“What is that odd-looking tank above my water heater?”“Somebody told me that I needed an ‘expansion tank’ on my water heater! Why?”“There is an odd fluctuation in my water pressure when I first turn on my faucet. What’s going on?”
If any of these questions are yours, read on! Here is everything you should know about the oft-overlooked expansion tank.
Note: This article is for informational purposes only. Plumbing work should only be done by a licensed, insured professional. Legacy Plumbing is not responsible for any damage or injury resulting from the application or misapplication of this information. Your access, use, and reliance on any information provided on our website is solely at your own risk.
What does a thermal expansion tank look like?
The common residential expansion tank looks something like a small propane tank. The most common colors are off-white or blue.
On the top, it has an air valve like you see on most tires called a Schrader valve. On the bottom, there is a threaded pipe connection. They are usually about the size of a basketball – more or less depending on the size of the water heater(s) they serve.
How does a water heater expansion tank work?
When you heat up a kettle of water on the stove, eventually the kettle will begin to whistle as the pressure developing inside it blows out steam.
Although the water in your water heater doesn’t reach the boiling point and turn into steam, it does expand when heated up. This is called thermal expansion.
This extra pressure will just get absorbed by the municipality’s water supply system and become negligible. No problem. However, if a check valve or a pressure regulating valve (PRV) is installed on the water supply line coming into your home, it keeps all that excess pressure trapped in your home’s plumbing system which stresses your supply lines, fixtures, and appliances.
That’s where the expansion tank comes in. Properly installed, it absorbs this excess pressure. Half of the tank is filled with water from the main water system of your home. The other half is filled with compressed air. There is a butyl rubber bladder in the middle. As the water in your water heater gets hotter and expands, it pushes against that bladder and further compresses the air on the other side.
When is it required?
An expansion tank is always highly recommended if you have a ‘closed-loop system’ caused by any kind of check valve or pressure regulating valve installed on your house’s water supply line.
A common illustration compares having high water pressure in your home to having high blood pressure. It usually doesn’t have immediate negative consequences. However, the long-term wear and tear of this excess pressure can reduce the life expectancy of everything in your plumbing system. The weakest parts of the system are usually the toilet fill valves, the supply lines, and the solenoid valves found in washing machines, dishwashers, and ice makers. These are much more likely to fail prematurely when exposed to high water pressure.
If you have excess pressure over 80 P.S.I. supplied to your home by the city water supply, an expansion tank by itself does nothing to correct this. That’s the job of the pressure reducing valve (PRV). When the pressure supplied to your home is at the proper level between 40 and 80 P.S.I. and you have a PRV or check valve, that’s where the thermal expansion tank comes in. It protects against the constant fluctuation of high pressure caused by thermal expansion upon a closed-loop system.
Depending on where you live, an expansion tank might be required by the city regardless of the water pressure in order to pass plumbing inspections – such as you would get upon the installation of a new water heater. For instance, in Frisco, Texas, a PRV and expansion tank are currently required on all new homes. Also, at the time of writing this, an expansion tank is required on all water heaters installed or replaced in Little Elm, TX. This is because the water meters come with check valves which create a closed-loop system.
Check with your city’s plumbing inspector to be sure of the applicable plumbing codes in your area.
Do I need more than one if I have multiple water heaters?
You need a thermal expansion solution that is adequately sized for your plumbing system.
One 50-gallon water heater, for instance, can be served by a 2-gallon expansion tank. If you have more than one heater on the same or connected system you can either have an expansion tank for each heater or a single, larger expansion tank. Again, check with local plumbing code amendments and with the expansion tank manufacturer’s specifications to make sure you have accurate information for your situation.
What is the life expectancy of an expansion tank?
The life expectancy of an expansion tank is very unpredictable.
There are many variables that can factor into how long one lasts, such as quality of the tank installed, water quality, and proper inflation of the tank - to name a few. We have seen some tanks fail in as little as two years that have caused major water damage and some that have lasted eight years or longer. As a precaution, we would recommend replacing them no more than a year after the manufacturer warranty expires.
