Hot water timing – a waste of effort.

It goes against our nature to be told there is nothing one can do about something but sometimes logic drives us to that conclusion. Energy saving heating controls are sometimes like that. Some controls are worthwhile: others are not. Despite apparently well-qualified sources saying the contrary, if one has a modern hot water cylinder heated by oil or gas, it saves almost no energy to have the hot water timed, rather than have it turned on continuously.

Here is the energy label from typical recent hot water cylinder.

This label shows that the cylinder has a volume of 150 litres and loses heat at the rate of 60 watts (60 joules per second).

Calculating the energy loss
Losing energy at 60 joules per second means 60 x 3600 = 216,000 J per hour.
Overnight (from 10 pm to 6 am) or during the day (from 8am to 4 pm) are 8-hour stretchs, so during that time the total energy loss is 8 x 216,000 joules = 1,728,000 joules = 1.78 MJ. That’s 1.78/3.6 kWh, about half a kilowatt hour, ie about 3 p if you heat your hot water by gas.

So, every morning, or every evening, if the hot water has been turned off for 8 hours, the boiler uses 3 p worth of fuel just to top up the losses during the day. That’s 6 p per day, or £22 per year. Is there the possibility of saving some of that energy wasted?

As the tank loses energy it cools down
The tank contents are 150 litres (150 kg) of water. We can calculate how much its temperature falls.
Energy = mass x specific heat capacity x temperature change.
1,728,000 joules = 150 kg x 4200 J/kg K x temperature change
temperature change = (1,728,000)/(150 x 4200) = 2.74 °C (Physicists label that 2.47 K)

If the hot water is turned off, the temperature drops a small amount.
If your controls turn the hot water off for an 8-hour period, your hot water drops in temperature by about 3 °C, say from a set temperature of 65 °C down to 62 °C. That means its average temperature is 63.5 °C.
The first effect of the heating system when the time turns the hot water system back on is to heat the water back up to 65 °C. That’s where the 3 p worth of energy is used.

If the hot water is left on, the temperature doesn’t fall
If you leave your heating switched on, the hot water stays at 65 °C. Whenever the thermostat senses that the temperature dropping, it turns the heating system on to top it back up. So instead of having to reheat the water when the system turns back on, it is continually topping up the energy loss – by an amount pretty much the same as the 3-p-worth which it needs if it switches the hot water off.

But surely you save some energy by turning off?
If you keep the hot water on, it is at a steady 65 °C, that is steadily 45 °C above the 20 °C surrounding room.
If your controls turn the hot water off for an 8-hour period, the tank temperature drops from 65 °C down to 62 °C, an average of 43.5 °C above the surroundings. This is not very different from leaving the hot water on and therefore the heat loss is pretty similar.

If, when switched off, the temperature above the surroundings has dropped in the proportion 43.5/45 compared with leaving the hot water turned on. The energy loss, and the cost of the energy loss, has dropped in the same proportions.

So the heat losses during switched-off times go from £22 a year to = £22 x 43.5/45 = £21.27 a year. That means that the reheating costs drop by 83p a year.

If you have oil or gas central heating and you control your hot water heating and turn it off for periods during the day, you might save £1 a year, a trivial saving against the convenience of hot water at any time or the simplifications of your heating control system.

In fact the savings are even less
So far we’ve assumed that all the ‘losses’ from the hot water cylinder are waste. This is not always the case. At times of year when a house is being heated, energy ‘lost’ from the boiler is useful in heating the house. If the house is being heated by gas, a 60 W loss from the hot water tank is worth as much as about a 20 W heating from the gas boiler during the heating season, so the potential savings of switching off drop to about 70p a year. If you are heating by on-peak electricity, the 60 W ‘lost’ is just as good as 60 W produced by your electrical heating, so the savings are about 42 p per year.

