B M T 0 5 20 25 30 10 15 B M T 0 5 20 25 30 10 15 B M T B M T 0 5 20 25 30 10 15 0 5 20 25 30 10 15 Scenario four is a problem of unwanted cooling entering the building via a defective VAV. This typically occurs when the VAV airlow controller or actuator diaphragm is defective. The problem is not easily detected, as you can see comparing scenarios two and four; the room temperature is identical. The heating function hides the VAV's defect. The heating input in scenario two compensates for perimeter heat loss: the heating input in scenario four compensates for unwanted cooling via the defective VAV box. In scenario two no energy is wasted. In scenario four the waste can be severe, similar to opening your windows at home and turning on you furnace to compensate for the unwanted cooling.. SCENARIO ONE (normal operation) The thermostat is set at (21°C). The room is 21°C. The thermostat branch signal is at 8 PSIG, forcing the 3# to 6# heating valve closed and the 8# to 13# VAV box to only allow minimum ventilation. There is no heating/cooling conflict. SCENARIO TWO (normal operation) The thermostat is set at (21°C). The room temperature is 20.1°C. The thermostat branch signal is at 4 PSIG, modulating the 3# to 6# heating valve open and holding the 8# to 13# VAV box to only allow minimum ventilation. There is no heating/cooling conflict. SCENARIO THREE (normal operation) The thermostat is set at (21°C). The room temperature is 21.75°C. The thermostat branch signal is at 11 PSIG, closing the 3# to 6# heating valve and modulating the 8# to 13# VAV box to 60% of maximum cooling airflow addressing normally expected heat gains. (solar, body, lights, etc.) There is no heating/cooling system conflict. SCENARIO FOUR (serious problem) The thermostat is set at (21°C). The room temperature is 20.1.°C. The thermostat branch signal is at 4.0 PSIG, modulating the 3# to 6# heating valve, compensating for the defective VAV which is causing unwanted cooling to enter the occupied space. There is heating/cooling system conflict. The heating is active, compensating for unwanted over-cooling via the defective VAV box. 5.43A VAV BOX VAV BOX AIR FLOW CONTROLLER 8# - 13# MAIN AIR THERMOSTAT SET 21˚C VALVE 3#- 6# NO THERMOSTAT SET 21˚ C MAIN AIR VALVE 3# - 6# NO AIR FLOW CONTROLLER 8# - 13# ROOM TEMP. 21˚C MINIMUM VENTILATION TO SPACE 100 CFM FROM FAN FROM FAN TO SPACE 100 CFM MINIMUM VENTILATION PERIMETER HEAT NO FLOW PERIMETER HEAT MODULATED FLOW VAV BOX VAV BOX FROM FAN FROM FAN TO SPACE 280 CFM TO SPACE 900 CFM AIR FLOW CONTROLLER 8# - 13# MAIN AIR MAIN AIR THERMOSTAT SET 21˚C ROOM TEMP. 20.1˚C ROOM TEMP. 20.1˚C ROOM TEMP. 21.75˚C THERMOSTAT SET 21˚C PERIMETER HEAT NO FLOW MODULATED COOLING VALVE 3# - 6# NO VALVE 3# - 6# NO HIDDEN ENERGY LOSS: VAV WITH DEFECTIVE AIR FLOW CONROLLER OR DIAPHRAGM . CFM IS OVER MAXIMUM DESIGN DEFECTIVE VAV: EITHER DEFECTIVE DIAPHRAGM OR AIRFLOW CONTROLLER EXAMPLE: DESIGN MINIMUM = 100 CFM DESIGN MAXIMUM = 400 CFM DEFECTIVE FLOW = 900 CFM EXAMPLE: DESIGN MINIMUM = 100 CFM DESIGN MAXIMUM = 400 CFM EXAMPLE: DESIGN MINIMUM = 100 CFM DESIGN MAXIMUM = 400 CFM EXAMPLE: DESIGN MINIMUM = 100 CFM DESIGN MAXIMUM = 400 CFM
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