The tanks installed in this area normally have a one- or five-year warranty, so replacement should be every two to six years.
Can my expansion tank leak?
Yes, like every other part of your home’s plumbing system, the expansion tank is subject to failure with use.
They normally fail in two ways. First, the rubber bladder inside them wears out and the tank ceases to function as a way to mitigate thermal expansion. Second, the point of connection between the water piping and the tank can corrode and begin to leak.
One thing you can do to ensure damage is not caused by the expansion tank is to have it properly installed by a skilled, licensed plumber. A good plumber will install the tank, making sure it is properly supported and has a good connection. He will also position it (when possible) over the water heater pan or somewhere it will minimize the risk of damage if it leaks.
Another thing you can do to boost the longevity of your thermal expansion solution is to purchase a quality tank with a five-year manufacturer warranty. Good tanks have a stainless steel threaded connection and are made with high-quality materials: thick butyl rubber, polypropylene, heavy gauge carbon steel, etc.
Most manufacturers recommend checking expansion tanks yearly when new and more often as the tank gets older.
How can I test if my thermal expansion tank is working?
NOTE: Before you begin testing, confirm that there is a functioning shut off valve prior to the expansion tank. If a catastrophic failure happens while you are testing, it could cause severe damage. We recommend a licensed professional do any testing. If you see any signs of corrosion on the expansion tank connection (see pictures) then shut off the water supply and call a licensed professional. Legacy Plumbing does not accept any responsibility for damages caused by improper testing or failure of fittings during testing.
There are several things you can do to tell if your expansion tank is still operational.
Monitor Pressure: Easy
The simplest is to monitor your water pressure visually. Without a working expansion tank on a closed loop system, you will be able to notice the water pressure fluctuate under certain conditions. Whenever there is a period of heavy hot water usage followed by a period where no water is used, it will allow pressure to build up in the system.
Turn on the hot water to a faucet and watch. It will start coming out with noticeably high pressure and then begin to drop down. When the excess pressure is bled out, the flow will level out and remain steady. Just be aware - because of other factors that affect flow and pressure, this is not a definitive test of your expansion tank.
External Tapping: Easy
The second way you can tell if your expansion tank is working is by tapping it with a metallic object. Although also not a full-proof test, it is a very easy way to check. The bottom half of a properly functioning expansion tank is full of water from your house plumbing system. The top is filled with pressurized air.
By tapping alternately on the top and bottom of the tank with a metallic object, there should be a distinct difference in the sounds produced. Where the air is, there should be more of a hollow ring instead of a clank. If there is no difference in sound, you might have a compromised expansion tank.
Check the Valve: Easy
Another, much more trustworthy way to determine if your expansion tank has been compromised is to bleed a little air out of the Schrader valve on the top. If air comes out, then the rubber bladder is still intact. If water comes out, then it has been ruptured.
Pressure Check: Advanced
Finally, after you have tried the previous tests and your expansion tank has passed, there is a definitive way to determine if your expansion tank is in good condition and is properly adjusted. It will require a water pressure gauge and an air pump such as you would use for a car tire.
Turn on an outdoor water faucet with a hose connection and let it run for about 15 seconds. This will relieve any potential excess pressure created by thermal expansion and leave you with just the pressure your PRV is set to.
Hook your water pressure gauge up to that faucet and determine the water pressure that is currently on your system.
Turn off the water to your house with the main shut-off valve.
Open up a faucet and drain all of the pressure off of your system.
Go to the expansion tank and connect the outlet of your pump up to the Schrader valve.
The gauge on your pump will tell you how much air pressure is in the tank. Most manufacturers dictate that the pre-charge of their tanks should set the same pressure as the previous water pressure reading you took in step 2.
If the pre-charge of your tank does not match the incoming water pressure, you can adjust it with your pump to the proper P.S.I.
After disconnecting your pump, you can restore the water supply to your house and double 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.
Do You Still Have Questions?
Here at Legacy Plumbing, each one of our service technicians is trained to diagnose any expansion tank issues and install them correctly. If you have any questions or concerns about your expansion tank and it’s installation, just reach out to us and we would be happy to help you.