I did the same to mine 4 years ago when the heat exchanger in the fire started to leak, and had no problems whatsoever. Pity though as it used to give me a lot of 'free' hot water.
Depending on the type of heat exchanger (saddle/2 bits or 1 bit or top), having disconnected the pipes from the back of the burned to the Hot Water Cylinder/CH system you could remove the heat exchanger/s which will give a better heat from the burner ~ don’t forget to block up the holes though!!
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