Petcock-type bibs are uncommon plumbing parts often found in older homes. They were once used regularly to release pressure or to bleed plumbing, but they have since fallen out of favor for many plumbing uses. Petcocks still have their place, however. They are being used more frequently in automotive and engineering applications to control the flow of air or gasoline.
By definition, any type of faucet can be a bibcock or petcock. The term is usually applied to a small drain that is used to bleed water from a plumbing valve. Petcocks are generally 3/8 inch or smaller and made of brass. They do not have handles but are often opened using a screwdriver or pliers. Petcocks on shut-off valves look like small brass knobs and sit directly below the valve handle.
Types of Petcocks
Plastic petcocks are often used for plumbing urinals. They help control the amount of water that flows into the plumbing fixture between flushes. Brass petcocks are the most common type of petcock in household use. These fittings contain a small valve that can be opened or closed with household tools. Tightening the valve down closes the petcock entirely, and loosening it allows a line to drip or be fully bled.
Petcocks were often used in conjunction with air chambers to control knocking, which was a common problem in early plumbing. The air chambers helped to keep pressure even on the pipes, but when they became waterlogged, the petcocks could be opened and the system drained easily. Petcocks are still used for draining, but they are found in conjunction with pipes exposed to extreme conditions in the outdoors, such as with sprinkler systems.
Most motor homes are outfitted with petcocks in their various systems so that all fluids can be drained completely for winterization. Hobby greenhouse operators sometimes use them in homemade irrigation systems to help purge water and excess air, depending on the season. Petcocks are also used in automotive and scientific settings to control the flow of gasoline or to bleed air pockets.
- Dictionary of Building Preservation; Ward Bucher
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- Candler Connection: When Mother Nature Gives Us the Cold Shoulder
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- ChestOfBooks.com: Standard Practical Plumbing -- Hot and Cold Water Supply Part 5