Frozen Drain Pipe: How to Tell, How to Thaw It Safely, and When to Call a Plumber

If a sink, tub, or washer stops draining during a hard freeze, the drain pipe may have ice in it. Stop sending water down it, warm the room or the exposed pipe gently with a hair dryer, heat lamp, or small space heater, and never use an open flame. If it's the main line, a septic system, or a spot you can't reach, call a plumber; the City of Kimberley, British Columbia, says to call one when a sewer line still isn't draining after you've tried to thaw it.

This page covers drains: the pipes that carry water away. If water barely comes out of the faucet, the supply pipe is frozen instead; see frozen pipes: what to do. For an ordinary clog in warm weather, see slow or clogged drain.

Frozen Drain or Frozen Supply Pipe?

Supply pipe. The American Red Cross says that if you turn on a faucet and only a trickle comes out, suspect a frozen pipe, likely against an exterior wall or where the water service enters through the foundation. That's the pressurized pipe bringing water in, and it has its own steps: frozen pipes: what to do.

Drain. Water comes out of the faucet fine but won't go down, or goes down and comes back up somewhere else. Signs that point to a drain or sewer line rather than a supply pipe:

• Water rises at the lowest opening. Wentzville says a backup overflows out of the lowest possible opening, which in homes with basements can be the floor drains.
• It backs up only while you use water. Wentzville says that if a backup happens only when you use water and slowly drains when you stop, the problem is most likely in your sewer lateral, the pipe from the house to the street.
• A washer won't drain. LG says a frozen drain hose keeps water from draining out of the drum.

Most Likely Causes

1. A drain pipe in a cold space. The Red Cross lists unheated basements, crawl spaces, attics, garages, kitchen cabinets, and poorly insulated exterior walls as places pipes freeze.
2. A slow trickle into the drain. The University of Minnesota's Onsite Sewage Treatment Program says a leaking toilet or showerhead sends a thin, slow film of water down the pipe that can freeze and eventually block it solid, and that a high-efficiency furnace or humidifier can do the same with the small amount it discharges. (For the furnace's own line, see condensate drain line clogged.)
3. A sag or low spot in the line. UMN calls pipes without proper fall, or pipes that settle or sag, a common cause of freeze-ups: sewage collects in the dip and freezes.
4. An empty house or very light use. UMN says a home left empty, or one using very little water, may not send enough warm water through to keep the system from freezing.
5. Frost reaching a buried line. UMN says thin or compacted snow cover lets frost go deeper.

What You Can Fix Yourself

These steps are for a drain you can see and reach. A buried main line is a plumber's job.

1. Stop running water into it. Wentzville and Brantford both say to hold off on drains until the blockage is cleared. Bail standing water into a bucket.
2. Check the supply side too. The Red Cross says that if one pipe freezes, others may freeze, so open the other faucets. Weak flow means a supply pipe is frozen as well; know where your main water shutoff is before anything thaws.
3. Warm the space. Open cabinet doors so warmer air reaches the plumbing, moving cleaners out of children's reach, and keep the garage door closed if pipes run through it (Red Cross).
4. Add gentle heat to the pipe. NYC DEP says to direct a hair dryer or heat lamp at the frozen section, or put a small space heater nearby. The Red Cross adds an electric heating pad or towels soaked in hot water.
5. Keep electricity away from water. CPSC says to keep a heater away from water and never touch it if you are wet, and recommends GFCI protection in kitchens, bathrooms, garages, and unfinished basements. Keep a space heater 3 feet from anything that can burn, plug it straight into a wall outlet, never an extension cord or power strip, and turn it off when you leave or sleep (CPSC).
6. Use warm water only if the drain can take it. If the drain still moves a little, Kimberley's method is to fill the tub a quarter full of hot water and release it slowly with the drain plug while watching for a backup, repeating once or twice if needed. Use hot tap water, not boiling: Charlotte Pipe rates its PVC drain pipe for temperatures up to 140°F. If water rises anywhere, stop.
7. For a washer, LG says to pour warm water, 122 to 140°F, into the drum, close the door, and wait 1 to 2 hours for the drain pump to thaw. See washer not draining for other causes.
8. Watch for leaks as it thaws. Somerville's Water and Sewer Department warns that a frozen pipe may already be broken and will leak once it thaws.

