rad

A rad is a radiator, the heat exchange device that sits under windows or along walls in most buildings. It consists of a metal body (usually cast iron or steel) with internal passages for hot water or steam to circulate through, while air flows around the outside fins or panels to warm the room. The term is universal shorthand among plumbers, heating engineers, and building maintenance staff.

Most rads operate on one of two systems. In a wet central heating system, hot water from a boiler circulates through the rad via pipes at temperatures typically between 60 and 80 degrees Celsius, then returns cooler to be reheated. In steam systems, less common in modern buildings, pressurized steam enters the rad and condenses back to water as it releases heat. Steam rads require air vents and condensate traps; water rads just need inlet and outlet connections and a bleed valve to release trapped air.

Cast iron column rads dominated domestic heating for over a century; they are heavy, hold heat well, and last decades. Modern panel rads are lighter pressed steel with welded fins, more efficient per unit volume but less durable. Aluminum rads appear in some continental European systems. The choice affects how a plumber sizes the heating loop, calculates flow rates, and plans the pipework layout.

Common failures and sizing

Rads fail by corroding internally, often from acidic water or incompatible metals in the circuit, which produces brown sludge that blocks the small internal galleries. Limescale builds up in hard water areas. Physically, the most common fault is an airlock that prevents water circulation, which the bleed valve solves. Undersized or oversized rads lead to rooms that never reach temperature or waste fuel. A plumber determines the right size in watts or BTU per hour based on room volume, insulation, and desired temperature rise.

Balancing a heating system means adjusting flow through each rad so all rooms reach temperature at once. This involves lockshield valves (on the return) and often thermostatic radiator valves (TRVs) on the inlet. The name "rad" is simply practical shortening: easier to say "bleed the rad" than "bleed the radiator" on a site where you might mention it dozens of times a day. It appears in work orders, stock lists, and everyday conversation across the heating trades.