How Central Heating Works: The Flow and Return Loop, Boiler to Radiator

Central heating works by heating water in one place, usually a gas boiler, and pumping it round a closed loop of pipes to radiators in every room. Each radiator passes the heat into the room, the cooled water flows back to the boiler, and the cycle repeats. A timer and thermostat decide when the system runs and when the house is warm enough, and valves on each radiator fine-tune individual rooms. That is the whole idea; everything else in the system exists to keep that loop running safely and evenly. This article follows the water round that loop; for the wider picture, from the parts of a system to when a whole-system upgrade makes sense, see our complete guide to how central heating works.

Most UK homes have this kind of wet system, so it is the one described here. Other ways of heating a home, such as heat pumps, storage heaters and communal heat networks, work on different principles.

The Loop: Flow and Return

Every wet system is a loop. Hot water leaves the boiler on the flow pipe and comes back cooler on the return pipe, and every radiator sits between the two.

The water in the loop is not the water from your taps. It is the same water going round and round, topped up only when some is lost. That matters, because the water ages: it picks up dissolved metal and air over time, which is where sludge and cold spots eventually come from. How far the water cools between flow and return tells an engineer how well each radiator is working, which is the basis of balancing.

What the Boiler Does

The boiler burns gas to heat the system water as it passes through a heat exchanger. On a modern condensing boiler, the cooler the water coming back, the more heat it recovers from its own exhaust gases, so the rest of the system affects how efficient the boiler is.

Boilers also differ in how they make hot water for taps. A combi heats tap water on demand and has no cylinder. A system or regular boiler heats a stored cylinder of hot water as well as the radiators, using a valve to send heat to one, the other or both. Either way, the heating side works the same. Anything inside the boiler or on the gas supply must be worked on by a Gas Safe registered engineer.

The Pump and the Pipes

A circulating pump pushes the water round the loop; without it, hot water would barely move. On a combi or system boiler the pump is built into the boiler, and on older regular boiler setups it is often a separate unit near the cylinder.

The pipes are usually copper, in 22mm for the main runs and 15mm for the branches to each radiator. Some homes, particularly those fitted in the 1970s and 1980s, have narrow microbore pipes of 8mm or 10mm, which are neat but block more easily. The layout of the pipe runs decides which radiators get hot water first and which get it last, and that is why the radiators furthest along often struggle.

How Radiators Heat a Room

A radiator is a steel panel filled with hot system water. Despite the name, most of its heat reaches the room by convection: air warmed against the panel and its fins rises, and cooler air is drawn in underneath, setting up a slow circulation round the room.

Each radiator has two valves. The lockshield sets how much flow the radiator gets, and is set once when the system is balanced. The other, usually a thermostatic radiator valve (TRV), opens and closes on its own to hold that room at the setting you choose. A radiator's heat output depends on its size and type and on how hot the water is, which is why a radiator that suits a room on a gas boiler can be too small on a lower-temperature system.

Controls: When It Runs and How Warm

The controls turn a heater into a heating system. A timer or programmer sets when the heating can run, a room thermostat switches it off when the house is warm enough, TRVs control each room, and a boiler interlock stops the boiler firing when nothing needs heat.

The thermostat does not make the house heat faster when you turn it up; it only changes when the heating stops. Our guide to which heating controls a house needs covers what each control does and what a well-controlled system should have. Smart thermostats combine the timer and thermostat in one device and add scheduling from your phone, but they sit on top of the same basic loop.

Pressure, Expansion and Keeping the Water Clean

Water expands as it heats, so every system needs somewhere for that expansion to go. How it does this is the difference between a sealed system and an open-vented one.

A sealed system is closed off from the air. It is filled to a set pressure, shown on a gauge on or near the boiler, and an expansion vessel absorbs the extra volume when the water heats. An open-vented system has a small feed and expansion tank, usually in the loft, that tops the water up and takes the expansion. If you are not sure which you have, our guide on telling a sealed system from an open-vented one walks through the signs.

Either way, the water needs protecting. A chemical inhibitor slows corrosion inside the radiators and pipes, and a magnetic filter on the return pipe catches the iron oxide that corrosion produces before it reaches the boiler. Our explainer on what a magnetic filter does covers how it works and how often it needs cleaning.

Where Central Heating Usually Goes Wrong

Most whole-house heating problems come from the loop rather than the boiler: air collecting at high points, sludge building up in radiators, or water taking the easy route through the nearest radiators so the far ones stay cool.

Each of these has a different fix. Air is bled out. Sludge is flushed out, then kept out with inhibitor and a filter. Uneven heat is corrected by balancing, and sometimes by resizing radiators or improving the controls. A new boiler fixes none of them on its own, which is why a system survey is often a better first step than a boiler quote when the heating is uneven. If your system has several of these problems at once, our central heating system upgrade service looks at the pipes, radiators, controls and water quality together rather than one symptom at a time.