Geoff

Your right, I plan to put a pipe thermostat on the outlet pipe of the log
burner and have this switch the pump on, it'll probably need a relay near
the pump as there's about a 50 foot run to the back of the boat and at 12v
there would probably be a lot of loss unless I used thick cable. I though of
using a timer delay relay that keeps the pump running for a time after the
stat has gone open to stop the pump from cycling on and off. That should
stop any more air getting in and then once it's all bled it will hopefully
stay that way.

many thanks

Steve

"Geoff" wrote in message
...
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?