Personal space heaters work by either heating the surrounding air mass or by using radiant energy to heat people and objects, or via a mixture of both modes. This heating tutorial goes over the pros and cons of convection versus radiant heating. This page looks at different types of heaters.
Discussing different types of heaters can be a bit confusing; heaters can be classified based on how they heat but also their physical configuration. Panel heaters are a case in point. A panel heater may be a convection heater or a radiant heater. The thing both types will have in common is that they look like a flat panel.
Difficulties aside, we will attempt to describe the major types of personal heaters in use today:
• Compact Radiant Panel Heaters;
• Micathermic Radiant Panel Heaters;
• Ceramic PTC Heaters;
• Heated Floor Mats;
• Oil Heaters;
• Panel/Convection Heaters;
• Fan Heaters;
• Traditional Radiant Heaters;
• Portable Reverse Cycle Air Conditioner Systems.
Compact Radiant Panel Heaters
Radiant heaters generated focused infrared energy that heat the user directly rather than the surrounding air. Since they do not try to heat a large volume of air, radiant heaters avoid the high-energy losses that occur when convection heaters are used in large or draughty spaces. Radiant heating is efficient at heating people. Modern compact panel heaters and infrared heaters are much safer than traditional high temperature radiant heaters (described below) such as bar radiators, strip heaters and quartz infrared heaters.
Compact radiant panel heaters are small enough to be placed under desks (make sure to provide a safe clearance around the heater). These panel heaters have a low energy density. This means that it can be positioned close to the user without risk. The result is that most of the energy output provides comfort for the user. The high temperature of a strip heater means it has to be placed some distance from the user. Much of the energy from a strip heater warms up nearby furniture and other objects rather than the user.
Compact heaters work by using a simple resistance element. This element is backed with a material which has reflective properties. It is then given an insulation layer so that it projects infrared heat on one side only of the heater. The heating element is not substantial in size and as a result it only takes a few minutes to get fully hot (this is generally much faster than that achieved by a conventional oil radiator).
As these heaters have a low wattage they do not generally need to have a built in thermostat. Typically an on=off switch is all they need. A case could be made for building a timer into these types of heater to counter people who forget to turn them off at night. Otherwise wise they are a very efficient form of heating.
Micathermic Radiant Panel Heaters
These heaters tend to be larger than compact panel heaters mentioned above. A typical size might be approximately 600mm long and 600 wide and 60–70mm thick. Maximum energy output of the model chosen for single person application is usually around 1000W.
A micathermic heater generates an energy density of in the region of 2500W/m². This is actually more than the panel heaters discussed above. However, it is a lot less that from other forms of heater.
As with the compact panel heaters a micathermic panel heater is able to be positioned near the user. In doing so they are good at providing quality and efficient individual heating.
Micathermic radiant panel heaters use a special technology, second generation micathermic II to direct infrared heat in one particular direction. They are efficient in that they are able to turn approximately 80% of the input energy into radiant heat, The remaining 20% creates convection heat.
Once switched on micathermic heaters can reach maximum temperature output in a short time, as little as a few minutes.
A micathermic panel heater will normally come with several temperature settings. A thermostat is usually incorporated into the heater due to the convection element to the heating provided.
The heaters also tend to incorporate a number of safety features such as the guarding against overheating or damage done to the surroundings should they be tipped over. A micathermic will often come with a timer.
Micathermic heating elements can be arranged to create to column heaters or other forms of double-sided radiant heater. A double-sided heater could be useful if two people were sitting side by side at a desk or seating area. The column heater spreads the infrared rays across a wide zone. This could be helpful if a person is moving around in a workshop or small office.
Ceramic PTC Heaters
A typical ceramic PTC heater has a ceramic heating element and a fan, usually with a couple of heat and fan speed settings.
The primary heating mode is via convection and so they have the inherent weaknesses of convection heating in large draughty spaces.
The heating element in a ceramic heater is made from titanate ceramic (with additional ingredients to achieve the design specifications – see below). The ceramic element can be configured into a honeycomb structure or otherwise designed so that a relatively large surface area is presented to the airflow created by the fan. Ceramic elements heat up quickly which means there is little delay before the heater delivers warm air (unlike an oil heater).
PTC materials experience an increase in electric resistance when their temperature is raised (PTC stands for positive temperature coefficient). PTC materials can be made into self-regulating heating elements that in effect switch off the current when a pre-set temperature is achieved.
By using the right combinations of materials, PTC heating elements can be made operate efficiently at temperatures below the ignition temperature of highly flammable materials, like tissue paper, and turn off very quickly once this temperature is reached.
As a result they can be made to be inherently safer than conventional fan and convection heaters (see below). The risk of fire or a burn injury is reduced, but common sense is still needed when using these heaters.
Fan heaters have a slight advantage over other convection heaters in that they direct a warm of stream air at the user. This stream of warm air can provide some useful heating (or reduction in body heat loss) for the user before going on to generally heat the air around them. The subsequent general heating can, however, be inefficient due to temperature stratification and losses from such causes as draughts.
