Energy, Heat and Heat Transfer can be a confusing topic. For us to be able to improve the energy efficiency of our homes, it is important to understand a some basic science: The fundamentals of ENERGY, HEAT and HEAT TRANSFER. In this article we have put together everything you need to know in one place. We think this is the best article on this topic aimed at consumers and non-professionals on the internet. Let us know what you think!
This is a long article, so here are the headings which link directly to the relevant section. But for a complete explanation, you can read chronologically.
What is Energy?
Energy is not a “thing” – it is a property that objects can have.
Energy can take many different forms. At the most basic level, all objects have “kinetic” energy or “potential energy” or some combination of the two.
Under these headings we can describe many different forms of energy. ELECTRICAL energy, CHEMICAL energy and THERMAL energy are 3 types of energy that we are particularly concerned with in relation to domestic energy efficiency and home energy systems. But there are many other forms of energy beyond these three types.
What is Heat?
What is heat?
- Heat is the amount of thermal energy in something – measured in Joules (J).
What is temperature?
- Temperature tells us how hot or cold something is. It is measured in degrees Celsius (°C)
Here is another way of defining HEAT energy:
“kinetic energy of the microscopic motion of particles”
Heat Energy
The Law of Conservation of Energy
This Law of Physics states that “Energy can be changed from one form to another, but it cannot be created or destroyed”.
Energy can be transferred:
- Mechanically (when a force moves through a distance)
- Electrically (when a charge moves through a potential difference)
- By radiation (eg. light, microwaves, sound)
Example 1: Solar panels are converting energy from the sun into electrical energy.
Example 2: An electrical inverter can send electrical energy into a battery where it can be stored as chemical energy.
Example 3: A solar diverter & and immersion heater can convert electrical energy into thermal energy and this thermal energy can be stored in a hot water tank.
Example 4: A PASSIVEHAUS can store thermal energy, by retaining that thermal energy within the building envelope
How does heat move?
Heat can be stored when it is contained within something that is highly insulated. But no matter how well insulated the object is, it won’t be perfectly insulated, and so heat will move away from that object.
Heat always moves from hotter to colder things.
Heat moves in three ways: RADIATION, CONDUCTION and CONVECTION.
If you touch something hot, you can feel the heat moving into your body – Conduction.
If you lean over a convection heater without touching it, you can feel hot air rising – Convection.
Heat can also travel as infrared waves through the air or empty space – Radiation.
Important: Heat can move by more than one method at the same time. It is perfectly possible for radiation to be occurring at the same time as conduction, for instance.
Some basic science
All matter is made from atoms — either single ones or those bonded in groups known as molecules. These atoms and molecules are always in motion. If they have the same mass, hot atoms and molecules move, on average, faster than cold ones. Even if atoms are locked in a solid, they still vibrate back and forth around some average position.
In a liquid, atoms and molecules are free to flow from place to place.
Within a gas, they are even more free to move and will completely spread out within the space in which they are contained.
Heat transfer by RADIATION
Heat can move by travelling as infrared waves.
You will be familiar with the electromagnetic spectrum; infrared waves have a longer wavelength than visible light.
Infrared waves can travel through a vacuum, travel at 300,000,000 m/s and can be reflected. They cannot travel through opaque materials.
Heat is transferred from the Sun to the Earth by infrared waves. So we instinctively know that infrared heat does not need to transfer its energy through a material – since this example of infrared heat transfer occurs in a vacuum.
In fact, all hot things radiate heat to cooler things. When the infrared waves hit the cooler thing, they make the molecules of the cooler object speed up. When the molecules of that object speed up, the object becomes hotter.
Consider this:
What colour an object is affects both its ability to absorb heat energy and to emit heat energy.
- Dark matt surfaces are better at absorbing heat energy than light shiny surfaces.
- Dark matt surfaces are better at radiating heat energy than light shiny surfaces.
Domestic “radiators” are often painted with gloss paint, but they would be better at radiating heat if they were painted with matt black paint instead. They are painted white to make them look nicer.
However, despite their name, domestic “radiators” actually transfer most of their heat to a room by convection, NOT radiation.
Normally radiant heaters must be at least red hot before they are effective.
Electric Radiation heaters
Now consider two types of electric heater. Both of these heaters are actually using the properties of radiation to transfer heat to the room.
The electric bar heater used to be quite common in the mid-twentieth century. They are very simple in concept. When they are turned on, an electric current is passed through a special resistant element, which converts electrical energy into heat energy. This heat energy is radiated out. The heat provided is instant, and very high temperature.
Electric Bar Heaters are an incredibly inefficient way to use electricity and very expensive to operate. Their use is only ever suitable in small rooms, hence they were quite popular in bathrooms, or were used in portable heaters. Also, they are either on – or off. There is limited ability to control the temperature.
