Energy Advice > Saving Energy > Urban Myths

Energy Advice Urban Myths or Simple Truths?
(or everything you wanted to know about saving energy but were afraid to ask!)

Here are 10 popular hints on how to save energy - or perhaps they are misleading advice that could end up costing the earth! Ian Byrne, the Foundation's Deputy Director, has been collecting popular wisdom about ways to save energy for several years, and has come up with some surprising answers...

1. It uses less energy if you leave fluorescent lights switched on

MYTH

This one has been around almost since fluorescent (strip) lights first appeared. It is certainly true that in their start-up phase, they use a lot more energy than when they are running continuously, so it is not a good advice to keep switching them on or off. But if you are going out of a room for short time, how long should you be away before it makes more sense to switch the light off, rather than leaving it on?

The answer may surprise many people. Although it depends on several factors, including whether it has an electronic or mechanical (condenser) starter, the period is generally in the region of a minute and a half, and never more than 5 minutes. Modern fluorescent lights, with narrower tubes (T8 or T15) and electronic starters generally should left on unnecessarily for no more than a minute or so.

Consider the current being used in a 40W strip light. Suppose also that it was being left on for 24 hours - in which case it would use 0.96kWh (almost a unit of electricity). Now if it didn't save money to switch this off, it would have to use the equivalent amount of energy in the start-up phase. Typically this last no more than 3 seconds. A light using energy at the rate of 0.96kWh in 3 seconds on a 240V supply would be drawing, in simple terms, around 4,800 Amps - enough to fuse not just the lighting circuit, but also the electricity sub-station!

2. Use less power - take a shower!

Sometimes a Myth, but sometimes True

The Government pushed this one heavily in the mid-1990s; I have a nice badge showing a dinosaur taking a shower. But in practice the answer is a lot more complex. The easiest way to test this out is to put the plug in the bath next time you have a shower, and see how much it fills up. If, at the end of the shower, there is less water than you would usually have in a bath, then you will probably save money by taking a shower. But it's not quite that simple...

The simple answer assumes that you are using the same source to heat water for the bath and the shower. This will be the case if you are using your central heating boiler or an immersion heater. But a lot of British homes have a "Power Shower" that uses electricity to provide a powerful and instant shower. Not only do they use water more quickly than a shower fed from a hot water system, they also use on-peak electricity that costs around 7p a unit, compared to 2p for gas. At this point it gets complicated, as to give a firm answer you would also need to know the efficiency of your heating boiler, the amount of insulation on the hot water tank and the losses on the pipework between the tank and your shower.

In fact, it may not make any difference either way, if you are using the central heating boiler to warm the water. For in most cases, especially in summer, we leave the boiler on for a fixed period of time, allowing it to heat up the tank of water. If the bath or shower is drawn off after the end of a water heating period, it will not affect the heating cost on the day it was used (for that has already happened) but on the next heating cycle. And if the tanks cools down then it will not be significantly affected by how much hot water was used after the previous cycle.

3. It's wisest to leave your immersion heater on 24 hours a day

MYTH

This one is easy. It is always best to place the water heating on a timer, as the energy lost from a hot water tank depends on the temperature difference between the surface of the tank and its surroundings. It's a common myth that it somehow takes more energy to keep heating up a tank than to maintain it at a high temperature.

Of course, as with all the urban myths on this page, there are a few "ifs" and "buts". If the tank is highly insulated (so standing losses are very low) and there is an effective thermostat on the tank, then the losses through leaving it on can be much reduced. And some people have a need for large quantities of hot water all day long, in which case they may have no alternative to leaving the immersion heater on. But in general, it is much better to install a timer - a heavy duty one, suitable for immersion heaters, should cost less than £20 and if you can fit it yourself safely, could pay for itself in a few months: an excellent energy efficiency investment.

4. If you have Thermostatic Radiator Valves you don't need a Room Thermostat

MYTH

This idea became quite widespread about 15 years ago, when it was being supported by an official government institution. However it's not true.

