board allows you to test the branch line and valve diaphragm (s) while you calibrate the induction unit thermostat. These thermostats are typically found on induction unit systems (see page 7.58) where the valve being controlled receives hot water in the winter mode and chilled water in the summer mode. Opening the valve in the winter gets heat, while opening the valve in the summer gets cooling; therefore, the thermostat has to reverse its logic for each mode of operation. These thermostats operate similar to the day/night thermostats. They have one direct acting bimetal and one reverse acting bimetal, where the day/night thermostats have two identical bimetals. The means of changing from one bimetal to the other, reversing the thermostats’ logic, is also via altering the main air pressure to the thermostats. The summer/winter pressures used vary to the same extent as the day/night systems. The change-over is normally manual and must always match the summer/winter mode of the mechanical system. The calibration method is the same as the day/night calibration. You can control to a lower temperature for winter than summer by offsetting the control point pressure. If the thermostat’s sensitivity causes a branch change of 2.5 PSIG per Fahrenheit degree and you wish to control two Fahrenheit degrees cooler in the winter, you adjust your winter control pressure five pounds higher than the pressure used for summer control pressure when calibrating. If the system is, for example, a 15 PSIG winter and 20 PSIG summer with a change-over point of 17.5 PSIG and air leaks cause the main air in an area to drop below 17.5 PSIG, the thermostats in that area will lose control, as their logic will be completely backwards in addressing the requirements of the area. I remember one building where we were on a contract. The building had many leaks not associated with our work causing the main air pressure to drop below the change-over point in some areas. We advised the building operator to find and correct the air leaks in order to correct the situation. He chose to run around the building with a magic marker reversing the dials, which read “Cooler/Warmer”. Obviously he corrected nothing and acquired no control in those areas. Hopefully, when he fan out of magic markers he considered our suggestion to fix the air leaks. There are models which look similar to a day/night thermostat. There are models which have thermal capillary element attached directly to them. There are models which have three ports on the thermostat, being the main and branch ports, as well as a sensing port, which is tubed to a remote bimetal controlling the pilot pressure of the thermostat’s relay. THERMOSTATS REGARDING ENERGY CONSERVATION While manually or automatically controlling a building, providing a healthy environment, comfort and best energy performance, a continuous input of information is required, allowing logical positioning of heating, cooling and ventilation functions. 3.26
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