Immersion Heaters

Simple advice to save energy with your immersion heater.

Heating water accounts for around 25% of heating energy usage in a UK home. There are a large number of homes in the country that are fitted with hot water cylinders but unfortunately the majority of people do not understand the costs of heating them. Many cylinders can be heated either with the gas boiler or by an electric immersion. Here we will focus mostly on immersion heating which is usually the more expensive option.

If your immersion does not have a thermostatic control, it will heat the water to potentially boiling point. Firstly there is the risk you may burn yourself with hot water (60oC is enough to burn you within 6 seconds) and secondly you are wasting heat and your money.

A typical electric immersion heater for you hot water tank is 3kW (Kilowatt). With a typical daytime electric tarriff of 15p per kWh (kilowatt hour), it costs 45p per hour to heat your hot water with an immersion. Heating with gas will be significantly cheaper. Gas is currently 5p per kWh and assuming you have an old inefficient boiler running at 80% efficiency, it will be costing you 7p per kWh.

You can halve the cost of heating your hot water tank by simply using gas compared to daytime electricity.

If you are on an economy 7 or similar cheap rate tarrif, the off-peak price of electricity will be about 5p per kWh, however your daytime rate will be considerably higher, probably 20p or more per kWh. Electric heating of water is close to 100% efficient so using an off-peak electricity tarriff if you have it will probably be cheaper than heating with gas.

If your immersion has a thermostat, it will switch off when water in the hot water tank reaches the temperature. By setting the temperature at a lower level will save you money. If you house has a small 120 litre hot water cylinder, and the water starts off cold (4oC) and you heat it to a nice 55oC with your immersion, you will use 7 kWh to heat the water. With the daytime rate this will cost you £1.05 whereas gas will cost you nearly 50p. If you did this every day of the year, you would save £191, simply by using gas.

If you dont have a thermostat and you let the water heat to 75oC with the immersion, you will use 40% more energy so it will cost you £1.47 instead of £1.05 with th thermostat. Over a year this would cost you £153 extra per year.

You might now be thinking that 55oC is too hot for you personally so set the thermostat to a lower temperature, well don't! Below about 50oC, the chance of legionella increases and they multiply significantly at temperatures between 40oC and 50oC. Legionella is a very bad bacteria so dont take the chance!

There are over 2 million homes in the UK that dont have gas so electric heating of water is common. If you leave your immersion always switched on, think about when you use hot water. If you have a well insultated cylinder it will lose about 1 kwh per day. That is it loses the heat equivalent to a 1 kw heater being on for 1 hour. Assuming you are paying 15p per kwh this equates to 15p per day or £54 per year. If the immersion is on just before you need the hot water rather than heating up hours before then the standing losses will be reduced. A simple way to save £54 is turn the immersion on 2 hour before you need the hot water.

A 3kw immersion will heat 1 litre of water by 1oC in 1.3 seconds. Assuming the water in the tank is all from the mains and is at 4oC, and you have a 120 litre tank, the tank needs to rise 51oC. It will therefore take just over 2 hours to heat the tank fully.

Immersion thermostat If you want to use the electric immersion, maybe because you dont have gas, check whether you have an immersion thermostat installed. Installing one is straight forward if you have basic electric skills and could save you £150 per year and cost you just £10. If your immersion already has a thermostat, it's worth checking what temperature it has been set to, simply turning it down to 55oC will save you money. A zero cost saving. Inside the Immersion
If your immersion doesnt allow for an immersion thermostat rod to be installed, it is still possible to add a cylinder or tank thermostat to turn the immersion on/off for the correct temperature. These are attached to the copper of the tank to measure the temperature. They need to be installed near the bootom of the tank - remember heat rises so the water at the top of the tank may be hot but the water at the bottom can be cold. If you have an instualted tank, some of the foam insulation will need to be removed so that the thermostat can touch the copper. Be careful not to puncture the tank when cutting away the foam. These typically cost £ 10-15 to buy and are relatively easy to install - just check whether the thermostat is rated to switch >13A . These can also be used with gas heating. If you have an old gravity fed boiler, it may not be possible to add a thermostat. You will need to speak with a plumber to see whether it is possible to adapt your boiler installation. Tank Thermostat
Potterton tank thermostat If you are frequently not at home, it may make sense to use a more sophisticated tank thermostat. The Potterton PTT2 tank thermostat has a boost button. When the button is pressed, the immersion (or gas) is turned on to heat a whole tank to the desired temperature. Once the temperature is reached, it doesnt come on again until boost is pressed again. These thermostats cost around £ 25 and could pay for itself in less than 1 year if you regularly leave the immersion on but are not home for long periods such as a 2 week holiday.
Mechanical immersion time switch If your lifestyle is more predictable and you know when in the day you need hot water from the tank, you can install a time switch to turn the immersion on a few hours before you need the hot water. This will reduce standing losses. A mechanical immersion time switch should cost about £15 and give 15 minute granularity of when to heat hot water. An electronic immersion time switch will be more flexible allowing different heating schedules on weekdays and weekends but will be more expensive at around £30. As previously mentioned, standing losses from the tank are likely to be £54 per year so payback for the cost of the immersion time switch should be within 1 year. Electronic immersion time switcvh
If you are going to do any electrical work yourself, make sure that the electricity is switched off to the immersion before you start and use heat resistance cable. Some older immersion elements contain asbestos so be careful you dont damage them. Unless the immersion is dead, dont worry about the asbestos - it will do no harm unless disturbed.

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