Protein build up is quite normal during the brewing process, the Grainfather High Performance Cleaner is specifically designed to remove burnt on proteins.
If using another cleaner, use as directed on the cleaner.
Fill the boiler up with 7.5 L (1.98 US Gal) of water. Add 30 ml (1 US fl oz) of Grainfather cleaner (if using another cleaner, use as directed on the cleaner instructions); remember to set the temperature to 55°C (131°F). See details under the cleaning section of the Grainfather manual.
Also, don’t be scared to give the bottom and sides a good scrub with a soft bristle brush. If need be you can increase the temperature to 65°C (149°F) and soak for 30 minutes to soften the buildup. It is best to scrape the element every now and then during the boiling process to minimise protein build up. If you don't, you may cause the element to trip or cut out during boiling.