On 19 July, 15:07, dog-man wrote:
What's the best stuff to clean a secondary heat exchanger?

Periodically, mine needs a degunge and I use normal bog standard
descaler such as a kettle descaler.

It does work, but I was wondering if there is anything better which is
safe to use?

Steve...........

The local heating guy told me to pour sulfuric acid (the drain
cleaning stuff) down the CH flow or return connection until it come up
the other side and then leave it overnight before rinsing all the crap
out with lots of water, I've used citric acid in the hot water side.
Make sure its stainless steel though!