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What Does an Expansion Tank Do On a Water Heater and How to Fix

what does an expansion tank do

I was told I needed an ‘expansion tank‘ in my water heater! Why?

The first time I opened my tap, my water pressure fluctuates weird. What is this about?

If these questions are yours, read on! Here is everything you need to know about inflated cans that are often overlooked.

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What is a thermal expansion tank like?


Common residential expansion tank looks like a small propane tank.

At the top, it has an air valve, like the one seen on most tires called Schrader. At the bottom, there is a threaded pipe connection. Usually they are the same size as basketball, depending more or less on the size of the water heater they serve.

How does the expansion tank of the water heater work?

When a pot of water is heated on the stove, the kettle finally begins to whistle because the pressure generated inside will blow the steam.

Although the water in the water heater does not reach the boiling point and turns into steam, it swells when heated. This is called thermal expansion.

This additional pressure will only be absorbed by the municipal water supply system, becoming insignificant. No problem, however, if a check valve or pressure regulator (PRV) is installed on the pipe entering your home, all excess pressure will be trapped in your home pipe system, which will press your pipes, lamps and household appliances.

This is where the expansion tank comes in. Correctly installed, it absorbs such excess pressure. Half the tank is filled with water from the main water system of your home. The other half is full of compressed air. There’s a butyl rubber bladder in the middle. As the water from the water heater warms up and swells, it pushes into the bladder, further compressing the air from the other side.

When do you need it?

If you have a “closed-loop system” because any type of check valve or pressure regulator is installed in your home water supply line, it is always strongly recommended to use expansion tanks.

A common analogy compares hypertension at home with hypertension. Usually it does not have an immediate negative impact. However, long-term wear of this excess pressure can reduce the life expectancy of everything in the plumbing system.

If you have more than 80 AD pressure supplied to your home by urban water supply, the expansion tank will not make any corrections. This is the work of pressure relief valves (PRV). When the pressure supplied to your home is at a suitable level between 40 and 80 PS and has a PRV or check valve, the thermal expansion tank enters. Prevents constant high pressure fluctuations caused by thermal expansion in closed-loop systems.

Depending on the place of residence, the city may need an expansion tank, regardless of the water pressure, for example, when installing a new water heater. For example, in Frisco, Texas, all new homes currently require a PRV and an expansion tank.

Consult your city plumbing inspector to ensure the applicable piping code for your area.

If there are several water heaters, do I need more than one?

You need a thermal expansion solution suitable for your plumbing system.

For example, a 50-gallon water heater can be supplied by a 2-gallon expansion tank. If you have several heaters in the same system or connected, you can equip each heater with an expansion tank, or you can use a single larger expansion tank.

What is the life expectancy of expansion tanks?

The life expectancy of expansion tanks is very unpredictable.

There are several variables that affect the duration of the tank, such as the quality of the tank, the quality of the water and the proper inflation of the tank, etc. We saw several tanks that failed in just two years and eight months, causing serious damage to the water, and some for eight years. As a precaution, we recommend replacing them within a year after the expiration of the manufacturer’s warranty.

Tanks installed in this area usually have a warranty of one or five years, so the replacement should be every two to six years.

Can my expansion tank leak?

Yes, like the rest of the plumbing system in your home, the expansion tank can fail when used.

They usually fail in two ways. First, the rubber bladder inside them is worn out, and the tank stops as a way to relieve thermal expansion. Secondly, the connection point between the pipe and the tank will corrode and begin to leak.

To make sure that the expansion tank does not cause damage, one thing you can do is get a licensed qualified plumber to install it properly. A good plumber will install the water tank, making sure that it is properly supported and has a good connection. It will also place it (if possible) in the pot of the water heater or somewhere, minimizing the risk of damage in case of leaks.

Another thing you can do to extend the service life of thermal expansion solutions is to buy quality cans with a five-year manufacturer’s warranty. Good tank with stainless steel thread connection, made of high quality materials: thick butyl rubber, polypropylene, heavy carbon steel, etc.

Most manufacturers recommend checking new expansion tanks once a year and more often as tanks age.

How can I test if my thermal expansion tank is working?

There are a few things you can do to determine if your expansion tank is still running.

The easiest thing is to control the water pressure. Without working expansion tanks in a closed-loop system, you will be able to notice fluctuations in water pressure under certain conditions. When heavy water is used for a period of time and there is no water, the system generates pressure.

Turn on the water tap and watch. A noticeable high pressure will begin to appear, and then it will begin to fall. When excess pressure flows out, the flow will be smooth and stable. This is not the final test of the expansion tank due to other factors that influence the flow and pressure.

The second way to determine whether the expansion tank works properly is to hit it with a metal object. Although it is also not a complete test, this is a very simple way to check. The lower half of a working expansion tank is filled with water from your home plumbing system. The top is filled with pressurized air.

When touching the top and bottom of the tank alternately with metal objects, the resulting sound should be significantly different. Where the air is, there should be more hollow rings than a jingling. If the sound does not differ, you may have a damaged expansion tank.

Another more reliable way is to flow a little air from the Schrader valve on top to determine if your expansion tank is damaged. If air comes out, then the rubber bladder remains intact. If the water comes out, then it’s broken.

Procedure to be Followed

Finally, after you have tested the above test and your expansion tank has passed, there is a clear way to determine if your expansion tank is in good condition and make the appropriate adjustments. It will require a hydraulic meter and an air pump, as it will use for car tires.

  • Open the outside tap with hose connection and let it run for about 15 seconds. This will relieve any potential overpressure generated by thermal expansion, leaving only the pressure set by the PRV.


  • Hang the meter on the tap to determine the water pressure in the current system.

  • Close the water to your home with the main shut-off valve.

  • Turn on the tap, drain all the pressure from the system.


  • Go to the expansion tank and connect the pump outlet to the Schrader valve.


  • The pump gauge will tell you how much air pressure is in the tank. Most manufacturers prescribe that the preload of their tanks should be set the same pressure as the previous water pressure readings that are in step 2.


  • If the tank preload does not match the inlet pressure, you can use the pump to adjust it to the appropriate P.S.I.


  • After disconnecting the pump, you can restore the water supply of the house and carefully check for leaks.

After performing this procedure, you can be assured your expansion tank is now working as it should. A common mistake for plumbers (especially construction plumbers) is to forget to pre-charge the expansion tank to the proper pressure when it is put in. This procedure can correct that installation error if the tank has not already been damaged.

If you are noticing that washers in plumbing fixtures are deteriorating rapidly, or water is dripping from the relief valve on the water heater, it may be wise to add an expansion tank. It can be low-cost insurance against more costly damage to your home’s plumbing system.

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