Just as a building has heat loss, so does the human body.
While other more conventional heating systems address the heat loss of the structure (satisfy the thermostat), radiant heating systems address the heat loss of the human body.
The human body basically has its own hydronic heating system; Heat is distributed through the blood pumped through the heart. The first priority of the body’s heating system is to maintain a central body core temperature. From there, the blood is sent to the extremities. Since the feet are the furthest from the core, they are the last to get heat, and the first to recognize the discomfort of a cold environment.
Where the body’s hydronic heating system fails, radiant floor heating systems excel.
Radiant floor systems apply direct conductive heat to the feet, and to objects in the room that, in turn, re-radiate the heat. Because radiant floors directly address the heat loss of the human body through conduction, people are typically more comfortable at a lower thermostat setting around 65 degrees Fahrenheit. This is another reason why radiant floors are more energy efficient.
A significant portion of the heat transfer from radiant floor heating systems results from radiation. Radiant heat does not heat the air, but heats solid objects.
For example, heat from the sun is radiant heat. Outer space is cold, but the earth gets its heat from the sun; radiant heat from the sun warms the earth, but not the air in between. And so, radiant floor systems heat people and other furnishings to a comfortable temperature.
The air temperature remains at 65 degrees Fahrenheit; the air temperature is approximately equal from floor to ceiling. With high ceilings, the temperature will actually be lower near the ceiling.
When the floor surface temperature reaches 7 degrees Fahrenheit (14 BTU/ hr/ sq.ft.) above the room setpoint temperature, convection will occur, so that the air temperature is often slightly higher than the setpoint.
The goal of the ideal heating curve is to achieve a temperature just below skin temperature at the floor. Moving towards the ceiling from the floor, the temperature lowers to about 65 degrees Fahrenheit. That temperature is maintained to slightly above the average height of an adult, and increases from there.
The common forced air heating system heats air to the temperature necessary to overcome the heat loss of a building. Hot air is then forced into the occupied space through duct work and noisy blowers. The heat loss of the building determines the temperature of the air that the occupants must endure.
If the heat loss is high, the air temperature must be uncomfortably high to maintain the room setpoint temperature. If the heat loss is low, the setpoint temperature can be maintained by either blowing low temperature air into the occupied space, leaving occupants subject to windchill, or short blasts of excessively high temperature air. In any case, you are left with an uncomfortable, uncontrollable situation.
Forced air heating systems are unable to match the profile of the ideal heating curve due to one of the basic principles of physics — hot air rises. Hot air heating systems do not distribute heat where the body needs it most. In order to meet the comfort needs of the body’s lower extremities, the air must be heated to a level that is uncomfortable to the upper body. The choice is cold feet, or hot heads.
Overall, the temperature at the floor never reaches the desired level, and the temperature at the ceiling is too hot. The shaded air between the forced air heating curve, and the ideal heating curve represents wasted energy and additional cost reflected in monthly utility bills.
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