Updated: Aug 21, 2020
Expansion tanks absorb pressure
An expansion tank is a device that is used in conjunction with a hot water tank to control pressure fluctuations in domestic water distribution systems, in this case inside your property. The idea is simple, as water gets hot its molecules expand and as it cools water molecules compress. Let's get to the bottom of it.
A residential property has what’s called a closed plumbing system and such system can create dangerous high water pressures. Therefore, when such dangerous pressures exist, an expansion tank can actually help absorb them. An expansion tank is simply a metal tank with a rubber bladder inside of it that uses air as a cushion. Think of it as a pillow. When you rest your head on it you are creating a pressure and the pillow is happy to compress and make you feel good. Well, that's the whole point of an expansion tank because it likes to help your water pressure system stay controlled.
How they should be installed?
For general knowledge an expansion tank holds about two gallons in a residential system but the size can also vary depending on the actual volume of the hot water tank they serve. Expansion tanks have labeling on them that specify the design pressure for which they were built, usually around 150 pounds per square inch (PSI) for residential use. A home inspector will generally inspect that they are properly supported specially if they are installed horizontally. Horizontal installations require proper strapping and bracing. With that being said, a home inspector will also look to see if the expansion tank is installed well above the hot water tank to ensure that water will easily flow back down in the direction of gravity. Another well known aspect to be checked are for any signs of corrosion. A corroded tank will lead to leaking and loose efficiency, so make sure you have a professional help you to either replace it or fix before something more serious can happen. Additionally, the best way to install them would be to position them in a downward direction just as shown on the above picture. Needless to say, many people wonder if they should have an expansion tank and for that matter it is best to hire a licensed home inspector, or a licensed plumber to help you.
An expansion tank is not a safety device, you need more!
Your hot water tank must be equipped with a protective device called a TPR (Temperature pressure relief) valve. A TPR valve will bleed water whenever the pressure system reaches above 150 PSI or the temperature surpasses 210° F. Water will be discharged in a safe manner. The purpose of a TPR valve is not to allow water to become superheated which is usually above 212° F. That’s why it is very important that homeowners check their TPR valves periodically at least once a month. When a TPR valve fails, the consequences can be very damaging. The pressure inside the tank will find the weakest point where it can rupture the vessel and once the superheated water is released into the air an explosion will follow propelling the tank like a missile. The resulting damage can range from property destruction, death and body injuries.
Remember that an expansion tank should be well supported and installed in a proper location. And always look for signs of leakage and corrosion.