stevet wrote:
Geoff

A bit more info about the system, it's an open system with a header tank
that's inside the boat, it can only hold about 5 litres. It's not at the
highest point in the system though as the pipe from the burner goes up and
right over the top of the cabin and is probably a couple of inches higher
that the tank. There's also a vent outlet that goes out through the roof
that is permamently vented, it has a fitting with some fibre washers in it
that seem to allow air through. The other heath robinson bit is that the
rads are connected in series so you can't turn any one of them off or down
as it would stop the flow completely.
As to getting air in the sytem, If you let the burner get too hot without
the pump running then it vents steam out through the vent valve and the
system looses a lot of water and air 'gets in', it then takes ages to bleed
everything again, hence the desire for an automatic valve.
My plan is to put a pipe thermostat on the outlet pipe from the burner and
get it to turn the pump on if the water gets too hot.
The other suggestion I like from BigWallop is to extend the bleed valve near
the pump up higher so that it can accomodate more air before it affects the
pump operation.
Or I could reposition the pump so that it's before the loop and use an
automatic valve.

thanks for all your help.

STeve



Hi Steve, I'll need to spend time pondering your post, but, I do feel
your going about this all wrong.
In my opinion you 'must' prevent air from entering the system. If your
getting boiling up into the header then you must take steps to prevent
this, bloody dangerous if nothing else, can you not have the pump
running whenever the stove is alight?