Wherever wasted heat occurs in or near a building, its energy can be recaptured by exposing the escaping Btus to a thin container of circulating water that absorbs the heat, then carries it to a more useful location in the building. A few heat sources that lend themselves well to these exchanges are flue gases, warm graywater, light fixtures, electric motors, refrigerators, and internal combustion engines. Removing such heat can also improve the source's performance: e.g. lights operate more efficiently and last longer if they are cool, and air conditioning is more efficient the more its machinery runs cool. Thus such thermal hitch-hiking often has a double energy value.
An excellent method of retrieving wasted energy is with a heat pump. The pump's cold end is immersed in the rejected heatflow, whether it be air or liquid, then the retrieved Btus are carried to the pump's hot end, which is immersed in the fluid that needs heating. The technology behind such thermal transfer is the same as that of the decades-old refrigeration industry. Such systems usually require considerable cleverness to actuate —thus they are a good example of substituting imagination for money.
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