When descaling a machine we use either straight white vinegar or 1-2oz of citric acid in 64oz of water.
These compounds will not harm you or your machine if fully flushed from the system.
Plumbed-In Espresso Machines
With a plumb machine we remove the water line from the water source and submerge it into a picture of the solution. Allow the machine to fully heat then pull about 8 oz through the group then pull about 6-8 from the hot water wand. Let the machine fill and reheat and sit about 1 hour then repeat every hour until the pitcher is empty. Allow your machine to cool if your machine has boiler drains. Then open the drain plug to drain the boiler/boilers. Close/seal the drain plug. If your machine does not have drains then turn off the machine and open the water wand allow the steam pressure to push out as much water as possible. Fill the pitcher with fresh water. Allow your machine to fill and heat. Flushing you will pull about 8 oz through the group then pull about 6-8 from the hot water wand let the machine fill and reheat you do not have to wait an hour in between. Flush until the water runs clear and smells clean. It is important not to drain more than 8oz from the hot water tap at a time you can risk damaging the element."