Thermostatic Radiator Valves (TRVs) will only switch the flow to a single radiator on or off. They do not stop the boiler from firing (and so using energy). They are useful, but tend to be a rather crude control of temperature in a room, as they are affected by siting (please don't put the sofa directly in front of them!) and are often not set at the right temperature.

In contrast a room thermostat is accurate to within a degree or so. What's more, if it has been wired up correctly (in what may be called an "interlock") it should send a signal back to the boiler to switch itself off if there is no demand for heating for either the heating or hot water circuits. This stops the boiler firing when the internal water temperature has slipped - ie. it stops so-called "dry cycling", and definitely saves energy.

There are other advantages to modern room thermostats. They can contain an optimum start temperature sensor, which delays the boiler firing on relatively mild days, again saving energy. And if combined into full zone control, they can set different temperatures for different floors of a home - meaning that living rooms can be snug, without over-heating bedrooms.

One final warning: don't put a TRV on a radiator close to the room thermostat, as the TRV's operation will mislead the room stat. For more information about TRVs and room thermostats, please look at our controls page.

5. It's cheaper to use an immersion heater in the summer than to heat water from your central heating boiler

Sometimes a Myth, but sometimes True

Maybe from your boiler, but not from mine! The answer to this one depends very much on how efficient your boiler is, especially when it is operating at part load, as it does in summer when there is no central heating demand. Broadly, if you have a modern high efficiency boiler (say rated 'D' or better on the SEDBUK scale) linked to a hot water tank thermostat, then it will be better to use the boiler to heat water, year round. But, providing you have a timer and well-insulated tank, it may be cheaper to use the electric immersion heater in summer if your boiler is not a high efficiency model.

Recently, a number of people have picked up on this, believing it to be a problem that only affects condensing boilers. They point out that in summer, when the boiler is only heating water, it operates at part load and does not normally enter condensing mode. "Aha!" they say, "in that case it must be wrong to use a condensing boiler only for water heating." Actually, that's not true. They are right to point out that the boiler may not be in condensing mode, but even so the larger (or second) heat exchanger unit in a condensing boiler still means that it will operate at a higher efficiency than a conventional boiler (unless it has a very high internal thermal mass). So a condensing boiler is still likely to be cheaper to run than a conventional one in summer. The Government's A-G boiler energy rating (SEDBUK) is based on average seasonal efficiencies, and takes into account the lesser performance of boilers in summer, so an "A" rated boiler will use less fuel over the year than a "B" rated one, and so on...

6. Replacing windows with double glazing will stop mould growth

Usually True but sometimes causes problems

We've all seen the adverts showing how double glazed homes are warmer and healthier, with less mould. This is true, providing they are also adequately ventilated, and can maintain a reasonable room temperature. Ventilation is the key; if a home is too airtight, moisture (from baths & showers, cooking and even breathing!) will stay inside the house and create a damp atmosphere in which mould can grow. For this reason, good double glazed units include trickle vents that be used to add controlled ventilation. Be careful about selecting double glazed units that have metal (aluminium) frames; if they do not have a very good thermal break, you are likely to get some condensation on the metal, even though there will be none on the glass itself.

There were major problems a few years ago in a large Scottish city, when the local council installed double glazing in some of its tenants' flats. Unfortunately they could not afford to improve the heating at the same time, and the residents kept their expensive to run old-style storage heaters. The result should have been predictable; low internal temperatures, together with cold walls (the flats were unsuitable for cavity wall insulation) and reduced air flows, led to a huge increase in damp and mould. What was worse, many tenants were drying damp clothes inside their flats, as there had been a spate of thefts from balconies and the designated drying rooms.

Of course, in a properly heated home, modern low-E double glazing (to the latest FENSA standards) will save energy and make you feel more comfortable.

