Traditional electric heating compared with Herschel Infrared

Three popular statements abound about electrical heating:

• It is electric: so it must be expensive;

• All Electrical heating is already classified as 100% efficient because…

• … “A kilowatt of heat in is a kilowatt of heat out” (which isn’t true and overlooks how well you use that kilowatt output)

• … “There is no difference between a kilowatt of convected heat versus a kilowatt of radiated heat” (this also is not true and is similar to comparing a ton of coal with a ton of feathers. The unit of measure is the same but the statement ignores all other relevant physical properties such as heat wavelength, watt density, temperature and transmissivity, all of which will vary considerably within a given kilowatt to produce very different types of heat and transmission effectiveness).

Current building specification regulations, for example, consider all types of electrical heating as a single category in terms of efficiency because all electric heaters are already 100% efficient. Is this position correct?

In a simple domestic or office room of 60m3, traditional popular types of electric heater that could be used are:

• Bar fires
• Fan heaters
• Electric radiators (mobile or fixed)
• Storage heaters.

Let’s look at the comfort and energy characteristics of each of these heaters in this 60m3 space.

Bar fire:

Produces heat in the medium infrared band (typically 400 – 600C, 2-4 microns) using a reflector to direct energy in one direction. Heat transfer method is therefore radiant. This configuration implies high intensity heat close to the heater relying on distance to decrease its intensity to “comfort” levels and gradually warm-up objects in the target area. Areas outside the heating arc remain cold. The classic use of these heaters was in churches and public halls and as top-up heating in old, cold houses. The “Long throw, narrow spread” of bar fires works well for large distances but not for covering wide areas unless many heaters are used. These characteristics make these inappropriate in more general, domestic comfort use although their radiant heat effect implies lower overall running time (as residual heat remains when the heater is turned off).

Daily requirement for comfort heating a 60m3 room:
2.4 kilowatts running 5 hours a day (thermostat mandatory in comfort situation).
Approximate daily cost = 168 pence.

Fan Heater

Produces heat in same medium infrared band as a bar fire, but at less wattage and instead of radiating the heat, a fan is used to direct heat in one direction. Heat transfer method is therefore convection. This configuration implies less input energy to produce the heat, but more energy to blow it. Comfort levels are acceptable nearer the heater than with bar fires, but close proximity can still become irritating. There is poor warming of objects within the heat arc (convected heat is not radiant and heats objects in its path poorly) and areas outside the heating arc also remain cold. Fan heaters consequently require more energy and longer overall running time than bar fires, as no residual heat remains when the heater is turned off. Classic domestic use is as top-up heating directly aimed at a specific individual. Larger heaters are better applied for zone heating (e.g. in an office).

Daily requirement for comfort heating a 60m3 room:
3 kilowatt heater running constantly through the day (8 – 10 hours) or higher wattage heater running more sporadically.
Approximate daily cost = 378 pence

Electric radiator

Produces heat in the lower far infrared band (60C, 8-15 microns). Although these are called “radiators”, this is a mis-nomer, as at such low surface temperature relative to human body temperature, most “useful” heat comes from warming the air and very little is perceived by the body as radiant. (For a 1kW panel of 0.55m2 surface, the convection/radiation ratio is approximately 69:31). Heat transfer is therefore primarily convection with little spread or throw of radiation or long-term warming of objects. However, comfort levels are acceptable very close to the “radiator” and warming effect is not directional, unlike the above two: making for a better comfort heater.

Daily requirement for comfort heating a 60m3 room:
2.4 kilowatts running 8-10 hours a day.
Approximate daily cost = 302 pence

Storage heater

Produces high intensity (medium infrared) heat to warm up a thermal brick during cheap overnight rates, for gradual release during the subsequent day. Heat production is therefore radiant, although some modern units have fan-assistance to help disperse heat more evenly. Close proximity to storage heaters can be uncomfortable and a clear zone is also required in the immediate vicinity of the heater both for comfort, safety and proper function of the heater.

Daily requirement for comfort heating a 60m3 room:
3.1 kilowatts running 7 hours at overnight rates.
Approxmiate daily cost = 173.6

Herschel Far Infrared (panels)

Produces heat in the upper far infrared range (100°C, 5-12 microns) wavelengths, the heat transfer method is radiant, with negligible convection and low-enough “watt density” of heat to create acceptable comfort levels close to the radiator panel, with a non-directional “spread and throw” of radiated heat to objects in the room. This allows the whole room itself to warm up over time with overall temperature and running time controlled by a thermostat.

Daily requirement for comfort heating a 60m3 room:
1.5 kilowatts running 5 hours a day (if governed by a thermostat).
Approximate daily cost = 105 pence.

General principles established:

For a fixed size of room, each of the different heating characteristics of the above heaters implies significant differences in wattages of heater, runing times and cost. Why?

A kilowatt of heat produced from each different type of heater whilst implying the same electrical unit of measure (e.g. a kilowatt), do not produce the same mechanical equivalence in terms of:

• heat Intensity Vs spread and throw (high intensity, narrow zone vs low intensity wide zone: both equal a “kilowatt” but with very different mechanical properties);
• heat transfer method (radiant, convected or a combination of both);
• resulting levels of comfort;
• resulting operating times and therefore cost.

Radiant-based heater types seen in the above examples (Bar fire, storage and Herschel Infrared panel) are always cheaper to run than any unit using convection as their heat transfer method – so long as proper controls are also used. This is because of the long-term warming effect of objects in the environment allowing either lower wattage or shorter running times or a combination of both.

A room of 60m3 is not small, but is not massive either – being  5m x 5m with a ceiling of 2.5m and therefore easily a domestic living room or small office room.  In a room of such proportions, the solution that provides the greatest levels of comfort – without compromising by creating clear areas to avoid the heater – and which provides an effective level of radiance without requiring excessive wattage or any form of convection assistance – and resulting in the least wattage for the least operating time, appears to be Herschel Far Infrared.

Returning to where we started:

• Electrical heating doesn’t have to be expensive;

• The treatment of all electrical heater types as a single group that are all 100% efficient is quite clearly wrong;

• A kilowatt of heat comprises a wavelength (temperature) and flux (spread and throw). Different combinations of which produce very different mechanical effects. “A kilowatt of heat is a kilowatt of heat” is a meaningless statement. “A ton is a ton” is the same sort of truism – but it could be coal or feathers.