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Maintaining thermal comfort

Panel location can significantly affect the magnitude and distribution of room surface temperatures (MRT) and thereby affect required heater capacity necessary to achieve a given comfort level. When units are properly-sized and located, a higher MRT for the occupants is produced which then permits a lower air temperature for equal comfort conditions.

However, if the radiant heat is too concentrated such that the asymmetric temperature (difference between the plane radiant temperatures of the opposite sides of a small plane element is too much felt by the occupant then (local) discomfort occurs. Normally, discomfort should not be experienced by occupants in spaces heated by radiant systems if thermal comfort equations are satisfied and the asymmetric temperature is limited to 9°C .

Energy efficiency

Radiative transfer between the occupant and surrounding surfaces benefits from the difference in the fourth power of the temperatures as compared to the heat exchange by convection between the occupant and the adjacent air, which varies linearly with temperature difference. A study made by Kilkis (1992) showed that radiant heating can also increase the efficiency of a heat pump system. Zmeureanu et al. (1988) found out that the heat load and peak load of a radiant heating system was lower (77% and 80%, respectively) than conventional systems at the same level of thermal comfort. Since part of the sensible thermal load is handled by radiant ceiling panels, volume of supplied air can be reduced which in turn can reduce air transport energy (by 20%). This saving reflects a total energy consumption of 10% less than a conventional convective system. Further savings can be benefited with the use of radiant heaters by means of installing fast-acting surface
mounted-radiant panels. Watson et al. (1998) used a multi-sized ceiling-mounted radiant heater with higher watt density of 50 W/ft2 sized to the nearest 100 W of heated area and found significantly lower retrofit installed and maintenance costs compared to other types of heaters.

However, since radiant heating systems heat surfaces instead of the air in the room, higher surface temperatures (wall, floor, glass) occur and produce greater heat losses through the surfaces to the outside (transmission losses). This can be compensated by ensuring that the heated space is well-insulated.

Reduced air temperature gradient

Since radiant heating systems heat surfaces, there is very little air motion resulting in a more uniform room air temperature distribution. This can lead to a more uniform distribution of thermal comfort (in terms of PMV values) within the occupied zone and reduction of energy requirements.

Healthier air

Utilisation of thermal radiation to condition air reduces the dependency on air as the thermal transport mechanism while passing indoor air quality requirements. Thus, allergens (e.g. mold spores, dust, insects, pollens) and disease-causing microorganisms usually carried by the heated air medium can be reduced if not totally avoided. This advantage gives radiant heating systems an edge to wider range of applications, from residential and commercial buildings to buildings requiring higher indoor hygiene (e.g. hospitals, clinics, nursing homes, etc.).

Convenient operation

Complications attributed to circulating high volumes of air (e.g. more wiring, pipes, ducts and other installations) are avoided with radiant heating systems.

Efficiency of space use

The space consumed by a radiant heating system, be it hydronic or electrical, is
less than that of a variable-air-volume (VAV) system.

Zoning

Radiant heating panels can be installed in such a way as to provide zoning or conveniently placed in a location that needs radiant compensation (Simmonds, 1996).

REFERENCES

Dudkiewicz, E. and Jezowiecki, J. 2009. Measured radiant thermal fields in industrial spaces served by high intensity IR. Energy and Buildings 41 (2009): 27-35.

Kilkis, B.I. 1992. Enhancement of heat pump performance using radiant floor heating systems. AES 28: 119-127.

Simmonds, P. 1996. Practical applications of radiant heating & cooling to maintain comfort conditions. T ASHRAE 102 (1): 659-666 .

Watson, R.D., Chapman, K.S., and DeGreef, J.M. 1998. Case study: seven-system analysis of thermal comfort and energy use for a fast-acting radiant heating system. T ASHRAE 104 (1): 1106-1111 .

Zmeureanu, R., Fazio, P.P., and Haghighat, F. 1988. Thermal Performance of Radiant Heating Panels. T ASHRAE 94(2): 13-27.