There are a wide range of personal or portable electric heaters available these days. Picking the right one can be a tricky task. We have created this site to help you pick the best space heater for your particular circumstances and needs. Different models claim to be more efficient, or in some way better than the others. This article explores how electric heaters work and, looks at the question of efficiency. It also examines the pros and cons of the two main forms of personal heating – convection and radiant heating.
How Heaters Work
Electricity is a common source of power to create heat in many different types of space heater. We look at the three main ways in which electricity is used to heat an individual or group of individuals in a particular space:
Convection heating
Convection heating is the process of heating the air around the heater. Air, when heated, either rises or gets moved with fans so that it mixes into the air around. This warm air then raises the overall temperature of the air making the room feel warmer to its occupants. The reduced temperature differential between the occupants of the room and the air in the room means that each person will lose less heat to the surrounding environment. These type of heaters include oil filled radiator heaters, PTC and fan forced heaters. Some radiant heaters heat the air, although this process tends to be indirect – see further discussion below.
Radiant Heating
If you move from a shady area into the sunlight you immediately feel the warmth from the sun. This warming is as a result of the sun’s radiant heat. The rays from the sun contain the full spectrum of light, including visible, ultraviolet and infrared light. It is the infrared light that heats. Infrared light is also used to generate heat in radiant heaters. These “light” waves travel outwards from the heat source, heating everything in their path, including importantly people in their path.
The energy from infrared light does not heat the air, but because in warming the objects surounding the heater they in turn become mini convection heaters transferring some of their heat into the air surrounding them.
Conductive Heating
Conductive heating occurs when the heat energy in an object is passed to another object in close proximity to it. As an example, if you were to place your hands on an oil filled radiator your hands would immediately feel warm. This warming is the heat transferring to your hands through convection.
Conductive heaters also include electric blankets in the home and heated car seats outside the home. Similarly conduction is the way in which you are warmed by a hot bath. One good type of personal heater is a heated floor mat. When placing your feet on such a mat some of the heating is as a result of conduction, however this does not tend to be a significant part of the total heat provided.
Do Heaters Warm People ?
This may seem like a strange question. Most heaters, especially oil heaters and other convection heaters, do not heat people at all. That is they do not transfer energy from the heater to the person. Instead they reduce the rate of heat loss a person experiences and this makes the person feel more comfortable. Radiant heaters are a bit different – they do put a bit of heat into a person’s body, and this can help the individual reach a comfortable thermal balance with their environment.
Check out our review of the best infrared space heaters
The human body has a number of mechanisms to maintain its temperature at around 37°C. If it is losing too much heat to the environment to maintain the target temperature in all parts of the body then it reacts to reduce the loss. Blood circulation to parts of the skin and extremities is reduced. Circulation to the feet is curtailed first and the next step is to reduce circulation to the hands. These changes make us feel cold. Generating more internal heat through shivering is also part of the body’s response to cold conditions (or more correctly – thermal imbalance).
Now if we sit in front of a fan heater the air being blown onto us feels warm. The air from the fan heater is not actually heating us unless its temperature is above 37°C when it strikes our body. Instead it is significantly reducing the heat loss from the part of our body in the warm air–stream. If this reduction is enough to compensate for losses from our head, our hands and elsewhere, then we feel comfortable.
Similarly an oil heater by raising the overall temperature of the air in a room reduces the rate at which we loose heat from our body. It does not actually heat us. We feel warm, but not because the air is warmer than our skin. We feel warm because our heat loss is reduced and our circulation system has allowed warm blood to come close to the surface of the skin.
This may all seem a bit academic but it is important background to understanding how we should deal with winter and the pros and cons of different heating methods (outlined below).
Heater Efficiencies
There are a lot of claims and counter claims about heater efficiencies. The issue to consider when talking about efficiency is “what is the heater efficient at doing or achieving”. All electric heaters are virtually 100% efficient at turning electrical energy into heat. It is what they do with the heat that matters. If the ultimate objective is to heat air then all portable heaters will eventually put their heat output into the surrounding air.
If the objective is to heat the air at a given point in a room to a specific temperature with least energy consumption then some differences between heaters are evident. These differences will revolve around whether the heater has an effective fan to mix the warm air from the heater with the cooler room air. Another point of difference will be the heater control strategy. How well can the heater detect the air temperature and maintain it within close limits?
Check out our review of the most efficient space heaters
Heater efficiency is often linked to how quickly a heater heats, or starts to heat the surrounding air mass. On this score a ceramic fan heater can be said to be more efficient than an oil heater. The large thermal mass of the oil heater means a long lag time between turning the heater on and getting some useful heat out. The ceramic heater delivers heat quickly. Using the speed with which a heater starts to generate useful heat as a criterion, a micathermic radiant panel heater would also be deemed highly efficient.
