I’ve been on a plumbing DIY kick lately, and after recently replacing a hot water recirculating pump that had been installed incorrectly (upside-down!) by a professional plumber, I’ve been reminded that it’s never a bad idea to periodically test things inside your own house — even if they were installed by a “professional.”

Now, I don’t want to bag on all professionals. Some, and probably most, do fine work. But paying someone to do something for you isn’t necessarily a guarantee that the job’s been done right, which is why I decided today to test the pressure on the potable hot water expansion tank at our Utah house. It wasn’t showing any signs of trouble. I just felt like testing it… and I discovered that it was dead. Any normal person would be bummed. Me? I was stoked — because that meant I got to perform a plumbing project I’d never attempted, and write a new blog post about it! 🙂

What’s a Hot Water Expansion Tank?

A hot water expansion tank is a small metal tank (usually 2-5 gallons) that’s installed just “downstream” of the cold water inlet valve that feeds your residential water heater. The expansion tank’s purpose is to protect your house’s hot water system from excessive pressure, which can cause damage to fixtures, your water heater, lines, and/or fittings.

Inside, the tank has a rubber diaphragm at around the mid-point of the tank; pressurized air goes below the diaphragm in the bottom half of the tank, and water flows in and out of the top half of the tank.

To visualize how an expansion tank works, imagine that the diaphragm is actually a rubber ball like you’d find on a playground. The rubber ball is partially filled with air, so that it completely fills up the tank. When the water in your system is “stronger” than the rubber ball, it pushes against it squeezes it smaller — allowing some of the water to force its way into the tank. But when the water pressure outside the tank returns to normal, the rubber ball is once again “stronger” than the water, and shoves the water back out of the of the tank.

In plumbing terms, the air pressure below the rubber diaphragm acts as a “cushion” against water hammer shock, and allows the tank to absorb excess water pressure caused by the expansion of heated water in the system. It’s far better to have pressurized water push against the “rubber ball” inside a reinforced tank, rather than push against your pipes and fixtures… or even the inside of your water heater. Expansion tanks are very cheap insurance against potentially dangerous and expensive damage to your plumbing. If you don’t have an expansion tank in your system, you really should get one.

Expansion tanks will normally wear out after anywhere from 5-10 years. To extend their service life as much as possible, it’s vitally important to match your expansion tank’s air pressure to your house’s water pressure.

Why Expansion Tank Pressure Is So Important

In order to for the shoving match between the water and the rubber diaphragm to work ideally, the air pressure below diaphragm the tank before it gets squeezed should be the same as the normal water pressure in your system. That air pressure is called the “pre-charge.”

Having the tank’s pre-charge set lower than the water pressure will allow expanding water to shove its way into the tank more often it should, which wears out the diaphragm prematurely. But having the tank’s pre-charge higher than the water pressure will prevent the expansion tank from allowing water into the tank, thereby rendering the tank pointless. It could also rupture the rubber diaphragm since it’s not designed to be inflated beyond 80 psi.

Most expansion tanks are set to a pre-charge of 20 psi when they leave the factory, while the majority of residential water systems have significantly higher water pressure than that. This means that whoever installed your tank was supposed to manually increase the  tank’s pre-charge to match the water pressure at the time of install.

But did they?

The only way to be sure is to test it yourself. Follow the simple test method below to make sure your installer set your expansion tank’s pre-charge properly, and verify that your tank is still operating as it should. If it’s not, you can also follow the instructions in this post to replace your expansion tank yourself. Don’t worry — it’s really easy.

 Tools You’ll Need

To test the water pressure in your house, I recommend using something like a Pasco 1431 or a Rainbird P2A water test gauge. It simply attaches to your system anywhere that a hose would connect and tells you the water pressure at the fixture. Because I didn’t want to wait for Amazon, I picked up the Pasco at a local plumbing supply store for $12. It’s worth having one of these in your toolbox for a number of projects.

To test the pre-charge of your expansion tank, all you need is a standard tire pressure gauge. If you don’t already have one in your car’s glove box, you really should. I used a standard dial type tire pressure gauge (about $9 from any local auto parts store). My tire gauge has a bleeder valve, which makes removing excess pressure really easy.