Modern Infrared heaters that are sold domestically are termed Far Infrared Emitters (FIR), and also use infrared radiation to transmit their heat energy. They use low watt ceramic plates that remain cold, while still emitting far infrared radiation. These type of heaters do not heat the air within the room itself and only heat the objects on which the radiation lands – the people, the walls the floor, etc.
Since the ceramic plates operate at a much lower temperature, they are relatively efficient, certainly when compared with electric bar heaters.
Infrared heaters are making a bit of a comeback in the UK. You can now buy infrared heaters for your home that come in a variety of formats. They can be embedded in mirrors, and ceiling panels.
Herschel Infrared Heaters: UK No. 1 infrared heating panel manufacturer (herschel-infrared.co.uk)
The principle change that homeowners need to reconcile with is that for the infrared heater to work, it needs to be highly visible within the room. Therefore, the infrared heater needs to be in a prominent position – such as on the ceiling in the middle of the room. Herschel have developed a range of stylish heaters that are unobtrusive. They are a viable alternative to other “conventional” heating systems in a domestic setting. Their range of mirrors are an excellent product, highly suitable for bathrooms and hallways.
Heat transfer by CONDUCTION
Conduction is another way that heat moves.
“Heat conduction is the transfer of energy from a region of higher temperature to one of lower temperature by the interaction of close particles.”
Conduction
Remember the first rule of heat transfer:
Heat always moves from hotter to colder things.
The molecules and atoms within a substance are moving, and the more heat energy they contain the faster they move. As these atoms or molecules move, they collide with other atoms or molecules, making them move too. These molecules and atoms then bump into other molecules and make them move, too. In this way, the heat is transferred through matter.
Conduction usually happens in solids.
Thermal conductivity is a measure of how well a material conducts heat.
- Materials that are good at conducting heat are known as CONDUCTORS
- Materials that are poor at conducting are called INSULATORS
Metals are good conductors of heat because:
- Metals have atoms inside them and lots of free electrons
- These free electrons can move around and vibrate
- The heat energy is passed on by neighbouring electrons vibrating along the metal
Wood is a poor conductor of heat.
Example 1:
If a piece of wood (eg. a table) and a piece of metal (eg. a chair leg) in your home are both at room temperature, when you touch wood with your hand it feels warmer than when you touch a metal.
This is because of conduction. The metal conducts the heat away from your hand quickly, whereas the wood does not.
Non-metals are poor conductors of heat, because heat is only passed on by neighbouring particles vibrating – there are no free electrons. We can say that non-metals are insulators.
Some materials are so poor at conducting heat, that this feature makes them ideal for use as an insulator. Polyisocyanurate (PIR) is a material that is specifically used in domestic settings for its insulating properties.
Liquids. Liquids are poor conductors of heat (and good insulators).
Gases. Gases are poor conductors of heat (and good insulators).
The particles in liquids or gases are farther apart than in solids. This makes it easier for gas and liquid molecules to move around. Thus, liquids and gases more often transfer heat through convection.
Example 2:
Put a pan on a stovetop and turn on the heat. The metal sitting over the burner will be the first part of the pan to get hot. Atoms in the pan’s bottom will start to vibrate faster as they warm. They also vibrate farther back and forth from their average position. As they bump into their neighbours, they share with that neighbour some of their energy.
As a result of collisions with their warmer neighbours, atoms start moving faster. In other words, they are now warming. These atoms, in turn, transfer some of their increased energy to neighbours even farther from the original source of heat. This conduction of heat through a solid metal is how the handle of a pan gets hot even though it may be nowhere near the source of heat.
Heat transfer by CONVECTION
“Convection happens when particles with a lot of thermal energy in a liquid or gas move, and take the place of particles with less thermal energy.”
Convection
Convection is a very important way that heat moves on Earth. Convection happens when a substance that can flow, like water or air is heated in the presence of gravity.
Liquids and gases expand when they are heated. This is because the particles in liquids and gases move faster when they are heated. As a result, the particles take up more volume, since the gap between particles expands while the particles themselves stay the same size.
The heated air or water becomes less dense. It rises up until it gets to air or water with the same density as it has, and when it gets there, it pushes the air or water that was there originally out of the way. At the same time, new air or water fills the space that was vacated when the heated molecules rose up.
The air or water that gets pushed out of the way falls down. This sets up a circular motion. Air or water is heated at the bottom, travels to the top, cools, gets denser, falls, is heated again and the whole cycle starts again.
In this way, convection currents that transfer heat from place to place are set up.
Convection does not occur in space because there is no gravity.
Example 1:
In this classic GCSE Science experiment, CONVECTION in a liquid can be observed by putting a crystal of potassium permanganate into a beaker of water and gently heating it with a Bunsen flame.
