Short definition
A drain cock is a small valve installed at the low point of a piping section — or on equipment like a water heater — to allow draining the line for service or winterization. The most common form is the brass “boiler drain” with a hose-thread outlet, which is what’s screwed into the bottom of every residential water heater.
What it is
Two common forms in residential plumbing:
- Boiler drain. A 1/2-inch male NPT inlet with a 3/4-inch male hose-thread outlet, usually with a multi-turn handle. The standard fitting at the bottom of a residential water heater. Connects a garden hose for tank flushing.
- Compact ball or globe drain valve on a manifold, PRV, or pressure-tank tee. Smaller, sometimes a quarter-turn ball valve.
The job is the same — open the valve, water flows out by gravity (or air pressure if the line is being blown out for winterization). When the line is empty, close the valve, the line is ready to be left empty (winterized) or refilled.
Why it matters to a homeowner
Drain cocks are how you actually drain a line. Three high-value applications:
- Annual water-heater flush. Open the drain cock at the bottom of the tank, attach a garden hose, drain a few gallons of sediment-laden water out the bottom. Sediment shortens water-heater life; flushing once a year buys substantial extra service.
- Vacation-cabin winterization. Open drain cocks at every low point after shutting off the supply. Combined with blowing the lines out with compressed air, this prevents freeze-burst when the cabin is left empty over winter.
- Repair drain-down. Need to swap a fixture or open a line for a fix? Close the upstream shutoff, open the drain cock, and the trapped water drains out instead of dripping for hours after you’ve cut the line.
The classic WA failure mode is the plastic-bodied boiler drain on cheap water heaters — they crack and leak after 3 to 5 years. Replacement with a brass boiler drain ($10–$20 part, 15-minute swap with the tank cold and drained) is a worthwhile small upgrade.
Common variants and what a drain cock isn’t
- Drain cock vs. stop-and-waste valve. Stop-and-waste combines a shutoff with a drain port — closes the upstream side and opens a drain to atmosphere on the downstream side. Drain cock is just the drain port.
- Drain cock vs. hose bib. Same threaded outlet, different intent. A hose bib is a service spigot for connecting a garden hose; a drain cock is a maintenance fitting.
- Boiler drain (the water-heater spigot) vs. drain cock generic. The boiler drain is the most common single application of a drain cock.
Washington note
Cabin and second-home culture in WA — Hood Canal, San Juans, Whidbey, Methow Valley, Leavenworth, Olympic Peninsula — depends on drain cocks at strategic low points for fall winterization. The drain-down protocol typically: shut off the inside main, open every faucet to relieve pressure, open every drain cock at every low point, and (for properties with longer runs or buried hose-bib supply lines) blow compressed air through the system to clear standing water from horizontal sections.