We have an Aga with back boiler in kitchen which is connected to a direct HW cylinder on the first floor. It works via thermosyphon and has done for years.
It is apt to kettling the cylinder at times, usually during the night when the lids are shut. We always have an excess of very hot water when the aga is running at cooking temperature.
Adjacent to the kitchen & behind the Aga, we have an un-heated utility. Is there any way that a small radiator could be plumbed into the thermosyphon pipes so as to both heat the utility somewhat, and also provide abit of a heatsink to the aga to prevent it kettling the cylinder???