|
|
|
Personal Heating Tutorial
|
| Introduction
There is a wide range of personal or portable electric heaters. Different models claim to be more efficient, or in some way better than the others. This section explores how 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 used in a heater to create a heat source. There are three basic ways this heat source can heat a person or working space:
- Convection: The heat source heats air directly. The hot air rises or is blown by a fan and mixes with the surrounding air mass. The temperature of the air in the room rises and this makes the room feel warm to an occupant.
The warm room reduces the rate at which the occupant's body loses heat to the environment. Oil heaters, convection heaters and fan heaters use this principle. Radiant heaters can also heat the air – indirectly as explained below.
- Radiation: When we step out of the shade into direct sunlight the warmth we immediately feel is due to radiant heat from the sun. Along with ultraviolet and visible light, the suns rays contain infrared radiation. Radiant heaters also generate invisible infrared energy. This form of "light" travels directly from the heat source through the air and heats people and objects in its path. Infrared energy barely heats the air it passes through, however. Instead air is heated when objects in front of a radiant heater are warmed up and transfer heat to the surrounding air.
- Conduction: Heat is directly transmitted from one material to another. When you place your hands directly on an oil or panel heater for a few seconds to warm them up, the heat transfer mechanism is conduction. Conduction also occurs with heated car seats and electric blankets. A hot bath heats you via conduction. Floor heating mats use conduction to a degree, but generally this form of heat transfer plays a minor role with personal or portable heaters.
If you work in an air conditioned office, or a workspace that receives a tempered air supply, or one that has
a reverse cycle air conditioning unit, then you are experiencing convection heating. The air in the room will be heated either by hot water pipes, an electric in-line resistance heater or hot "refrigerant" coming from a reverse cycle air conditioning unit.
BACK TO TOP
|
| 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.
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).
BACK TO TOP
|
| 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?
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?"
BACK TO TOP
|
| Convection Heating
Introduction
Given the typical characteristics if non air conditioned spaces at UNSW, portable convection heaters are neither energy efficient nor effective. They consume large amounts of electricity to make a person comfortable. In large offices they simply cannot heat the air mass enough to do the job. They have four major shortcomings:
- Air stratification from temperature gradients;
- Major energy losses through windows, walls and ceilings;
- Energy losses from draughts; and
- Inability to heat large spaces.
Air Stratification
Imagine a person sitting at their desk 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 creates a problem, namely temperature stratification. Warm air rises from the heater and a strong vertical temperature gradient develops. The air just under the ceiling is a lot hotter than the air at desk and floor level.
The convection heater will hopefully have a thermostat. This control may only be 40–50 cms 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 narrow band of air between 40 and 1100 cms above the floor will be at the right temperature. The air volume in the top half of the room will be overheated. A lesser volume just above the floor will still be too 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 conditioning systems try to overcome this problem by using various techniques to ensure the air in a room is reasonably well mixed. These systems do a better
mixing job in summer, with cool air coming out of the ceiling registers, than they do with
warm air in winter.
You can achieve a degree of air mixing in a non–air conditioned office 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 21°C
is a much more energy eficient proposition then having to heat over half the
room to 25°C or more).
Major Energy Losses
Non air conditioned offices are often located in older buildings with high ceilings. In such cases you have to heat a massive volume of air to a high temperature to obtain a comfortable environment. Using a fan mitigates this problem
but other factors present in old and even some relatively new buildings tend to
wipe out the gain.
The warmer an air mass is the greater the rate of heat loss to the cold
outside air. Most buildings at UNSW are uninsulated and lose a lot of energy
through the ceilings, walls and old single pane windows. The types of blinds
typically used in offices, namely Venetian or roller blinds, have little
insulating value. Metal window frames are another significant source of heat
loss. The bottom line is that heating a large volume of air in a poorly insulated
building is not a efficient way to make people comfortable.
Draughts and Open Offices
Unfortunately old buildings (and some newer ones as well) are draughty. This means that warm inside air leaks out of the work space and is replaced by cold outside air that now has to be heated. Keep your office door closed as much as possible and call for maintenance (telephone FM Assist 55111) if your windows are broken, or they don't close properly etc.
This will help, but fundamentally convection heaters are not well suited to draughty spaces.
