Having made a housing for my water heater, I needed to test it and the pump that I have chosen. I wired the heater to a mains plug and used an RCD for safety. Then I dipped it into a large pan of water, turned it on and waited for signs of life.
Within a few seconds bubble started forming on the surface of the element where localised boiling was starting to happen. The photo doesn’t show it well, but the water also started moving as the hot area caused convection.
The pump is a cheap Chinese 12v food grade thing from ebay. It seems a lot of the cheap pumps are intended for aquarium/fish pond use and have quite a low temperature limit. I chose this one because it claims to go up to 100°C. While I was getting everything ready in the kitchen, one of our children asked “how do you make electricity move water”, so I had to take it to bits to show them.
Its a very simple design, with a single moving part that is allowed to get wet. No seals between the motor and the impeller. It is also cheap, not very well made, and vibrates madly when there’s no water in it. It has a 20,000 hour life, but we’ll see about that.
Here it is in action. It was tricky to prime the pump: it isn’t self priming, and can’t pull water up an air filled pipe. This won’t be a problem in the finished heater since it will be a closed loop that will be bled when its filled. We did a quick flow rate measurement by pumping into a second container for a measured time, then we emptied the container into a measuring jug. It came out at about 400 litres/hour, about half the stated rate.
The final test was to run everything together. Here the pump is on the cold side of the heater.
I had to turn the pump voltage down to “barely running” before I could feel any warmth in the water outlet, but it was definitely working. Next step is to connect it to the heating coil.
The eagle eyed will notice that this is not the same heater housing I described earlier. This is Mk II, which is all soldered. Mk I had a pin hole leak and I couldn’t face the hassle of heating up such a huge lump of brass and doing the soldering all over again!