How Long Before Pipes Freeze Without Heat in a Power Outage

A house loses heat in proportion to the gap between inside and outside, so it cools fast at first and then slows down. We modeled that with a simple cooling curve, starting at 68 F, and timed how long the room air takes to reach 40 F. We use 40 F as the act-now point because pipes in outside walls and under sinks on outside walls run colder than the room air around them.

Drafty older home: about 20 hours at 30 F outside, 13 hours at 20 F, 10 hours at 10 F and 8 hours at 0 F.

Average home: about 40 hours at 30 F, 26 hours at 20 F, 20 hours at 10 F and 16 hours at 0 F.

Tight, well-insulated newer home: about 80 hours at 30 F, 53 hours at 20 F, 40 hours at 10 F and 32 hours at 0 F.

These are estimates; wind, sun and how much of the house is below grade all move them. Put a thermometer in the coldest room, often a corner bathroom or a room over the garage, and trust it over any chart. A min/max thermometer shows how low it went overnight.

Pipes in unheated spaces freeze first: crawl spaces, attics, garages, outdoor hose bibs and supply lines in exterior walls. Kitchen and bathroom sink lines on outside walls are the usual indoor victims, because the cabinet keeps room heat away from them.

Pipes rarely burst where the ice forms. Ice blocks the pipe and pressure builds between the blockage and the closed faucet, so the split often shows up a few feet away. That is why a dripping faucet helps: it relieves the pressure.

First hour: open cabinet doors under sinks on outside walls, close off rooms you do not need, and keep the garage door shut. Let cold and hot taps on outside walls drip a thin stream.

If the heat will be off longer than your house can hold out, shut off the main water valve, open every faucet starting at the top floor, and flush toilets to drain the tanks. Turn off a tank water heater's power or set a gas one to pilot or vacation before draining it, and never run its burner or element with the tank empty.

If a pipe is already frozen, keep the faucet open and warm the pipe with a hair dryer, a heating pad or warm towels, working from the faucet back toward the frozen spot. Never use an open flame or a torch. If you cannot find or reach the frozen section, call a plumber.

A gas or propane furnace only needs electricity for the blower and controls. A small inverter generator or a 2 kWh power station runs it, and a furnace transfer switch connects it to the hardwired furnace. That keeps every pipe in the house warm, not just the one you are watching.

For pipes that are always at risk, like a crawl space line or a well house, self-regulating heat cable is the permanent fix; see our heat tape guide. It only works while there is power, so pair it with backup power in outage-prone areas.