Unfortunately Inlet and Outlet are exactly the same level.
Oh dear, not well designed, this makes you totally reliant on the efficiency of the water pump and the frictional losses of the hoses and matrix core, you may have a partial air lock in the heater - try a bit of a power flush with a hosepipe via the feed pipework
All the Smiths round heaters had inlet and outlet adjacent and at same level.
They relied on water pump typically taking hot feed from cylinder head often with dedicated union at rear of head to shorten pipe run. This feed was via manual gate valve in engine bay or cable operated eqivalent to allow control within vehicle. Return was to pump inlet.
They work fine providing matrix isn't clogged up. Less can be said for the demisting abilities on my Land Rover though
I note mine has a T piece at the outlet with short pipe and bung to allow bleeding.
