Two popular statements exist about electrical heating:

  • It is electric: so it must be expensive;
  • All Electrical heating is classified as 100% efficient because “A kilowatt in equals a kilowatt (of heat) out and the law of conservation of energy states you cannot lose energy”.

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

Unfortunately two flaws exist in this argument:

  • Most electric heaters do not convert 100% of their input energy into heat.
  • A kilowatt of convected heat has substantially poorer heat transfer properties than a kilowatt of radiated heat and is more prone to loss by air circulation. So whilst both units of output are a kilowatt, their physical heat transfer properties differ markedly. You don’t need as many kilowatts of Infrared heat and you don’t need to run the heaters as long.

Most electric heaters cannot convert 100% of their energy into heat:

Perhaps the most obvious example of this is to consider a forced convection (fan assisted) heater in which an amount of the input energy is used to power the fan, rather than produce heat. However resistance based heating elements can also vary widely in their conversion of input energy into output heat. Factors which contribute to the actual conversion efficiency of a resistive element that absorb energy without producing heat include:

  • combination of metals used in the wire (a combination of conduction and resistance are required. Top efficiency cannot be maintained over all temperature ranges);
  • number of connections and their own electrical efficiency;
  • oxidation / corrosion of the wire over time;
  • the gauge of the wire and the length of the wire;
  • expansion, vibration and production of noise in the wire;
  • electromagnetic emissions which are not heat producing (e.g. light and radio waves);
  • on-off cycling of the current (e.g. according to a timer or thermostat) which produce different combinations of current (electricity flow) versus resistance (production of heat) at different times of the cycle.

The science of producing resistive-based elements has progressed considerably since electric heaters were originally deemed already 100% efficient 40 or more years ago. Efficiency is a target that all manufacturers will strive to improve-upon, but cost of materials and production techniques (and indeed operating conditions) weigh heavily against producing the perfect element and consequently considerable difference still exists between heater elements.

Herschel Infrared heaters operate in the high 90% efficiency range and this is verifyable by analysis of the radiant output of the panel versus the power being consumed. It is also a considerable advantage that Herschel Infrared heaters are designed to work only at one temperature – for which the internal architecture can be optimised.

Different heat transfer properties between Convection and Radiant heat:

See our more detailed topic on comparison of convected heat versus radiant heat. However in summary it is possible to say:

  • Radiant heat has a higher rate of heat transfer per kilowatt than heated air. This means you require less capacity to bring something up to temperature and lower running times to keep it there.  This is one of the key reasons that radiant heat is preferred over convected heat in manufacturing processes.
  • Radiant heat establishes “thermal mass” in a room quicker and more effectively that heated air. The room becomes a 360° radiator.
  • You can “zone” radiant heat to heat only one place if you want. If you heat air, you have to heat all of it.
  • Control over radiant heat is more accurate and precise than control over heated air.

So whilst the law of conservation of energy in itself is a truism, it is incorrectly applied when considering both “a kilowatt in equals a kilowatt out” (varying amounts of input energy don’t end up as heat) and when comparing a kilowatt of convected heat to a kilowatt of radiated heat (have markedly different heat transfer properties).

Practical comparison:

A practical comparison of the above in a real-world scenario is useful. 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.

Electric heater types

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 (one kilowatt consumed definitely does not result in one kilowatt of heat output). 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 complete mis-nomer, as at such low surface temperature, most heat is convected, very little is radiated. (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 primarily convective and 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 pence.

Herschel Far Infrared (panels)

Produces heat in the upper/mid far infrared (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, running times and cost. Why?

A kilowatt of heat produced from each different type of heater whilst implying the same electrical equivalence, 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 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 in equals a kilowatt out” is a meaningless statement. “A ton is a ton” is the same sort of truism – but it could be coal or feathers.

 

Publisher: Herschel Far Infrared