Pressuretrol
A pressuretrol is a control used to control a steam boiler, by setting when the boiler should begin and end firing based on steam pressure. The pressuretrol is generally secondary to another control, such as a thermostat(generally in smaller buildings) or outdoor-reset controller- If this controller is not calling for heat, the pressuretrol will not activate the boiler.
Settings
Pressuretrols have 2 settings: A main cut-in or cut-out setting (depending on model), and a differential setting (which is either additive or subtractive, and usually stated on the controller). The main setting is accessible from the outside of the controller, but the differential is often located inside the controller box, and typically the cover must be removed by removing a screw to access the differential control. If the main setting is a cut-in setting, then the differential setting sets the cut-out setpoint by adding to the cut-in setting(additive), and if the main setting is a cut-out setting, then the differential sets the cut-in by subtracting from the cut-out setting (subtractive).
Domestic hot water
Note that in systems in which the boiler provides domestic hot water as well as steam, the pressuretrol does not control the DHW temperature. This is controlled by an aquastat which can control the boiler independently of the pressuretrol to meet DHW demand.
Purpose of a pressuretrol
The pressuretrol starts and shutdown the boiler according to adjusted pressure it also controls its operating range. For example, an average operating range of a steam boiler may be 240 to 250 psi. When the steam pressure in the boiler drops 10 psi, the boiler will start up automatically. When the pressure reaches to 250 psi, it will shutdown the boiler. In case of modulating pressuretrol it controls the boiler firing at high and low within adjusted range with the help of modulating motor linked with fuel valve.[1]