Engine Coolant Expansion Tank Question

[Beausoleil]

I'm finally getting around to plumbing the engine coolant connection to my recently-installed Isotemp water heater. My engine manufacturer, Westerbeke, recommends "if any portion of the heating circuit rises above the engine's closed cooling system pressure cap, then a pressurized (aluminum) remote expansion tank must be installed in the circuit to become the highest point."

The existing expansion tank next to the engine is plastic, but I suppose that's because it's connected to the expansion valve of the pressure cap on the engine. When the water heater is plumbed in, there's no pressure relief valve mounted near it, hence the need for a more robust expansion tank. Does anyone know where I can find such a valve, and more importantly, would it just be easier to bite the bullet and fork out more cash to Westerbeke for their aluminum expansion tank kit? Their water heater connection kit is already expensive enough! I'd hate to think what they charge for an aluminum expansion tank...

[colemj]

It doesn't have to be aluminum. Get any old engine coolant tank out of a junk yard, plug off one of the inlets, mount it higher than the engine/water heater, put a new pressure cap on it and tee it into the cooling system using the other inlet (fill it with coolant, of course). This will now become your main expansion tank, and the one on the engine will just be part of the cooling system (so don't open the pressure cap on the engine or coolant will flow out from the header tank).

Hunt around old marina yards with junk engines lying around.

The plastic tank is for overflow, and can't be used as a pressurized header tank.

Mark

[anjou]

I agree with colemj The plastic tank is only an overflow catcher and not rated for temps and pressures of normal engine running. You may have to change the spring cap on the old header for a non spring one and make the new one on the header a spring loaded pressure relief.

[James S]

I'm not so sure...is the water heaters exchange coils higher than the highest point of the engine cooling?
The coils on my Isotemp are on the bottom when its lying on its side...I installed it that way to avoid having any of its coils higher than the engine.
I think if the expansion tank is to be installed at the highest point in the system its main job is to be an accumulator type tank in which case it will be under the same pressure as your engines cooling system.
What happens when you open the coolant cap on the engine...does the coolant that is higher drain out?
There is a lot to be said for having the WH lower.

[Chief Engineer]

I have seen boats with a simple header tank at the high point....it is "Teed" into the incoming line to the water heater. The Pressure cap remains the same on the engine.

[redcobra]

Yep, had to do the same thing when I repowered. ALways wondered why the expansion tank was in the cockpit for the old westerbeke now I know. Luckily I didn't throw it away and just reused it.