Insta Hot, Part 4

New Hot Water Dispenser Faucet
This one does not swivel and the finish is some fancy, antiqued,
matte tin color which does not match the faucet, but hey, it works.

Our Insta Hot hot water dispenser has been dribbling for the last few months. If you released the handle so the valve snapped closed it seemed to shut off more reliably. The spout swivels and I often swing it over the counter so  I can fill my cup with water with one hand while I am stuffing my face with donuts with the other. If the valve doesn't shut off completely when I am done filling my cup, it dribbles water on the counter top and then someone has to mop it up, an odious chore if there ever was one.
    Yesterday Osmany and I set about to replace this defective device with a new one, which I just happened to have on hand because Insta Hot sent me a new one because I made a big stink the last time I had trouble with it. The tank wasn't leaking and the heater seemed to be working fine, so we just replaced the valve assembly and left the tank in place. Go it all hooked up and everything seems to be, no leaks, so we declared victory and went home.
   This morning I am filling up my mug and the Insta Hot is spitting. It is delivering plenty of hot water, but this spitting is not cool. The water is darn near boiling and I don't like being spattered with drops of boiling water. Seems our victory was not quite complete.
    My first suspicion is that the thermostat somehow got turned up and the water is actually boiling. I turn the water on and let it run for a minute until all the very hot water has been purged from the quart-sized tank, but it's still spitting. Could we have an air leak somewhere? If we had an air leak, you'd think we'd also have a water leak, but we don't, so off to the internet I go where I find this bit:

I just talked to a service tech at Insinkerator. He said there is an in internal aspiration valve in the tank that could have gone bad, and since it is not fixable, he will send me out a new tank.

There is a lot of drivel out there about how the thermostat is set too high, which would be good advice if the original question hadn't specifically stated that the problem existed even with cold water.

    Anyway, not a problem for us, we have a new tank (that came with the new valve), we swap it out and the problem goes away.
    But Osmany isn't satisfied, he wants to know what's wrong with tank, so we open it up. This required taking the cover off and extracting the tank from it's form fitting styrofoam insulation. The tank itself is not surprising, but what's this little plastic thing-a-ma-bob sitting on top of the tank? It sort of reminds me of the overflow tank for an automobile radiator. A little more looking and we discover something rattling around inside it. This will not stand, this plastic overflow tank will have to be opened. Some serious work with a one-inch wood chisel along the seam forces it open and we discover a small plastic ball, about a quarter or 3/8 of an inch in diameter.

Damaged plastic check valve ball

The surface of the ball is encrusted with what appear to be calcium deposits making it very rough. There is one circle that is smooth. I suspect that as long as the tank wasn't disturbed, the ball did not change position and so it was working properly. When we, or rather Osmany, were under the sink working on the valve, the tank was undoubtedly jostled, which led to the ball not sitting perfectly centered on its smooth face anymore, which meant it wasn't sealing properly, which is how we got air in the system, which is how we get water spitting out of the nozzle.
     I do not understand exactly how air from our 15 PSI atmosphere manages to get inside a tank pressurized with water at 90 PSI, but I assume that it could be explained by a principle with a fancy Italian name like Venturi or Bernoulli.

I originally wrote this a week ago, but I didn't find time to finish it until today. More photos from this op can be found here.