I too am completing a channel 31 and was advised by the Marine Engineer doing my engine installation that the calorifier is in the wrong position. Apparently it should not be higher that the highest point of the engine otherwise you get airlocks!
I am thinking of modifying mine and putting the calorifier lower down so am very interested in what you find out. Have you tried the Hunter Association website or Simon at the Factory?
Once the system is bled of air initially you should not have any problems even if the tank is above the engine. Keep the flow and return lines as short as possible and fill your cooling system from the calorifier hoses by disconnecting one of them and pouring the coolant in using a funnel - that should get all the air out of the coil. Alternatively disconnect the return to the block, keep your finger over the block spigot and run the engine while topping up the header tank - needs two people for this!. As a precaution you can fit either an automatic bleed in the highest point or a stopcock which you can open to bleed out any air. Another alternative is to fit a circulation pump in the feed pipe between the engine outlet and the calorifier switched through your ignition. The calorifier on my Sadler is mounted well above the engine but does not give any problems with air locking (I have a circulation pump as my engine is raw water cooled and drains a bit when stopped but the circulation pump forces the water round the coil pushing out any air once the block is full again)
“Artificial intelligence is no match for natural stupidity”
Dont forget to put a u-bend in one if not both pipes,as this will stop thermo syphoning thro. the engine,when at rest.If you do what "cliff" says and it still is no go try running the engine at highish revs.for a while as this sometimes works.
Now that's interesting. I've had a (Johnson - ceramic impeller) pump in my calorifier circuit for years, giving piping hot water from my raw water cooled Bukh 20. After removing the engine for a complete strip down (new pistons, liners, rings etc etc) and replacing it in exactly the same way i.e pipe runs to/from calorifer as before, I've now got no hot water even though the pump's working as before.
Boat's only been back in the water a couple of days, so haven't had much time to fiddle about, but did check with Bukh re the pipework runs. All ok, though they reckon the pump should be on the flow from the engine (i.e. before the calorifier) whereas I've always had it on the return line after the calorifier, and it's worked perfectly before. The calorifier inlet is higher than the engine and it's outlet is slightly lower than the return connection on the motor.
Have tried separately bleeding the circuit as you have described (with the pump running water powers out of the return!) but the 'hot' water only gets about 200mm along the pipe from the engine once it's reconnected to the block. It's almost like there is a contra flow stopping it. The engine is spotless internally with no furring up or other restrictions.
It's really weird, everything as before, but now it doesn't want to work. So, any ideas??