Frozen pipe repair, before the thaw makes it worse

A frozen pipe is a problem with a deadline. Right now it is an inconvenience — one tap that will not run. In a few hours, when it thaws, it is either still an inconvenience or it is a flood, and which one depends largely on what happens next.

How a pipe actually freezes, and why it splits

Water expands as it freezes, by roughly nine percent. That expansion is not usually what breaks the pipe, though — copper and PEX both tolerate a surprising amount of it. What breaks the pipe is hydraulic pressure.

Ice forms first at the coldest point of the run and grows into a plug. Water beyond that plug is now trapped between ice and a closed faucet, and as the plug keeps growing it compresses that trapped column. Pressure climbs far past what the system was designed for and the pipe lets go wherever it is weakest — a solder joint, a crimped fitting, a spot thinned by years of hard water.

Pipes rarely burst where they freeze. They burst where the pressure has nowhere to go.

Which explains two things that puzzle homeowners. First, why opening the faucet helps so much: it gives the trapped column an escape route, and a pipe with an open end has a much harder time building destructive pressure. Second, why the damage often turns up in a room away from the cold wall.

Where it happens in Colorado houses

Freezes here are less about record lows and more about where the pipe was run and what the wind is doing. A dry, windy 8°F night with a north-west blow does more damage than a still night at -5°F, because moving air strips heat out of a rim joist far faster.

• Unheated crawlspaces, especially with a vent that was never closed for winter
• Supply lines through a garage ceiling to a bathroom above
• Runs inside north and west exterior walls, behind kitchen sink cabinets
• Hose bibs with a hose still attached in November
• Rim joist penetrations where a drill hole was never sealed
• Basement laundry lines on an exterior wall in a partly finished basement

There is also a housing-stock pattern. Homes built through the mid-century Front Range boom often have plumbing tucked into places that made sense with a coal or gravity furnace running constantly. Turn the thermostat down to 58°F while you are away, close a few interior doors, and a run that has been fine for sixty years finds its limit.

Frozen, or already split?

These are different jobs and it is worth knowing which one you have before you start heating things.

What you can safely do before anyone arrives

Stop and call if the pipe runs inside a finished wall, if it is a drain rather than a supply line, if the meter is moving, or if you have thawed it and lost pressure elsewhere in the house. Those all point at damage rather than a simple blockage.

How the repair goes

A pro arriving at a frozen line works in a fixed order, and it is worth knowing so you can follow along.

First, control: main off or ready to close, and the affected zone isolated. Second, controlled thawing — often a purpose-built thawing machine that passes low-voltage current through a metal pipe to heat it internally, which is dramatically faster than warming a crawlspace and does not put an open flame anywhere near your framing. See pipe thawing for detail on that step alone.

Third, test. The system is brought back up to pressure and watched. This is the step that finds the split the ice was hiding, and it is done deliberately with the shutoff in hand rather than by turning everything on and hoping.

Fifth — the part people skip — the cause. A pipe that froze once in that spot will freeze again in that spot. Insulation, heat cable on a thermostat, sealing the air path that was feeding cold to it, or rerouting the run out of the exterior wall entirely are all cheaper than doing this again in February.

Stopping the repeat

The prevention list is short, cheap and boring, which is why it does not get done.

Disconnect hoses before the first hard freeze and drain the bibs — a connected hose holds water against the valve and freezes it from outside in. Get the irrigation blown out properly in autumn rather than hoping; see sprinkler blowout and winterization.

If your plumbing is old galvanised steel, freeze damage tends to be the event that finally forces the conversation about repiping. Corroded, thin-walled pipe fails at lower pressures than new pipe, so the same cold night that a modern PEX system shrugs off can open two or three pinholes in a 1950s system.

Mountain, plains and metro are not the same problem

On the Front Range, most freezes are single-run failures in occupied houses, caught within hours, and repaired the same day. In the mountain communities, the dominant pattern is the unoccupied house: a second home or a rental where a furnace failed, and the discovery comes days later. That changes everything about the visit, because the question is no longer one pipe but which pipes survived.

On the eastern plains, wind is the variable. Sustained wind on an exposed north wall pulls heat out of a wall cavity relentlessly, and freezes happen at temperatures that would be survivable in a sheltered Denver neighbourhood. Well houses and pressure tanks in outbuildings are a common failure point out there, and they are usually the coldest structure on the property.