there are two types of heat input to worry about. a submerged pump adds both types, while a air cooled pump only adds pump heat. here is an article I found
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"The pump uses some amount of electrical energy. This energy must end up somewhere. A relatively small amount of it is dissipated as heat from the motor. The overwhelming majority of it is converted from electrical energy to mechanical energy in the form of a rotating shaft that does real work on the water.
This energy ends up in the water by increasing its temperature. It is called "pump heat" and can be very significant.
An Eheim 1048 is rated at 10 watts, almost all of which ends up in the water.
An interesting aside for non-believers: This is also why excessive use of a blender to mix up frozen orange juice results in the juice not being as cold as expected. Also, nuclear power plants use primarily pump heat (from three or four 6,000 HP pumps) to heat up almost 75,000 gallons of water from 200 degrees F to about 550 degrees in about six hours or less.
The point here is that there is a trade off in how big a pump to use to increase the
flow rate. More flow is beneficial. It is best to achieve the desired flow with as small a pump as possible and flow paths with minimum flow resistance.
Bottom Line
If you increase flow rate with the same pump, your temperatures will trend in the direction of goodness. If you increase flow rate by going to a bigger pump, you will reach a trade off somewhere where the pump starts putting too much energy into the system and temperatures will start increasing.
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And to prove this, I (Happy) started two 5 megawatt Reactor Coolant Pumps at the Prairie Island Nuclear Power Plant Unit 1 last night, and heated up about 1500 tons of steel and water 60 degrees on my shift. Someone else will heat the reactor up another 350 degrees today. These pumps draw 6000 amps at 4160 volts on startup, which is enough power draw to dim lights in nearby towns. And, the night before last, I placed the 570 Megawatt Unit 2 generator on line, and after doing that, I reduced Southeast Minnesota's typical outlet voltage from 123.8 volts down to 121.2 volts, nominal. Every day, I change the voltage that comes out of the socket down here, but the frequency is 99.999 percent of the time dead on 60 Hz. Heck, they even pay me to do this, which I then turn around, and toss the money into the bottomless pit known as my reef.
