The Heatrae Sadia Streamline water heater is a 'vented' water heater range. This means that as the water is heated, its expansion is taken care of through the tap. On the 'Oversink' models, this ihappens through the prefitted spout, but for the undersink models, you MUST make sure you have vented taps which can cope with this venting (these are shown as related products on each of the product listings). Failure to use an appropriate tap will result in damage to the heater. More details on this are given in our advice centre article (on the right hand side of any product listing). Also note that a vented water heater can only supply water to one sink. If you are in any doubt, please call one of our experts on 01934 253600.