A running tap does help prevent frozen pipes, but a slow drip is often too little. A pencil-thin stream keeps replacing the still water in the at-risk line with warmer water from underground, and if ice starts anyway, the open tap relieves the pressure that splits pipes. Open the tap fed by the risky line, hot and cold if both are exposed, during the coldest hours.
- Moving water works two ways: it brings in warmer water from the ground, and an open tap gives trapped water an exit so pressure cannot build.
- Utilities ask for a steady stream, not drops: Toronto and Calgary say pencil-thin; Winnipeg's post-thaw flow fills a 250 ml cup in about 15 seconds.
- A drip of one drop a second replaces the water in a 6 m run of ½-inch pipe only every four hours or so, slower than a bare pipe can freeze in deep cold.
- The trickle only protects the pipes between the meter and the open tap; open the tap that the risky line actually feeds.
- Running water costs money on a metered supply, but a week of a thin stream usually costs far less than one thaw call.
01 /Why does running water stop pipes from freezing?
Running water stops pipes freezing in two separate ways. First, it keeps replacing the water sitting in a cold wall with water that has come through the underground service, which in winter is usually a few degrees above freezing. The water in the at-risk section never stays still long enough to lose its heat. Second, if ice forms anyway, the open tap at the end of the line relieves the pressure pocket between the ice and the fixture, which is what actually splits pipes.
The Government of Canada's Get Prepared guidance for winter storms includes running a trickle of water to prevent pipes from freezing, and municipal utilities from Toronto to Edmonton give the same advice. The detail they add is the size of the stream, and that is where most dripping advice goes wrong.
02 /How much should I let the tap run?
Run a steady, pencil-thin stream, not a slow drip. The City of Toronto describes a pencil-thin stream, and the City of Calgary's Frozen Pipes Prevention Program asks affected customers to leave one inside tap running about the width of a pencil around the clock. After a City thaw, Winnipeg asks for a flow that fills a one-cup (250 ml) measuring cup in 15 seconds, which works out to about 1 litre a minute.
The arithmetic shows why drops are not enough. A ½-inch copper or PEX line holds roughly 0.13 litres of water per metre. A 6 m branch from the basement to a kitchen sink holds about 0.8 litres. Compare how often different flows replace that water with the time a bare pipe can take to freeze:
| Flow at the tap | Approx. flow | Water in the run replaced every | Water used per day |
|---|---|---|---|
| Slow drip, about one drop a second | 0.003 L/min | about 4.4 hours | about 4 L |
| Fast, steady drip | 0.03 L/min | about 26 minutes | about 43 L |
| Pencil-thin stream | about 0.5 L/min | under 2 minutes | about 720 L |
| Winnipeg post-thaw flow (250 ml in 15 s) | about 1 L/min | under 1 minute | about 1,440 L |
In this site's freeze-time model, a bare ½-inch pipe in still −20 °C air freezes through in under two hours, and in a draft in under one. A slow drip that replaces the water every four hours loses that race. A pencil-thin stream wins it easily. Measure yours: time how long the stream takes to fill a measuring cup, and aim for well under a minute.
03 /Which tap should I leave running?
Open the tap that the at-risk pipe feeds. A running tap only moves water through the pipes between the meter and that tap; every other branch stays still. If the kitchen sink on the north wall is the one that freezes, the trickle has to come out of the kitchen sink, not the laundry tub.
- To protect a branch line: run the fixture at the end of that branch, such as the kitchen sink on an exterior wall or the bathroom over the garage.
- To protect the service line and meter: run a tap close to where the water enters. Toronto suggests the lowest point in the house, usually a laundry sink or tub, and Calgary's program uses one inside tap.
- Hot and cold: if both lines run through the cold spot, both need flow. On a two-handle faucet, open each a little. On a single-lever faucet, set the lever to the middle so both sides move.
- Toilets and appliances: you cannot trickle a toilet supply or a dishwasher line. Protect those with heat, sealing and insulation instead.
04 /Why did my pipes freeze even though the tap was dripping?
Pipes freeze despite a dripping tap for a handful of predictable reasons, and they point straight to the fix.
| What went wrong | Why it failed | Fix |
|---|---|---|
| Drip instead of a stream | Water sat still between drops long enough to freeze | Open to a pencil-thin stream |
| Wrong tap | The at-risk branch was not on the path to the open tap | Run the fixture the risky line feeds |
| Only one side open | The hot line froze while the cold ran, or the reverse | Run both, or set a single lever to the middle |
| Started too late | Ice had already narrowed the line before the trickle started | Start before the coldest night, not after |
| Extreme draft on the pipe | Heat loss outran the incoming water | Seal the draft; insulate; add heat cable |
| Service line already cold | Water arriving from a shallow service was near 0 °C | Talk to your utility; protect the meter area |
A trickle also cannot help a section sealed between two ice plugs, which is one reason heat and sealing come first on the permanent-fix list. If the same pipe needs a stream every winter, the where pipes freeze guide will help you find the cause.
05 /What does running a tap all night cost?
On a metered supply you pay for the water and usually for sewer charges on the same volume, and Toronto notes plainly that you will be charged for the water used. The volume is modest compared with the risk. A pencil-thin stream of about 0.5 litres a minute uses roughly 0.7 cubic metres a day, or about 5 cubic metres over a week of cold.
Water and wastewater rates vary widely across Canada, so check your own bill. As an illustration only, at a combined rate of $3 to $5 per cubic metre, a week of that stream costs roughly $15 to $25. Where a municipality asks specific customers to run water all winter to protect a vulnerable service, it may bill differently: Calgary's program, for example, charges metered residential customers a flat 7 cubic metres a month for water and wastewater during the program.
For comparison, the thaw call-out range on this site's prevention cost page is higher than a whole winter of cold-snap trickling for most homes, before counting any repair.
06 /When should I start and stop the trickle?
Start before the first very cold night, not after a tap slows. The Pipe Freeze Risk Checker adds a trickle to its action list for any score above 4, and it recommends running it during the coldest hours. For most houses that means from evening until mid-morning while the cold snap lasts, and around the clock for a line that has frozen before.
Stop when daytime and overnight temperatures return to normal for your area and the risky spot has warmed; a remote thermometer in the cabinet or crawlspace takes the guesswork out of that call. If you live somewhere the utility has asked you to run water to protect the service line, follow their letter: Calgary's program asks residents not to turn the tap off until a follow-up letter arrives.
FAQQuestions people ask
How fast should I drip my faucet to prevent freezing?
Faster than a drip. Aim for a steady pencil-thin stream. As a check, Winnipeg's guidance after a thaw is a flow that fills a 250 ml cup in about 15 seconds.
Should I drip hot or cold water to prevent frozen pipes?
Whichever line runs through the cold spot, and both if you are unsure. On a single-lever tap, set the handle to the middle so both lines move.
Does a dripping tap work if the pipe is already frozen?
It will not thaw the ice, but an open tap still matters: it relieves pressure behind the plug and shows you when water starts moving during thawing.
Which faucet should I leave running in a cold snap?
The one fed by the pipe most at risk, usually a kitchen or bathroom sink on an exterior wall. To protect the service and meter, run a tap near where the water enters the house.
Is it wasteful to leave taps running in winter?
It uses water, which is why the goal is a thin stream only from the tap that needs it and only during cold snaps. Sealing drafts and insulating the pipe reduce how often you need to do it.