Radiant Heat Fundamentals

To understand why hydronic radiant heating is so effective it is important to understand how heat transfer takes place.

Radiant Heat Knowledge Guide

The science of hydronic radiant heating

Heat transfer can occur in three different ways: conduction, convection and radiation. Hydronic heating works so effectively in keeping your home more comfortable versus conventional forced air systems because it utilizes conduction and radiation along with a liquid which is a superior conductor of heat.

Forced air heating systems rely solely on convection and air movement. Even the warmest air can feel cool when it is blown throughout a space. Air is such a poor conductor of heat that it is often used as a means of insulation, as can be seen by its use in between the panes of energy efficient windows.

Science of Radiant Heat

Thermal Conduction

This is the movement of heat through objects that physically touch, where heat moves from the warmer object to the colder. Standing barefoot on a beach with hot sand or a cold kitchen tile floor are good examples of this process. Density affects an objects ability to be conductive, which is why liquids are much better conductors than gases. This is the reason why we get colder much faster immersed in cool water, as opposed to standing in outside air temperatures that are the same.

Science of Radiant Heat

Thermal Convection

This occurs when fluids or gases transfer heat while they are being circulated from one area to another. Traditional heating systems that use forced air are perfect examples of this type of heat movement. Just picture the warm air that flows out of the vents in a room to understand this process. Unfortunately, air and other gases, as mentioned above, are poor conductors of heat.

Science of Radiant Heat

Thermal Radiation

Thermal radiation is heat that travels in invisible waves through empty space. It is not something that can be blown away by the wind or moved. It is simply absorbed by the person or object that is in the path of the beam of energy, and is a far more effective means of transferring heat. The effects of thermal radiation can be felt when standing in direct sunlight.

Radiant Heat Knowledge Guide

Components of a hydronic heating system

The heating emitter in a hydronic system is a closed-loop system comprised of:

  • Tubing made of a flexible polymer (typically a cross-linked polyethylene with an oxygen barrier) with spacing of 6 to 12” to a manifold for distribution.
  • Copper pipe or PP-R (polypropylene) to a low temperature radiator, baseboard heat or fancoil
  • Mechanical room piping, hi efficient condensing boilers needs a primary loop, low loss header or buffer tank. The boiler exchanger is at a higher pressure than the rest of the system and therefore requires a point of no pressure change to the zoning (secondary side).
  • There is a greater pressure loss within the boiler exchanger than the rest of system therefore need a point of no pressure change to the zoning (secondary side).

System circulation and actuation

The system circulates a mixture of hot water and propylene glycol (antifreeze), similar to the mixture used in a car radiator. The fluid is warmed to temperatures of 85F to 180 F to provide sufficient heat for the heating emitter required.

Pumps or a combination of pumps, zone valves or actuators are used to circulate to each zone via a control system (thermostat and zone controller).

Hydronic Radiant Heat vs Forced Air Heat

Radiant Heat Boiler Forced Air Furnace
Warms the objects in a room Heats the air in a room
Consistent, uniform heat Hot & cold spots (stratification)
Conduction & radiant heat transfer Convection heat transfer
Quiet Fan noise
Freedom to design room layout Registers required at each window
High energy efficieny & lower utility costs Medium energy efficiency & higher utility costs
Hypo-allergenic Low dust control
Higher installation cost, lower life-cycle cost Lower installation cost, higher life-cycle cost