One of the mysteries of the English language finally explained.
1Designating a system of hot-water central heating in which radiators take water from and return it to the same pipe, which runs in a complete circuit from the boiler and back to it again.
2Designating a system of plumbing in which waste from sinks, etc., is conveyed to the sewer by the same pipe as that from lavatories (i.e. the waste pipe and soil pipe are effectively combined).
Late 19th century; earliest use found in The Manufacturer and Builder. From one + pipe.
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