It sounds like you may have a leak somewhere in your system. This would cause the system to refill with fresh water, if it has an autofeed regulator. Fresh water has air in it. Do you heat your hot water with the boiler? Is your pressure high in the system? Does the relief valve seep water?
Similar thing happened to me. We switched to coal and had suburbans propane tank. It was a 500 gallon tank and we were only using a gas range. I bought a 100 lb tank and called them to come get their tank. They said that they didn't have a tank there. After a few weeks I threatened to drag...
Are you doing this install yourself? If you aren't then run away from who told you that. That's what mixing valves are for. It mixes the cool return water with the 160-180 degree boiler water to lower the supply temp down to 100-115.
Being in NEPA, you should look hard at the coal option. The combo coal/wood burners generally aren't efficient. Anthracite needs a deep firebox with air coming up underneath it. Most outdoor wood boilers don't have that and are more suited for bituminous coal. I have a 24 year old heatmor 200ss...