Improving energy audits with technical retro-commissioning

Using technical retro-commissioning to understand building operation can significantly improve energy audit reports.

09/21/2011


Figure 1: Retro-Cx teams typically use flow hoods to measure ductwork airflow. Courtesy: McFarlane Inc.Technical retro-commissioning (retro-Cx) is the systematic hands-on process that determines building system performance during actual operation. The technical retro-Cx process corrects ventilation- and energy-related problems in existing buildings and is also used to obtain actual operating data for audit savings calculations.

There are two commonly used energy auditing approaches: process audits and technical retro-Cx audits. The process audit approach relies on the written work of others and includes review of the test and balance reports, the temperature control sequences shown in the contract documents, a review of the plans and specifications, and a brief facility walk-through to determine corrective action. ASHRAE calls this a Level I audit.

Audits of this type usually have simple corrective actions that, in many cases, can be quite predictable. The typical recommendations for facility improvement measures (FIMs) typically include:

  • Replacing T-12 lighting for more efficient lighting
  • Installing occupancy sensors to turn off lights when not in use
  • Installing VFDs on constant volume equipment
  • Turning off unused equipment
  • Lowering temperatures in winter; increasing temperatures in summer
  • Install weather-stripping at doors
  • Replacing single-pane windows for double-pane windows
  • Adding roof insulation.

Some firms use boilerplate energy audits that use the same wording and format but change keywords for each building. Although these approaches can reduce energy usage in many instances, simply applying these FIMs to every building can, in some cases, increase humidity and energy usage, and reduce indoor air quality. Typically with Level I audits, implementation costs and projected savings are not calculated. These types of audits are less expensive and can produce savings of around 10%.

Retro-Cx audits are more thorough

The more accurate technical retro-Cx audit relies on the retro-Cx team to determine the entire building operation. ASHRAE refers to these more stringent audits as level II or III audits. In the technical Level II audit, the retro-Cx audit team checks the entire building and all of the energy-using components. These audits focus on the electrical and lighting systems, building envelope, and the entire HVAC operation. The Level III audit adds an energy-modeling component to determine savings outlined in the Level II report.

Electrical systems are verified for proper grounding. Thermal imaging is used to determine loose connections on all electrical panels, disconnects, and motor connections. Power quality analysis is used to determine low voltage, low power factor, or phase load imbalance on the electrical systems. A small 10% reduction in airflow or water flow has the potential to reduce the horsepower of a fan or pump by 33%. Revised motor horsepowers are calculated and smaller, more efficient motors are installed where practical.

Lighting systems are evaluated for efficient use of ballasts, lamps, occupancy control, and daylight harvesting. A Level I audit may simply change all T-12 bulbs to T-8 or lower. The more technically advanced technical retro-Cx audit checks for lighting levels, modifies these levels to meet code or occupant-required lighting levels, and then changes the lamps and ballasts to more efficient ones. A Level I audit may simply change the existing lamps but leave the space overlit or underlit and in the same condition as found.

The building envelope is evaluated for air infiltration, moisture intrusion, building pressurization, and glazing efficiencies. The Level I audit may look for cracks at doors and leaks at windows to determine areas of infiltration. However, the more thorough technical retro-Cx audit will perform building pressurization tests on the envelope using blower-door testing kits or using the actual air handling units themselves to pressurize the building in order to determine leakage.

Building pressurization testing

In this test, the building is pressurized or evacuated to two or three different levels of pressure between 0.1 in. wc and 0.3 in. wc. The air infiltration or exfiltration at these pressures is measured to determine the actual air leakage. A statistical analysis of the data is completed to determine the airflow leakage in cubic feet per minute of leakage per square foot of building surface at a pressure of 0.3 in. wc.

Various industry-specific sets of data are available at this pressure. These data show that high-performing buildings leak less than 0.1 cfm/sq ft of building surface. Medium-performing buildings leak about 0.3 cfm/sq ft, and leaks in a poorly performing building exceed 0.4 cfm/sq ft of building surface.

Thermal imaging of the building surface while under these test pressures shows where the leaks are. Because of extreme winter temperature in the ASHRAE/U.S. Dept. of Energy Climate zone 7s, our firm has found buildings constructed with leakage rates in the 0.1 cfm/sq ft to 0.3 cfm/sq ft range at 0.3 in. wc of pressure. We have measured building leakage rates of more than 10 cfm/sq ft in ASHRAE/DOE Climate zone 5 at this same pressure.

