of the discharge air being supplied to the space through the supply air diffusers. Surprisingly, I am finding this is often not the case which can have major implications in system performance.

Figure 1: Controls diagram for VAV box with DAT sensor.

Benefits of DAT Control

One of the primary reasons for controlling to a resetting DAT setpoint is that too warm of discharge air, coupled with the common arrangement of ceiling mounted supply diffusers and return grilles, allows for warm air to hug the ceiling and short circuit into the return duct. Excessive temperatures can be witnessed when DAT is not measured and allowed to limit the reheat valve. Such excessive DATs can result in the following:

  • Poor temperature control
    • I have personally witnessed 125°F supply air unable to satisfy the space heating setpoint of 68°F due to extreme stratification of the room air temperature. The heating control loop becomes unstable at a point (i.e. space heating demand rises, control valve opens more, DAT increases, supply air is not properly mixed, space temperature continues to drop, heating demand further increases, and on and on).
  • Decreased ventilation performance
    • The air supplied into the space includes at least a portion of fresh, outdoor air. If too warm of supply air hugs the ceiling and never makes it to the breathing zone, the V in HVAC is not really being achieved.
  • Increased energy consumption
    • Return air heading back to the AHU is actually warmer than the space temperature. Some of the return air may be exhausted at the AHU as part of an economizer sequence and made up for with higher quantities of cool, outdoor air than needed for minimum ventilation requirements. This will increase thermal energy consumption for the air system.

With a resetting DAT setpoint approach, a range for the DAT setpoint is defined. Using a high end setpoint of say 90°F can prevent the issues listed above from occurring. Additionally, it essentially places a limit on the amount of heat that can be added to any given box’s airstream. Some have advocated this eliminates the need for hydronic system balancing.1

These benefits of a resetting DAT setpoint approach apply to any duct mounted reheat coil, whether it is for a variable or constant air volume application. These benefits are well documented, and several ASHRAE standards2,3 have thus set limits on how much higher the supply air to the space can be above the space temperature.

DAT Stratification

For proper implementation of the resetting DAT setpoint control approach, you need to have temperature readings that are representative of the discharge air coming out of the diffusers. Note I used the word “representative,” not “accurate.” You certainly need accurate sensors as well, but inaccurate DAT sensors are far less prevalent than poorly placed sensors in my experience. Those accurate sensors need to be measuring air temperature at a location within the air stream that is “representative” of the air coming out of the downstream diffusers. Achieving such representative readings is easier said than done, due to excessive temperature stratification that is often present within the duct on the downstream side of reheat coils, especially reheat coils downstream of a modulating VAV box damper.

Research published at the 2023 ASHRAE Winter Conference studied this phenomenon4. They placed a grid of 25 temperature sensors at 1.5’, 2.5’ and 3.5’ downstream of an active reheat coil. In one trial which was representative of findings throughout, extreme stratification was witnessed 1.5’ downstream of the reheat coil, with readings ranging from 76°F to 112.2°F. Temperature stratification decreases the further downstream the air travels, but it is still excessive 3.5’ downstream with a range of 76.4°F to 102°F. I encourage readers to look at this paper as it contains many figures which help further demonstrate the extent of the stratification they recorded.

In addition to the DAT sensor’s distance downstream from the VAV box, the research found that the extent of stratification also depended on:

  • Upstream damper position
  • Hot water supply temperature

Lower-Than-True Readings

When temperature stratification within the duct results in the DAT reading being lower-than-true, all the benefits listed above for implementing the advanced resetting DAT setpoint approach are lost, and your space may be susceptible to space temperature stratification.

Such instances of lower-than-true readings are rarely identified as a lot of these issues of inefficient heating and ventilation performance don’t get to the point of being noticeable by occupants.

Higher-Than-True Readings

The real pain comes when DAT readings are higher-than-true. Imagine the maximum DAT setpoint used in the control logic is 90°F. Then, due to temperature stratification within the duct downstream of the heating coil, the DAT sensor reads 90°F when the actual temperature of the air coming out of the downstream diffusers is ~72F. The heating valve will not open any further since it thinks DAT is already at 90°F. This will result in the space heating setpoint possibly never being achieved on colder days. And if you are implementing aggressive nighttime setbacks with your space temperature setpoints, it may take 6 hours or more to raise the space temperature from its unoccupied heating setpoint of 62°F to its occupied heating setpoint of 69°F, making for cold mornings for occupants. If the duct’s temperature stratification is really bad, the served space may never get to 69°F for consecutive weeks in the dead of winter!

Does this above scenario sound awfully specific? That is because this happened on a building I had just finished functionally testing! A large number of spaces had temperatures in the building in the mid-60s, yet none of their reheat valves were above 20% open!

