Heat: the Three Ways It Moves, How It Is Measured, and How It Is Kept In

Conduction needs a solid. Convection needs a moving fluid. Radiation needs nothing at all — which is how the sun reaches us across empty space.

The three methods of heat transfer

Heat always moves from hotter to colder, by three methods. That direction is not a rule anybody made; it is simply what heat does, and it means you can never add cold to something — only take heat away.

Conduction — through a solid

Passed from particle to particle. Metals conduct well; plastics do not.

In plumbing: heat passing through the wall of a copper pipe, or through a cylinder coil into the water.

This is why copper is used for heat exchangers and coils, and it is also why a copper pipe burns you long after the torch has gone.

Convection — through a moving liquid or gas

Warm fluid rises, cool fluid falls, and a current forms.

In plumbing: a radiator warming a room, and the hot water sitting at the top of a cylinder.

Convection is the same density effect as gravity circulation in the properties of water lesson — warm fluid is lighter, so it rises, and the movement carries the heat with it.

Radiation — as rays through space

Needing nothing to travel through.

In plumbing: the warmth you feel on your face standing in front of a hot surface, and how the sun heats a solar panel.

That is the distinguishing feature and it is what questions test: radiation crosses a vacuum. Conduction needs a solid to pass through and convection needs a fluid to move, but radiation needs nothing at all.

The word “radiator” is misleading

Worth knowing, because it sounds like a trick and it is a real fact.

Most of a radiator’s output is actually convection, with only a small part genuine radiation. It warms the air against it, that air rises, cooler air is drawn in at the bottom, and a current circulates around the room.

Which is why a radiator with a shelf right above it, or a sofa pushed against it, performs badly: the convection current is what is being blocked.

Temperature measuring devices

• Thermometer — liquid in glass, or digital.
• Thermocouple — produces a small voltage that varies with temperature. You will meet these again on gas appliances.
• Thermostat — measures temperature and switches something on or off. That switching is what makes it a thermostat rather than a thermometer.
• Infrared / laser thermometer — reads a surface temperature without touching it. Useful for checking a pipe or a radiator quickly, and for finding which pipe is the flow.

Temperature scales

• Celsius (°C) — water freezes at 0 and boils at 100. The everyday scale.
• Kelvin (K) — the SI scale, starting at absolute zero, the coldest possible temperature. Water freezes at 273 K.

A rise of 1 K is the same size as a rise of 1 °C, so a difference in temperature can be written either way.

Surfaces and finishes

The surface of an object changes how well it radiates and absorbs heat.

• Dull surfaces — especially dark ones — are good radiators and absorbers of heat. Which is why a solar panel absorber is matt black.
• Shiny surfaces are poor radiators and absorbers, and good reflectors. Which is why foil behind a radiator reflects heat back into the room instead of letting it soak into the wall, and why a cylinder jacket has a shiny face.

The neat part is that it works both ways round: a good absorber is also a good emitter. Matt black takes heat in and gives it out well; shiny does neither, which is exactly what you want on something you are trying to keep hot.

The benefits of insulation

Insulation slows heat transfer down. Its named benefits are:

• Energy efficiency — less fuel is needed to keep the water or the room at the temperature you want.
• Prevention of heat loss — heat stays where it is wanted instead of escaping into a loft or a cupboard.

In plumbing it also does two other jobs worth knowing, and both come up constantly on site:

• It protects pipework in unheated spaces from freezing — lofts, garages, outbuildings.
• It stops condensation forming on cold pipes and dripping onto a ceiling. A “leak” that turns out to be condensation on an uninsulated cold pipe is a call-out you will make more than once.

The line to remember

Conduction needs a solid. Convection needs a moving fluid. Radiation needs nothing at all — it crosses a vacuum, which is how the sun reaches us.