Once areas of infiltration are located, proper sealing can be implemented. Leakage points are sealed and retested until an acceptable level of leakage is attained. Controlling the wind-induced infiltration by improved sealing at wall roof joints, wall floor joints, doors, and windows is an important step in reducing the amount of unconditioned air that can enter a space. Window composition and construction are evaluated to determine if window-shading coefficients or U-values can be improved.

Optimizing the HVAC systems

Figure 2: Running static profiles on air handling units is a significant part of technical retro-Cx. Courtesy: McFarlane Inc.The retro-Cx team determines the current operation of the HVAC systems. Using the technical retro-Cx process, the team looks for areas where air and hydronic flows, temperatures, pressures, and run times can be optimized. The team also looks for ways to decrease fan airflows and remove restrictions that increase the static pressure in duct systems (see Figure 2). While a 10% reduction in fan airflow has the potential to reduce total horsepower by 33%, reducing static pressure by removing restrictions or by simply lowering the static pressure setpoint can also reduce energy usage. A 10% reduction in static pressure at constant airflow reduces horsepower by 11%.

Many design engineers use the ASHRAE 99.4% weather data and then add safety factors of 10% or more to determine the design flows and equipment for the project. While this may be an acceptable approach to ensure the project has sufficient capacity, actual operating flows can be reduced. This reduction is completed by analyzing the actual heating and cooling load in buildings at the 98% ASHRAE weather data and then removing the safety factors from the various airflows and water flows. Actual building trend data can also give a real-time building load profile.

This exercise usually results in a significant reduction in the maximum flow requirements. It is important to reset the variable air volume (VAV) terminal units to these lower values. Leaking VAV reheat valves or arbitrarily high air handling unit discharge temperatures will cause the VAV boxes to revert to the higher position if the boxes are not reset to the new revised conditions. Minimum position settings can usually be reduced and still maintain acceptable indoor air quality.

Toilet exhaust systems are an area that are often ventilated well above code required minimums. Savings can be generated by slowing the exhaust to code-required minimums, shutting the fans off when the space is not occupied, and adjusting minimum fresh air as required to maintain proper building pressurization.

Ducts that are not reinforced to the proper SMACNA standards can collapse. Collapsed ducts impart a restriction on the duct system and usually leak at broken seams and joints. HVAC systems that operate above the SMACNA duct design pressure specified to construct the ductwork can also split ducts open and cause loss of airflow.

Open end caps, open duct collars, duct work that is collapsed or has split seams, and unsealed ductwork provide opportunities to lower fan speeds and reduce airflow once the problems areas are repaired (see Figure 1). Restrictive duct fittings, elbows without turning vanes, elbows with debris on turning vanes, and choked fittings impose unneeded restrictions that increase static pressure, horsepower, and energy usage.

In order to check out the duct and hydronic systems, ceiling tiles are removed to enable the entire duct system to be reviewed for proper sealing, duct reinforcement, fitting, and construction. The inspection team looks for missing end caps, open duct collars, collapsed ducts, split seams, and leak points that cause major temperature or pressure differentials in ceiling plenums.

Systems are analyzed for proper zoning. Mixing interior and exterior spaces on the same terminal box or the same air handling system causes both comfort issues and increased energy usage. The technical retro-Cx process finds and corrects leakage, restrictive fittings, and zoning issues.

The actual airflows, water flows, temperatures, and pressures are measured. The actual water flows and airflows are compared to the new 98% ASHRAE design requirements or the operating requirements of the building. The percentage reduction in flows provides the necessary information to determine energy savings from flow reductions.

Actual temperature measurements of air and hydronic heating and cooling temperatures will outline the potential for energy reductions. Heating systems can usually be reset between the temperature required at design heating days and the lower hot water temperatures that can be used during warmer outside conditions. This lower hot water temperature is a function of the type of boiler and the capacity of the heating coil.

A typical hot water reset schedule in northern climates may reset the water temperature from 180 F at 0 F to 140 F or lower at 50 F. Attempting to run boiler water temperatures below 140 F to save energy should be approached with caution. Condensation can occur in the boiler. If the boiler is not a condensing boiler design, corrosion can occur and ruin the boiler if it is run at lower temperatures for extended periods of time.

A similar reset schedule for cooling would provide 45 F chilled water at design conditions. However, the chilled water temperature should be increased to 50 F or higher, depending on the cooling coil’s ability to provide properly dehumidified air at cooler outdoor conditions.