As expected, I was immediately asked by the team “didn’t you just test these boxes?” Luckily for me, I had documented these DAT stratification issues on my Action List the month prior, noting that several of the sampled VAVs had higher-than-true DAT readings due to improper location of the sensor (about 24 inches downstream of the reheat coil). Unfortunately, gaining buy-in of the necessity to move the DAT sensors further downstream was difficult for the following reasons:

  • Most in this industry don’t even know the reasons for the resetting DAT setpoint logic. The immediate tendency when this situation arises is to revert back to a conventional control approach controlling the reheat valve directly to space temperature. One member of the controls team on this particular project told me I need to think of DAT sensors as binary, only there to prove heating is occurring, not to quantify it. He stated analog DAT sensor readings have never been intended to be trusted. Needless to say, he didn’t convince me.
  • The design documents had no language on minimum distance downstream of the reheat coil that DAT sensor must be placed.
  • The spaces were already occupied and everyone on the project team was ready to move on.

All these things considered, the contractor’s change order proposal to move all the sensors further downstream was cost prohibitive. Instead, the “cowboy solution” was taken. The maximum DAT setpoint, originally allowed to be 20°F above space temperature, was allowed to go to 30°F above space temperature for all boxes in the building. This improved the heating performance of the system, but we will never know the extent of inefficiency it presented for the boxes who may have had lower-than-true DAT readings.

Table 1: Issues resulting from unrepresentative DAT sensor readings.

 

Improving Your Projects’ Chances for Success

The above example was not the first or last building our firm has seen this on. The “cowboy solution” is an easy, though not correct, fix to a situation that should never present itself in the first place if the DAT sensors were placed in the correct location. I propose the following steps be taken in the commissioning process to mitigate the chance of your projects running into these same issues:

  • Design Reviews
    • Recommend to the designer that proper specification language is included for DAT sensor placement. This needs to be both in the control diagrams and controls specification sections.
    • ASHRAE Guideline 365 recommends placing sensors a minimum of 5 duct diameters downstream of the reheat coil. I personally have seen even that to be insufficient. I recommend 7 duct diameters at the very minimum. ASHRAE Guideline 36 also recommends the DAT sensor must be upstream of any takeoffs to diffusers. I disagree. If the design includes diffuser takeoffs closer than 7 duct diameters downstream of the reheat coil, engage the design team with your concerns of DAT stratification resulting in wildly different temperature air streams being sent down the various duct paths. If they are unable to adjust, I would still recommend placing the DAT sensor 7 duct diameters down the duct path with the majority of airflow running through it.
  • Submittal Reviews
    • Mark up submitted controls shop drawings reminding the contractor of the requirements for DAT sensor placement. The installers of such sensors will rarely look at the design specifications but will certainly be referencing their approved shop drawings.
  • Installation Site Observations
    • Make the controls team aware that DAT sensor location will be a point of interest during your site observations. Include all members of the controls team on the correspondence (PM, installer, programmer, etc.) as to not rely on their internal communication channels, or lack thereof.
    • Attempt to coordinate site observations when the controls installer is onsite. Engage them and educate them on the need for proper sensor placement.
  • Functional Testing
    • Perform point-to-point on the DAT sensor in two scenarios. Place your handheld temperature sensor in the supply air diffuser. Compare readings from your handheld device to the BAS’ DAT sensor both with the heating valve closed, and with the valve stably controlling to maximum DAT setpoint. The first reading will be testing sensor accuracy, the second scenario will quantify if DAT stratification is negatively affecting the DAT sensor reading.
  • Additional Measures for Improved Readings
    • Aggressive static pressure setpoint resets will allow the air system to operate at its lowest possible static pressure, and the VAV dampers will settle out at a more open position. As found in Wendler et al, this will decrease the prevalence of DAT stratification.
    • Aggressive hot water supply temperature setpoint reset will allow for the lowest temperature hot water needed to properly serve the building. As found in Wendler et al, lower temperature supply water can decrease DAT stratification.

Conclusion

DAT stratification resulting in higher-than-true readings can create a lot of confusion when it results in an inadequately heated building. People question why the heating system is holding itself back, and naturally want to revert to conventional control logic which controls the reheat valve directly to space temperature. As commissioning providers, we need to advise them not to throw the baby out with the bath water. Educate the team on why a resetting DAT setpoint logic is needed. Be able to provide alternative solutions for resolving such instances. And, if you are involved in a new construction commissioning process, make every effort you can to avoid this situation from occurring in the first place!

References

  1. Taylor, S. J. Stein, G. Paliaga, H. Cheng. 2012. “Dual maximum VAV box control logic.” ASHRAE Journal (12).
  2. ANSI/ASHRAE Standard 90.1-2019, Energy Standard for Build­ings Except Low-Rise Residential Buildings.
  3. ANSI/ASHRAE Standard 62.1-2022, Ventilation for Acceptable Indoor Air Quality.
  4. Wendler, P., Raftery, P., and Cheng, H. 2023. “Variable Air Volume Hot Water Reheat Terminal Units: Temperature Stratification, Performance at Low Hot Water Supply Temperature, and Myths from the Field.” ASHRAE Winter Meeting, Chicago, January 2023. ASHRAE Transactions 2023, Vol. 129 Issue Part 1, 8p. January. https://escholarship.org/uc/item/6b9590qr
  5. ASHRAE Guideline 36-2021, High-Performance Sequences of Operation for HVAC Systems.
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