Apply self-regulating heat tape (not constant-wattage) spiraled or straight along the pipe. Plug in and monitor. Physics: Resistive heating (typically 5-15 W/ft) raises pipe temperature to just above freezing. Self-regulating tape reduces power as temperature rises, preventing overheating. Pros: Safe for plastic, can be left on for 24+ hours. Also serves as permanent freeze prevention. Cons: Slow (4-12 hours). Requires access to wrap tape. Not effective if pipe is encased in insulation without direct contact. Verdict: Ideal for known problem pipes in crawlspaces.
A slow drip in a sink can lead to a "layering" effect where ice builds up in the drain pipe over time until it's solid. thawing frozen drain pipes