Air handling unit discharge temperatures can be reset based on outdoor design conditions or the number of zones calling for heating or cooling. Typical outdoor reset schedules adjusting the mixed air or discharge temperature would have a discharge temperature of 55 F at 65 F outside air and a 65 F temperature at 0 F outside air. This reset depends on the ability of the interior space terminal boxes to handle the cooling load, the dehumidification capabilities of the coiling coils, and space humidity requirements. The increased discharge temperature reduces the amount of reheat required. The increased discharge temperature and reheat savings should be compared to the increased horsepower due to the increase in airflow to arrive at the proper balance.

After the restrictions outlined in the above paragraphs have been removed and the actual duct system pressure is known, the duct discharge pressure can usually be lowered until the box requiring the greatest static inlet pressure is controlling at the 90% to 95% open range. This pressure is then set as the maximum static pressure required for the air handler and is typically found in the summer. This pressure can usually be reset to a lower static pressure when boxes go to the heating mode in the winter. However, if the majority of the building spaces are interior zones, these lower pressures may not be attainable.

In order to set the lower static pressure limit, all boxes that require heating are set to a call for heat. The static pressure required for the most restrictive box to meet the reduced heating airflow becomes the lower duct static pressure setpoint. This duct discharge pressure reset is also verified by the number of boxes calling for cooling. Duct static pressure is reset between the high and low settings based on the number of VAV dampers at 100% open.

Many control sequences will specify that the hydronic pumping systems have a pipe differential pressure set at 6-10 psig. In many cases, this is excessive. Similar to air systems, hydronic systems are set up so that the most restrictive coil valve operates at the 90% to 95% open position at the lowest required static pressure. Lowering system pressures reduces energy costs.

Temperature control testing

Figure 3: The technical retro-Cx team ensures that VFDs are calibrated and operating properly. Courtesy: McFarlane Inc.The technical retro-CX team also verifies the temperature control operation of the system and sequences. Actual run times, control response times, and trends of all loops are recorded for two weeks to one month to confirm normal system operation before changes are made.

One of the easiest changes that can be made to provide immediate savings is to compare the run times of equipment to the actual occupancy of the building. Systems do not need to run for up to 3 hours after the building is closed. Fresh air dampers do not need to be open when exhaust systems are shut off in the unoccupied mode.

Turning boilers, chillers, pumps, fans, and air handlers off when the building is unoccupied and shutting outside air and relief air dampers saves energy. However, these energy-saving strategies should be tempered when extreme weather conditions occur. It may be unwise to completely shut off cooling on design cooling days even when the building is unoccupied due to dehumidification requirements. Similarly, heating equipment may need to stay energized on a design day to keep recovery times on the warm-up cycle reasonable.

The team compares operational sequences to known energy-efficient control sequences. Occupied and unoccupied times are compared to actual building occupancy. Building pressurization sequences are reviewed to determine that the building is adequately pressurized at 0.02 in. wc to 0.05 in. wc. Return fan tracking sequences used to control building pressurization are notoriously poor control sequences for maintaining proper building pressurization. In cold climates, buildings under negative pressure tend to be drafty due to window leaks or leaks at the floor/wall and roof/wall joints. In colder climates, occupants sometimes compensate for these drafts by placing electric plug-in space heaters at their feet. These plug loads increase electrical energy usage.

Control loop testing

Current control sequences are compared to the aforementioned reset schedules. The energy savings seen in these pressure and temperature reductions can be accurately calculated. Perhaps most importantly, the technical retro-Cx team physically verifies each HVAC system control loop for proper operation. This ensures that all dampers, valves, VFDs, and related equipment open and close fully, control at setpoint, and respond to changes in an acceptable amount of time (see Figure 3). Valves that do not fully close leak either hot or cold water into the coils. This leakage is an energy waster due to extra pumping energy and additional reheat if the cooling coil leaks, or additional airflow when the heating coil valves leak.

Dampers that do not open fully create energy-wasting restrictions. Dampers that do not shut fully allow hot or cold outside air into the space in the unoccupied mode. Return dampers with excessive leakage can reduce the amount of time that economizer systems can run effectively. Because of return damper leakage, the mixed air temperature can be elevated causing the mechanical cooling systems to be energized.

Technical retro-Cx auditing is hands-on building systems evaluation conducted by professionals skilled in engineering, design, and building operation. A technical retro-Cx team with a good track record can produce energy savings of more than 30% by tuning building systems, thereby reducing pressures, temperatures, and run times.        

McFarlane is vice president of Technical Commissioning Inc. He was previously president of McFarlane Inc., Grand Forks, N.D., for 32 years. His expertise is in Cx and retro-Cx.



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