This product has the same function as the everyday tea kettle, providing hot water for instant beverages, soups, hot cereals, for thawing frozen foods, etc. Unlike the tea kettle, of course, this product is always ready to go. You activate the handle, and there it is - steaming hot water. Most models have a storage capacity of about two quarts. Even folks who understand the concept and purpose of an instant hot water dispenser, however, are sometimes puzzled about its construction. Why, for instance, if the dispensing valve is designed to handle a single fluid, are there often three or four different tubing connections involved in the process of hooking it up? The answer is related to the water's high temperatures. Without some means of handling the expansion of superheated water, a simple tank would act as a locked chamber, creating a dangerous situation. The solution is to provide a tank in which the heated water can be stored without being pressurized. Such a system necessitates routing the incoming supply water first through a valve in the dispensing head (above the counter). When activated, this valve permits water to pass down into the bottom of the storage tank, where it displaces heated storage water. This heated water then passes up through the tank's outlet tube to the spout where it is dispensed. In other words, as long as the activating valve remains off, there is no supply pressure on the tank; but once the valve is activated, water is permitted to flow through the tank and out to the spout.

All heating tanks are built with a provision to handle the expansion of the water when heated (volume can grow as much as 5% when brought to the near-boiling level). Before we explain that, let's look at what would happen if there were no such provision for expansion. If you filled the tank with cold water right up to spout level and stopped, what would happen as the water began to heat? The expansion would cause the water to slowly drip out the spout until the entire tankful reached the maximum temperature. Needless to say, most folks wouldn't care for this, so manufacturers provide a means of handling expansion internally. Usually this is accomplished through the use of an expansion chamber - in some cases vented to the atmosphere - in other cases contained within a flexible bladder (also acted upon by atmosphere). With such a chamber, the increased volume of water is given a place to go without dripping from the spout.

So far, so good - we found a place to put our expanded water during the heating cycle. Now, how do we get it out of there to make room for more? Very simply, the expansion water gets “sucked” out of the chamber by the primary flow of water during the next activation of the system. This process is called aspiration. So that explains the number of tubes involved in the installation. One goes directly to the dispensing head valve from the supply line; one goes from there to the bottom of the tank; one from the tank back up to the spout, and in the case of vented models, one more connects the vent outlet of the expansion chamber to the dispensing head.

The design of the dispensing heads (faucet component) for instant hot systems has been evolving in recent years to include a cold water option (often for filtered and/or chilled water), as well as more decorative high-end constructions.