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Mini Heaters Costing Pennies to Run -what is the evidence?

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There are plenty of adverts like this around but do they hold up. Part of being Rubbish STEM trained is looking through the jargon and pseudo science and asking. Where is the evidence? 

The language is very persuasive and sounds scientific but when they put pictures like this in. A big red flag. Their heater turns the room from blue to red??

‘The advert says ‘A Revolutionary ceramic piezo technology’ ‘ with ‘induction coil’    in reality it is a rock that is heated by a coil of wire  up and air is blown over it. Something basking reptiles on sun warmed rocks have known all about for millennia! Ceramic heaters have been around for about 40 years

It claims to be 99.8% efficient ‘losing almost no power to energy leakage’.  What is this energy leakage and what happened to the 0.2% ? All heaters are 100% energy efficient in physics terms as if you follow energy transfers always leads to warming the environment.  Sound waves from a heater create vibrations that warm the environment. Light rays emitted eventually warm the environment, fire a gun and the environment is warmed, jump up and down and the room is warmed.

It has also been advertised as a powerful heater that can heat your home almost for free – Highly misleading !! Or add a proviso if you hardly ever turn it on!

What is definitely a rip off is that these unbranded heaters can be bought for less than £10 without the fancy marketing and psychological tricks to make you believe you are getting a bargain paying up to 5 times as much as you might.

There is lots of talk of ‘heat’ in adverts a terrible word for physicists. Heat can mean different things, it can be an indication of the temperature, how hot something is measured in degrees centigrade/celsius or it can mean the thermal energy of a body. or volume measured in joules.  A hotter body may have less thermal energy than a larger cooler one. Take a hot cup of coffee versus a warm bath, the coffee has the higher temperature, but the combined energy of all its molecules are less than that of the bath. If you do not believe this try pouring a mug of boiling water into a cold bath and see what a small difference it makes to the temperature of the bath.


Energy is measured in Joules

A joule is defined as the work done when a force of 1 Newton moves through a distance of 1 metre. Think a small apple thrown to a height of 1 metre. 

Power is the rate that energy is transferred and is measured in watts. A  watt is a joule per second think throwing the apple up once a second. 

Heaters have a certain power or wattage. Anything from 100 watts to 2000 watts  or 2 kilowatts. The more powerful the heater, the faster it will heat your room but the more it will cost you. A 1 kilowatt (1000 Watts) heater left on for an hour uses what electricity companies measure as 1 kilowatt-hour (kWh) or 1 unit This is what your electricity meter numbers mean. Every unit costs around 35p so. You can check the price per unit with your electricity company.

A 1 kW heater left on for an hour transfers 1 unit and costs 35p

A 500 W heater is half as powerful so will only cost 17.5p per hour

A 2000 W or 2 kW heater is twice as powerful so transfers twice as much energy per second as a 1 kW one as so costs twice as much to run at 70 p an hour

Many adverts for heaters say ‘ Cost pennies to run’ but nearly all of them cost less than £1 an hour ( A 3kW heater may cost more than this) so this statement is true for all of them. Everything costs pennies if you don’t keep them on for long! However leaving a 2kW heater running for 10 hours a day will cost you over £200 a month. Always buy a heater with a thermostat so it turns itself off when a certain temperature is reached as these will be more economical.

There is no cheaper electrical heating option!!

I repeat there is no cheaper option if you are heating with electricity to heat  the whole room up

In order to heat a room up to a certain temperature we need to transfer a certain amount of thermal energy. In a well insulated room it does not matter how quickly or slowly we do this, the total energy needed to be transferred and hence the cost is the same. The higher powered heaters will get the room up to temperature faster, but the smaller less powerful ones will not be cheaper to heat the whole room they will just take longer.

What might happen if?

If the room is poorly insulated and draughty the low power heaters may be ineffective at raising the temperature of the room as the rate of loss may be similar to the rate of gain. It is helpful to use a ‘what might happen if?’ approach here and take things to the extreme. What might happen if the heater was super powerful?  The room would be heated instantly. What if it was very weak it will make virtually no difference.

All heaters are not the same 

 Some heaters are more appropriate than others depending on the rooms and needs. The mini ceramic heaters are useful under certain conditions.

Gas central heating is still likely to be the most economical option to heat a whole house and even a single room if thermostatic valves are on the radiators which they should be. Turn the radiators off in all the rooms but the one you are using.

Heat pump technology is the future and there will be a blog about this soon

Different types of heaters for different purposes

Fan heaters 

These blow warm air in a direction and so are useful to heat part of a person. A small heater under a desk (care taken to avoid fires) does a good job of keeping feet and legs warm. Fan heaters usually have a quick start up time. The ceramic ones are good and make the temperature of the room fairly consistent. They are not great for allergy sufferers as they circulate dust. They also go cold quickly after they have been turned off. Use these for personal heating close to you, but also be aware they may get very hot and become a fire hazard.

Radiant Heaters

All warm objects emit infra red radiation.  You can feel this as the warmth from the Sun. Infra-red is part of the electromagnetic spectrum, like visible light . Think of it like a warm red light.

Older readers will remember the coil of wire that glowed red when switched on and a reflector that was often the only heating in the bathroom.  I moved into a house that had bar radiant heaters as the only form of heating. They were a fire hazard as they got extremely hot and if they hadn’t been used for a while switching them on was accompanied by the stench of burning dust. You could feel the warmth directly in the same way as from the Sun on your skin. More advanced versions of these are still available including infra red panels and even wallpaper. It is very difficult to compare how efficient these are as they are designed to make you feel warm directly rather than raise the temperature of the room. Think of feeling the warmth of the sun on sunny but cold day, the more clothes you wear the less you will feel this radiation so if you are not moving this targeted heater is a good option.

A ceiling mounted infra – red heater over a bath can give very targeted warmth 

Convector heaters and Oil filled Radiators.

The common way of talking about convection is ‘hot air rises’ though really warmer areas in a fluid become less dense as their molecules are moving faster, hence they collide more often. Less dense objects (or volumes) will rise for the same reason that objects float on water. Think of a mosh pit before the music starts there is little motion and people are close together. When the music is playing people are bouncing off each other and are further apart on average.

Convection only takes place in fluids (gases and liquids)  as the warmer molecules themselves are moving on mass as opposed to vibrating in fixed positions as happens in solids.  

Holding your hand above and below a convector heater will make it very evident that the area above it is much warmer than below. So convector heaters make the top of rooms warmer than the bottom, so are a bad choice in rooms with high ceilings and poor insulation. Unless you sit on a very tall chair. 

Radiators are actually wrongly named they do radiate as do all warm objects, but mostly they are convectors. 

Oil filled radiators are primarily convectors take a while to get hot, but also to cool down. They are safe in that they do not get very hot so that may be appropriate for children’s bedrooms.

Heating the Person not the room

Heating air molecules with a heater in order to make them transfer thermal energy by hitting the skin of a person is a really inefficient process. Devices that warm you by direct contact are much cheaper to run. These include electric blankets and electric vests.

If you have any questions please add them to the comments below

A Video explainer is HERE

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