Total Building Commissioning

As noted in The Genesis post, we will be sharing a lot of stories from the field.  This example seems like a great place to start.

On a recent project, the Variable-Air-Volume (VAV) air handler controls sequence did not have supply air temperature reset control.  This is acceptable according to the local energy code (ASHRAE 90.1-2007) which only requires supply air temperature reset for constant volume air handler systems, but it uses additional energy. These air handlers are VAV-type systems with four stages of cooling; outside air economizer dampers (OA Econ), in-direct condenser water cooling coil (ICC), direct evaporative cooling (Evap), and a chilled water coil (CHW).  This system is commonly called an IDEC (In-Direct, Evaporative Cooling) system.  The cooling sequence is as follows;

Stage	OA Econ	ICC Valve	Evap	CHW Valve	OAT
1	0 – 100%	0%	Off	0%	OA < DAT StPt
2	100%	0 – 100%	Off	0%	OA > DAT StPt
3	100%	0 – 100%	ON	0%	OA > DAT StPt
4	100%	100%	Off	0 – 100%	OA > DAT StPt

At first glance, someone would look at this and be thinking that these are expensive air handlers, and they would be right that it does cost more to add the in-direct and direct evaporative sections to a regular air handling unit. So it is important to note that this project is located in Salt Lake City, UT which is a high mountain desert.  The design conditions are 97°F DB and 63°F.  If you do a BIN data plot on a psychometric chart,  it is fairly easy to see that this system will operate on mechanical cooling between 100-200 hours a year verses about 2000 hours a year on a traditional mechanical cooling only air handler.  The following BIN data plot shows the number of hours that the system will operate at each outdoor air condition throughout the year (based on a five-day work week).  The number of hours that the cooling will be reduced are the hours between the first and second lines.

An IDEC type cooling system requires the supply air temperature to be 55°F or lower whenever the evaporative cooling stage is on.  This constraint is meant to ensure that the indoor humidity will not rise above 40%. The engineer specified that the supply air temperature set point be set at 53°F.

Even though a constant supply air temperature set point is acceptable for a VAV system based on the local energy code (ASHRAE 90.1-2007) it was causing the system to use large amounts of reheat energy most of time, even when the outside air temperature is as high as 80°F.  It was also causing many occupant comfort complaints especially when the outside air temperature was cold or moderate.

There are multiple methods to provide Supply Air Temperature Set Point  (SAT StPt) Reset Control.  Two of the most common methods are based on Outside Air Temperature and on the maximum VAV box cooling signal.

Each reset method has its advantages and disadvantages:

Outside Air Temperature Reset Method

Advantages:

• Relatively simple to program
• Better than nothing

Disadvantages:

• It is “open loop” control.  Open loop control does not provide feedback into the control loop and therefore does not adjust based on system response.  Open loop control is highly discouraged because of this.  An example of open loop control in your car would be speed control based on the type of street driven on.  This method would press down the accelerator a certain percentage based on the street type.  This control would not give any feedback to how fast the car was going, what the traffic was like, whether you were traveling downhill or uphill, or even, the actual speed limit.  It is easy to see how this would be very inaccurate and undesirable.
• No very accurate.  A building operator has to watch the system over a year of two and try to adjust the reset parameters to get it to match the building characteristics.  Even when this is done right, it will only be a fair estimation.

VAV Box Cooling Demand Method

Advantages:

• The system responds directly to the building load and is able to use less energy by using the highest supply air temperature as possible.

Disadvantages:

• It is more complicated to program.

This particular project’s building owner operates well over 100 buildings and has a very knowledgeable, full-time control system engineer who monitors the buildings and can deal with building control issues as needed.  Based on this, the owner chose to use the outside air temperature reset control method, with the understanding that the controls engineer would need to monitor the building over the next one to two years and tweak the set points to get the supply air temperature set point (SAT StPt) reset adjusted for this building.

The following Supply Air Temperature Reset Control sequence was used for this project:

Reset the air handler SAT StPt of each air handler based on the outside air temperature.  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:

End of Sequence

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.  Another item addressed in this sequence was graphic design that would provide simple and intuitive access to the operators for monitoring system operations and adjusting system settings.

In my next post I will provide an example of the SAT StPt reset control based on VAV box cooling demand. Later blog posts will explore control system graphics design.

 

 

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.