From bigclivedotcom.

The main advantage of undersink water heaters like this is that they store a preheated quantity of water for immediate delivery to the tap when it’s used. They do different sized storage tanks and also vented and unvented versions. There’s no waiting for the hot water to reach the tap. It’s there almost instantly. They also just use enough energy to keep the well insulated tank at the preset temperature, so they are very efficient.

The unvented version simply has a metal tank at full mains water pressure, and can feed standard taps/faucets. It does require understanding that while heating, the water will physically expand and it needs to be able to displace that expansion to a header tank or if fed directly from the incoming water main it needs an expansion tank and a safety pressure relief valve to prevent the tank from rupturing. If the tank did rupture, the unit is fed by full mains water pressure and would cause a flood.

The vented version has a tank that is only at low pressure, as it relies on an open outlet at all times, and the tap simply supplies water to the tank on demand, displacing the preheated water out of the open port. That does rule out the use of standard taps though, and you have to use a specialist vented tap that has a single cold water feed that can either be diverted straight out of the tap for cold water, or diverted to the vented tank and then back up to the tap spout. The proper taps are expensive and you don’t get much choice in style. One of the primary advantages of a vented system is that even if the tank fails it can only release the water that was in it at the time. So there’s much less water damage.
One quirk of the vented systems is that the taps drip while the water is heating and expanding. Once you get used to it you stop trying to turn the tap off harder!

If you like the steampunk look you can make a very nice copper pipe vented system with a lever valve to send water to the heater and a big spout as the water outlet.

Another major advantage of the under sink storage heaters is that they can be plugged into a standard socket or wired into a spur. Their power requirement is similar to a small kettle, and only while they top up the heat.

These systems are not suitable for filling a sink or bucket with hot water. They only store a small quantity which is best suited to casual dish and hand washing. But for many applications they are perfect.

I think the silicone smeared around the heating element is to seal it against accidental water ingress between the metal sleeve and ceramic, as that can cause earth leakage tripping issues.

As usual the sellers have misdeclared the tank capacity. No great surprise for something from eBay.

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