Radiant heat warms objects and surfaces without heating the air in between. Those objects warm up directly and radiate back.
Convection warms the air, which is a triple inefficiency. The air has to:
- Warm up. Air absorbs heat poorly;
- Travel (usually upwards to the ceiling or through doors and windows);
- Deliver the heat to something. Air transfers heat out poorly;
Degree for degree °C, radiant heat delivers 4x the heat transfer capability of convection.