B M T 0 5 20 25 30 10 15 B M T 0 5 20 25 30 10 15 NIGHT DAY NIGHT DAY NIGHT DAY NIGHT DAY COMMERCIAL OFFICE BUILDING - Actual outside air temperature. - Design reset water temperature based on outside air only. - Actual water temperature required by the coolest room. - Night set back feature active. PUBLIC HIGH SCHOOL - Actual outside air temperature. - Design water temperature based only on outside air. - Coolest room heating demand signal. - Actual water temperature required by coolest room. - Night set back feature not active. The average required supply water temperature was 26.9F˚cooler than the average design water temperature. Perimeter water reset based on demand of coolest room. Average temperature = 123.1° F Perimeter water reset based on only outside air temperature. Average temperature = 150° F Scenario “A” illustrates a building circuit resetting the perimeter water temperature based on the heating requirements of the coolest area, after it has modulated its cooling, via the VAV box, back to minimum ventilation. This assures that the coolest room eliminates excessive cooling via the fan system before that room requests heat. Scenario “B” illustrates a building circuit resetting its perimeter heating water temperature based only on the outside air temperature. As per the two graphs below, reset from only the outside air temperature will provide more heat than required by the coolest room. When unwanted heat is forced into a building, the VAV boxes provide cooling to remove the unwanted heat from the rooms. This is similar to starting your furnace: providing too much heat: and then opening your windows to regulate the room temperature. Caution must be exercised, regarding impact on the boilers; if night set back is part of your control strategy. Consider the impact of a large volume of cool secondary water entering the primary loop at the changeover from nights to days. Proper control logic gradually increases the secondary loop temperature, keeping the boilers above their minimum allowable firing temperature. Caution must be exercised in selecting the sample signals, determining greatest heating demand. 27.7°F 95.9°F 165.6°F 126.4°F 65.76°F 181.6°F 5.40 HIDDEN ENERGY LOSS: VAV WITH PERIMETER HOT WATER OSA RESET "A" VAV BOX "B" VAV BOX FROM FAN FROM FAN MINIMUM VENTILATION TO SPACE TO SPACE MODULATED COOLING AIR FLOW CONTROLLER 8# - 13# MAIN AIR THERMOSTAT SET 21˚C ROOM TEMP. 21˚C PUMP V1 PUMP V1 NC C NC C PUMP NO NO PUMP PERIMETER HEAT CONTINUOUS FLOW PERIMETER HEAT CONTINOUS FLOW MAIN AIR THERMOSTAT SET 21˚C ROOM TEMP. 21.75˚F AIR FLOW CONTOLLER 8# = 13# CONTROL PRESSURE 12 PSIG CONTROL PRESSURE 8 PSIG BOILER BOILER
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