SectionsThey are cast one by one. They are joined side by side with threaded nipples.
Thermostatic valveIt senses the room's air temperature and controls this one radiator.
Bleed valveIt is opened with a radiator key. Air trapped inside keeps a radiator from heating properly.
Lockshield valveIt is set once to balance the heating system. Then it is left alone under its cap.
Supply pipeHeating books also call it the flow pipe.
Return pipeThe water leaves cooler than it came in. It has given its heat to the room.
Car radiator: tanks and cap 6
Radiator capPressure raises the coolant's boiling point. A valve in the cap vents anything over the limit.
Filler neckCoolant is poured in here; the overflow tube leaves from its side.
Overflow tubeCoolant or steam escapes through it if pressure goes over the limit.
Upper tankIt spreads the incoming coolant across the top of the core. Also called the inlet tank.
Lower tankFrom here the cooled coolant goes back to the engine. Also called the outlet tank.
Drain cockA small valve for draining the cooling system.
Car radiator: core and hoses 5
CoreAir passing through it carries the engine's heat away.
TubesRows of small tubes join the upper tank to the lower tank.
FinsThey take heat from the tubes and pass it to the air.
Inlet hoseIt joins the engine to the upper tank. Also called the upper radiator hose.
Outlet hoseIt joins the lower tank to the engine. Also called the lower radiator hose.
How this list was compiled: Every name checked on 2026-09-30, a name kept only where two independent sources use it. Room radiator: John Donaldson, Modern Hot Water Heating, Steam and Gas Fitting (1911), full text on archive.org (https://archive.org/details/modernhotwaterhe00donaiala); Allen and Walker, Heating and Ventilation, read as passages through archive.org's full-text search (https://archive.org/details/heatingandventi01walkgoog); Collins Complete Plumbing and Central Heating (2004) and Arthur Baker, DIY Central Heating (1989), both read as passages through archive.org's full-text search (https://archive.org/details/collins-complete-plumbing-and-central-heating and https://archive.org/details/diycentralheatin0000arth); US Department of Energy, Energy Saver, Heat Distribution Systems (Internet Archive copy of 2025-06-01, https://web.archive.org/web/20250601023856/https://www.energy.gov/energysaver/heat-distribution-systems); Wikipedia, Radiator (heating). Car radiator: US Army TM 9-8000, Principles of Automotive Vehicles (1985), chapter 9 (https://archive.org/details/tm-9-8000-principles-of-automotive-vehicles-1985); Richard F. Armento, Automotive Cooling System Training and Reference Manual (1979) and a car maker's 1978 technical service manual, both read as passages through archive.org's full-text search (https://archive.org/details/automotivecoolin0000arme and https://archive.org/details/amc-1978-technical-service-manual-vol-1); Wikipedia, Radiator (engine cooling). No maker is named on the page. Licence: own work
The drawing: Drawn for this site on 2026-09-30 from plain geometry; schematics of no maker's radiator, nothing traced. Licence: own work
How it works
The parts of a radiator depend on which radiator you mean. The first drawing is a hot-water room radiator with its valves and pipes. The second is a car radiator, with a piece of the core enlarged to show its tubes and fins.
Read the full guide
Room radiator first, then the car radiator
Numbers 1 to 6 are on the room radiator. Hot water comes up the supply pipe and through the thermostatic valve into the sections. It leaves past the lockshield valve and goes down the return pipe. The bleed valve is in a top corner. Numbers 7 to 17 are on the car radiator. Coolant from the engine comes through the inlet hose into the upper tank and runs down the tubes of the core, where the fins pass its heat to the air. It collects in the lower tank and goes back through the outlet hose. The cap sits on the filler neck, the overflow tube leaves beside it, and the drain cock is at the bottom.
A plain handwheel valve can stand where the thermostatic valve is. It turns the flow of water on or off by hand.
This car radiator is the down-flow kind. In a cross-flow radiator the coolant moves crossways and the tanks are at the ends of the core.
Questions and answers
What are the parts of a room radiator called?
On a hot-water radiator: the sections that hold the water, a handwheel or thermostatic valve at one end, a lockshield valve at the other, a bleed valve in a top corner, and the supply and return pipes.
What does the bleed valve on a radiator do?
It lets out air trapped inside. Air in a hot-water radiator keeps part of it from heating properly. The valve sits in a top corner and is opened with a radiator key.
What are the main parts of a car radiator?
Two tanks with a core between them. The core is rows of small tubes with thin metal fins between the rows. A filler neck with a pressure cap sits on the upper tank, and hoses join the tanks to the engine.
What does a radiator cap do?
It holds the cooling system under pressure, which raises the coolant's boiling point. A pressure valve in the cap vents anything over the limit through the overflow tube, and a vacuum valve lets air back in as the engine cools.