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




"Geoff" wrote in message
...

I've been helping a friend get her canal boat 'ship shape' and the
heating sytem is most odd, there's a log burner with some sort of back
boiler and pipes that run from one end of the boat to the other, via the
radiatoirs. The pump (a small 12v affair) is at the rear in the engine
bay and there is a loop of pipe that goes up and over before going to the
inlet of the pump. This loop seems to alway get air in it, it does have a
bleed valve but it's a pain to keep having to bleed it. Also you have to
bleed it with the pump off otherwise the pump just sucks more air in.
My question is this, can I put an autmatic vent valve onto this loop,
bearing in mind it's on the inlet side of the pump and only 6in away from
it, or will it just result in more air being sucked into the system. If
it's not a problem then it will make life easier in keeping air out of
the
An automatic vent has to be on outlet side, ie positive pressure. You need
to find where the air is getting into the system and cure that, and, or
move the pump down-stream to a position where it wont suck air. Air, or
rather oxygen in the system on a continuous basis is a 'very-bad-thing'
it'll rot your radiators in no time. There is some mechanism in the system
to allow expansion of the heated water I hope, is air entering thru that,
as when the system cools.
g