This leads to an important point; is the real objective to heat air? When dealing with an office environment and taking an energy efficiency perspective, the issue is “how can we make people comfortable with least consumption of electricity” (or alternatively – with minimum carbon footprint).
Heating the air in an office is only one way to make people comfortable (another way is to encourage people to dress appropriately for winter). An efficient heating solution is to use radiant heaters to heat people directly. These heaters reach their full infrared output within a minute or so. In draughty and poorly insulated rooms, they use less electricity to make a person comfortable than convection heaters. On this basis, radiant heaters could be said to be the most “efficient” form of portable heating.
The important point is that whenever you are confronted with an efficiency claim, ask “efficient at doing what?”
Convection Heating
Most homes are not air-conditioned and share similar characteristics, as a result portable convection heaters are not particularly energy efficient or effective. In order to raise the temperature sufficient to make a person comfortable they need to use a large amount of electricty. For larger rooms they are really unable to heat enough air quickly enough to make them practicable. They suffer from four major problems:
- Air stratification from temperature gradients;
- Too much energy is lost through your windows, your walls and the ceiling;
- Draughts causing resulting in warmed air to be lost; and
- Their inadequacy in heating larger spaces.
Air Stratification
Imagine a person sitting with a convection heater, an oil heater say, in their room. Convection heaters work by heating the air to a temperature that a person to feel comfortable. This approach generates a particular problem, that of temperature stratification. As we know, air warmed by the heater rises but this creates a vertical temperature gradient, which sounds complicated but simple means that the air near the ceiling is hotter than the air lower down, and more importantly the air near the person who needs warming up.
The convection heater will hopefully have a thermostat. This control may only be 3 ft above the floor. If a person tries to use the heater efficiently they will usually adjust the thermostat so that they are comfortable when seated. This means a band of air between 3 ft and 7 ft above the floor will feel to be a good temperature.
But unfortunately the air in the upper space in the room will be too hot and of little benefit. Finally at feet level the air will still be quite cold. People that use oil heaters will often have cold feet and legs. They deal with this by placing the heater right beside their legs in the hope of getting a bit of radiant heat from it. Oil heaters are inefficient at generating this type of heat.
Fan heaters and convection heaters with fans create a bit of local air mixing but stratification still develops and a band of hot air forms in the top half of the room. This is a problem because the band of high hot air exacerbates heat losses from the room through uninsulated walls and ceilings and single paned windows.
Overcoming Stratification
Air conditioners are used to try to negate this problem by using techniques which mix up the air in the room. Ironically air conditioner systems are actually better at this air mixing process in the summer months than they are able to do when the mixing is more necessary.
You can achieve a degree of air mixing in a non–air conditioned room by using a ceiling fan switched to winter mode. In winter mode the fan pushes air up and across the ceiling. The air then descends beside the walls and rises back up the fan. This mode generally does not create troublesome draughts for the room occupants (using a fan to get most of the air in room heated to a comfortable 70°F is a much more energy efficient proposition then having to heat over half the room to 77°F or more).
Of course the big problem with this is that not every home has air conditioning. In such a case there is little the average homeowner can do to overcome temperature stratification.
Major Energy Losses
You may live in an older building with higher ceilings. If so then it is likely that in order to achieve a comfortable temperature you will have to heat far more air than you would like. Using a fan can help to mitigate these problems old and indeed some newer building also have other problems that mean that such gains are rarely sufficient to fully overcome the main problem.
The warmer an air mass is the greater the rate of heat loss to the cold outside air. Most homes are insulated but still lose a significant amount of energy via ceilings, the walls and even windows, especially if they are single glazed. Some types of window coverings such as a Venetian or Roman blind, provides little in the way of insulation. If you have window frames made from metal you can also lose heat quickly through heat conduction. This heat loss can be significant.
All in all it remains a stubborn fact that if you have a poorly insulated building then convection heating is not a particularly efficient way to keep people in that building at a comfortable temperature.
Draughty Rooms and Homes
Many older buildings (and indeed some newer ones too) are quite poor at preventing draughts. Draughts allow warm from inside a room air leaks and gets replaced by air from outside which is much colder. You can mitigate some of this by closing your doors and repairing broken windows or catches as soon as you notice them. Fundamentally however, it should be clear that convection heating is not ideal for heating draughty areas.
Convection heaters do not do a particularly good job larger open areas and/or workshops. Due to many of the problems identified above a convection heater will often continue to heat the air because the air in the room never reaches the threshold necessary to trigger the thermostat. The warm air gets lost, leaking out from open areas or through opening doors whenever that occurs.
