How heat moves by conduction, convection and radiation through pipes, radiators, cylinders and rooms, with latent heat and insulation.
Heat always passes from a hotter region to a cooler one, by conduction, convection and radiation. Identify the particular part of the heat path: several modes can act in the same system.
- Conduction: Energy passes through a material without its bulk movement. It occurs in solids, liquids and gases. Copper conducts well, which is useful when heat must cross a heat-exchanger wall or spread along a pipe. Air conducts much less readily, but its conduction is not zero.
- Convection: Moving water or air carries energy. Density differences can drive natural circulation, such as warmed water rising around a cylinder coil or warm air rising past a radiator while cooler air moves in to replace it. Pumps and fans produce forced convection. An open vent does not by itself tell you whether circulation is natural or pumped.
- Radiation: Electromagnetic waves carry energy between surfaces, including across a vacuum. At domestic heating temperatures this is mainly infrared. You can feel a warm panel across a gap without waiting for the surrounding air to warm, the way you feel warmth from a fire.
For revision, link each mode to an example: conduction through a pipe, convection round a radiator, radiation off a hot surface.
Why a panel radiator heats the air
A typical panel radiator with fins has a large surface available to warm the air moving past it. Convection commonly accounts for most of its room-heating output, with radiation also heating nearby surfaces and people. The proportions depend on construction and temperatures; use the particular emitter’s data when a numerical split is needed.
All three at once: why insulation matters
Most plumbing components use all three modes at once. A hot water cylinder loses heat by conduction through its wall, convection of the air around it and radiation from its surface, which is exactly why a good insulating jacket cuts the losses.
Why water carries heat so well
Water takes in a comparatively large amount of energy for each degree its temperature rises (a high specific heat capacity), so a modest flow of water can carry a lot of heat from the boiler to the radiators. That is one reason water, rather than air, is used to move heat around a wet central heating system.
Change of state: latent heat
Latent heat is energy taken in or released during a change of state. For a pure substance boiling or condensing in equilibrium at fixed pressure, the temperature stays at its transition value while the phase change proceeds. Sensible heat instead changes temperature without a change of state. Some refrigerant blends have temperature glide during boiling or condensation, so a constant temperature is not a rule for every refrigerant.
Condensing boilers recover energy as water vapour in the flue gases condenses; that does not mean the whole flue-gas mixture stays at one temperature. Refrigeration circuits also use the energy exchanged in boiling and condensation. Surface evaporation can occur below the boiling point and remove energy from the remaining liquid, cooling it when that loss is not replaced by heat from elsewhere.
Reflective foil needs an air gap
Place a low-emissivity panel behind a radiator with an air space facing the room. Its reflective face redirects some infrared energy away from the external wall. If the foil is pressed against another surface, heat can pass by contact instead, so much of the reflective benefit is lost.
Metal clips can bridge the insulation
A metal pipe clip in direct contact with a hot pipe at one end and the building structure at the other conducts heat straight past the lagging, forming a small thermal bridge. Each clip only loses a little, but a long run has many of them, and they mark exactly where continuous insulation is interrupted — a suitable insulated support reduces this heat path while carrying the required load. Ordinary soft lagging must not be relied on as a structural support.