Convection heaters are also not suited to heating large open plan offices, receptions and foyers, and workshops. They will operate in these spaces continuously and to no avail. The
surrounding air will never get warm enough to trip the thermostat. The warm air
will rise away in tall foyers, leak out sideways in open receptions or flow
through self opening doors. People that try to use oil heaters and other
convection heaters in open plan offices and other large tall areas, invariably huddle right beside the heater. They are trying to use a convection heater as a radiant heater, a mode it was not designed for.
BACK TO TOP
|
| Radiant Heating
Introduction
Traditional high temperature radiant heaters, called strip heaters, bar radiators or quartz infrared radiant heaters are not suitable
as personal or portable heaters at an institution like UNSW.
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
- Efficient conversion of electricity to radiant energy.
Modern radiant heaters are configured as flat panels. They are thin, ranging
from 25 to 75 mm in thickness. they are larger in area than traditional radiant heaters and the heat source is not as intense. While they are hot
to touch they will not usually cause a burn. Most of the radiation is emitted from one side of the panel. This means the back side can be placed against a wall or
close to a piece of solid office furniture. Typically about 80% of their heat output is infrared radiation. The balance is convection heat. Energy outputs range from around 160W 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 range in size. The model chosen for use at UNSW is about 500mm long and 450 mm tall.This is an ideal size to heat the legs of a person seated at a desk. A large proportion of the radiant heat produced by the unit will strike the person and be effective.
The energy output of the compact model is 160W. The energy density is only 800W/m² (front surface area) compared with 10,000–15,000W/m² (reflector area) for a strip heater and this makes the compact radiant panel heater much safer than a strip heater. The low energy density also means the heater can be placed relatively close to the user. This results in most of the output energy being used to provide comfort. 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.
The compact heaters use a simple resistance element backed by reflective material and insulation to project infrared heat out one side of the panel. There is not a lot of material to heat so they only take about 5 minutes to reach full heat output (much quicker than an oil heater)
The low wattage radiant mode of these heaters means that thermostats are not needed. They usually just have an on 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 Radiant Panel Heaters
These heaters are a bit bigger than compact radiant panel heaters. They are around 600mm long and 600 wide and 60-70mm thick. Maximum energy output of the model chosen for single person application at UNSW is 1000W (more powerful units may be suitable for large reception counters where people are standing and moving around). They have an energy density of about 2500w/m², more than a compact panel heater but much less than with a strip heater. Once again these heaters can be placed relatively close to the user to create effective and efficient personal heating.
Micathermic radiant panel heaters use a special technology, second generation micathermic II, to project infrared heat in one direction. They convert about 80% of the electrical energy input to radiant heat and 20% to convection heat. Micathermic heaters reach full power output in a matter of a minute or two.
Micathermic panel heaters typically have several heat settings. Because they produce a degree of convection heating, a thermostat is worthwhile and is usually built in. The heaters have a range of protection devices to guard against overheat or being tipped over. They often have a timer.
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 a reception counter. 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 reception foyer.
Radiant Heating Pros and Cons
Radiant heaters have a number of pros and cons when compared with portable convection heaters:
- They heat people efficiently across a range of conditions; but
- They have range limitations and can be tricky to configure;
Radiant heating can deal with most of the shortcomings of convection heating. It can efficiently heat people working in large open spaces or draughty environments. More of the energy
a radiant heater produces actually heats the person and less disappears as losses through uninsulated ceilings, windows etc. Nonetheless radiant heaters do eventually heat the air to a degree so using ceiling fans in winter mode and taking steps to improve the insulation and reduce draughts are still useful measures.
A person needs to be in front of a radiant heater to get the best result. Sometimes reconfiguring furniture, location of waste paper bins etc to achieve this can be a bit tricky. Generally after a bit of experimentation, most people are able to make some some small adjustment and find a good, safe heater location that works for them.
A broader problem with portable radiant heaters is that when the person moves away from the heater they lose its heat. After a while a radiant heater will heat the air in small to medium sized offices to a degree and take the sting out of the air. When using any heating system you should dress for winter. If you are suitably dressed, then short excursions way from the heater (to go to the photocopier or to talk to someone) should not be a problem. If you are using an oil heater in a large space it will not be able to heat all the surrounding air
anyway. In this case if you move away from your desk you will be no better off than if you had a radiant heater
(but if you swapped to a radiant heater, you would have an effective heater when you are at your desk).
BACK TO TOP
|
|
|
|
|
|
