I have read that limescale in pipes makes them way less energy efficient, so that 50-80% of energy is wasted for example in a boiler. How would I get rid of this? I am looking for some product or liquid that I can pour down the pipes to disintegrate the limescale.
For cleaning limescale from my kettle I use Citric acid (lemon/lime salt?) and let the kettle boil with water.
Sometimes if I want to clean something else from limescale, I just pour the boiled water on that thing and it helps cleaning it.
If you have easy access to the pipes and their material is fine with with a bit of acid, so it can be a good and easy method. The reason I prefer it over vinegar is that the vinegar can leave taste and smell where used. But if you can wash it after passing vinegar, maybe it doesn't matter at all.
All limescale does is slow down heat transfer a bit. All the energy still ends up in the water, so there's no efficiency loss.
The slower heat transfer might pose a problem in tankless water heaters where the water spends only a small amount of time in the heater: the final water temperature will be a bit lower.
A boiler is usually a device that heats up a large tank of water. The water will be in there long enough to reach the required temperature.
Your boiler is connected to your water supply. Disconnecting that so you can pour e.g. vinegar into the water circuit is a lot of work, and afterwards you'll spend more getting all the loose limescale out of your water system.