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Old 12-29-2010, 05:43 AM   #17 (permalink)
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Join Date: Jan 2008
Location: Vancouver, BC, Canada
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Hi. Crazy hot water tank insulating guy here. You may be interested to know that the above hot water tank only requires about 65W on average to keep the water at 45 degrees C for showers each morning. It could literally be heated with a light bulb but I use a 520W element and just run it 3 hours each morning on a timer.

A couple points of interest related to the crab tank discussion:
1. The efficiency of any kind of light bulb as a heater can be considered very nearly 100%. With an incandescent bulb you might get around 95% heat and 5% light "at the bulb" but the light will be converted to heat when it is absorbed by a surface. A 20W fluorescent bulb or a 20W incandescent bulb or a 20W anything will all put exactly 20W of heat into a closed container if you don't let any of the light escape. That's the law of conservation of energy.
2. Temperature rise (difference between internal temperature and ambient temperature) is proportional to power (Watts). So if your tank gets 10 degrees warmer than the room when you use a 40W bulb, it should get 15 degrees warmer than the room with a 60W bulb or 25 degrees warmer than the room with a 100W bulb.
3. If you're starting from zero insulation, adding a little insulation will go a long way. Even just some cardboard taped to the sides of the tank could raise the temperature enough.

Good luck.
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