Check out our look at the best garage and workshop heaters
Often, those using oil-filled radiators or other convection heaters in such areas only really feel warm if they huddle up close to the heater. Elsewhere in the room the effect of these heaters is difficult to determine.
Radiant Heating
Traditional high temperature radiant heaters, called strip heaters, bar radiators or quartz infrared radiant heaters are often not suitable as personal or portable heaters in some homes.
These high temperature heaters typically have one or more very hot infrared elements backed by reflectors. Sometimes a radiant heater will contain other types of heating element (such as ceramic elements) and they may also have fans to create a degree of convection and well as radiant heating. The problem with all these types of heater is that the high temperature heat source is inherently dangerous and creates a fire risk and burn hazard.
There are modern safe radiant heating alternatives and these are described below. Generally they have three characteristics:
- Low intensity radiant source;
- Unidirectional radiation; and
- Efficiently converting electricity into radiant heat energy.
A typical modern radiant heater will be configured as a flat panel. They also tend to be thin, being between 2 to 6 inches thick. They tend to be larger in overall surface area than more traditional model of radiant heater. In addition the heat generated is not quite as intense. This means that when you touch such a heater you feel it as hot, but it should not be hot enough to burn you. The design of these panel heaters is usually such that the heat is created on one side of the heater only.
This allows convenient placement with the non-heating side being placed against a wall or other piece of furniture safely. With these types of heaters approximately 80% of the heat created is through infrared radiation. The remaining 20% or thereabouts is down to convection heating. Energy outputs range from around 160 W to over 1 kW. There are two main types of modern, safe radiant heaters:
- Compact radiant panel heaters
- Micathermic radiant panel heaters
Compact Radiant Panel Heaters
These heaters come in a range of sizes. A typically useful sized model is about 18 inches long and 15 inches tall. This should easily be able to warm the legs of a person who is sitting in a chair or at a desk. Most of the heat generated will be absorbed by the person sitting there and warm them effectively.
The energy output of a typical compact model is 160W. The energy density should be around the 75 W/ft² level, this compares with around 900–1,400W/ft² for many strip heaters. This means that a compact radiant panel heater is a lot safer to use.
Because they use a relatively small amount of energy they do not get overly hot to the touch. This allows them to be positioned close to the person needing heat without fear. Much of the heat generated is directed at this person, making them very efficient. 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.
Because they require a low wattage thermostats do not normally get included. Indeed all they really require is an on and off switch. 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 they are a very efficient form of heating.
Micathermic Panel Heaters
Micathermic panel heaters tend to be a bit larger than the compact panel heaters mentioned above. They are around 24 inches long and 24 inches wide and 2 inches thick. Maximum energy output of the model chosen for single person application may be 1000 W (more powerful units may be suitable for large rooms where people are standing and moving around).
They produce an energy density of approximately 230 W/ft². This is more than that of the compact panel heater although a lot less than many other types of heater. As a result you can position these heaters close to where you need the heat without fear of burning or damage.
Micathermic panel heaters allow you to select a number of different heat settings. A thermostat is normal as they also generate some convection heat. You will also find other safety features such as a timer and tip over auto shut off protections.
Micathermic heating elements can be arranged to create 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 say, a table. The column heater spreads the infrared rays across a wide zone. This could be helpful if a person is moving around at a workshop or large room.
Radiant Heating Pros and Cons
Here we look at various pros and cons relating to radiant heaters, comparing them with convection heating:
- They provide a good degree of heating in many everyday conditions; but
- They are unable to provide effecting heating further away from the heater; and
- They are more complex to use;
Many of the problems associated with convection heating can be dealt with by using radiant heat. They sidestep the issues of draughts or use in large open enviroments. In addition they tend to be efficient as they warm people close to the heater but little else. Over time, some of the air in a room will be heated through convection and this will suffer from the problems associated with convection heating but the problems are significantly less severe.
A person sitting directly in front of the radiant heater will experience the best heating result. Sometimes re-configuring furniture is a bit tricky. However, with not too much experimentation a good configuration is usually found and people position their heater where it works best for them.
The main problem is that as you move away from the heater the effect of the heat generated drops sharply. In smaller rooms the heater will eventually warm the air somewhat, but in larger rooms the effect of this convection warming is less noticeable.
As a final point, it is worth remembering that before using any heating you should first consider dressing more appropriately for the cold weather. By doing so you become far less dependent upon the heater for your comfort.
The post Understand Electric Space Heaters and What Makes Them Efficient appeared first on Space Heater Pro.
No comments:
Post a Comment