Did you ever wonder whether it was cheaper to heat your hot water tank with your immersion heater or your oil/gas boiler? Well we have done the sums so you don’t have to. Below is a comparison between various different ways of heating your hot water tank.
Background: Boilers heat water and pump it through the heating pipe network of the house. Part of this pipe circuit runs through the hot water tank and indirectly heats the water in the tank; there is no mixing of water between circuit and tank. By contrast an immersion heater uses electricity to heat the water in the tank only.
Results of calculations: Based on the calculations below, standard rate electricity is the least efficient way to heat water in your home. The cheapest way is to use night saver electricity; however this will require the installation of an additional electricity meter before the night saver rate can be availed of, i.e. extra cost. Therefore for most people, using their oil/ gas boilers is the best option, especially in winter when the heat lost from the hot water pipe work between the boiler and water tank will help to heat the house. However this will only be true if people ensure that their boiler is serviced every year to ensure high efficiency levels. Of course if anyone has access to free timber (from cuttings or scrap), then a wood burning stove with a back boiler, would provide free hot water.
Summer valves: A summer valve isolates the boiler from the radiator circuit and therefore less water in the pipes is heated if the boiler is turned on to heat the hot water tank. Basically then a summer valve increases the efficiency of using the boiler to heat water in the hot tank.
My own experience: My house doesn’t have a summer valve, so our boiler heats our water in the winter months along with the house. We also have a wood burner stove. In the summer months we usually just use the immersion to heat the water as we use little enough hot water; we have an electric shower for washing and I am happy enough to shave with cold water! The immersion is usually only used for baths and if we are washing up after a party or something, otherwise the dishwasher can handle the load. I might use the boiler more in the summer after doing these calculations though.
Note: By their nature, the calculations below are approximate averages as they are based on various assumptions and complexities. Every situation is different and the purpose of this exercise is to just give an indication of what system is most efficient. For a specific house, the calculations would need to be redone with the house system particulars.
Click the following link to view a PDF of the calculations, Boiler or immersion heater to heat hot water, or below for an image of lesser quality.