Hot Water 2

Topic 2. Domestic Hot Water: Heating Water Efficiently


This topic guides you through ways to improve the efficiency of generating hot water using electricity including interaction with solar PhotoVoltaic (PV) systems.

Using electricity to generate heat can be expensive and carbon intensive because of the higher cost of electricity and the higher carbon emissions compared to gas. Therefore it is best to ensure that you are only generating as much hot water as you need, only when you need it and reducing the heat that escapes. We’ve outlined some key considerations for improving the efficiency of hot water generation.

Energy Savings Opportunities

Insulating hot water tank and pipes:

Insulating your hot water tank and pipes reduces the amount of heat lost, so you spend less money heating water up and hot water stays hotter for longer. Most tanks have some insulation attached as standard but if you can afford it, spending a small amount on increased insulation will save energy.

The image on the right shows a hot water tank with uninsulated, hot, pipes coming from the top. The tank is insulated, but you can see that it is still giving off heat. Insulation reduces heat loss, but will not eliminate it completely. New cylinders will have 50mm of insulation as standard. Topping up existing insulation so that the tank is surrounded with 10mm is a good idea.

A hot water cylinder jacket costs about £15, and fitting it is a straightforward job if you follow the manufacturer’s instructions.

 Pipe insulation consists of a foam tube that covers the exposed pipes coming out of your hot water tank, reducing the amount of heat lost and therefore, keeping your water hotter for longer. It’s usually as simple as choosing the correct size from a DIY store and then fitting it around the pipes. If other sections of pipework are also uninsulated make sure you cover these up too. It can also help to keep cold water colder too.

Point of Use Generation is a great, efficient way to provide hot water to individual sinks. Point of use systems only heat the water required as and when it is needed. For example a kitchen sink might have a small electric immersion heater under the sink which heats water when the tap is turned on. This is similar to an electric shower. Point-of-use (POU) wastes less energy as there is less heat lost in the pipes between the tank and the tap, less hot water is stored and potentially less hot water generated in the first place.

It is worth considering this if your hot water demand is small and localised, for example your hot water tank is only feeding the bathroom sink taps and a kitchen sink in a self-catering accommodation. Changing to POU also means you no longer need a large hot water tank, and so this can free up storage space in your building. You can install POU under your kitchen sink and also in your bathroom.

The image below shows a point-of-use hot water heater underneath a standard kitchen sink. It holds and heats 10 litres of water at a time and is rated at 2kW, which is the same as a kettle. Which means if it were running at full power for 1 hour it would use 2 kWhs of electricity. You can just about see the dial on the unit, to the right of the central bar of the kitchen unit. Using this dial you can adjust the temperature of the water, about mid-way is fine for washing up. The unit can be turned off over night using the main switch on the back of the cupboard to the right of the image.



Consider: What do you need hot water for, and when do you need it?

It’s worth considering the timings for your hot water usage, so that you can optimize your hot water usage, and take advantage of any solar PV or solar thermal generation, alongside any dual tariffs you may be using. If you have solar PV and an electric immersion heater, make sure you have a timer set for when the most sun hits your panels. This way you can make use of the free electricity being generated.

It is possible to buy systems that will do this automatically. These are called solar power diverters or sometimes ‘solar PV optimisers’. These systems monitor when surplus electricity is being generated and  the system diverts the surplus electricity to your hot water immersion heater.

Always make sure your tank is well insulated with at least 100mm of insulation, so that the tank stays warm until the water is needed in the evening.

If you have a duel tariff and pay a lower rate for your night-time electricity, and you are sure the volume of your tank is large enough to supply a full day of hot water demand, then set your timer for over-night to make the most of the lower cost electricity. If you do this, make sure it is well insulated. See below for more information.

If you do not have solar PV or solar thermal panels then set your timer to run the immersion heater for when you or your business needs hot water. This could be a few hours in the morning and a few in the evening for a typical accommodation business.

Immersion heaters don’t have to be on all the time!

It’s a common myth that it’s better to leave the hot water on all the time rather than turning it on and off. Your immersion heater will heat up hot water which is stored in a tank, and as long as the tank has a good insulating jacket, it will keep the water hot all day, without needing to be constantly reheated[1].

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 (ideal temperature around 60 degrees Celsius), then the losses through leaving it on can be much reduced. But in general, it’s much better to install a timer – a heavy duty one, suitable for immersion heaters, which are generally quite cheap[2].

If you have an economy 7 tariff then set your timer to run the immersion heater overnight. Here is a image of a typical controller for an immersion heater. It includes a boost control so you can add hot water when required without having to change the time settings.

Check: Do you know what temperature your tank is set to? Do you know when it runs?


Next Steps

Having read the information above, think about the following questions in relation to your business. If relevant, have a conversation with all the staff to find out the answers:

  • Your current water heating practices: e.g. times of heating, times and volumes of use, what it’s used for.
  • Are the pipes and tanks effectively insulated? If not, decide who can do this and where would you buy the insulation material?
  • What opportunities there are for reducing the electricity consumption for water heating?
  • Do you have effective timers on your immersion heaters so that you can control when it turns on and off? Is the thermostat set at an optimum temperature?
  • Do you need to replace your hot water tank for any reason? Should you consider installing point of use heater instead of a new tank?

If you have carried out one of these actions communicate this to the guests. Try explaining to guests that heating hot water is one of the biggest energy consuming activities on the Isles of Scilly but that your business is helping to reduce this. Finally, describe what you have done.

Activity: Write a paragraph of text explaining to guests why you are working to reduce the energy consumption of your hot water system.


Still have questions?

Head over the forum to post questions, comments or your experiences.

Further Information

Energy Myth buster

Home energy myths

The Energy Saving Trust – Tank and pipe insulation