If a fan heater is used at all in a space, then the preferred type of fan heater is a ceramic PTC heater designed to keep its element temperature below the ignition temperature of highly flammable materials like tissue paper. If you use a fan heater make sure that it has this feature (not all "ceramic" heaters do). It should also have tip over protection and a thermostat.
A ceramic fan PTC heater might be suited to a small room or an office that can contain the warm air produced and not lose excessive heat, namely a room that:
• is free of draughts (no faulty windows or doors);
• does not have a large amount of glass or window area (these cause heat loss);
• has an overhead fan that can be set to winter mode to disrupt temperature stratification.
One of the problems with ceramic heaters and other convection or fan heaters is that they are overpowered resulting in excessive load on circuits. Users also overheat their rooms – to temperature that would be a cause of complaint (too hot) in summer.
Ceramic heaters should only have a moderate heat output. There is at least one model on the market with 1800W maximum output and a 900W setting. In all cases the thermostat should be set to achieve a reasonable room temperature -20 to 21°C. The heater should not be left on whilst unattended.
Heated Floor Mats
These consist of a low wattage (~70W) heating element sandwiched between two robust carpet squares. They normally have over-heat shut-off and often come with two energy settings. Heating mats provide radiant and conductive (contact) heating.
They are very useful in ground or basement spaces where the concrete floor slab will quite often be cold. They can neutralise the tendency of the concrete to draw heat from the occupant's feet. In less challenging environments they can warm feet, especially if the occupant can/is willing to remove their shoes.
One shortcoming of the heated floor mats arises when people are using foot stands. It can be hard to secure the mat to the top of the foot stand. Often the mat is larger than the stand and it is undesirable to have the mat overhang the edge of the stand. We are trying to source smaller mats to deal with these issues (remember, to be effective a floor mat only has to be big enough to cover the area where you place your feet).
Heated floor mats are a low tech, low energy heating technology. They are efficient at achieving their intended purpose, namely countering cold feet. They can be used on their own if cold feet are the only problem or in combination with other sources of heating such as compact or micathermic radiant heating panels.
Oil Heaters
Also known as oil filled column heaters or oil filled radiators. These have a hollow steel shell body with fins. The body is filled with oil which surrounds an electric resistance element. The element heats the oil which in turn heats the body and fins. Air is warmed as it rises past the fins. The main heating mode is convection.
A small amount of radiant heat can be felt at very close range. Some oil heaters are enclosed in cowl or panel structure and have a fan to speed up the transfer of heat to a room (as a result, modern oil heaters are increasingly looking like convection panel heaters).
While modern oil heaters may have features like a fan, thermostat and timer, they still have a number of serious shortcomings. The problem most people experience is that they take a long time before they deliver any useful heat. When they are turned off at the end of the day they continue to transfer heat to the room until they cool down. In other words they do not heat you as soon as you want and they heat the room when you are not there. They are not efficient.
The only reason they have any currency is because they are a relatively low temperature form of heating and, compared with a bar radiator, they are viewed as reasonably safe.
Oil heaters heat mostly via convection and therefore have all the problems of convection heating. People often use them in large rooms or draughty situations where they are ineffective. To overcome ineffective performance, people huddle right beside the heater and try to use it as a radiant heater.
While oil heaters are often sold as "oil filled radiators" they are very inefficient as radiant heaters. A compact or micathermic radiant panel heaters will do a much better job.
Partly in response to their ineffectiveness, people buy high wattage oil heaters thinking big is better. The large current draw of high wattage heaters can trip circuit breakers and cause inconvenience to others.
Panel and Convection Heaters
These typically have electric resistance wires that directly heat the surrounding air. There are usually grill openings at the top and bottom of the heater to allow air to flow (hot air rises principle) across the heating wires.
The sides of the heater are often flat sheet metal (hence the use of the term panel heater). Heat exchange usually takes place over a fairly long length of heating element (combined with a bar radiator). This combined with the generous clearance between the heating element and the panel sides prevent the sheet metal from becoming too hot.
The main heating mode is convection although a small amount of radiant heat may be felt at close range. These heaters may also have a fan that blows air vertically through the heater to speed up heat transfer.
Panel heater heat mostly via convection and therefore have all the problems of convection heating. People often use them in large rooms or draughty situations where they are ineffective.
To overcome ineffective performance, people huddle right beside the heater (as with oil heaters) and try to use it as a radiant heater. Once again, panel heaters are designed for convection heating and are inefficient as radiant heaters.
While modern panel heaters will usually have thermostats and often have timers, they are not very effective in large rooms and draughty spaces. In spite of tip over protection and overheat protection, they can still create a fire risk and particular care must be taken to ensure that the top of the heater is never obstructed (by an item of clothing, sheet of paper etc).
