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So, What Is An Expansion Tank, And Do I Really Need One?

Thermal Expansion Tanks

At this year's Home and Garden Show we found a lot of curiosity in the expansion tank atop our water heater display. So, what is an expansion tank, and do I really need one?

Well, the short answer is yes.  Almost any plumbing system benefits from an expansion outlet and ever more cities are requiring one. The benefits aren't just for safety, but also save money.

Firstly, the problem we are addressing with the tank is thermal expansion.  When water is heated it expands. For example, water heated from 90°F to a thermostat setting of 140°F in a 40 gallon hot water heater will expand by almost one-half gallon. This is because when water is heated, its density decreases and its volume expands (see below). Since water is not compressible, the extra volume created by expansion must go someplace. During no-flow periods in a system, pressure reducing valves, backflow preventers, and other one-way valves are closed, thus eliminating a path for expanded water to flow back to the system supply. Hence, system pressure increases. Temperature vs Density

What is closed loop system? A closed loop system is any domestic water system that does not have a way for water to flow back towards the meter or city supply. This is done when you or your city places a backflow devise in the domestic system after or in your water meter. These are more common than you might think. Today's newer homes have PRV’s pressure reducing valves which only let water flow one way. Double check valves which do the same thing for your sprinkler systems to keep you from poisoning your self by adding lawn chemicals to your lawn only allow water to flow in one direction also. Some cities place check valves in the water meters to protect the public water supply.

In a closed system this extra water pressure from thermal expansion can cause the most damage because the extra water pressure has nowhere to go since it cannot push back into the city water. Most cities require that expansion tanks be installed in closed systems. If you have some form of backflow preventer installed on the main water line you may be required to also have a water heater expansion tank installed. Check the local building code or call your local plumbing contractor to acquire more information.

So if you have a closed loop system your water heater will heat the water in the water heater and the water pressure will increase. This is thermal expansion. This not only affects your water heater but all your appliances. When your water heater heats the hot side of the system it also puts pressure on the incoming cold side of the system to. This can lead to all kinds of system failures like faucets, toilets, washer machines, ice makers, dishwashers and any thing else connected to the water supply in your home.

A water heater expansion tank is designed to help prevent fluctuations in water pressure, thus preventing damage. The water heater expansion tank serves as an overflow receptacle by absorbing excess water volume that the water heater creates when heating water. The PRV or check valve regulates incoming pressure from your water supply, now your closed system relies on the expansion tank to avoid added pressure from the water heater (thermal expansion).

 

If you do not have a one way valve installed on the main water line, and therefore do not have a closed water system, you may still benefit from a water heater expansion tank. Expansion tanks are normally installed above the water heater on the cold side before the water comes into the water heater. The extra hot water from the thermal expansion can be stored in the expansion tank and when hot water is demanded it is put back into the water heater. Having an expansion tank helps prevent dripping faucet and running toilets by keeping the extra built up pressure from reaching the fixtures and prematurely damaging

them.

 

Water heater expansion tank installation prices vary according to areas and job scope, although the tank is typically easy to install with a normal 3/4” threaded connection. A tee will have to be installed above the water heater if one is not there already. If you are hiring a plumber to have a water heater installed it’s a good idea to get a price for the installation of an expansion tank also. It may not be much more than you are already paying if it is done at the same time. If you are installing a water heater yourself consider installing an expansion tank even if it is not required since the benefits far outweigh the work and cost involved.