Last month (October 2009), this column addressed the relationship between commissioning and energy conservation. I strongly believe that, in general, a commissioned building will be more energy-efficient than a non-commissioned building. However, commissioning alone does not guarantee that all of the owner’s energy conservation goals will be achieved.
The following are project examples of how an energy conservative design does not necessarily result in an efficiently operating system. These are areas where a diligent third-party commissioning professional can be very helpful in identifying the issues before they become problems and/or before the owner accepts systems that appear to be operating well but doing so inefficiently.
Submittal ReviewsThere have been situations where the design team member reviewing equipment submittals during construction is not aware of (or does not remember) the importance of certain aspects of the design requirements for energy conservation. As such, equipment was approved that limited or precluded certain energy efficient modes of operation.
One common example of this is approving equipment without the requisite communications interface for integrating with other system components. Many energy conservation strategies require multiple pieces of equipment to coordinate with each other. The loss of integration leads to less precise control of the systems to meet their respective heating, cooling, and ventilation requirements.
A specific submittal-review example is the approval of hundreds of one-row terminal reheat coils when two-row coils were scheduled on the design drawings. The two-row coils were required to provide sufficient reheat capacity with low-grade hot water (approximately 100°F) during dehumidification operation. The 100° hot water was to be provided by a heat recovery chiller designed specifically for that application.
The problem was not discovered until the first dehumidification season, when the building was too cold with the reheat control valves fully open. The solution was to fire up the central heating hot water boilers to provide high-temperature hot water to the reheat coils. This rendered the substantial capital expense of the heat recovery chiller useless, and you can imagine the interminable “discussions” regarding whether the design team or construction team was responsible for the error.
Chiller Plant StartupThere was one project where the variable flow chilled water system could not be kept operational during the contractor’s startup. The problem was with a primary flow, variable speed chiller system where the chillers have a minimum water flow requirement. The chilled water system served AHUs with airside economizer control, and the control valves were all two-way.
In the initial switchover from economizer cooling only to economizer cooling with supplemental chilled water cooling (i.e., when the outdoor air temperature was only slightly above the discharge air temperature setpoint), the demand for chilled water at the air handlers was not enough to result in the minimum required water flow through the chillers. The solution was to open the air handler chilled water valves fully and reset the chilled water supply temperature based on outside air conditions.
This solution solved the immediate problem of the chillers shutting down under low-flow conditions but resulted in the complete loss of variable-speed pumping and far less reliable temperature control at the air handlers. The latter most likely resulted in increased reheat energy at the terminal units, as the terminal units compensated during times of over-cooling at the air-handling coils. In addition, the extra capital expended by the owner for variable-speed chillers was for naught.
Gas-Fired Rooftop StartupSimilarly, during startup of a large gas-fired rooftop air handler, the contractors were unable to maintain sufficiently low discharge air temperatures during mild weather (e.g., around 35° to 50°) when only a little heat was required. The modulating burner did not perform to its advertised turndown ratio. The solution was to modulate the outside air dampers open when the burner was at its lowest firing rate in order to load up the burner and achieve the setpoint discharge air temperature.
Clearly, this solved the overheating problem, but at the expense of heating far more ventilation air than was required. In addition, given the active building pressure controls in the facility, the increased ventilation air resulted in an increased call for exhaust air. Therefore, there was a resultant increase in exhaust fan electrical energy consumption.
An alert and conscientious commissioning team can help identify such situations and inform the rest of the project team when modifications are being considered that will negatively impact the ability of the systems to perform as efficiently as expected. However, only the other project team members (including the owner) can make the decisions and take the steps necessary to return the systems to their intended energy-efficient operation.ES