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 heat entering the building via defective control valve seats. This typically occurs when the perimeter heating water is controlled via only outside air reset. The valves are forced to modulate just barely cracked open, causing wear and tear increase on the seats and discs. The problem is not easily detected, as you can see comparing scenarios three and four; the room temperature is identical. The cooling function hides the heating valve’s defect. In scenario three no energy is wasted. Scenario four the waste can be severe, similar to turning on your furnace at home and opening windows to dissipate any unwanted heat. Resetting the heating water temperature based on the area with greatest heating demand, after its control valve is fully open, will tend to save the seats on your control valves and will save energy in reducing heat loss. 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 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. The heating valve’s defective seat is allowing unwanted heat gains equal to the solar, body, lights, etc. heat gain in scenario three. There is heating/cooling system conflict. The cooling is active removing unwanted heat injected via the defective valve seat. 5.42 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 FROM FAN FROM FAN TO SPACE MINIMUM VENTILATION PERIMETER HEAT NO FLOW PERIMETER HEAT MODULATED FLOW VAV BOX VAV BOX FROM FAN FROM FAN TO SPACE TO SPACE AIR FLOW CONTROLLER 8# - 13# AIR FLOW CONTROLLER 8# - 13# MAIN AIR MAIN AIR THERMOSTAT SET 21˚C ROOM TEMP. 20.1˚C ROOM TEMP. 21.75˚C ROOM TEMP. 21.75˚C THERMOSTAT SET 21˚C PERIMETER HEAT NO FLOW PERIMETER HEAT VALVE SEAT PASSING MODULATED COOLING MODULATED COOLING VALVE 3# - 6# NO VALVE 3# - 6# NO HIDDEN ENERGY LOSS: VAV WITH DEFECTIVE REHEAT HEATING VALVE
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