Heat is initially transferred through the glass wall of the beaker by conduction. The water in the area of the Bunsen flame is heated. This water expands, becomes less dense and rises.
This risen water is replaced by the cooler, denser water which surrounds it. This water is in turn heated, expands becomes less dense and rises.
As the process continues, a convection current is set up and heat is transferred through the liquid.
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Heat transfer in a house
In your home, you probably experience all three forms of Heat Transfer – Radiation, Convection and Conduction.
Conduction. Your home will lose heat via Conduction. Heat is transferred from inside where it is warmer to the outside where it is cooler via CONDUCTION.
Convection. Convection occurs inside each room. Warmer air rises from a heat emitter such as a convection heater towards the ceiling and cooler air sinks towards the floor.
Radiation. Heat energy is transferred from the Sun through the windows of your home by Radiation.
Ventilation and infiltration
Unless your home is perfectly sealed (it won’t be) your home will also lose heat through the movement of air.
This comes in two forms:
The intentional movement of air – Ventilation. We ventilate our buildings for many reasons. We expel humid air from our bathrooms, we open our windows to allow fresh air into the house and we expel air in our kitchens to expel smells and humid air.
The unintentional movement of air – Infiltration. The air in our homes changes all the time. Modern building regulations stipulate that a room will have no more than 0.5 air changes per hour, but in an older building with an open fireplace, this might be 2 air changes per hour. Air infiltrates because the front door isn’t well sealed, because your windows have trickle vents or because the wind is blowing outside and air is being forced into the building via gaps in its construction.
This is really a separate topic from Heat Transfer itself, but it is nevertheless highly significant when considering Energy Efficiency in buildings.
What is Solar Gain?
Thermal Energy from the sun in the form of radiation strikes your Window.
Some of that energy is reflected by the window, some is absorbed but most is transmitted through the window.
Remember that radiation can transfer through a vacuum – that is why it can move from the Sun to the Earth. So it transmits through double and triple glazing.
The amount of heat that radiates through a window is defined by its G-value. The G-value is a number between 0 and 1 (sometimes it’s shown as a percentage) which describes what proportion of the radiant heat falling on the outside of the window will get through the window to the inside.
If the G-value is 0.5 then this means that half of the radiant heat gets into the building. No glass is perfectly clear, so the G-value can’t be as high as 1. The highest the G-value can be for a standard double glazed window is around 0.78, for triple glazing this value is reduced to about 0.71 because the extra layer of glass blocks some of the radiant heat.
We can design our home to benefit from Solar Gain during the winter, but at the same time we must be cautious about not being subject to too much solar gain during the summer. Unfortunately you only need to take a look at modern British housing development to realise that consideration of Solar Gain is rarely considered, with the orientation of the house in relation to the Sun largely ignored.
What are U-Values?
The U-value describes the rate at which heat is lost by conduction.
All parts of a building’s fabric have U-values, walls, floors, roofs, doors and so on.
Lower U-values show that there is less heat that is lost by conduction.
A U-value of 1.4W/m²K means that if the temperature difference between inside and outside is 1°C then the window will let out heat at a rate of 1.4 Watts for every square meter of window area. If the temperature difference is 2°C then the heat lost will be twice as much (2.8 Watts for each square meter) and so on.
For more information about U-Values, check out our in-depth article here.
Does a Radiator radiate heat?
Radiators are grossly mis-named! Radiators are primarily a convection heater. They work by heating air, and then have that air circulate around the room. They draw cooler air in at the bottom of the “radiator” and then the hot air escapes out of the top.
Where you see “radiators” with fins attached, this is precisely to increase the surface area available to heat the air (rather than to increase its radiation potential).
Because the warm air from the “radiator” rises, the floor is the coolest place in the room, whereas the ceiling is the warmest place.
In order to be effective, the “radiator” needs to be significantly hotter than the air in the room in order for the convection effect to work properly. When you operate a “low temperature heating” system, the “radiators” need to be much bigger to achieve the same result, since the convection effect is clearly diminished.
What about underfloor heating?
With underfloor heating, the pipes containing the warm water (usually between 30-35°C) circulate within the sub-surface screed layer of the floor. The thermal energy is transferred from the pipe to the screed via Conduction.
The screed then heats up, again via Conduction, all the way to the floor surface.
The heat is then applied from the floor to the room via Radiation. The floor radiates heat throughout the whole room. Because the temperature difference between the air in the room and the floor isn’t large (perhaps only a 10 degree temperature differential), the effect of conduction or convection is minimal.
One of the reasons that people often prefer underfloor heating is that it distributes the heating throughout the entire room evenly. You don’t get hot spots or cold spots.
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