P B M IN OUT IN B M T B M T M B HWR HWS The existing design caused the VAV to over ventilate at a minimum airflow of 50%, when the actual ventilation airflow required was only 25%. Heat, via the heating valve, had to be added compensating for the over ventilation in addition to the space’s normal heat loss. The suggested solution allows the VAV to modulate airflow down to the actual ventilation minimum rate of 25% if cooling is not required. If heating is required, the thermostat opens the heating valve fully: then increases the airflow from the minimum 25% to a maximum of 50%, as required, delivering the desired heat. AS EXISTING The airflow controller has minimum, as well as maximum, airflow settings. The maximum setting addresses the maximum airflow requirement at 100% design cooling load. The minimum setting normally addresses the required ventilation rate (example 25%) for the area being served, respecting air quality. When a reheat coil is associated with a VAV box, the minimum setting relates to the quantity of air required carrying heat to the space (example 50%). This is normally much more than required for air quality maintenance. This arrangement requires that the heating be available all year, as the space will over-cool at the minimum setting of 50%, if no heat is available to warm the air. SUGGESTED SOLUTION This arrangement allows the VAV box to be set at the ventilation rate (example 25%) as a minimum setting. As the room cools, the thermostat will modulate the VAV box back to the 25% minimum ventilation position. As the room continues to cool, the thermostat opens the reheat valve. When heat is available and if the room continues to cool, the thermostat, through the reversing relay and high selector, increases the airflow through the VAV box from 25%, variable as needed, up to 50% airflow, delivering the required heat. If no heat is available, as determined by the supply heating pipe temperature controller, the reversing relay will not receive main air, thus the VAV box is disallowed modulation from the minimum ventilation rate of 25% up to the maximum heating rate of 50% airflow. This circuit provides the most effective energy use while still attaining the best comfort conditions in the space, both summer and winter, while allowing the boiler to be turned off during the summer. 5.43 NOTE: The Kreuter RCC1502 reversing relay allows the required bias. Predetermine if the unit intended for use can achieve this. TO MAIN PORT OF OTHER REVERSING RELAYS SET 0 PSIG =MAXIMUM HEATING CFM REVERSING RELAY HIGH SELECTOR AIR FLOW CONTROLLER 8# - 13# TO SPACE FROM FAN VAV BOX THERMOSTAT FROM FAN TO SPACE AIR FLOW CONTROLLER 8# - 13# THERMOSTAT TEMPERATURE CONTROLLER DIRECT ACTING 100 F MAIN AIR MAIN AIR VAV BOX MODULATED EXISTING COOLING MODLATED COOLING SUGGESTED VAV/REHEAT ENERGY PERFORMANCE IMPROVEMENT VALVE 3# - 8# VALVE 3# - 8# HWS HWR VAV AIR FLOW 100% AIR FLOW 75% AIR FLOW 50% AIR FLOW 0% AIR FLOW 25% AIR FLOW MAXIMUM CFM COOLING MODULATED CFM COOLING VENTILATION CFM REQUIREMENT VAV CLOSED (NO AIR FLOW) HEATING CFM REQUIREMENT
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