Don't use water from a C.H.system. When I worked at a British Gas call centre, they justified their seemingly high price for a power flush by including a chemical that inhibits rusting of the internal pipework and so preventing further clogging up of the system. If this chemical can inhibit rusting at temps up to 30 degrees C for up to 5 years minimum, then it may be safer to distill your own urine than risk that stuff getting into your system FWIW