How do boilers and radiators heat rooms?

1. It is a frosty morning and the radiator under the window is cold. It has no flame and no plug. So how does a metal panel on the wall warm a whole room?
2. The heat comes from the boiler. Inside, flames warm a coil of pipe with water flowing through it. The water does not boil. It just gets hot, about as hot as a cup of tea.
3. A pump pushes the water round the loop. Hot water leaves the boiler along the flow pipe and travels under the floors to every radiator in turn.
4. Hot water flows through the radiator and warms its metal. The metal warms the air touching it. Warm air rises, cooler air slides in to take its place, and soon the whole room is stirring with warmth.
5. Having given up some heat at each radiator, the water is cooler. It flows back along the return pipe to the boiler, is heated again, and sets off on another lap.
6. A valve on each radiator lets more or less hot water through, so a bedroom can be cooler than a bathroom. A thermostat tells the boiler when the house is warm enough.

Tap each part and follow the water round its loop.

• Boiler. The boiler heats the water. Inside it, flames warm a coil of pipe, and the water flowing through the coil picks up the heat.
• Pump. The pump is a small electric motor with a spinning wheel inside. It keeps the water moving round the loop. Without it, the hot water would just sit in the boiler.
• Flow pipe. The flow pipe carries hot water out of the boiler and off toward the radiators. It is usually hidden under the floor.
• Radiator. A radiator is a hollow metal panel with ridges. Hot water flows through it and warms the metal, and the metal warms the air in the room.
• Radiator valve. The valve is a knob at one end of the radiator. Turning it lets more or less hot water in, which makes that room warmer or cooler.
• Return pipe. The return pipe brings the water back to the boiler after it has given heat to the rooms. The water in it is cooler than the water in the flow pipe.
• Expansion tank. Water takes up a little more space when it is hot. The expansion tank has a rubber sheet with air behind it, which squashes to make room.
• Bleed valve. Air bubbles can get trapped at the top of a radiator, leaving it cold up there. A grown-up opens this tiny valve with a key to let the air hiss out.
• Thermostat. The thermostat measures the room's temperature. When the room is warm enough, it tells the boiler and pump to rest. When the room cools, it starts them again.

Light the boiler and start the pump. Follow the water and compare how hot it is when it leaves and when it comes back.

Experiment: Water leaves the boiler hot. What is it like when it gets back?

1. Switch the boiler on, then the pump.
2. Watch the color of the pipes change along the loop, from the flow pipe to the return pipe.
3. Now stop the pump and watch the radiators.

What we found: The water comes back cooler, because each radiator let some of its heat out into a room. When the pump stops, no new hot water arrives, so the radiators cool down even though the boiler is still hot.

Turn on JavaScript to try it yourself.

The bathroom wants to be warm, the bedroom cool and the living room cozy. Set the three radiator valves, watch where each room is heading, then check.

Water is a wonderful carrier of heat. A small pipe of hot water can carry as much heat as a big duct of hot air. That is why many homes are warmed by water that travels round a loop of pipes.

The loop starts at the boiler, where a flame heats the water. A pump pushes it along the flow pipe to the radiators. A radiator is a hollow metal panel. The hot water inside warms the metal, the metal warms the air next to it, and that warm air rises and spreads through the room while cooler air moves in to be warmed.

After giving up some of its heat, the water is cooler. It flows back to the boiler along the return pipe, gets heated again and sets off on another lap. The same water goes round and round for years.

Valves on the radiators let you choose how much hot water each one gets, so different rooms can be at different temperatures. Our train comparison breaks down in one way: a train drops off whole parcels, but the water gives up only part of its heat at each radiator and arrives a bit cooler at the next. The model here uses one loop that visits each radiator in turn. Many homes use two pipes, so every radiator gets water straight from the boiler.

Key words

Boiler

The appliance that heats the water for a heating system. The water gets hot but does not usually boil.

Radiator

A hollow metal panel that hot water flows through, passing its heat to the room.

Flow and return

The flow pipe carries hot water out to the radiators. The return pipe brings the cooler water back.

Convection

Heat carried along by moving air or water. Warm air rises and cooler air sinks.

Pump

A machine that pushes a liquid along a pipe.

Specific heat capacity

The energy needed to warm one kilogram of a material by one degree.

Check your thinking

1. What carries the heat from the boiler to the radiators?

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   Hot water flowing through pipes. A pump pushes hot water from the boiler through pipes to each radiator.

2. The boiler is hot but the pump has stopped. What happens to the radiators?

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   They cool down, because no new hot water reaches them. Heat has to be carried by moving water. If the water stops, the radiators give away their heat and are not topped up.

3. Why is the water in the return pipe cooler than the water in the flow pipe?

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   It gave some of its heat to the rooms on the way. Each radiator lets heat out into its room, so the water leaves a little cooler every time.

4. How does a radiator warm the far side of a room?

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   Warm air rises from it and moves around the room. The radiator warms the air next to it. Warm air floats up and cooler air moves in, so the air keeps stirring.

5. In a single series loop, you turn the first radiator down. What happens at the next radiator?

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   It receives hotter water than before. The first radiator now takes less heat out of the water, so the water arrives at the next one hotter.