Kit - do you really mean 30l/min, or is that a typo? That sounds more like a bath tap flow! I doubt if my system is pushing much more than a few litres a minute through the system. I think the pump is rated at something lkke 10L/min at 12V and from watching the speed of odd bubbles running through the pipes, I would guess at rather less than half that at 5V.

So, trying to put a bit of science behind these finger in the air estimates, I come up with the following. Does this make any sense?

Let's assume motor running flat out, absorbing a total of 2.2KW. As a rough rule of thumb, an electric motor might hit around 75% efficiency, so about 550W is wasted and appears as heat in the motor, the rest being absorbed by the load. 550
W (and here I have to go back to my schoolday units which is where I was supposed to have learned this stuff) is about 132cal/sec. Starting with an assumed flow rate of 1lit/min, that's 1000/60g/sec, that is 16g/sec. That heat input into that amount of water suggests that the water will be heated by 132/16 = 8degC (I'm rounding a bit here). So, with this low flow rate and the motor running flat out, the water coming out will be around 8C hotter than the water going in. I'm probably running at about 5l/min, so temp differential in/out through motor will be about 1.6degC. Given that I have around 8-10m of pipework (my cooling pipes run through the cable chains) and the surface area/volume ratio of the pipes is pretty high, I'm probably dissipating a lot of heat through the pipe walls anyway. And that is with a motor running at max load, which I doubt many of us achieve in a home machine. Looking at those figures, I'm not surprised that my motor case temp never seems that much above ambient!

Does this make any sense? There are other drivers, of course - pump availability, and the problem of different fitting sizes on pump and motor. I made use of a local yacht chandler who stocked a wide range of plastic tube of various sizes to find one size to fit the pump, another to fit the motor, and which fortunately also fitted inside each other with the aid of some small jubilee clips.