I recently came across (while looking for something else) a product called UMAX, which is an additive for the water in a central heating system that is said to

"change the way water boils, making the water in your radiators hotter without turning them up. As water in a central heating system passes over the heat exchanger in the boiler, a small percentage of that water will boil on the hot metal surfaces. During this boiling a layer of irregular bubbles forms which eventually join together and act as insulator between the gas firing and the water within the radiator circuit. Umax changes the way water boils so that only micro bubbles form which do not coalesce greatly increasing the heating efficiency and the transfer of heat from the boiler into the radiator circuit. (see: https://www.youtube.com/watch?v=9mKxfzIuQLE)"

It sounded a bit far fetched, but I found some on eBay for just £3.99 post free, which was hardly going to break the bank, and lo and behold, it seems to work, insofar as the radiators really are a lot hotter - almost scaldingly hot.

I don't really understand why this means the boiler thermostat doesn't cut out earlier - but I pass this on, being interested to see if anyone else has had experience of this product.

Keith