Freeze risk in older buildings

Pipes fail in unheated and under-heated space: vacant units, stair towers, crawl spaces, unconditioned corridors, and anywhere a heating zone has quietly stopped working without anyone noticing. In an occupied apartment somebody complains when the heat fails. In a vacant one, nobody does.

The failure comes on the thaw

Water expands as it freezes and can split a pipe or force a fitting, but the split does not release anything while the contents are still solid. The water arrives when it melts. That is why the call comes the morning after a cold snap ends rather than during it, and why a mild afternoon following a hard freeze is the moment to have someone walking the building.

Where to look before winter

• Vacant units. Confirm heat is on in every one, not that it is scheduled to be. Thermostats get turned down during a turnover and stay down.
• Stair towers and vestibules. Often minimally heated, frequently containing standpipe or sprinkler lines.
• Crawl spaces, cellars and unconditioned corridors, particularly where a line runs along an exterior wall.
• Roof tanks, mechanical rooms, and any exposed line in unconditioned space, and whether its insulation is intact rather than present.
• Heating zones themselves. A zone valve that has failed closed is invisible until the space it serves is measured.

Where a line runs through unconditioned space, insulation slows heat loss but does not add heat. If a line is genuinely at risk, it needs heat, heat tracing, or to be drained for the season.

Sprinkler and standpipe systems

Wet sprinkler systems in unheated space are a particular exposure, because a failure discharges continuously until somebody isolates it rather than releasing the contents of one pipe. If part of a system runs through space that can drop below freezing, that is a question for whoever maintains the system, and it is a better question in October than in January.

If a line has frozen but not burst

Call the same day. Thawing under controlled heat and then assessing the pipe is a fraction of the cost of a burst riser in an occupied building. Do not use an open flame, and do not leave a frozen section unattended in the hope it resolves quietly, because the moment it thaws is the moment you find out whether it split.

If it has already burst, the priorities are the same as any water loss: isolate the supply, note the time, photograph before anything is moved, and call it in even if the visible area looks small.