You, sir, are both arrogant and (apparently) unable to read. You are also, it appears, not knowledgeable about the warm air system about which you chose to offer faulty advice.
You would do well to withdraw and consider your position.
In the FIRST LINE of the original posting, the OP stated that it was a warm air system with a separate heat exchanger for hot water. This would cover several types, MOST of which must NOT be treated with anything that could affect the quality of the DHW.
Your latest offering:
It sounded as if it has a copper coil heat exchanger for warm air take off
is evidently an attempt to hide your blushes rather than an honest misinterpretation of what the OP actually wrote.
From my own direct, personal experience of a number of J&S and similar warm-air systems installed within less than a mile of where I live, Primatic cylinders are a common method of providing hot water. Amongst other work, I have in fact REMOVED one such Primatic cylinder and replaced it with a pumped, indirect cylinder, a) because Primatic replacements are ridiculously expensive and b) to improve hot water recovery time over the previous gravity system.
So again - please withdraw your offensive comments, apologise for your error, or do what I suggested elsewhere!
I think this thread has now been adequately covered - always assuming the OP has read it - and I have nothing more to add whatever you post in response.