One thing with an open non pressurized storage tank is that it is vented, not by much and the vent configuration tends to minimize venting of vapor but they are vented. My AST tank has a 1/2" PVC vent and I use it to hang a temperature probe down into the top of the tank so I had not checked the water level in the tank in a few years. I was down prepping for the season and I removed the sensor and "sticked" the tank. First try with a short stick didn't reveal any water when I pulled it out. I got a longer stick and did find a water line but it looked real low so I repeated, I got the same result. I was about 10" down on water level. Dooh! I figure I was down at least 80 gallons maybe a bit more. Every BTU stored counts especially since I cant justify swapping over to radiant baseboard. I ran with the low tank for the first run of the season but need to get the hose out and top it up.