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Common Radiant Floor Heating Terms Explained

Common Radiant Floor Heating Terms Explained

13/7/2006

This article covers the following terms and what they mean for choosing radiant floor heating for your home; "Wattage Density", "Duty Cycle" and "Heat Sink".

Wattage Density

The wattage density is the number of Watts of power drawn per square metre of flooring.

It is actually better to have a radiant floor heating system high in wattage, rather than low. This is because a high-density system will achieve room comfort temperature levels quicker (offsetting the higher wattage) and still only consume energy relative to the heat loss of the room thus achieving efficient operation. Another reason for this is explained below in under Duty Cycle.

As a guide, the following are suggested values for wattage densities.

Living Areas

100 W/m2 for background radiant floor heating (using another heat system for comfort).

120 - 150 W/m2 for primary, comfort heating levels. (No backup system is required).

Bathroom / Wet Areas

160 - 180 W/m2 (must be controlled with floor probe to prevent floor temperature overshoot).

Duty Cycle

Duty Cycle is the proportion of time a system is drawing current opposed to being switched off.

A system lacking the grunt to reach the set temperature, will inevitably consume more energy as it may be unable to "Cycle"; especially during colder weather, where the heat loss of a room is higher.

Because of this, disastrous running costs have occurred in radiant floor heating systems where insufficient wattage has been applied. It's important to remember, you get what you pay for and the radiant floor heating industry is no exception.

Most people have little or no idea what specification is considered to be efficient. A well-specified system can achieve a low 30-35% duty cycle, which is a desirable level.

Performance will always be relative to variable factors such as external temperature, the energy efficiency or "Heat Loss" of the building itself and the internal temperature the system is being run at. The system must be able to offset the steady state heat loss of the building and in the case of radiant floor heating, also cope with it.

Heat Sink

Heat Sink is a phenomenon where heat energy is absorbed into approximately 55% of the depth of the concrete slab under a house.

This affects heat up time, which also depends on the thickness and type of floor covering and the moisture content in the concrete, which is naturally higher in a new construction.

It is common for the average new dwelling to contain six tonnes of excess moisture in the concrete slab. Thus, because of heat sink, a central heating system, be it Gas, Reverse Cycle A/C or Radiant Heating, can consume up to 1.6 times the normal energy rate during the first six to eight weeks of operation.