The temperature and pressure relief valve is a safety device that is installed at the factory. It is designed to open on a residential water heater, for your protection, when there is excessive water temperature or excessive water pressure. There are many things that result in conditions causing a T&P to release water. Thermal Expansion in a closed system is a common cause that is often the last to be considered.
When water heats it expands, just as mercury in a thermometer expands rising up the scale to show you a person’s body temperature. If the expansion of the mercury were to exceed the capacity of the thermometer the thermometer would explode. When water heats in a water heater it also expands. The expanding water will typically push back into the supply line preventing excessive pressure from building inside the water heater.
When a check valve (backflow preventer) or pressure reducing valve is installed without a built-in bypass water is not allowed to push back into the supply line. Pressure instead builds within the water heater. At 150 psi the T&P will begin releasing water to reduce the pressure within the water heater for your protection. Should this occur we recommend that you install a properly sized expansion tank. An expansion tank helps by absorbing the excess water pressure in a diaphragm tank. The diaphragm allows for water to flow into the expansion tank as it is being heated and expanding and flow back into the water heater at other times. An expansion tank will prevent wasting water released from the T and P during expansion, as well.
Why would a backflow preventer/check valve be installed in the first place? To prevent your home’s water from backing up into the main supply. Removal of the backflow preventer/check valve may not be an option, as it is likely required by codes in your area. Removal of a pressure reducing valve is also not a likely option, as controlling the water pressure into the home may be necessary.
Other causes of the T and P dripping or dumping water include: