The pump is essential for a good flow, it do not need to run fast, but one should consider the quality of the pump in regards of durability and especially its food grade quality. As many have posted the “Brown Solar Pump” as a good mashing punp, I also tried this one together with a 5v phone charger and a 12v light dimmer! This combo ran fine….but…..!
But my experience is it gave a metallic taste and this was showed from running it in tap water samples and perceived by both me and my wife. Where or not this is an issue for all this type pf pumps in unknown…. there is a bunch of resellers on ebay from China.
Instead I brought a “Magnetic Pump” from Brewferm (seen at the price of 75-100 EUR), as I felt this is not an area to compromise. This is rather powerful pump and way more powerful than I need, hence, one must damper the output by either lower the electricity going into it, or dampen it physically.
Physically damping I do no think is good for the durability/life of the pump.
I have used a 220v light-dimmer, but have recently added a second SSR in my craftbeerpi controller for controlling the pump as the light-dimmer/pump was interfered by the surge from the draw of the hot-plate and hence the pump went up and down, sometimes stopping complete with temperature going insane then. So if you do not make a CraftbeerPi controller with a dedicate SSR for the pump control, you will need to physically restrict the pump. Might be fine, maybe not….I am not a pump expert!
In regards of getting a stabil temperature the foremost important is the pump is running at a stabil speed, and best advice is to make use of a choosen speed and let it be! Secondly, if it is just give a tiny over-flow combined with ofcouse a over-flow system, you get the best results!
If fiddling to much with pump, or flow from pump goes up and down, eventually stopping, the temperature will change too. This will confuse the CraftbeerPi controller giving a higher need for adjustments in the sense of turning heater on and off, hence, less precision in temperature. Therefore a steady flow-rate is giving best results.
Other pump(s) to consider:
The later is what I likely would go for, as it rating is sufficient and seems to be the pump various sellers sells as the re-placement pump for Grainfather!