"ALL energy is converted to heat", wrong. a pump moves water by converting somewhere around 20% (varies greatly) of the electrical energy into fluid momentum, the rest creates heat. [edit: bigben beat me to it, my dial-up is buggy so it took a few tries to upload]
some of you might recall my fan duct for my ehiem 1250. i did a test. I fixed all my other fans typical operating RPM, turned off the pump's 80mm Antec fan, ran SiSoft CPU burn-in for 3hrs, & took temps. then turned on the pump fan to 30% duty cycle (~30% max RPM) & did the same:
w/o pump fan:
h2o=33.4C
ambient=24.6C
diff=+8.8C
w/ ducted 80mm pump fan @ 30% speed:
h2o=31.0C
ambient=25.2C
diff=5.8C
that's 3C cooler just by moving a small amount of air over the pump. so i may have proved that inlines need airflow, because without air-flow the heat builds up quickly on & around the pump & water temps generally will rise as a result. for even the most powerful inline pumps, a small amount of air flow is all that is required to sustain low temps, so the benefits of a high CFM pump can be realized (more CFM=more water velocity=more turbulence=better cooling). further, about 10CFM directed on the ehiem1250 is enough to peel the heat off & dilute with cool-air, your exhaust fan(s) will do the rest.
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