there is an elson tank located in an airing cupboard, directly above a potterton kingfisher mf50rs boiler, the pump runs, all the rads heat up, the over heat stat and thermister have been checked yet whenever the boiler is left on for over an hour it srarts to produce steam from the elson tank, which fills the room! and sugestions on probable causes?

many thanks

Possibly pumping over.

considered this, pump was set on 2, dropped pump speed to 1 but still the same problem. the pump does look as though it has been changed at some point, is it possible that to big a pump has been fitted? any recomendations for pump size on a gravity hw / pumped ch system with arround 1.2m of head?

Check the pump when fitted was not reversed (put in wrong way round) and is on lowest setting. When running, is water flowing from heating vent pipe, and if you can check heating cold feed for water movement (stick you hand in cistern) If its the vent pumping over it may be easier to extend the height of vent pipe than try to find an alternative pump. (assuming a standard circulater has been fitted. Good Luck