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Radiant Heat Transfer v Convection Heat Transfer

This page explains the fundamental comparisons between radiant heat v convection heat, and the transfer of  the heat generated. Comparing 2 heating systems: the traditional pumped hot water filled convection Central heating system versus an Electric Radiant Heat heating system.

A kilowatt of energy does not produce the same amount of Heat Transfer, this is because each type of heating system has different heat transfer properties. The system with the lowest heat transfer capabilities will require increased capacity and will take much longer to reach the required target temperature than the system with the highest heat transfer capability.  These factors impact on the amount of energy used, and therefore the costs.

Conduction  | Convection  | Radiant Heat

Conduction: Not suitable for space heating. Conduction is very efficient as the heat transfer is by direct contact for example Saucepan on the Hob.

Convection: Currently the commonest form of space heating, the transfer of heat by the movement of  liquid or gas using air as the medium. Convection occurs when air (or liquid) is heated, the hot air is less dense & lighter than cold air, and as such rises from its point of origin. This hot air cools as it moves away from the heat source, following the process of cooling the air becomes more dense/heavier and falls back to ground level. This creates the convection current. In the traditional Central Heating system therefore, hot water is heated in the boiler, pumped throughout the system incorporating pipes and radiators. The heated radiator transfers its heat to the air which then travels by convection up to the ceiling, cools and returns cold to the floor.

Disadvantages of Convection. Heat Transfer; Convection vs Radiant Heat

Air movement cannot be easily controlled, heat transfer moves from hot to cold. If a draft is created by door opening, or poor window fitting warm, air will be sucked or blown away.

Convection currents only move upwards. So apart from the radiant heat transferred from the surface of the radiator, no benefits can be experienced from the sides of the radiator.

Air does not absorb or transfer heat well, it is difficult to zone as it disperses, and rapidly cools when the thermostats switch off the radiators.

Radiant Heat: The transfer of heat through electromagnetic radiation (See Infrared Wave Analysis)

This system has considerable advantages over convected heat for comfort heating because:

  • The rate of flow of radiant heat energy across a given area (flux) from an infrared heater is much higher than the equivalent output convection radiator. Therefore with an IR heater less energy is used for less time to achieve the same heating result as a traditional convection radiator.
  • IR heater heats objects, not the air between objects. The heated surface area of these objects transfer radiant heat, creating a fully heated environment. This effect is minimal with the convection system.
  • Residual energy within heated objects is retained much more than heated air, which cools rapidly.
  • Thermostats may be set at a lower temperatures with IR heaters as the environment heats up first not the air.
  • Minimal convection current with radiant heating meaning very few airborne allergens. Clean air for maximum Comfort Heat.
Kilowatt Comparisons. Heat Transfer; Convection vs Radiant Heating

We are charged per kiloWatt of energy, however this does not mean we get the same environmental heating effect regardless of the type of heat transfer method.

Here we compare 4 different types of  heater, each rated at 1kW.

Convection Panel Radiator

Wet CH radiator output diagram.Heat Transfer; Convection vs Radiant Heating

A traditional water filled radiator panel has a BTU (British Thermal Unit) value rather than a Kilowatt value; 1 kW is approximately 3400 BTUs.
The 900mm x 620mm (0.55m2) single panel convector (3433 BTUs) above is the closest match to a 1kW radiant heater. This panel has a front area of 0.55m2 and at 60C will radiate 311 Watts at 8 – 15 microns (i.e. cool- see infrared wave analysis) and convect the remainder (689 Watts). When estimating rooms for the size of convection panel radiator, it is considered that 40 watts is required per cubic metre. Thus a 1 kW convection heater will heat a room 25 m3, or approx 3mW x 4mL x 2.1mH.

Far Infrared Panel Heater

Radiant Heat v Convection Heat: Herschel Infrared Panel output diagram.

A Herschel Far Infrared panel of 1m2 area at 90C radiates 0.9kW at 5-12 microns at the panel surface. The panel’s larger radiating surface emits up to 40% higher temperature for the same kilowatt input. The radiant heat is felt directly on the skin in the “red” zone in front of the heater. Heat will emanate out, to heat the building or objects in the orange portion of the heat bloom.
Heat transfer occurs as the emitted radiant energy meets objects which absorb the energy. This is a more efficient form of heat transfer compared with convection, because a very high percentage of energy is transferred into target objects which retain the heat instead of into the surrounding air which looses heat rapidly.
Consequently a lower capacity IR heater is required, and runs for a shorter time.

When estimating rooms for the size of a Far Infrared Panel, it is considered that 25 watts is required per cubic metre. Thus a 1 kW Far infrared panel heater will heat a room 40 m3, or approx 4.5mW x 4.2mL x 2.1mH.  Note that a 625 Watt Far Infrared panel heater will heat a 25 m3 room which requires a 1kW convection panel radiator.

Medium Wave Infrared Space Heater

Radiant heat v convection het: Herschel Advantage IR2 output diagram.

The Herschel Advantage IR2 space heater comprises 2 x 650W ceramic emitters giving a total output of 1.3kW, emitting 400C at 2 – 10 microns. (8 micron bandwidth). See Infrared Wavelength Analysis . This space heater is designed to heat people in open spaces up to 9 square meters, from 2.3 to 2.5m away. The heater runs at high temperatures, as its intended use is in open spaces such as  a warehouse/workshop/packing station where draughts and vehicle movements have to be compensated for. It is a very efficient heater in certain space heating applications where the above heaters would not be effective.

Shortwave Quartz Infrared Space Heater

Radiant heat. Quartz IR heater output diagram.

A 1kW quartz heater emits 1500C at 1-2 microns. (1 micron bandwidth) or shortwave. 1-2 microns is extremely hot (1500C) and is suitable for cooking meat and welding plastic. This type of heater is particulary useful where you need to throw heat, for example in outside patio & terrace areas where there is a lot of movement and weather. These types of heater would not be appropriate in an enclosed or small room, as the heat would be too intense  and uncomfortable.

Conclusion: Heat Transfer; Convection vs Radiant Heating

There are a wide range of physical properties created by different types of heater rated at 1kW.  Heaters are optimised by manufacturers to suit certain situations. Our website is designed to guide you to the correct choice! Poor selection results in the inefficiency of the use of that energy. Choose wisely. There is an infrared heater for every application. If you require any advice, please contact us by e mail or telephone and we will happy to take your call.