Building automation systems can be useful in diagnosing problems in a building's HVAC systems. Here are some typical trouble spots that arise.
From the Jan-Feb 2015 print issue.
Building Automation Systems (BAS) have become common in larger buildings over the last few decades, but many building owners and operators do not use the BAS’s capabilities to their full advantage. Once configured, the systems are often rarely viewed and only accessed when there is a complaint or failure. Yet a BAS can help you find energy conservation measures. Also, using the BAS and relatively simple strategies it is possible to diagnose issues in the HVAC system before the components have failed and complaints arise.
The best way to find potential problems is to do energy reviews when the building is unoccupied because often the activities of building users can alter the buildings operations and make it harder to detect problems. However, because it is not always possible to do the reviews when the building is unoccupied we have often used the BAS system’s trend logs. The logs enable us to document potential problems and check on them in the future.
Points in manual override
A common clue to a potential problem is when points are found in manual override. Points that have been overridden and locked in manual are not allowing the building to operate under its intended parameters. Sometimes the points are overridden to alleviate a temporary problem and are never released. At other times the operation of the building or space requires modifications to the parameters, but instead of updating the schedules or set points someone permanently overrides them. Or someone has overriden the point to test something and then never released it back into automatic. In general, manual override should only be used as a diagnostic tool, and any points should be released to automatic and the parameters should be updated.
Outside air dampers
Outside air dampers are exposed to large temperature fluctuations and frequent modulation. Due to these and other factors they often fall out of calibration or get stuck which can result in over ventilation. If you check them during periods of cooler weather and review the mixed air temperature, return air temperature, and outside air temperature points, you can find what the ratio of return air is to outside air through a simple calculation. If the damper positions don’t reflect the ratios there is a good possibility that they are not properly calibrated.
During periods when the building is unoccupied or has low occupancy, you can also manually override the return damper to full open and the outside air damper to full closed positions; then let the system run for a few minutes and review the stabilized air temperature. If there is a significant difference in temperature between the return air and mixed air temperature when the outside air damper is closed, then it is likely that the damper is not closing completely. The reverse can also be done with the return air damper closed and the outside air damper open. While this latter approach does not have as much of an impact on energy use, it will affect the system’s ability to free cool and it will impact the indoor environmental quality. As a caution, it is advisable not to open the outside air damper to full open position when the outside air temperature is below freezing as this can lead to frozen coils. After you have checked and recorded the temperatures, release all the points back into automatic operation.
Carbon dioxide sensors are commonly used to detect CO2 levels in a zone and to modulate the damper to provide no more outside air than is required. While CO2 sensors alleviate the over ventilation issue, they also create a maintenance point. During the lifespan of a sensor they can be knocked out or drift out of calibration and should be periodically reviewed to see if they are still reading accurately.
Heating and cooling
Heating and cooling valves are also subject to frequent modulation and large temperature variations so should be checked periodically. While these valves are configured to fail to full open or full closed, faulty actuators, poorly calibrated travel and valve seating failures often go by unnoticed. A simple way to verify their operation is to command the valves shut and review the mixed air and supply air temperatures. After manually commanding the valves closed and allowing some time for the coil temperature to stabilize, the mixed air and supply air temperatures should be recorded. If there is a temperature rise it is likely that the heating valve is passing, and conversely a drop in temperature would suggest that the cooling valve is passing. Beyond the waste of energy of the passing valve, the problem will also create a load that the other coil has to counter, so it effectively doubles the energy used from the failure. These failures will typically only be identified during peak loads and can easily be missed.
One of the most common wastes of energy in building automation systems is that the occupancy schedules do not match the building’s occupancy. Most building automation systems place systems into a setback mode and reduce outside air during periods of non-occupancy. But often the building will be operating in occupied mode even when there is no-one inside. The schedules sometimes get changed to accommodate a special event or to alleviate a cold complaint when staff came in early, and then are never returned back to normal. Ideally a smart start or building warm up routine will be incorporated into the system to allow sufficient time to bring the space to the occupied temperature settings. Building schedules should be monitored frequently, including reviewing calendars to update them for holidays.
Another common point of adjustment is the setback temperatures. When the building is unoccupied it is possible to reduce the temperature requirements in order to save energy. Often when staff come in before the scheduled occupancy they complain that the space is too cold and so the building operator adjusts the night time setback. While this may be better than switching the building into occupied mode, it also eliminates any savings created by the setback. The range of setbacks varies depending on the system, but typically they are in the range of 2-3°C.
Dead band optimization
Depending on how the BAS is configured it may be possible to adjust both the heating and cooling setpoints independently. If these values are too close together, or worse, overlapping, then it can result in a large amount of energy being wasted as the system frequently shifts from heating to cooling. In addition to the energy consumed, this problem results in additional wear on equipment. As a minimum there should be 2°C separation between the heating and cooling during the occupied and unoccupied periods. It is common in many systems that mechanical cooling is disabled during unoccupied periods, although free cooling is often still enabled. Trend logs will reveal if the heating and cooling valves are operating simultaneously or with very small gaps.
PID controller tuning
Proportional, Integral, Derivative (PID) controllers sound more complicated than they are. Their purpose is to modulate smoothly the operation of control valves, resets, dampers, etc. and eliminate hunting. Hunting is when a system modulates around a setback that it is trying to achieve. For example, if the supply air temperature setpoint was reset to be 25°C, the heating valve was at 0% and the mixed air temperature was 17°C, the system would call the heating control valve to open. As it modulates open the PID controller monitors how far below its setpoint it is, how much it has changed since the valve started opening, and how quickly the temperature is increasing. The goal is to avoid the valve opening past what is required and then closing off, opening, closing and so on. Not only does this waste energy, it can result in the actuators failing prematurely, as well as comfort complaints from people feeling “drafty” due to the frequent temperature modulations. By reviewing trend logs it is possible to see if the PID loop is operating as intended.
Economizers use larger volumes of outside air to provide free cooling when it is required by the system and air conditions outside are cooler than inside. The simple way of checking the function of an economizer is by reviewing a trend log of the economizer, indoor and outdoor temperatures. If the economizer is operating when the outdoor air temperature is higher than the indoor temperature there is a problem.
Depending on the skill sets of the building operator some of the above points can be easily remediated but others may require outside experts to do the repair. Being able to identify, articulate and precisely state the scope of work will make it much easier to bring in specialists. Also, although it is rarely done, keeping a log of the testing and any changes made to the building automation system is advisable. Then in the future you will be able to find settings that have been changed. cce
Kyle Hasenkox is a principal with Rocky Point Engineering in its Victoria, B.C. office. He is also ASHRAE Chapter #145 president, secretary of TC 9.7-Educational Facilities, corresponding member of TC 7.5-Smart Building Systems and TC 9.9-Data Centers, and Region XI-YEA Regional Coordinator with ASHRAE.