Infrared is radiated heat: the feeling of warmth from the sun on your face; the heat from a coal fire, or a toaster. It is even the same form of heat emitted by your own body. It is the most basic form of heating known to man.
Used by cavemen to heat themselves by fires; by Romans in their hypocausts, by log burners and tile stoves, Infrared heating has been favoured for millennia because like the heat of the sun on your surrounding environment – even during winter – Infrared heats objects, which then radiate back and keep the environment warm around you. Radiant heat does not heat air – which holds little heat and rapidly disperses.
Infrared waves travel through the air and when they touch a surface, heat energy is released regardless of the surrounding air temperature. That heat energy excites the molecules in the object it meets which being to vibrate and gain energy (and warm up). Water absorbs Far Infrared specifically well, and as our skin is 80% water, we are perfectly adapted to Far Infrared (Robinson, 2014). Far Infrared – unsurprisingly – the same band of infrared that the human body itself emits.
All objects (including people) absorb and emit infrared and whether one is absorbing or emitting depends on the difference in temperatures between objects in an environment. If objects in an environment are warmer than you are, you will warm up from them. If you are warmer than objects in an environment you will radiate out to them and feel cold. (This Infrared emission is why police Infrared cameras can see fugitives trying to escape detection). But it is also why we can still feel cold in centrally heated rooms, which only heat the air and don’t heat the objects within a room.
If you are in a centrally heated room at 21°C with your back to an outside wall at 17°C, you will be radiating heat out to that outside wall and you will therefore feel cold: regardless of the room’s “comfortable” air temperature. This underlies a fundamental difference between infrared and “convection” heating.
An experiment at the John B. Pierce Laboratory, USA, clarified the different human perceptions of heat:
“Test persons in a room with a temperature of 50°C (122°F) of warm air and cooled walls froze deplorably; when in a room with a cool air temperature of 10°C (50°F) and warm walls, they broke into an unpleasant sweat.”
(source: Techn. Info “Strahlungsenergie – die Ur-Energie, neu entdeckt, TT Technotherm GmbH, Nürnberg).
Feeling warm has nothing to do with air temperature. It is all about absorption of infrared from our surroundings (warming up) or stopping ourselves losing radiation (cooling down) to a “colder” outside.
But in the last 60 years, we have forgotten about radiant heating: not because a better technology replaced it, but because fossil fuels that powered central heating made it so cheap just to heat air.