Good morning Pops, water usage will depend on the temp that you run the boiler at, I personally run them at 190 degrees F plus, what I tell my customers is run it as hot as you can without seeing visible steam coming from the vent tube, if you see any steam from the vent tube then back the temp down by 5 degree increments until it stops. That being said, I always tie a domestic make up water line to the manifold and outdoor wood boiler side of the heat exchanger to avoid dragging a cold garden hose out to the boiler and having to deal with draining it when done. all it takes is to tee the domestic water into the boiler line, put a ball valve on each side of the tee so you can direct the water to the boiler either on the supply or return side separately, by doing this you can eliminate any air in the lines. feel free to call if you can't understand my jibberish