Steam condenses in the loop and forms a water seal, so the gauge or pressure switch sees boiler pressure but not steam temperature, which would ruin its bellows or diaphragm. The pigtail must be kept clear, because sludge or scale plugging it leaves the control blind and the boiler free to overpressure. Standard drawings call it a siphon; everybody in the boiler room calls it a pigtail.
Pressuretrol isn't seeing pressure, pull the pigtail and blow it out, it's probably packed with crud.
Fill a new pigtail with water before you screw the control on, so the first steam of the cycle never reaches the bellows.
Also heard as
- siphon
- siphon loop
- gauge siphon
Related terms
Pressuretrol
Shop name for the pressure switch that starts and stops a steam boiler's burner to hold pressure in a set range.
Operating control
The automatic-reset control that cycles or modulates the burner to keep steam pressure or water temperature in range.
High limit
A safety control that shuts down the burner if pressure or temperature rises above the normal operating range.
Same word, different worlds
Short length of wire spliced to several conductors so only one lead goes to a device terminal.
HVAC & RefrigerationA short jumper wire spliced into a group of conductors so they can share a single terminal.
Fiber OpticsA short fiber with a factory connector on one end and bare fiber on the other, spliced onto a cable to terminate it.