I just had mine apart. My instahot was sputtering hot water and I found that it was drawing air from the plastic tank at the top. Still don't know exact fix but I suspect that it involves clamping off this third tube.
Starting on top with the faucet there are 5 tubes going into the Insinkerator faucet:
- 2 copper tubes for cold water (One each to hot and cold valves controlled by the faucet handles)
- 1 blue line that that is connected to the hot side copper tube when the hot valve is opened. This blue line supplies cold water to the bottom of the heating tank.
- 1 rubber line from the center of the heating tank back to the faucet. This is connected directly to the spout without any valves in the middle.
- 1 plastic tube that is the subject of this query and is connected to the upper tank of the instahot.
What I found is that there is a one way valve inside the faucet between the rubber line supplying the hot water and the plastic tube which will return water back to the upper tank. If the spout were to become completely blocked the water would circulate inside the faucet and return to the upper tank and from there drawn into the lower heating tank through cold water inlet some sort of suction caused by the running cold water.
When the hot valve is opened, the cold water goes throught the valve and then via the blue line to the left inlet shown above. At the same time it is drawing water from the plastic holding tank at the top. That water is guided through a tube inside the tank to the bottom and as it is pressurized it pushes the hot water out the top of the tank through the center line
I still don't know the exact purpose of this plastic right line. I suppose it is to drain the spout when the water is turned off (although I am not sure why this is necessary).
I found that my instahot is drawing air from this plastic line somehow. I suspect it involves something in the faucet but I don't know why air is entering that line. If the hot water is running, and a vacuum were on that plastic line it should be drawing the hot water running up the center tube. As long as their is water in the upper tank I am OK, but as soon as that is sucked dry air starts entering the lower heating tank and causing the sputtering.
My current "fix" is to simply clamp off that third line to prevent air (and water) from coming down from the faucet. No idea what problems that will cause.
If anybody is interested, I re-sealed the two halves of my tank with a 5 3/8 ID x 1/8 CS O-ring A568-253. You might do better with 5 1/4ID (A568-252) as mine was just a little large. I also found that the O-ring on the cold water inlet (which seals the quick connect) had failed and that is 5/16 ID x 1/16 O-ring A568-011.