Tri-cock

The two valves marked "H" are Tri-cocks.

Definition from American Model Engineering Supply:

This valve is designed to be used on a boiler as a indication of water level, either to check the water gauge indication or as a method of checking water level when the water gauge is inoperative. Normally, one valve is installed on the desired operating level and one just below the high water level and, most important, one just above the minimum safe water level. In most full sized boilers the minimum level was what was considered as the minimum safe level over the crown sheet.

External Links

• "Try 'Tri' cocks?", Chaski.org