Comfort

The main purpose of a building’s heating system is to provide comfort for its inhabitants. By definition, comfort is a condition whereby a person feels neither too cold nor too hot. The human body achieves comfort when it can “dump” the heat created by bodily functions at the same rate as by which it gains heat from its immediate environment. A person undergoing light activity will generate, and have to get rid of, approximately 400 btus (422 kJ) per hour in order to feel comfortable. If there is too little heat supplied to that person from the building’s interior spaces, that person will feel cold; too much and they will feel hot. If they increase their activity level, they’ll need to get rid of more heat, and a cooler surrounding air temperature would be required in order for them to not feel overheated. As you can guess, a building’s heating or cooling system does not have the ability to anticipate the comfort levels of every person within it, as we all have differing metabolisms and levels of activity. The best we can do, as heating professionals, is to provide a constant, uniform temperature within the building so that each individual may tailor their activities and attire to meet their own needs.

The manner by which a person receives the external heat it requires is also a major contributor to the level of comfort expected. The human body loses heat by five means: radiation, convection, evaporation, conduction and respiration.

A body loses the majority (approximately 48%) of the heat it generates by way of thermal radiation. Radiation is the transfer of heat via infra-red waves through the air from a warm object to a colder object. In the winter, if a person sits close to an outside wall, they will feel colder than if they were sitting nearer to the middle of the room, even though the air temperature in the two locations is the same. This phenomenon has been labeled “cold 70”.

Heat lost by convection (air currents) accounts for approximately 30% of the body’s total heat loss, and approximately 22% is lost through evaporation (sweating). If a person is clothed and wearing slippers or shoes, conduction losses are very minimal, such as would occur through bare feet on a cold floor. As well, respiration (breathing) losses are so small that they are not considered when designing heating systems.

Radiant Heating vs Convection Heating

Because radiation losses from a body are the highest of all the methods of heat transfer mentioned, it makes sense that, by putting heat back to that person in the same manner, they should feel most comfortable. The “ideal heat curve” of the human body would see the floor temperature at approximately 80°F (approximate skin temperature), to approximately 68°F at a point 5 feet above the floor (average head height), and to much less near the ceiling, where heat is generally not needed. A radiantly-heated floor panel system matches almost perfectly those heat layers which the human body needs for most comfort.

On the other hand, forced warm air furnaces supply hot air to a room through grilles in the floor. This hot air rises and collects near the ceiling, which intensifies the heat loss high up in the room. The floor is cold to the touch, while the air temperature at face level may be close to the desired setpoint of the thermostat. Coupled with the fact that forced air systems try to achieve their results by adding heat to an insulator (air) and distributing it, forced air systems typically operate as one big zone, with a thermostat located in a hallway or central part of the house. While the hallway may be comfortable, nobody typically spends time there and the living areas of the house suffer the consequences. The rooms at the south end of the building are overheated while the rooms on the north side are generally colder. There are far more advantages than disadvantages to the use of hydronics, in particular hydronic radiant systems, over the use of forced air systems.

Btus

Heat is described and measured by its intensity and quantity. Most people know that they are experiencing heat when they touch a metal object whose other end is embedded in a flame and it feels hot to the touch. In actual fact a quantity of heat is being applied to the end of the metal piece within the flame, and this results in a change in intensity of heat (temperature) in degrees Fahrenheit or Celsius between the molecules of the metal in the two opposite ends of the metal. This is evidence of heat transfer via conduction.

If the end of the metal farthest from the flame gets hot to the touch, the person is experiencing the intensity of heat within the metal object. It is the quantity of heat that we are most concerned with when we design a building’s heating system. That quantity is measured in btus or, more specifically to our heating industry, in btus per hour (btuh).

The language of heat within the heating industry in North America is predominantly in Imperial terms. Therefore, for the purposes of the information within this learning guide, quantities of heat will be referred to in Imperial terms as btuh, and intensities of heat will be referenced in degrees Fahrenheit (°F). It should be noted that some building departments or jurisdictions in Canada may require heat loss estimates to be in kilojoules (kJ) and degrees Celsius (°C). In those instances, heat loss calculations can be done in Imperial terms first and then converted into the desired outcomes.

One kilojoule (1 kJ) = 0.95 btus, or conversely 1.055 kJ = 1 btu.

For heat’s intensity, 1.8 °F = 1°C, or conversely 0.555 °C = 1°F.

To convert heat’s intensity to a temperature scale (thermometer), the formula to use is:

(°C × 1.8) + 32 = °F, or conversely (°F ÷ 1.8) − 32 = °C.

Media Attributions

• Figure 1 Theoretical ideal heating curve comparisons by ITA is licensed under a CC BY-NC-SA licence.
• Figure 2 Heat transfer via conduction by ITA is licensed under a CC BY-NC-SA licence.