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Old 03-15-18, 08:25 AM   #13
jeff5may
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Ok so if you have an uninsulated tank with a scaled up heat exchange surface, it doesn't affect your equation at all. The heating element is still burning away like an incandescent light or an Amish hearth heater. 95 percent efficiency at best due to natural losses. Current in wires and crap. The losses due to no insulation add to this overall bleeding of energy outside of the tank.

I bet you a buck a new insulated one would cost at least 20 bucks a month less to operate, even if you never draw any hot water out of it. Once the water starts flowing, the losses increase due to run time and lag. In fact, the best way to get more hot water out of the tank per watt input is to super insulate the tank. You can't tell me about the scale issue not affecting performance. Less run time is less run time, the end.

With a layer of scale, the heat transfer is impeded and a delta T increase is set up between the water and the element. This reduces raw BTU transfer rate and run time increases. The house wires and circuit breakers and thermostats and terminal connections and such all suck power while the element is running. The time lag also relaxes the thermostat controller loop and that lazy thermostat factor also acts as a vampire loss. That's mostly how your electrical efficiency is reduced from perfectly on paper form. The more scale, the longer the time lag and the higher the delta T between everything every heating cycle. The Q flow follows the change. Q spent over Q in the tank is your reference efficiency spoken of.

So from a math on paper perspective, one could relate the impedance of the scale layer to the parasites in the electrical system, and to the constantly exiting heat transfer through the tank surface area. Time is not your friend in this argument. Ten percent impedance in heat transfer rate equals 11 percent more run time, 11 percent more electrical vampire loss, and 11 percent more envelope heat loss, while the element is energized. I'm sure the geniuses at Whirlpool have to do the homework on this one every product cycle, and the industrial engineers do this as part of figuring out the budget.

I'm not that guy, but I can think that way when I try. This argument is a whole lot like the one about suction pressure versus temperature in a heat pump. The argument there is that cold refrigerant is more dense than warm refrigerant, so it should move more energy per unit of volume...

Last edited by jeff5may; 03-15-18 at 10:39 AM.. Reason: Soelling and grammer
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