Masona,
I had the same when I used a chemical cleaner on my brothers flat. I introduced the fluid into the CH header tank, then ran the CH/HW system for 1 hour on full heat (per the instructions).
The blockage was broken down a treat by the chemical, and the "cold" rad started working lovely. I drained out the chemical and fill/drained the system twice more to remove the chemical. I then put an inhibiter in to prevent corrosion and prevent scale build up.
Later that evening, his wife ran a bath and the hot water came out brown/black.
Inside the hot water cylinder is a coil of tube, through which runs "boiler heated central heating" water (controlled by a valve) as the Hot water part of the CH/HW system. The water for the taps in the cylinder gets heated indirectly by the "central Heating" water circulating through this coil (ie by contact with the hot coil which itself is heated by the water in the CH/HW circuit).
The chemical cleaner had "cleaned away" the corrosion which was on the outside/inside of this coil where it is soldered to the inside of the tank wall - allowing Boiler heater water to pass from the coil into the tank water. The corrosion was probably the only thing maintaining a seal inside the tank.
A repacement tank cured this defect.
Sadly, a couple of weeks later, the downstairs flat reported "light brown rain" falling in their lounge - an underfloor joint in the vacinity of the previously blocked radiator had also corroded through and (probably) as a result of being chemically unblocked - started leaking as a piece of corroded material became detatched.
It is often the case that in older, corroded systems, at some joints, the corrosion present actually prevents them leaking.
I used to joke that I once had a car that I wouldn't wash in case it was the dirt that was holding it together
.
Paul
