I added the loosening chemical to our closed central heating system a few weeks ago and am going to flush the whole thing a few times this weekend and add the protection chemical. My daughter's rad was gurgling the other day so I bled it. Strange smell to the air that escaped. Then while standing in the kitchen later I heard a short hiss from the top of the boiler, a Potterton combi 80, and smelt the same smell. No trouble since but is there an air escape valve up there or should I be worried??