As per above post.......
The Q&B stuff (for example) is not that strong in the form supplied (about 5% HCl / 95% water), so you can pretty much use it from the bottle. If you're worried, dilute it 50:50.
(Even at 5% it will however irritate the skin quite quickly so gloves / googles & running water to hand......)
HCL at this strength attacks Aluminium quite slowly so two or three 10 minute 'exposures' is unlikely to do much harm (unless the core has become paper thin over time), but DO NOT leave it in there for hours and hours (e.g. over-night).
As long as you have SOME flow through the core the acid can get ( and needs
to get) through the entire
matrix ( uncoiled it'd probably be about 8 or 10 feet long) in order to get to work on the limescale therein.
If you have NO
flow whatsover then the acid won't work beyond the first few inches of the heater pipework.....so don't waste your time.
Hydrochloric acid won't dissolve the rust deposits, just loosen the limescale that its all bound up in....hence the need to pump it through with a garden hose to blast out the red muck.
I needed to repeat this about 3 times before it finally stayed clear - which also seems to be the experience of many others who've posted here.
For reference, a clear / clean matrix allows a VERY substantial flow of water through - close to the flow rate of the garden hose itself - so if it "ain't pouring out" after treatment its probably still partially blocked.
Gently rinse everything down afterwards to get rid of any last traces of acid - cover up the electrics / intakes for obvious reasons.
Best of luck.........