Like oil heaters panel heaters are often over-sized in order to overcome their weaknesses. In technical terms they are sized beyond the point of diminishing returns. A lower wattage heater would provide a better balance of costs (including the problems they create) and benefits (comfort). In the few places where a personal convection heater may be OK, a ceramic PTC fan heater would be a better option.
Fan Heaters
Fan heaters are quite compact. The conventional types have a fan that blows air across high temperature heating wires (ceramic PTC fan heaters are discussed above). The air typically exits the fan almost horizontally so that warm air can be directed onto the user, typically onto their feet and legs.
The air is usually heated to a higher temperature than in a panel heater. If the air is hot enough (above 37°C) then a degree of heat conduction occurs as the air flows over the user's body.
The main effect of fan heaters, however, is to heat the air in room and consequently they have the same shortcomings as oil heaters and panel/convection heaters. Blowing the warm air horizontally (almost) might mean that fan heaters mix the air in the room a bit better than the other convection heaters. If so then this is a slight plus in their favour.
In the few places where convection heating may be OK, a ceramic PTC fan heater would be better (safer) than a conventional fan heater.
Traditional Radiant Heaters
Also known as strip heaters, bar radiators or infrared quartz heaters. These have long tube like quartz elements (typically one to four in number) that get very hot, glow red and radiate infrared energy.
They invariably have a reflector behind the element(s) to increase outgoing radiation and a grill in front of the element to reduce the risk of objects (and hand/feet/clothes/magazines) coming into contact with the element and burning. This grill is usually very hot itself but it mitigates the burning problem. The main heating mode is radiant.
Parts of the heater can get quite hot and these areas heat the air directly and produce a degree of convection heating.
Overtime the reflector element often becomes corroded or the chrome surface peels off. This causes the radiant efficiency of the heater to drop away. Consequently the heater casing often gets hotter and more of the energy is transmitted as convection rather than infrared radiation.
Bar radiators are an intense heat source (10,000 – 15,000 W/m² of reflector surface) and most people find they need to place them at a distance (a meter or more away) to avoid getting too hot. The radiant heat output is strong enough to ignite objects in front of the heater (in other words an object does not have to touch the grill or quartz infrared element.
Bar radiators were once a popular form of heating due to the quick heat up time, cozy glow and ability to heat people in draughty rooms. Safer radiant heating alternatives now exist and bar radiators are widely recognized as dangerous form of personal heating that has had its day.
There is still a lingering role for traditional radiant heaters. In some applications they can be seen fitted high up on a wall away from people or the risk of contact with inflammable materials. You will see them used in this situation in sports changing rooms and practice areas, workshops and sometimes receptions or other large spaces. Even in these areas better radiant heating technologies are available in the form of underfloor and ceiling radiant heating.
Portable Reverse Cycle Air Conditioning
Reverse cycle portable air conditioners are a modern variation on the window rattlers (aircon units installed through windows or walls). They are called "reverse cycle" because they can cool in summer and then be reversed so that they heat in winter.
They use the refrigeration cycle whereby a liquid under pressure is allowed first vaporize and is then recompressed back to a liquid. When the liquid vaporizes it absorbs heat from its surroundings. When it is compressed back to a liquid it gets hot enough to reject heat to its surroundings. A reverse cycle aircon unit can take heat from a room in summer and dump it outside. In winter it can be reversed so that it takes heat from outside and then brings it inside (even on a cold day the outside air contains heat).
Modern reverse cycle aircon units are freestanding appliances that be placed fully within the room rather than with part of the unit sticking through a wall opening (like the window rattlers). They need a connection to the outside air though. This is usually achieved via a length of ducting connected to a fairly discreet fitting that sits below the window sash (there are various conversion panels for different types of window).
As part of their normal operation the reverse aircon units condense moisture from the air. Some modern units get rid of this water by using surplus heat from the cycle to evaporate the water. A fan blows the moist air out through the duct. This approach actually improves the overall efficiency of the refrigeration cycle. Other units require a drain for the water or have a storage reservoir that requires regularly emptying.
One of the features of the refrigeration cycle is that more useful energy can be obtained from an air conditioning system then goes in as electrical energy. This is because the electrical energy is used to pump heat that is already present from one place to another. This multiplier effect can be significant - x2 or more in heating mode. So why isn’t more made of portable reverse cycle air conditioning units? There are several reasons:
1. They are many times more expensive to buy (x 5 for the same heat output) and more costly to install than other types of personal heater;
2. Practical problems often arise with installation especially with finding an opening for the duct outlet;
3. Their heating mode is via convection and this is not efficient in large, tall or draughty work spaces;
4. The smallest portable reverse cycle unit with auto water removal (4000w heating capacity) is often overkill for rooms where convection heating could be realistically considered; and
5. Since they can be used for summer air conditioning, it is quite likely they would end up being used for this purpose even if the thermal comfort of the space without air conditioning was satisfactory.
The energy efficiency of reverse cycle air conditioning heating would be quickly wiped out by summer use. This factor and cost considerations rule this technology out for general application.
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