Since I re-sited the bleed screw on my back boiler heating system and filled it with a 30% antifreeze solution, it's been working much better and I'm more relaxed about leaving the boat for the weekend in freezing weather.
However. It's now using water! The header tank level is dropping, perhaps by as much as a litre a week!
I know that antifeeze can get through a gap that water can't, I pulled up a couple of weeping joints when I first filled it, but I can't find any external leaks.
Pretty much all of the pipework is accessable for inspection. Only a small section to the calorifier is out of sight and there is no evidence of water in the bilge below that. Antifreeze leaks are usually detectable by the sweet smell, especially in the cramped still air of a boat, but no such smell is apparent. There is one slightly "wet" compression joint on a radiator that won't pull up, but no evidence of an actual drip.
I don't think the calorifier coil is leaking internally because surely if so the (pressurised) water in the calorifier would back fill the (unpressurised) water in the heating system and cause the header tank to rise, not fall.
I dont think the water is evaporating because the header tank, which is the only part of the system which is vented, never gets very hot.
I also don't think the back boiler is leaking into the stove, as there is no evidence of water inside the stove even when it's been left to cool down for a couple of days.
The stove has been run continually for days at a time this winter and has given no trouble other than the mysterious vanishing water!!
I really haven't a clue where it's going.......any ideas?