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For 'outside' storage, I suggest that you:
1. apply 'heat trace' wiring to the INLET line of the tank(s) or to the connection to the 'bottom' of the tank - home depot stuff. The small amount of heat from the 'tracing' will thermo-siphon into the tank, especially if the traced line is LOWER (in altitude) than the tank.
2. always keep the water 'flowing' when the temp is 'severe' .... just a few drops / cc's-per-minute of flow, and routed 'back' to the tank will help prevent a total blockage of ice.
Also applies to 'inside' tanks when the weather is extreme - just keep the water constantly flowing and 'dribbling' back to the tank. For bilge tanks the ambient outside water temp (under the ice) will be ~4°C (39°F) and if you keep the tank 'slightly stirred up' (slight 'recirculation' by 'dribbling back to the tank(s)) you wont freeze. If the tanks are touching walls where the ice is in contact, then you need to 'warm' the tanks.
Last edited by RichH; 11-11-2010 at 10:44 AM.