Built-in hot water dispensers consist of a small heater tank you need to install under your kitchen sink and faucet that can resist the pressure of boiling water. The temperature of the tank stays close to 200 °F as long as you keep the unit plugged in and you do not change the unit settings. This is far hotter than anything you can get from your regular water heater and is as hot as the water in coffee makers get. People install these because of the benefits they offer.
There are a number of benefits in having an under-sink hot water dispenser. First, under-sink hot water dispensers give you what you need instantly so you do not have to stop in the middle of something and wait for the water to boil. This is a serious advantage for those who spend a lot of time working in the kitchen. Next, contrary to the belief that keeping hot water in tanks is inefficient, built-in instant hot water dispensers actually waste less energy than other means of producing boiling water. For instance, with a kettle, you often have to heat more water than you need and the unused water ends up cooling down because there is little insulation. On the other hand, under-sink tanks have decent insulation so there is very little stand-by heat loss. Finally, under-sink boilers take no space on your kitchen counter while kettles or countertop hot water dispensers can prove to be annoyances.
With all the comfort they offer, hot water dispenser tanks can also cause some problems if you are not careful with them. For instance, some cheap units can be prone to leaking because of the immense pressure and corrosive effects of constantly boiling water inside. If they fail when you are away from home, there can be a serious damage to your property. To avoid such accidents, you would do well to drain and unplug any boiling water dispensers in your home if you plan to be away for more than a day. A more dangerous issue about these devices is the scalding risk. Liquids at 140 °F can cause full thickness injury in a few seconds and what comes out of an instant hot water tap is much hotter than that. Any person who puts a hand under that tap will suffer a serious injury and this is especially a problem for persons who cannot fully comprehend the risks. If you want to use these systems in homes with elderly people or small children, you must turn down the unit's temperature to cut the risk.
In conclusion, under-sink hot water dispensers are useful devices but you have to make sure they are suitable for your lifestyle before you get one and you need to be careful when using them.