People are warm-blooded exothermic heat generators, making our own heat. The function of indoor space conditioning systems is actually to control the heat loss of our bodies to keep us comfortable. And while designing mechanical systems for high building efficiency* is important, the first priority should always be on occupant comfort. So it's important to understand how occupants exchange heat with our environments.

*Radiant heating and cooling systems also help buildings to achieve high system efficiency, but that's not the focus of this webpage

Heat emission from the human body occurs via four modes of transfer (in relative proportions):

  • Radiation (infrared heat exchange with our surroundings) ~45%
  • Convection (transfer of energy through air movement) ~30%
  • Evaporation (giving off heat through breathing and perspiration) ~20%
  • Conduction (heat exchange with things we touch, like floors) ~5%

Our bodies radiate heat to any surface in line-of-sight which is cooler than our body's surface temperature, approximately 85ºF (29ºC). Cold interior surfaces, such as caused by large windows or poorly insulated surfaces (walls, ceilings, floors) that make up our surroundings, will increase human heat loss and reduce comfort. On hot days, excessively warm interior surfaces surrounding our bodies can decrease our heat loss and reduce comfort. Warm interior surfaces can also be caused from devices that give off heat, such as computers, lights, factory equipment, etc. and other people in a crowded space.

Traditional air-based heating systems surround people with hot dry air to offset the heat loss caused by cold surfaces in wintertime, while air-based cooling systems surround people with cold moving air to offset the heat gain caused by hot surfaces in summertime. Neither of these systems are ever truly comfortable, as they try to mask excessive or insufficient radiant heat transfer with overheated or overcooled air. Air is a natural insulator, not a conductor, so it takes great volumes of overheated or overcooled air to transfer enough energy to keep people close to a comfort zone, often resulting in excessive air speeds with noise and drafty sensations.

Mean Radiant Temperature/Operative Temperature

When a space is heated through exposed radiant surfaces, then the resulting higher Mean Radiant Temperature (MRT), which is the average temperature of the surfaces, reduces the radiant heat loss from human body. What we actually sense in a space is called the Operative Temperature, a combination of MRT and convective heat transfer with the surrounding air.

With a higher MRT, indoor air does not have to be so hot for comfort, so it can be cooler and fresher, with better health and comfort. Relative humidity stays higher. This also leads to greater building efficiency heat loss, since less hot air escapes the structure. Finally, when heat is delivered through pipes embedded in warm floors, temperature stratification is reduced, and the warmest air is actually just above the floor, instead of at the ceiling.

Hydronic radiant heating systems use large heated surfaces (floor, walls or ceiling) and can deliver enough thermal energy to heat a space with low fluid temperatures, typically less than 110ºF (43ºC) in cold climates of North America. The heated surface temperatures are even lower, typically lower than 80ºF (27ºC), depending on location and heat loads. Radiant systems are safe, gentle, steady and invisible. In a high-performance home or building, or any building with low heating loads, both fluid and surface temperatures are even lower.

When it's hot outside, solar gain impacts buildings through windows and heats the building envelope. Heat is also conducted through building surfaces from the hot outdoor ambient air, and through any air infiltration. For interior cooling, the most comfortable method to deliver thermal comfort is to surround occupants with cooler surfaces and increase the radiant heat loss from bodies. With radiant surface cooling, less cold air is required, which reduces the need for high air volumes and the resulting noise and drafts.

Depending on the climate zone, most buildings that utilize hydronic radiant cooling also cool and dehumidify incoming fresh air, keeping occupants comfortable with low air volumes. The dehumidified air actually reduces the need for the air to be so cold, while preventing any condensation on the cooled surfaces.

A typical radiant cooled space will use 75ºF (2ºC) air temperature with no greater than 50% relative humidity, yielding a low 54ºF (12ºC) dew point temperature. To maximize system capacity, supply-fluid temperature is typically set between 57ºF (14ºC) and 61ºF (16ºC). Control systems modulate water temperatures through the tubing to make sure that surfaces always stay above the dew point of the space.

With proper design and controls, most buildings designed to have radiant heating can also incorporate radiant cooling, switching from heated to chilled water as buildings loads change. It's important that all heating and cooling systems are properly sized and designed by professionals.

See Also