PNEUMATIC THERMOSTATS SINGLE BAND, SUMMER/WINTER, DAY/NIGHT There are many different pneumatic thermostat models presently in the field. Pneumatic thermostats come in single, two and three pipe configurations. A collection of the manufacturers’ data sheets on all makes and models of pneumatic thermostats would be overpowering; therefore, we are going to take a generic approach to troubleshooting and calibration regarding types of thermostats, developing a common sense approach to the subject. You should be able to trouble shoot and properly calibrate any thermostat even from a manufacturer you never knew existed. Most often specialized tools and gauge adaptors are required. Pneumatic thermostats come as relay and non-relay units. Page 3.20 contains functional illustrations and calibration instructions for single band one pipe non-relay thermostats. Page 3.21 contains functional illustrations and calibration for single band relay thermostats. The odd thermostats in the industry are the POWERS (SIEMENS) “D” stat. They do not have a pilot orifice. They operate off a double ball arrangement for the makeup and exhaust seats. There are non-relay two pipe thermostats in the industry which are basically two single ban thermostats with one controlling the heating and the other controlling the cooling, in a dead band arrangement. Thermostats are either direct or reverse acting, (see page one) based on the controlled medium characteristics (hot water/chilled water, hot air/cold air, etc.) and the normal position of the valve, damper, transducer, VAV box, bypass box or pressure switch being controlled. Some thermostats have a direct acting bimetals for winter operation and a reverse acting bimetals for summer operation. Each time you calibrate a thermostat set the branch pressure at mid range of the controlled device and breathe on the bimetal, witnessing the branch pressure of the thermostat. The signal should be responsive and in the correction direction for the application. The various combinations, obtaining the desired end result, are extensive. The best failsafe position for the controlled devices must always have first consideration. Usually heating fails on and cooling fails off; however, logical exceptions exist. Sometimes you are required to pick the least unsafe of a few choices. For example: If your choice is risking freezing a coil, or blowing up a building in an explosion-proof environment, you choose the risk of freezing the coil as the safest failsafe position. The vast majority of pneumatic thermostats are proportional, meaning they vary their branch (output) signal at infinite pressures between zero PSIG (pounds per square inch gauge) and the main air PSIG. Johnson Controls made a two position, T400 series, thermostat, meaning it can only produce a branch signal of either zero PSIG or the main air PSIG. It cannot modulate between those pressures. In over forty years in the pneumatic service field I’ve only seen one of these thermostats: they are rare. The pilot orifice (see illustration on page 3.20 for relay thermostats) is normally non-adjustable at .005”. The obsolete, Johnson Control, T400 series, to my knowledge, is the only pneumatic thermostat with an adjustable pilot orifice. The adjustment is a screw under the cap-screw facing directly toward you at the bottom of the large brass plate. A special Johnson Control gauge is required, which has a full range of 35”WG. The T400 factory pilot setting is 5”WG. 3.23
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