Total Building Commissioning

In the Air Handler Supply Air Temperature Control post, we discussed a method of resetting the air handler supply air temperature set point based on the outside air temperature (OAT).  This post will discuss another method of resetting the air handler supply air temperature set point (AH SAT StPt) based on VAV box cooling demand.

The advantage of resetting the AH SAT StPt based on VAV box cooling demand is that the supply air temperature set point would actually track the building demand.  It sounds pretty simple on the surface, but there are some things that can derail this strategy and make it fail.

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Keys to making this strategy work:

1. All zones must be able to be cooled most of the time by higher supply air temperatures.
   • A good rule of thumb is that the full cooling supply air flow delivered at the upper SAT reset limit should cool 50% of the design load.
   • Zones that are scheduled to turn off during unoccupied periods should be separated from zones that have a 24/7 cooling load.  This is often done by providing dedicated air conditioning units for the 24/7 rooms such as data closets, or grouping these zones together and serving them from separate air handlers.
   • Accurate, properly placed and calibrated temperature sensors are also key to making any control strategy work.

Conditions that will kill any supply air temperature reset strategy:

1. One or more zones has a constant high cooling demand and always requires supply air temperatures near the lower end of the SAT reset range.  Typical applications are data and telecom rooms or any place that has equipment running.
2. Failure to communicate how the reset control strategy works and the conditions necessary for it to work.  This is important to describe in the Basis of Design.

These items listed above point out good design practices that all designers know, but the designer needs to make some adjustments to the standard design in order to optimize any SAT reset strategy.

After determining each zone’s peak heating and cooling loads, the designer needs to then identify all of the zones that have high cooling demands that do not back off during non-peak conditions.  If there are cases where high cooling demand zones cannot be separated from intermittently occupied zones, the designer can design these high cooling demand zones for a higher supply air temperature.  This will increase the cooling airflow which will increase the size of the VAV box serving these zones.  For instance, a zone supplied by 55 degree air with 75 degree return air requiring 1000 CFM to cool the space would required 2000 CFM to cool the space with a 65 degree supply air temperature.  This will cost a little more up front for a larger VAV box and supply ductwork in this zone, but it will save significant amounts of reheat energy and increase comfort in the other system zones.  The argument could successfully be made that the air handler airflow would not need to be increased because during the cooling design load conditions the air handler SAT would be 55 degrees and so this zone would only require 1,000 CFM to cool this room.  The CFM would only increase during the periods of low cooling demand in the other zones.  As long as this is explained in the Basis of Design other people can quickly understand that the system has diversity and that the sum of the VAV box CFMs will be more than the air handler CFMs.

A real-world example of this strategy is the project that is located in Salt Lake City where the summer dry bulb design temperature is 97 degrees F.  The system resets the supply air temperature between 55 and 68 degrees.  The strategy has eliminated almost all of the reheating and reduces the overall system airflow and fan energy.  This was easy to see why, because before the reset strategy was implemented, 50 to 75% of the VAV boxes would be in reheat mode while about 2 – 5% of the VAV boxes would be in cooling mode when the outside air temperature was 70 degrees F.  By resetting the supply air temperature, the supply airflow is increased to the high cooling load zones while the other boxes that were in reheat are now operating at the minimum airflow set point which is significantly less than the heating airflow set points.

With that said, here is a Supply Air Temperature Reset Control sequence based on VAV box demand that could have been used in the project from the previous post.

1. SUPPLY AIR TEMPERATURE SET POINT RESET CONTROL:

Reset the air handler SAT StPt of each air handler based on the VAV box cooling demand.  If the air handler enters a cooling mode that involves the evaporative cooling (Stage 3), then the SAT StPt shall be set to the evaporative cooling mode set point. Each air handler will have its own reset schedule as follows:

1. If the maximum VAV cooling demand is below 85% for 5 minutes then the supply air temperature set point shall be raised by 0.5 degrees.
2. If the maximum VAV cooling demand is above 100% for 5 minutes then the supply air temperature set point shall be lowered by 0.5 degrees.
3. The initial supply air temperature shall be 60°F.
4. If the air handler enters a cooling mode that involves the evaporative cooling (Stage 3), then the SAT StPt shall be set to the evaporative cooling mode set point.

Air Handler SAT Reset Based on VAV Box Cooling Demand

It should pointed out that this strategy is called “Trim and Respond” because the set point is “trimmed”, and then the system responds and is allowed to settle out before trimming again.  The reason for using this type of strategy instead of a PID loop is that there would be interactions with other PID loops and it would be very difficult to keep them stable.  When you change the supply air temperature, then the VAV box PID loop will respond by increasing or decreasing the cooling demand signal.  This is fed back to the SAT StPt control loop.  By using a trim and respond sequence with a significant time delay, the SAT StPt control will be stable.

As it is noted in this post that there are many details that need to be addressed to make the VAV box cooling demand reset method work, I believe that the extra effort is worth it and can pay huge dividends in energy savings and occupant comfort.

Sequence Notes: A safety in this sequence was to make sure that whenever the evaporative cooling mode was operating that the SAT StPt would be 55°F or below.  Graphic design was also addressed to provide the operators simple and intuitive access to monitor system operations and adjusting system settings.

Todd Rindlisbaker

Todd Rindlisbaker, P.E., QCxP, LEED AP, HBDP, CCP, has been in the HVAC/plumbing design and commissioning business since 1993. He has extensive experience in project management, HVAC design, and energy studies and specializes in hydronic heating and cooling systems, controls optimization for comfort and energy efficiency and in commissioning. He has been involved in the design, installation, and commissioning of mechanical, plumbing, and building management systems throughout the United States and internationally.