Human Factors in Comfort Heating
Humans don’t perceive “temperature” as such. What governs our sense of comfort is an acceptable rate of heat loss or heat gain. Although our body core temperature is 38°C, once the insulating effect of fat and skin tissue is taken into account, our outer skin temperature is lower than our core temperature and when you then add clothing on top, our outer temperature judged by our clothing is in the low 20s °C, which is why we arrive at a figure of around 21°C as an ideal thermostat setting temperature. What we have done is just ensured our rate of heat loss is now stable and 21°C is roughly that environmental temperature that gives us equilibrium. It varies from individual to individual.
So, here’s the thing. You are sitting in a room heated to 21 °C according to the thermostat with your body about 1.5metres from an outside wall that has a temperature of 15 C. The question is: even though the air temperature is 21 C – would you feel hot or cold?
Well the answer is you will feel cold. Why?
Because even though the air temperature may be 21 C, of far greater influence on your body is the negative temperature of that outside wall. You are radiating out to it! Even in a centrally heated room you can still feel cold (and just by experience we know this to be true). This is what you are experiencing in those moments.
All objects absorb and radiate heat and emission always flows from hot to cold. This is as true for humans as it is for any other object above absolute zero. We feel absorption of heat as warming up, and we feel loss of heat as cooling down.
Humans can tolerate up to 5 degrees radiant heat difference under or over their ideal comfort levels and still feel reasonably comfortable. But beyond 5 degrees, people will feel uncomfortable (too hot or too cold) and they will start to adjust by:
– Adding (or removing) clothes;
– Adding (or reducing) levels of activity;
– Increasing (or decreasing) air temperature – and air movement (if that is possible).
But the fact remains none of those measures addresses the issue of the radiant temperature imbalance between the environment and the person.
Insulating the wall may or may not be possible or economical. You may not want to add more layers of clothing in a sitting room or office which should – after all – be comfortable; you won’t want to exercise during your favourite TV program (or if you’re preparing a presentation for your boss at work); and – being cost-conscious – you don’t want to “bump up the thermostat”.
Convection-based heating systems and wet central heating panels do not have a solution for increasing the mean radiant temperature of the environment without excessive additional expenditure of energy. As our page Infrared Fundamentals: Heat Transfer showed: these systems actually radiate little relative to their greater emission of energy to the air. Only systems that properly “Radiate” can address the issue, because that is how they heat the room: by heating objects in the environment (which raises mean radiant temperature), not the air.
The implication is: if you get your radiant heating right, you get your comfort heating right.
Read also “Preferred wavelengths for Comfort Heating” for more detail on the effects of different types of heat on human skin.