Operating nutThe five-sided nut on top. It turns the stem to open or close the hydrant. It needs a special wrench, which stops casual misuse.
BonnetThe dome-shaped top of the hydrant. In the US its color and the caps’ can show how much water the hydrant can deliver.
Hose nozzleThe smaller outlets on the sides, where fire hoses connect.
Pumper nozzleThe large outlet facing the street, where the fire engine connects. It is also called the steamer connection, from the days of steam fire engines.
BarrelThe upright body of the hydrant. In a dry-barrel hydrant it stays empty until the hydrant is opened.
Other 5
Nozzle capsScrew-on covers that keep dirt out of the nozzles. Firefighters take them off to connect a hose.
StemThe rod from the operating nut down to the main valve. Turning the nut moves the stem, which opens or closes the valve.
Lower barrelThe part of the barrel below the ground. It reaches down below the frost line to the main valve. Also called the riser.
Main valveThe valve at the bottom that lets water up from the main. It sits below the frost line so the water cannot freeze in the hydrant.
Drain valveOpens when the main valve is fully closed. The water left in the barrel drains into the ground so it cannot freeze.
How this list was compiled: Read 2026-09-28. (1) Wikipedia, Fire hydrant, https://en.wikipedia.org/wiki/Fire_hydrant : the dry-barrel design, the valve below the frost line connected by a riser to the part above ground, a valve rod extending up through a seal at the top operated with a wrench, a drain valve that opens when the water valve is fully closed so the body does not freeze, the special pentagonal-socket wrench against misuse, outlets and the large central steamer outlet, nozzle caps and bonnet colored by flow in the US (AWWA and NFPA). (2) U.S. Army Corps of Engineers, ERDC/CERL TR-13-20 (DTIC ADA587794), a cross-section of a typical dry-barrel hydrant: operating nut, bonnet, stem, upper and lower barrel, pumper nozzle, weep holes in the lower barrel that drain the upper barrel. Where the two name a part differently (riser and lower barrel; steamer and pumper nozzle; drain valve and weep holes) the label uses one and accepts the other. Licence: own work
The drawing: Drawn for this site by script, 2026-09-28. Own work, nothing traced. Licence: own work
How it works
These are the parts of a fire hydrant: a dry-barrel street hydrant, with the part under the ground cut open to show the stem and the valves. Start with the 5 main parts.
Read the full guide
Above the ground
The operating nut on top of the bonnet opens the hydrant. Fire hoses connect to the hose nozzles at the sides, and the fire engine to the large pumper nozzle at the front. Caps cover every nozzle when the hydrant is not in use.
Below the ground
Turning the nut turns the stem, which runs down the lower barrel to the main valve, below the frost line. Opened, water from the main rises up the barrel. Closed, the drain valve lets the barrel empty so it cannot freeze.
In the US, the colors of a hydrant’s bonnet and caps can show how much water it can supply.
Questions and answers
Why is the nut on a fire hydrant a pentagon?
So an ordinary wrench cannot turn it. Opening a hydrant needs a special wrench with a five-sided socket, which helps stop people wasting water by opening it.
What is a dry-barrel hydrant?
One whose valve is underground, below the frost line. The barrel above stays empty until the hydrant is opened, and a drain valve empties it after use, so it cannot freeze in winter.
What do the colors on a fire hydrant mean?
In the US, the AWWA and NFPA recommend coding the bonnet and caps by flow: light blue over 1,500 gallons a minute, green 1,000 to 1,499, orange 500 to 999 and red under 500.