Watch QETTLE's Todd and Dan explain how a boiling water tap works, and understand the difference between a true boiling water tap and an instant hot water tap. A QETTLE tap is connected to an undersink boiler tank and filter system. Boiling water taps are indispensable for everything from instant hot drinks to speedy pans of boiling water for pasta, rice and vegetables. The best boiling water taps are 4 in 1 taps, which in addition to boiling water, also provide filtered drinking water, plus normal hot and cold flows of water.