Our new Beta 60 hp engine is going well, but are baffled by cooling water overflows.
We have a water heater, mounted higher than the engine
We also have a cabin heater. It has a pump that pulls off the line to the water heater, through a total of about 50 feet hose, and puts the water back into the line flowing to the water heater.
The capacity of the whole system is 11 quarts. We put that in, which came to about half way up the 2 quart header tank
Ran at idle for a short while
Level dropped and we topped up to mid point on tank.
This happened twice, total addition 2 quarts
When we run under load, the engine heats up to 72 C (about 170 F) and the cooling water quietly overflows. NOT boiling or anywhere near it.
The header tanks is only about 120 deg F
There is a vent from the high point in the water circulation (at the water heater) to near the top of the header tank, so it seems to me that any air will get out that way.
If we start from cold, and the header tank almost empty, it still overflows when the engine is up to operating temperature.
When I calculate the volume of the expansion from cold (40 deg F here) to 170 F, I arrive at about half a pint, so that should not be enough to overflow.

Any ideas on how to resolve issue?