Tips For Handling Water Heater Expansion Tank Troubles

Here’s an overview on how to go about addressing customers’ water heater expansion tank problems

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Editor’s Note: Master plumber Anthony Pacilla will occasionally be writing “Now What?” features, where he sets up a scenario and uses his real-life experiences to provide problem-solving advice about it.

You arrive at Mrs. Jones’ house and find that the T&P (temperature and pressure) valve on the water heater is dripping. You know from experience and training to check her expansion tank. She does not have one. Easy day! You install the new expansion tank on the cold side of the system and restore water pressure. No leaks, T&P valve stopped dripping. Great job, you did everything right!

Two weeks later you get a callback to the same house for the same issue. You go to the expansion tank and it is completely full of water. A manufacturer defect possibly? Mrs. Jones was happy before but now she is questioning whether she really needed the work done, and is questioning your expertise.

Now what?

Let’s simplify and demystify the water heater closed-type compression expansion tank for new plumbers and maybe freshen up some details for the old veterans.

What Is It and Why Do I Need One?

An expansion tank (compression/closed type) is a tank that is used to protect a water distribution system from damage and over pressurization.

It works like this: The inside of the tank has a rubber membrane that splits the inside into two chambers. One of the chambers has compressed air, and the other half of the chamber is open to the water supply. The reason you need one on a water heater system is because when water heats up, it expands.

Mrs. Jones’ Scenario of No Expansion Tank

You check her water pressure using a water pressure gauge. She has an incoming water pressure of 100 psi coming into her water heater. The water heats up and expands. When the water gets hot and starts to expand, where does it go? That’s right, it has nowhere to go. It just keeps pushing on every fixture and every pipe in the entire house. The problem is that the incoming cold-water pressure was 100 psi and has grown to over 150 psi. All sorts of crazy things can happen now.

It can break the water meter, since most new water meters don’t handle back pressure very well. It can bust through washers in faucets, toilets, valve packing, etc. — anything that is holding water back. Most fixtures have washers that can only handle around 70 psi. You have now put over double that on that tiny washer. People will start to complain that everything is starting to leak in the house, one item at a time. The most dangerous thing that can happen is it can push open the relief valve on the water heater. This is a valve that has a temperature and pressure setting (thus called a T&P valve). For example, it could have a rating of 150 psi and 210 degrees F. That means the valve is designed to open and relieve itself if the water temperature gets above 210 degrees or if the pressure goes over 150 psi. Well, guess what? In this scenario, the pressure is 150 psi. The relief valve will open if it is working. If it is not working, you now have a highly dangerous bomb in your basement waiting to blow.

All of this can be avoided by having an expansion tank installed.

How It Fixes the Problem

Since the expansion tank has one side open to the water supply and the other side has a compressed-air chamber, when the water heats up and starts to expand, it will push the compressed-air chamber to equalize the system pressure. It gives the extra pressure somewhere to go. Now the pressure will remain constant, and your house will thank you for it.

Where to Install the Tank

The expansion tank (potable water type) should be installed on the inlet cold-water line. It can be installed anywhere in the building on the cold-water supply (after the regulator). It is usually installed by plumbers on the cold-water inlet of the water heater.


Expansion tanks are limited to a working pressure of 150 psi and up to 200 degrees F. Therefore, due to fluctuating city water pressure, you should install a water-pressure-reducing valve where the water enters the house. This will provide a steady manageable pressure to the entire water distribution system. The recommended inlet water pressure is 60 psi or less.

All expansion tanks (even the small 2-gallon tanks) should be supported in some way. There are a few options that plumbers use consistently. The first is a mounting bracket that can be placed nearby that uses stainless steel bands to secure and support the tank. The second common support system is to secure band iron to the ceiling and “belly strap” the tank to the ceiling.

You will need to adjust the air charge. Yes, the factory does precharge the tank, but usually only to 40 psi. You want to match the pressure in the expansion tank as close as possible to the water pressure of the house. Make sure you get a water pressure reading using a water pressure gauge on a cold-water spigot when the line is not already hot or you will get a false reading. You want to charge the tank using a manual type tire pump.

Bleed the Air Out

Once you have properly set up and installed your expansion tank, it is time to turn the water back on and bleed the air out of the water heater. Turn on the water supply to the water heater and let water run out of a hot spigot — preferably the laundry tub since it usually has no aerator; aerators will often get clogged when turning a building’s water back on because the water hammer stirs up mineral buildup and shoots it around the water distribution system. Run the hot water for about 15 minutes until the water comes out clean.


The expansion tank should be checked every time you do plumbing work in a customer’s house, the reason being diffusion. The membrane that separates the water from the compressed air is semipermeable and will allow a small amount of air to travel through the membrane at a rate of around 1 psi per year. If the tank is not installed correctly, the membrane could rupture. You can tell if the tank feels water logged or you can press the Schrader core and see if water comes out instead of air. If the tank has lost enough of an air charge over enough of a time period, it could rupture and be overtaken by the water pressure.


If the tank is water logged it needs to be replaced. If the tank is under or over charged, adjust the charge of the tank using the Schrader core. If the T&P valve is discharging, check the water pressure of the house and the expansion tank pressure. Check the size of the expansion tank. If the pressure of the house is below 60 psi, and the expansion tank is set up properly and is the correct size, then replace the T&P valve. When in doubt, replace both the T&P valve and the expansion tank. Better safe than sorry. Sometimes T&P valves that have discharged have a bad habit of not fully closing, leading to callbacks. 

About the Author
Anthony Pacilla is a registered master plumber for McVehil Plumbing in Washington, Pennsylvania. He has 22 years of experience in the plumbing and HVAC trades, and has a bachelor’s in business and economics from Thiel College. 


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