The impact of tubing sizes

[Cathar]

But I think there's a problem, there is simply no way the pump dumps ALL the heat it generates into the water, but here you've done pretty much exactly that.

Actually I didn't. I've measured the ratio of power draw to heat dump for a number of pumps. In general most pumps dump 85-90% of their power draw into the water directly as heat, and I actually factored that into the calculations.

I model the heat-dump of the pump into the water, and the related heat dump of the pump into the air as two separate entities.