What is This and What Does It Do?

The curly pipe commonly installed on many steam boilers is called a siphon or pigtail. The job of this pipe mounted on the boiler is to protect the steam pressure control. When steam is first generated, it enters the loop and when the pressure is high enough, the pressure control will shut off the burner. Once the burner stops, the steam condenses back to water and is trapped inside the siphon, becoming a water seal. Every subsequent time the boiler starts, the steam pushes the water trapped inside the loop against the internal components of the steam pressure control. If the boiler did not have a water seal, the steam would destroy the control in a relatively short time. 

There is a joke in the industry the job of the siphon or pigtail is to plug, and they do that very well. When they plug, you essentially lose control of the boiler as the pressure controls cannot sense the steam boiler pressure. In case you were wondering, that is not a good thing. The siphon should be removed and inspected at least once a year. If it is plugged, I urge you to simply replace it. The time it takes to try to clean it will outweigh the cost of a new one. 

Some contractors save money by using one siphon to feed both pressure controls. I recommend using a siphon for each control. This will eliminate the chance of one plugged siphon disabling both steam pressure controls and the boiler running wild. 

There  is a proper direction on the siphon. The loop should be perpendicular to the front of the control. In other words, if you see the loop from the front of the control, it should be turned to face the rear. 

One last suggestion is to install a union on the top of the siphon. If you have to inspect the siphon, the union allows you to disassemble the piping without having to unwire the control.