This is my experimental mini radiator for the downstairs loo made from scrap 28mm copper pipe and some spare fittings. I got the idea after finding an old photo of the bio-fuel heat exchanger I made almost 10 years ago. It's painted matt black to maximise radiation (finally found a use for O-level physics). So far it isn't working too well. Airlock is one problem I need to consider, but I also have a suspicion that the TRV valve may have stuck after more than a year screwed down closed. I'll investigate further.
UPDATE - Seems to have been an airlock which has cleared itself elsewhere in the system, hopefully right to the top!