What Not to Do

• No torches or open flames. Somerville warns of fire and carbon monoxide, and says overheating one spot can burst the pipe. The Red Cross also rules out kerosene or propane heaters and charcoal stoves.
• Don't keep running water to “push” it through. With a city sewer, water you add backs up at the lowest opening (Wentzville). With a septic system, UMN says not to run water continually to try to unfreeze it, and not to leave water running all the time.
• No chemical drain cleaner. Poison Control says splash-back can happen when too much drain cleaner is poured into a completely blocked drain, that caustic products burn skin and eyes on contact, and to buy one only if nothing else will do the job. For a septic system, UMN says not to add antifreeze, salt, or a septic additive. Already poured some in? Follow the warning box in slow or clogged drain.
• No boiling water. LG warns it can cause serious injuries.

When to Call a Pro

Call a licensed plumber, or for a septic system a licensed onsite professional, when:

• You can't find, reach, or thaw the frozen section (Red Cross, NYC DEP), or Kimberley's warm-water steps aren't helping.
• Several drains are affected or water is coming up a floor drain or tub. Fergus Falls, Minnesota, says to call Public Works as well as your plumber, so the city can check whether the blockage is in its main or your line.
• A septic system has frozen. UMN says to call a licensed onsite professional first; pumpers and installers use steamers, high-pressure jetters, and cameras.
• A pipe has cracked or is leaking. NYC DEP says a service line that freezes and breaks is the homeowner's to have repaired by a licensed master plumber. Shut off the water first: burst pipe: what to do.

Prevent It

• Insulate pipes in cold spaces. The Red Cross says to add insulation to attics, basements, and crawl spaces, and to consider pipe sleeves or UL-listed heat tape or heat cable on exposed pipes. UMN suggests heat tape in the pipe that receives a high-efficiency furnace's discharge. Our heat tape guide covers the options; install it exactly as its maker directs.
• Seal drafts. Somerville says to seal cold air leaks around wiring, dryer vents, and pipes, and to protect all pipes in unheated areas.
• Keep the heat on. When you go away in cold weather, keep the heat no lower than 55°F (Red Cross).
• Send warm water down in a cold snap. Kimberley says to run hot water through the sewer system once a day in cold weather, and that a load or two of hot-water laundry a day also helps. UMN gives septic owners the same advice: warmer, normal water use.
• Fix drips. UMN says fixing leaky fixtures and appliances helps prevent freezing.
• Protect a buried line. UMN says to keep vehicles and foot traffic off septic pipes and tanks.
• Drain an idle washer. LG says to drain the water left in its hoses when it sits unused in cold weather.

Before a cold night, our pipe freeze check reads your zip code's forecast.

Frequently Asked Questions

Can I pour boiling water down a frozen drain?

No. LG warns that boiling water can cause serious injuries, and Charlotte Pipe rates its PVC drain pipe for up to 140°F. Use hot tap water, and only if the drain still moves; if water backs up, stop.

Will drain cleaner thaw a frozen drain?

Don't try it. Poison Control says too much cleaner in a completely blocked drain can splash back, and caustic products burn skin and eyes on contact. Warm the pipe instead, or call a plumber.

Should I leave water running so the drain doesn't freeze?

Not into a drain that is already blocked. For prevention, Kimberley suggests running hot water through the sewer once a day in cold weather. With a septic system, UMN warns not to leave water running all the time, since that overloads the system.

Who fixes a frozen sewer line?

It depends on where it froze. Wentzville and Fergus Falls make the line from the house to the city main the owner's; the city handles its main. Call public works and a plumber.

The Bottom Line

A drain that quits in a cold snap is probably frozen where it runs through a cold space. Stop sending water down it, and warm the room or pipe with a hair dryer, heat lamp, or space heater kept away from water and anything that burns. No flames, boiling water, or drain cleaner. For a main line, septic system, backup, or a spot you can't reach, call a licensed plumber, and keep the heat no lower than 55°F when you're away.

Sources

Save to Pinterest