The effective operation of year round working air handling system above 5000 CMH connected to cooling or heating cycle, mostly neglected once the system is in operation for ensuring only the performance. But each and every part of the system requires careful attention in order to achieve the same performance the unit has been installed and commissioned. A detailed study as below provides some of the key points one should consider while the system is in use.
OPERATION AND MAINTENANCE CHECKLIST common to all air handling units
Verify proper operation of air dampers
Confirm that all AHU dampers are operating correctly. Have an operator to send a control signal to open and close all dampers _ and visually confirm that they are fully opening and closing. Also, have the operator open the dampers to about 50% to make sure they are modulating correctly. Repair any actuators or damper banks that are not functioning correctly. In some cases, modulating damper actuators will need to be installed to allow for changes in outside airflow rates if the current dampers are open/closed.
Dust control for ensured air quantity
The entire air system must be periodically cleaned for removal of air borne dust particles and specifically the filters, so that the required quality and quantity of air can be rest assured for maintaining indoor comfort conditions.
Regulate the untreated fresh air
The makeup fresh air into the air handling system, predominantly designed for constant volume make up, which adds dust and heat load. This can be modulated and controlled with demand control ventilation standards using a CO2 sensor through a motorised damper.
Verify proper operation of air-side economiser
Check the DDC system control sequence to see if the current control system is using an air-side economiser. Make sure the economiser is working correctly by viewing damper positions and outside airflow rates at different outside air temperatures. If no air-side economiser function is currently used, determine the applicability of enabling one. Outside air temperature sensors are required for dry bulb–based economisers and an additional outside air relative humidity sensor is needed for enthalpy-based economisers. Both of these sensors need to be working correctly for the system to operate as designed.
Eliminate duct leakage
Periodically trace the main heating, ventilating, and air conditioning (HVAC) duct runs and listen for air leaks in the duct system. Confirm that the duct static pressure set point is within the pressure class of the installed ductwork before repairing any leaks. If the distribution system is really leaky, consider replacing the ductwork or using a duct sealant system to seal the leaks.
Verify proper operation of heating and cooling valves
Check all heating and cooling valves for proper operation. Check the heating valve in the winter by shutting off the valve and checking the air handler mixed air and discharge temperatures. If the valve is closed and supply is still heated by the coil, the valve isn’t seating correctly _ and it is causing unnecessary simultaneous heating and cooling. The same procedure should be followed for cooling coil analysis.
Check the condition of heating and cooling coils and AHU filters
Visually inspect the condition of the heating and cooling coils and the AHU (Air handling Units) filters. If the coils are dirty or the filters are clogged, change out the filters and clean the coils. If the current maintenance schedule isn’t sufficient, revise the schedule accordingly.
If static pressure sensors are used to determine filter dirt loading, make sure the sensors are working correctly. Dirty filters and coils will increase the static pressure across the coil, increasing fan system energy use in addition to reducing the heat transfer coefficient ofthe coil, forcing the system to move more air over the coil for the same heating / cooling effects.
Eliminate 100% of outside air systems if practicable
One of the largest energy-wasting systems is 100% outside air ventilation in areas that don’t require it. Common areas that require 100% outside air are Operating Rooms (ORs) and labs. If 100% outside air AHUs are being used for differing space types that don’t require outside air, modulating outside air dampers and return air systems should be installed to reduce outside air.
Constant Volume Air Handling Unit
Adjust total airflow and head
Airflow rates are often significantly higher than required in buildings, primarily due to system oversizing. In some large systems, an oversized fan causes over-pressurisation in terminal boxes. The excessive airflow can often cause excessive fan energy consumption, excessive heating and cooling energy consumption, humidity control problems, and excessive noise in terminal boxes. Calculate the required airflow rate in heating and cooling mode to determine the correct flow rates. Replace match pulley sizes based on actual airflow. If the rate can be reduced, install a VFD and slow the fan down to the required flow rate. The VFD can also be used to slow down the fan during unoccupied hours, if the AHU has to stay on at night. In addition, make sure outside air is reduced accordingly.
Convert the constant volume system to a VAV system
A VAV system can significantly reduce HVAC system energy use. All constant volume AHUs should be identified and considered for retrofit to a VAV system.
Implement a supply air temperature reset schedule
The goal of a supply air temperature reset schedule is to minimise combined fan power and thermal energy consumption or cost. For single duct constant volume systems, maintain the supply air temperature not higher than 57°F if the outside air humidity ratio is higher than 0.009 or the dew point is higher than 55°F. When the outside air humidity ratio is lower than 0.009, the supply air temperature can be reset to a higher temperature over the temperature range of 55 to 65°F.
Variable Air Volume Air Handling Unit
Investigate duct static pressure
For VAV systems, review your duct static pressure set points _ and adjust them as low as possible while keeping all VAV dampers below 90% open. If VAV dampers are 100% open during periods, identify the reasons the space is calling for additional air flow _ and adjust system loads (i.e., relocate certain internal loads).
Reset the supply air temperature
Maintain the air temperature not higher than 57°F, if the outside air humidity ratio is higher or the dew point is higher than 55°F. Both humidity ratio and dew point can be determined using dry bulb temperature _ and relative humidity data. Maintain the supply air temperature no higher than 57°F if the fan air flow is higher than 70% of the air flow under the maximum load conditions. This is often significantly smaller than 70% of the design air flow. When the air flow is higher than 70%, increased air flow has a significant impact on fan power. For example, resetting the supply air temperature from 55°F to 57°F can potentially increase the air flow by 10%. This will increase fan power from 34% to 51% of the maximum value. When the outside air humidity ratio is lower and the air flow is lower than 50%, the supply air temperature can be modulated to maintain total airflow at 50% or lower.
If the air flow is lower than 50%, the supply air temperature can be increased. However, the supply air temperature must be lower than a high limit, which can be set to 65°F. The old air handling units once down for major repair or replacement, one should go in for energy efficient systems with all best options now available should be considered for reduced repairs and maintenance cost as well as energy efficient system with more power savings.
