PREFACE Proper calibration of thermostats is critical to comfort as well as proper energy control. Day/night and summer/winter thermostats are switched modes by a change in the main air pressure. There are several combinations of pressures achieving this change-over depending on the manufacturer and the particular building. Many thermostats are considered defective and replaced when the service person does not have a calibration change-over board illustrated on page 3.22. All that is required most often is an adjustment in the change-over point and recalibration of both operational modes causing the thermostat to perform as new again. On one occasion a client bought five hundred day/night thermostats to upgrade a whole high school. APS was hired to replace the thermostats with the school board’s control mechanic. Although the sealed boxes, containing the new thermostats, indicate “factory calibration”, we never trust that statement. Logically the calibration can only be accurate for one spring range and will be out of calibration for all others even if the factory calibration was done properly and the thermostat was treated gently from that point onward. On this occasion we tested three hundred thermostats for day calibration, night calibration and change-over point. Not one was accurate on all three considerations. We also found multiple other problems with the three hundred “new” thermostats. We very much advise to check every thermostat for proper calibration and operation before putting it into service, even though the box states “factory calibrated”. The signal from occupied space thermostats can control their own associated valves, VAV’s or damper motors and also be used to co-ordinate the heating and cooling provided by the base building heating and cooling equipment. The thermostats’ output signals from 0 PSIG to 20 PSIG create a ramp where functions can be selectively allowed in various segments of the ramp. This allows the system to minimize the over-lap of heating and cooling. Air leaks waste a lot of energy and may damage your compressor via extended run periods. Page 3.27 presents the impact of a thermostat signal air leak, while still maintaining enough pressure to control the temperature properly. A Johnson Control T4002 thermostat controlling a valve with a 3# to 6# range and a diaphragm leak causing the branch signal to drop from 20 PSIG to 10 PSIG will, while still maitaining temperature control, be consuming as much compressed air as 40 thermostats with no leaks. Control air compressors are normally sized to handle 25% to 33% run time: not large air leaks. i
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