I have a question that is a bit of a tangent, but also not entirely unrelated... my apology in advance if this seems like it is hijacking the topic. I just replaced my hot water heater (with a coolant/heat exchange to warm the water). In the process of disconnecting the old heater, and installing the new, I lost a fair amount of coolant (in the old heater, in the lines, etc.). I haven't started the engine yet (boat is being launched tomorrow), but obviously I anticipate I'll need to add coolant to replace what was lost (maybe a quart or two), and somehow bleed out air from the system. After doing the swap the other day, I opened the coolant fill cap on top of my engine (Yanmar 3ym30) to see how low the coolant was, and it appears to be totally full. The water heater is physically lower than the coolant fill, so obviously there's more than just gravity at work here (otherwise coolant would have drained entirely out of the water heater hoses while detached). Does the engine's thermostat control coolant flowing to the water heater loop? Assuming yes, do I need to run the engine long enough to get that to "open", and then add fluid, as the level falls in the coolant tank? I can't bleed the coolant loop if nothing's flowing into it... and I know these might seem like "rookie questions", but... can I add coolant to the overflow bottle to more easily "see" the coolant level while I'm running the engine to get coolant out of the coolant fill tank, and into the water heater, or do I need to add/monitor through the actual fill cap on top of my engine (not as easy to access/see especially while the engine is running. Not that it really matters, but I'm using Ethylene Glycol (green stuff) that's only seen one season of light use, so I'm not planning on flushing the system and starting fresh if I don't have to... planning on just topping off with more of the same (E.G.). Thanks for any advice/tips/clarification in advance.