7. Strapping on a gizmo to your boiler can save you at least 10% from your heating bills

MYTH

Ten years ago, there were a lot of high-pressure salesmen selling "Boiler Energy Managers" with this claim. (I know; I bought one.) But you could make the same savings by switching off your boiler for 10% of the time, and that is what most of these domestic boiler energy managers did. If they are fitted by the manufacturer, then they will save money, but retrofitted the same effect can often be achieved by turning down the thermostat by a degree or so. Both delay the firing of the central heating system. Modern boilers and with good controls should not need an additional strap-on energy manager.

So the salesmen have moved on to another product - the magnetic ring that aligns the gas molecules and achieves amazing improvements in boiler efficiency. Well, I would be amazed if these worked as well as they are claimed to, as in some cases they would end up giving boilers more than 100% efficiency. Preliminary investigations by Government scientists suggest the claims are just hot air - I expect that by the time we know for certain, the salesmen will be selling us something else again...

On the other hand, there is evidence that strap-on devices designed to stop scale forming in hot water pipes may be effective in hard water areas. These apparently work by precipitating out limescale into microscopic particles that stay suspended in the hot water, rather than coalescing as scale on the walls of the pipe. It's not clear how much energy they can save (and it's unlikely to be anywhere near 10%), but hopefully there will be independent test results available in the near future.

8. Big freezers cost more to run than little freezers

Usually True, but not always

In one sense this is obviously going to be true. But in practice, it often isn't! Small freezers are often upright models, which lose a significant amount of cold air whenever the door is opened. (It can take as much as 30 minutes for a freezer to regain its temperature are a door has been opened for a minute.) Chest freezers, with a lid opening - and typically thicker insulation levels - will often use only less half as much energy for a given volume of food storage. So a 4.5 cu.ft (125 litre) upright freezer will often use more energy than a 9 cu.ft (250 litre) chest model. And if you have empty space in either type of freezer, it's best to fill it, either with empty cardboard boxes (breakfast cereal packets are ideal) to stop air flow, or with loaves of sliced bread!

9. Leaving your PC screen switched on during the lunch break prolongs its life and doesn't waste much energy

MYTH (mainly) First things first. Leaving the screen on over lunch may prolong the life in theory, but in practice how many office PC screens are thrown away because they have broken down, and how many because they have become superseded by more modern technology?

So how much energy will be saved if screens are switched off? The answer is "enough". Whilst in operation, most CRTs (cathode ray tubes, not the more expensive flat screen type) use between 30W and 200W. Oddly, smaller 14inch screens often use as much power as 17 inch screens. In even a medium-sized office, this is going to add up to a fair amount of energy being used. Of course many PCs will go into sleep mode if they are not being used, but do not assume this will happen. Screens do not use less energy when they are in screen saver mode (that's just designed to stop the phosphor coating being damaged), and even when they power down into sleep mode (typically after more than 20 minutes) they still use some energy.

PC screen consumption can often represent at least a third of the electricity consumption in a modern office. Most of the wasted energy is given off as heat, and in an air-conditioned office half as much energy again can be used in getting rid of this waste heat in summer.

10. "With global warming we won't need as much insulation in the future"

MYTH

This quote is attributed to an MP. It's wrong for several reasons:

• Global Climate Change will not provide a nice even increase in temperature. It's likely to be accompanied by greater instability in weather, so homes will need just as much insulation to deal with cold spells.
• The most likely effect in the UK is to shorten the heating season slightly, not to reduce the need for heating altogether.
• Global warming will not happen overnight. It's expected to take around 50 years for the temperature to rise by another 2°C.
• Some scientists are concerned that melting icecaps will lead to the Gulf Stream shutting off or changing course, with a significant cooling effect for Britain and Europe.
• Even if global warming were to benefit the UK, it's rather irresponsible to encourage it when it is likely to have severe adverse effects in other parts of the world.

Conclusion

So that's 5 myths and 5 ideas for which there is at least a grain of truth. Energy saving at home and in the office need not be difficult, but we all need to think just a little more sometimes before acting on received wisdom. This website offers a number of practical suggestions for low cost and free ways to save energy, as well as a comprehensive range of energy saving measures and a free self completion Home Energy Check.