If you are in a hard water area of the country you will find that your heater elements will become coated with limescale during use. As the limescale gets thicker it will take more time and electricity to heat your baptistery. There are 3 ways of tackling this but they differ a little depending on which heater you have.
Remove 4 corner screws on the lid and carefully chip away the lime scale from the elements. In most cases this will come away quite easily.
Off-the-shelf products such as Killrock (or any other product used for de-scaling kettles). Again remove the 4 corner screws from the heater lid. For the 2.4 heater, slide the lid up the cable out of the way, mix de-scaler using manufacturer’s instructions. Put mixture in a bowl just deep enough to cover the element when heater is stood on its end. Make sure only the element is in the mixture. It will not hurt the plastic box, which will have to be partly immersed too.
For the 4.8 you will need a shallow container with at least one straight side as once you have removed the lower part of the box containing the elements, the elements will have to be immersed without touching any other components. The descaler is mixed as above.
When the limescale touches the mixture it will usually fizz for a while it’s doing its business.
Other remdies use household items such as vinegar. Whilst you could use an equal part solution of water and household vinegar, you will need a lot of it, as you’ll see. Mix 1 part vinegar with 1 part water and stand the elements in it for an hour. Then heat the solution with the heater. The heater needs to be covered by 10cm of liquid to ensure it is not damaged, so you will have to mix up large amount of vinegar and water.
Only attempt the descaling if you feel confident. If not go to option 3
You could combine service and de-scale as it does not cost a lot and extends the life of the heater.