The easiest way to understand radiant heat is to understand the feeling of warmth on your body from the sun.
On a sunny, yet cold day, even when the air temperature may be below freezing with snow and ice on the ground, you can still feel warm while in the sunshine if there is little wind. Out of the sun however, the feeling of warmth disappears, as the remaining sensation is now dominated by the freezing air temperature.
Efficient radiant heating systems heat large surface areas to a low temperature, warming a dwelling or building with evenly distributed radiant heat emitted from a floor, wall or ceiling. Using an eUFH wire or hUFH pipe within a floor, the floor warms the people and furniture within a room directly and not just the air.
This radiant heat allows the air temperature within a room heated by underfloor heating to be measurably lower than for a convection system,while still providing the same level of comfort.
In a typical system, the air temperature can be 1°C lower when using a radiant heating system that it can be with a convective heating system. With the average winter temperature in the UK being approximately 10°C and the average indoor temperature being 20°C, over an entire winter, a radiant heating system typically reduces heat loss by approximately 10%.
What is Convective Heat?
Radiators and forced convection units, like air conditioning systems, mainly work on the principle of heat convection and not radiation.
Standard radiator systems heat air to a high temperature, in excess of 50°C, using 75°C water. This overheated air is then used to distribute the heat around the room mixing in with the rest of the air as it flows up to the ceiling, across the room and then back down again as it cools.
Radiators are normally fitted on external walls beneath windows, to prevent excessively cold drafts causing significant discomfort. Placing a radiator on the wall opposite a window where the cool air is falling against the external wall and the window leads to uncomfortably cool drafts across the floor. However, wherever radiators are positioned, they will create cool drafts across the floor to a greater or lesser extent, it’s just how they work.
Fitting a radiator beneath a window is the least energy efficient position for them It places the hottest air in the room in direct contact with the part of the room that has the highest heat loss, approximately 10x the amount of heat that would be lost though a similarly sized piece of wall. If the windows are leaky, have trickle vents or are latched, the heat loss is further exacerbated as the 50°C air could be going right out the window.