A dip tube is the cold water inlet pipe on a water heater. A dip tube is specifically manufactured to be long enough to introduce cold water into a water heater near the bottom of the vessel and close to the heat source. The length of the tube is critical to the maintenance of the correct convection cycle within the heater. If the tube shortens due to breakage, the heater may no longer produce an acceptable quantity of hot water or only deliver tepid water. These tubes are also exclusively made of non-metallic materials such as heat resistant plastics so that they don't interfere with or damage the heater's sacrificial anode.
Water heaters rely on the principles of convection to efficiently produce an acceptable temperature and volume of output water. An efficient convection cycle in a water heater sees cold, incoming water concentrated around the heat source in the bottom of the tank. As the water heats up, it moves towards the top of the tank where it exits from the hot water outlet when one of the hot water faucets are opened. The heater thermostat is usually located low on the tank in the colder water area and, as the water heats up towards the bottom of the tank, reduces the heat or cuts off the heating elements altogether.
This positional relationship between cold and hot water is essential to the efficient operation of the heater and relies on the cold water being introduced at the bottom of the tank. If the cold water was, for instance, introduced near the top of the tank, the convection cycle would be defeated. This would have several negative effects, the most immediately noticeable being tepid water or a very brief flow of hot water from the faucets. Heating bills would also rise because the incoming cold water would simply destroy all the heater's work, thereby causing it to have to run longer to keep the water hot.
All of these factors mean that a dip tube of correct length is essential to an efficient water heater system. Any signs of the previously mentioned symptoms may indicate a degraded or broken off dip tube. Small pieces of plastic in appliance filters, faucet aerators, and shower heads may also indicate a degraded dip tube. Fortunately the replacement of a dip tube does not represent a major expense and is, for the most part, a fairly simple exercise for the do-it-yourself brigade. Many dip tube variants have a flared upper end which means a new tube can simply be dropped into the heater fitting and the inlet nipple fastened down onto it.
Dip tubes are made of heat resistant plastic to prevent damage to the heater's sacrificial anode and should always be replaced with tubes of similar construction. The sacrificial anode is a metal rod inserted into the tank which serves as a preferred target for corrosion, thus sparing the tank from excessive rusting. The use of metal, particularly copper, in the dip tube may damage the anode or negate its efficacy, thereby causing eventual rust-related leaks in the tank.