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NORA Oilheat Technicians Manual

12-16 show a vapor control and a pressure control. In both cases the switching mechanism is actuated by a diaphragm—the steam pressure counteracts the pressure exerted by the spring in the control. The tension of the spring is predetermined by the pressure adjustment screw, or main scale set point screw. It is important to read the pressure adjustment instructions for the particular pressure controller being adjusted. On some controls, the differential is subtractive, meaning that if the pressure cut out is set for 3 PSI and the differential is set for 2 PSI, the cut in point will be 1 PSI. On other controls, the differential is additive and if the cut in point is set at 1 psi and the differential is set at 2 PSI, then added together, it would give us a cut out point of 3 PSI. The cut out point is the pressure at which the oilburner will shut off. The cut in point is the pressure at which the burner will restart. It is very important to remem- pressure controls will also complete or break a circuit by opening or closing their contacts. Their contacts may be constructed of the metal to metal design, or they may be of the mercury tube type. The operating range of a pressure control must never exceed the design pressure of the boiler. All residential and most commercial boilers are of the low pressure type with a maximum pressure range not to exceed 15 PSI; therefore, pressure controllers must be limited to the 0 to 15 PSI range. Occasionally a steam system may be required to maintain pressure and the use of an operating controller may be necessary. In this case, two separate pressure controllers are necessary, one acting as the safety limit and the other as the operating limit. Residential steam systems require less than 2 PSI. Most pressure controls are not sensitive enough for the low operating pressures required for some residential steam systems. In these systems, you will get better results with a Vapor Control that operates on ounces of pressure instead of pounds. As the steam pressure changes, an expansion or contraction of a bellows actuates the switching mechanism. The cut-in and cut-out pressures can usually be independently set to meet any requirement. The snap-acting switch type does not require leveling. If mercury switches are employed, the control must be leveled. Figures 12-15 and Figure 12-15: Vaporstat 12-14 Limit Controls and Thermostats Figure 12-16: Pressuretrol Chapter 12 Limit Controls/Thermostats


NORA Oilheat Technicians Manual
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