Expansion question

I have two 1000 gallon tanks that are horizontal, with one above the other. My question is can I only fill, let's say 1800 gallons worth of water, and use the remaining 200 gallons of vapor space as an expansion area, or does a separate tank have to be used? Thanks in advance for any responses.

You bet. Need some way of knowing what the water level is, and of course need to draw hot water to loads from the right level and need to keep that level flooded.

It might make sense to do away with the external air separator on the wood boiler supply side and let any bubbles return to storage.

Great, I was hoping that could be the case. So some external sight tube could work or I could get fancy and install a level transmitter with indicator.

It would then look a lot like a Garn boiler.

You have to be fastidious with water treatment chemistry to have all that wet iron exposed to air all the time. But it can be done without too much trouble.