Hot Tanks – A Hot tip by Transition Town Letchworth

A hot tank is like a large electric kettle but is much more expensive to run. The following is irrelevant if you have a combination boiler (and hence no hot tank) or you may use your gas boiler to heat the water even in summer when your central heating is off.

Electric immersion heaters are expensive to run. A hot tank heated by gas via your central heating system can be very efficient as gas is around 4 to 6 times cheaper than electricity per kilowatt hour (kW-h). However, most tanks also have an electrical built-in immersion heater.

Immersion heaters use a surprising amount of energy even if they are well insulated. A 3 kilowatt immersion heater that is on for one hour will consume 3 kilowatt hours which at say 52p/kWh (the present ‘price cap’) costs around £1.56p an hour! You may be paying less than that price cap and your tank's thermostat will limit the running time. However, the annual running cost is still likely to be significant and more than that of your freezer.

Immersion heaters are typically rated at 12 amps of current, about the same as an electric kettle. Since a partly-filled electric kettle is only on for a minute or so at a time, it costs little to run (always boil the minimum quantity you are going to need), but if your immersion heater is left on 24 hours a day, that may be your biggest energy cost. Washing machines and dishwashers generally heat their own water, so your kitchen and bathroom sink taps only need warm water, not water at say 45 degrees. Hot water is only needed for showers (about a third of a tankfull) and for baths (about one tankfull).

So if you regularly use your immersion heater make sure that it is on a timer rated at 13 amps with a manual override. Set it to come on for around 15 minutes per day (preferably in off-peak times). For showers or baths use the timer and override facilities to minimise the on times. If the immersion heater is plugged in you can just insert a timer yourself; but if it is wired in you will need an electrician. An electrician will also be able to access the adjustable thermostat inside the immersion heater to reduce the maximum temperature so that you get the right temperature for showers and baths (but no lower than 55°C, to meet Legionella requirements!).