What about longer periods of switch-off
If you are away for a whole 24-hour day, that’s three times as long as the 8-hour periods we’ve considered. If you turned the heating off for that day away, the hot water temperature would drop by about 3 x 2.47 K, = 7.4 K, ie down from 65 °C to 57.6 °C, an average temperature of 61.3 °C, ie 41.3 K above the surroundings. This would take the heat loss down to 8.3p. So you can save 0.7 p by turning off the heating if you are aware for a day. Only if you are away for several days during which the tank temperature will drop very significantly, is it worth turning the hot water off. If you turn the tank off when you are away for a week, you will save about 60p. With the average household away for less than 4 weeks a year, turning off hot water when one is away can save at the most a couple of pounds a year.

If you have a combi boiler
Most combi boilers heat hot water on demand. With them there is no stored hot water and no ability to turn the hot water off. Some combi boilers have a small reservoir of water that is always kept hot so that the boiler delivers hot water even more quickly. One can save a small amount of energy by turning this pre-heat system off but manufacturers do not recommend it because the savings are small.

The only time is is worth controlling hot water heating
If you have off-peak water heating it is worth controlling the hot water heating. In that situation it makes sense to make sure that the heating of the hot water is done during the night. Let’s run the calculations.

In a flat occupied by 2 people, hot water consumption might be 100 litres (100 kg) of water per day. This water has to be heated from an average temperature of 10 °C to, 65 °C, an increase of 55 kelvin. The energy needed to heat this water is
mass x specific heat capacity x temperature rise
= 100 kg x 4200 joule/kg K x 45 K = 18.9 MJ = 18.9/3.6 = 5.25 kWh.

Heating this by on-peak electricity at 14.9p per kWh costs 14.9 x 5.25 = 78 p per day.

Heating this by off-peak electricity at 9.3 p per kWh costs 9.3 x 5.25 = 49 p per day.

So you can save 29 p per day, ie £106 per year, by using off-peak electricity.

Under those circumstances only is it worth controlling the time at which you heat the water.

Topping-up electrically heated hot water
Many electrically heated hot-water systems have a top-up facility by which one can turn an immersion heater on if one runs out of off-peak-heated water.

If one needs the top-up regularly, then certainly one is using more than 100 litres per day and there is value in considering a larger tank. There are savings in the order of £200 per year if one heats 200 litres of water a day with off-peak electricity rather than with on-peak.

On the other hand, if one tops up rarely, the easiest thing to do is to set the overnight heater to a high temperature and the top-heater to a lower temperature. It will rarely turn itself on and the cost of having hot water always available will be less than £10 a year if one found that one was turning the on-peak heater on less often than once a week.

There are exceptions to this analysis. If one wears hair shirts and likes to be nagged into reduced water consumption, rely on the overnight hot water running out and giving you the occasional cold shower so that you have less time in the shower and take pleasure in using less hot water and less energy.

In practice there are a number of confusing factors to this analysis. Experience shows that those households in which saving energy is a high priority are also those with drench showers that use most water…

Other reasons for switching the hot water off at night.
Sometimes the most numerate don’t like the feeling that there is nothing that one can do to effect a significant saving and they want to control the water heating whatever. Sometimes there is a light sleeper in the house disturbed by the noise of the hot water system…

Surely there is something you can do?
Modern hot water cylinders are much more effectively insulated. If your hot water cylinder is old, and by that I mean perhaps as little as 10 years old because insulation standards have increased rapidly in the last decade, it is worth considering a new hot water cylinder.

Hot water cylinder swap worthwhile?
Heat your hot water cylinder fully and measure the hot water temperature (say at the tap with a cooking thermometer). Then turn off its heating completely and measure the hot water temperature after 8 hours.
If the temperature falls by less than 5 °C in eight hours your cylinder is well-enough insulated. The greater fall than this, the more worthwhile it is swapping the cylinder. If the temperature falls by 20 °C in 8 hours, you’ll save well over a hundred pounds a year by upgrading to a modern hot water cylinder.

The green serenity prayer
As always it makes sense to change the things that matter, accept the things that don’t and have the wisdom to know the difference.