Plumbing · Lesson 9 of 13Beginner6 min read

Clogs, Frozen Pipes and Noisy Pipes

Most plumbing calls come down to a handful of everyday problems: a drain that won’t drain, a pipe that froze, pressure that dropped, or pipes that bang. Each has a simple bit of physics behind it.

Why drains clog

Drains are built to carry water, human waste and toilet paper, not much else. Kitchen drains clog with grease, which cools and sticks to the pipe walls, plus food scraps. Bathroom drains clog with hair and soap. Toilets clog with too much paper or things that should never be flushed. EPA’s septic program puts the rule simply: flush only human waste and toilet paper. Wipes, grease and similar items clog pipes and septic systems.

Many clogs form close to the fixture, in or near the trap, where a plunger or a hand snake can reach them.

Plungers and snakes

A plunger works because water barely compresses. Push on the cup and the push travels through the water to the clog; that’s Pascal’s principle, the same physics as a car’s brakes. Pulling back tugs the other way, rocking the clog loose. The cup needs water over it to seal, and other openings, such as a sink’s overflow, must be plugged so the push doesn’t escape. Use a cup plunger on sinks and tubs and a flange plunger, with its extra rubber sleeve, on toilets (Unclog a Sink or Toilet).

If plunging fails, a hand drain snake, or auger, is a coiled steel cable you crank into the drain to hook or break up the clog. For clogs deep in the building drain or sewer, plumbers use powered drain machines through a cleanout, and cameras to see what’s down there.

Same physics as hydraulic brakes. See Being an Adult 101 for the step-by-step.

StopChemical drain cleaners

EPA’s septic program advises avoiding chemical drain openers. Never mix drain cleaners or other products, and don’t plunge a drain that has chemical cleaner in it; call a plumber instead. If water backs up in more than one place, such as a tub when the toilet flushes, the clog is probably farther down the line: stop using water and call a plumber.

Why frozen pipes burst

Water is strange: it expands when it freezes. USGS says ice is about 9 percent less dense than water, so a given amount of water needs about 9 percent more room as ice. In a pipe, an ice plug forms first where the pipe is coldest. The water trapped between the ice and a closed faucet has nowhere to go as the ice grows, and the pressure soars until the pipe splits. That’s why pipes often burst a short distance from the ice, and why a split may not leak until it thaws.

Pipes most at risk run through cold spaces: outside walls, crawlspaces, attics, garages and unheated basements, plus outdoor hose bibbs and sprinkler lines.

Pipes often split away from the ice itself, where the trapped water has nowhere to go. Density figure: USGS Water Science School.

Keeping pipes from freezing

Insulate and protect. HUD’s standard for manufactured homes requires all piping exposed to freezing temperatures to be insulated or protected; where heat tape or pipe heating cable is used, it must be listed for that purpose. The Department of Energy recommends insulating pipes that must run in exterior walls, putting cavity insulation behind them, and sealing the wall cavity so cold air can’t flow around them.
Drain outdoor lines. EPA says to shut off the water to irrigation systems and drain the lines before freezing weather.
Turn off hose bibbs when you aren’t using them, as EPA’s checklist suggests, and disconnect hoses in winter.
Know your main shutoff. If a pipe bursts, close the main valve right away (Shut Off Your Home’s Water).

StopIf a pipe is frozen or has burst

If water is spraying or a pipe has split, close the main shutoff now and call a licensed plumber. If a faucet gives only a trickle in freezing weather, a pipe may be frozen and could split as it thaws, so be ready to close the main. Thawing a frozen pipe safely depends on where it is and what it’s made of; your water utility or a plumber can advise. Never use an open flame on pipes in a wall or near framing.

Low water pressure

Start by asking where. If one faucet or showerhead is weak, the cause is usually right there: a clogged aerator, or scale in the showerhead, which EPA says to clean with a cleaning product or white vinegar, or a fixture shutoff valve that isn’t fully open. If the whole house is weak, check that the main shutoff is fully open, ask your neighbors and your water utility, and think about the pressure-reducing valve (Lesson 2). In an older house with galvanized steel pipe, HUD’s inspection guide notes the pipes may be clogged inside by rust and mineral deposits. HUD’s gallon-jug test, about 24 seconds at the kitchen faucet, gives you a number to compare.

Noisy pipes and water hammer

The U.S. Army’s plumbing manual explains water hammer: when flowing water is stopped abruptly, it slams against the valve, and the result is noise, rattling pipes and sometimes leaky joints. Toilet fill valves, washing machines and dishwashers close quickly, so the bang often comes right after one of them finishes. The fix is a cushion of air: an air chamber, a capped pipe that holds air, or a manufactured water hammer arrester, installed near the valve. San Diego’s utility guidance adds that banging or a high-pitched whistle, often when the water is turned off, can mean a failing pressure-reducing valve and too much pressure.

Other noises have other causes: a rumble or pops from a water heater often mean sediment in the tank (Lesson 5), and ticking as hot pipes warm up is the pipe expanding against its supports.

From the U.S. Army’s plumbing field manual. Quick-closing valves in washers and dishwashers, and toilet fill valves, are common triggers.

Check yourself

Tap an answer to check it, or open “Show the answer.”

Why does a plunger work?
Show the answer
Water barely compresses, so the push travels to the clog. Pascal’s principle: a push on trapped water is passed along through it.
About how much less dense is ice than water, according to USGS?
Show the answer
9 percent. About 9 percent, so freezing water needs more room and can split a pipe.
Pipes often split…
Show the answer
where water is trapped between the ice and a closed faucet. The trapped water has nowhere to go as the ice grows.
Pipes bang right after the washing machine fills. What is it most likely?
Show the answer
Water hammer. A quick-closing valve stops moving water abruptly.
Only one faucet has weak flow. What should you check first?
Show the answer
The aerator on that faucet. A clogged aerator is right at the faucet and easy to clean.

Go deeper

Being an Adult 101: Unclog a Sink or Toilet Unclog a sink or toilet, step by step.
Being an Adult 101: Shut Off Your Home's Water Shut off the water fast when a pipe bursts.
OpenStax College Physics 2e, 11.5 Pascal's Principle Pascal’s principle in OpenStax.
USGS Water Science School: Water Density USGS: why ice floats.
U.S. Department of Energy, Building Science Education: Pipes in Exterior Walls DOE: pipes in exterior walls.

Sources

I finished this lessonSaved on this device only.

A free course from Teddy Rook Book. It explains how plumbing works in general and follows U.S. sources; your local plumbing code, the authority having jurisdiction and manufacturers’ instructions are the final word.