Expansion & Bubbles

Everything gets bigger when you heat it.  Fact of the universe.
Water is one of "everything", so when your engine warms up, the cooling water in it gets bigger - it expands.

In older cars, space was left at the top of the radiator for the water to expand into.  In newer cars, the radiator is filled to the brim, and there is an external expansion tank.  There is always one somewhere in the system - there has to be.

Expansion tank on Eolian's engine

It is no different for boat engines.  But.  No radiator, no expansion space in the radiator.  But there has to be an expansion space somewhere in the system.  Right?

OK, now second thought:  air bubbles.  Where do they collect?  Yup, you got it in one - at the highest point in the system, eventually.  In a car, we're back to that space in the radiator.  It's easy to replace the air bubbles in the system with more coolant if they collect in the expansion tank.  It's a good combination of uses.  And in any modern car, the expansion tank or exit to the external reservoir is at the highest point in the system.

Ah, but in a boat.  In a boat, the "radiator" (there isn't one - it's a heat exchanger instead) is unlikely to be the highest point in the system...  Do you have a water heater that gets its heat from engine cooling water?  I'll bet it is mounted considerably higher than the engine.  How about a Red Dot heater (Eolian has both)?  Same question.  So where do the inevitable air bubbles accumulate?

Well, at the highest point in the system.  Always.  On Eolian, that was the water heater.  So, guess how effective the heater was, given that the hot water coils were filled mostly with air?  Yeah, not so much.

Expansion tank, above the water heater

It's been years now, but I installed an auxiliary expansion tank/reservoir at the inlet for the engine cooling water at the water heater.  And collected a lot of air in it.  The expansion tank on the engine simply got filled completely with water - an ineffective and irrelevant (and now removed) bulge in the system.  I put a 14 lb radiator cap on the engine expansion tank, and moved the original 7 lb cap to the auxiliary expansion tank by the water heater.  That way the cap on the engine would never release, and the one on the auxiliary tank would.

Big change in water heater efficiency!

So where do the air bubbles in your engine cooling system accumulate?

Expansion & Bubbles