A question that NJ Home inspectors get often from clients is what is an expansion tank and why do I need one on my water heater? Thermal expansion is the tendency of matter to increase in volume when it is heated. This thermal expansion is especially important in homes because there is a water supply system and rigid pipes. Water heaters produce domestic hot water in most homes. For this discussion we are going to be focusing on standard water heaters both gas and electrical that are found often in homes.
Domestic water supply systems can be described as being open or closed. If the system in the home is open water can flow both in and out through the main water supply pipe. If the water heats up it can flow backwards negating this increase in thermal pressure. If the domestic water supply system is closed water cannot flow back as just described.
The majority of homes in the US have closed water supply systems. The elements that make a closed system is a check valve, pressure regulator or newer water meter with internal check valve installed.
A check valve is a back flow preventer that is installed on the supply piping into the home to prevent contamination of the municipal water supply system. If there is water contamination in the home the municipality does not want this contamination “back flowing” to the source. Some newer water meters have check valves built into them while most of the older meters in use do not have the valve.
A pressure regulator or a pressure reducing valve is a specialized valve also installed on the water supply pipe to the home (typically close to the meter). The purpose of the valve is to reduce high water pressure from the source so that high water pressure cannot cause damage to the piping, fittings and fixtures in the home. Not every home will have a pressure reducing valve, but they are very common. Between the check valve, pressure regulator and newer water meters, many of the home that NJ home inspectors look at will have closed water systems in place.
This is where a thermal expansion tank comes it. The expansion tank is installed in the cold-water supply (inlet) to the water heater. The purpose of the tank is to absorb the thermal expansion created by the heating of the water in the tank. The expansion tank it divided into two parts separated with a rubber bladder. Water flows into the tank from the top and the bottom of the tank is filled with compressed air. When the heated water enters the tank the air compresses allowing for the excess water to enter the tank, reducing pressure. The compressed air allows the tank to take up the thermal expansion from the water heater preventing damage and hazards.
In a closed system without an expansion tank the water heater can be damaged from high pressure. If the liner in the water heater cracks rust can form inside the tank and the tank will deteriorate quickly. High water pressure can also cause fixtures, fittings, pipes and valves to wear quickly and lead to leakage or flooding.
If the water heater does not have an expansion tank and the home has a closed water supply system pressure can build up to a point where it can cause damage and leakage. Often, we will find the temperature pressure relief valve on the water heater leaking. The temperature and pressure relief valve is a safety valve on the water heater that opens when there is too much pressure in the system. Often a leaking TPRV valve indicates a need for an expansion tank installation or replacement.
The next question is how to determine if the expansion tank is operating correctly? Testing expansion tanks is beyond the scope of a home inspection in New Jersey however there are a couple of things you can look at. The tank should feel cool to the touch and sound hollow when knocked on. A water-logged expansion tank will have a solid sound.
Another test is to push the air valve which is a Schrader valve on the tank. This valve looks like the valve on your auto tire. When pressed it should release air and not water. If water is expelled the tank is most likely going to need replacement. Testing these valves by New Jersey home inspectors is not recommended.
What inspectors can do is the knock test and to make sure the expansion tank is properly supported. Many times, the tank is installed horizontally with the only thing supporting it is its threaded fitting. This crates too much stress, and the weight of the tank on this fitting can cause leakage over time. Expansion tanks should be strapped and supported especially in the horizontal position.
New Jersey Home inspectors should look for the presence of an expansion tank on the water heater if there is a closed water supply system. They may want to recommend the installation of an expansion tank as a best practice recommendation even if the water supply system is open. It’s better to be safe than sorry.