How to Check Your Water Pressure

First, find the pressure of the water going into your water heater by attaching the water pressure gauge to your water heater’s drain valve at the bottom of the tank (the same place you’d attach a hose when you flush your water heater). Open the drain valve all the way. You don’t need to shut off any other valves or do anything else. Take note of the water pressure. Mine at the Utah house is 60 psi:

Testing residential water pressure with simple and cheap gauge

How to Check Your Expansion Tank Pre-Charge

Before checking the pre-charge on your tank, try this “quick and dirty” test to see if there’s any water in the tank: knock a few times on the side of the tank with your knuckles. You should hear the hollow “tink tink tink” noise of a metal tank that’s mostly full of air, rather than the “thud thud thud” noise of a metal tank that’s full of water. If your tank sounds full of water, then it’s possible that:

• the pre-charge is set too low, which is allowing the water pressure to “defeat” the diaphragm too easily and let too much water into the tank, or
• the diaphragm has ruptured or otherwise failed, so there’s nothing to keep the water out.

If you think your expansion tank may be full of water, unscrew the plastic cover on the bottom of the tank that exposes the Schrader valve, then and briefly press in on the valve’s pin (like you would to let air out of a tire). If water comes out of the valve, then the diaphragm in your tank is ruptured, and the tank needs to be replaced. If only air comes out,  continue with the test procedures below, but don’t attach your tire pressure gauge just yet!

Before you can accurately check the pre-charge (the pressure inside the rubber diaphragm in your expansion tank), you first need to relieve the pressure that’s pushing against the diaphragm from your house’s water system. Imagine, for example, that you want to check the air pressure of a yoga ball. But if you test the pressure while still sitting on the ball, you’ll get a higher number than you would if you checked the pressure while standing next to the ball. So to get rid of the pressure from your water system “sitting” on your expansion tank’s rubber “yoga ball,” do the following:

1. Shut off the cold water valve that feeds your water heater (it should be just “upstream” of your expansion tank). With these lever types, the valve is open when it’s parallel to the line, and closed when its perpendicular. This photo shows it closed:

Cold water valve turned off

2. Turn on a hot water fixture that’s closest to your water heater, such as a hot faucet. Keep it open throughout this procedure.

3. Turn the knob on your water heater to the PILOT position, to prevent the burner from firing:

Gas valve set to PILOT

4. If you have a hot water recirculating pump, turn it off and close the valve on the return line:

Turn off the recirculating pump and close the return valve

5. Connect a drain hose to the drain valve of your water heater (the same valve where you tested the water pressure), and open the valve all the way. Do a slow count to 10, then close the valve. That should remove enough water from the system to relieve any water pressure that’s pressing against the rubber diaphragm inside the expansion tank.

Partially drain your water heater to relieve water pressure

With the water pressure relieved, you can now get an accurate reading from your tank. Expose the Schrader valve (same valve you’d find on a bike or car tire) on the bottom of the expansion tank (you may have to unscrew a plastic cap) and firmly press your tire pressure gauge against the valve to get a reading. If the resulting pressure is higher than your house’s water pressure, then take some air pressure out of the expansion tank a little at a time by pressing in on the pin inside the valve (exactly as you would when lowering the pressure in your car’s tires). If the expansion tank’s pressure is lower than your house’s water pressure, you’ll need to fill the tank to the matching pressure, using the same type of air compressor you’d use to re-inflate your car’s tires.

If your pre-charge was set properly or only needed a minor pressure adjustment, then HOORAY! You’re good to go.

But, if you get no pressure reading from your tank (which is what happened to me today), then that probably means the diaphragm in your tank has ruptured or otherwise failed, meaning your expansion tank needs to be replaced. Here was my zero pressure reading today:

No pressure from hot water expansion tank

Replacing a Potable Hot Water Expansion Tank

If you’ve determined your expansion tank is bad, or it’s old enough that you just want to replace it anyway, installing a new one is really easy — especially if you’ve already followed the above steps to test the pre-charge (meaning you’ve already relieved pressure from the line and closed all the appropriate valves).

Generally, you should replace the old expansion tank with a new one of the same size. Going bigger is fine — going smaller is not. Most water heaters that are 50 gallons or smaller should use a 2 gallon expansion tank, such as an Amtrol ST-5 or a Watts PLT-5. But if you have two water heaters hooked together, or a water heater that’s larger than 50 gallons, go for a 4.4 gallon expansion tank like the Amtrol ST-12 or Watts PLT-12. Since I have two 40 gallon water heaters that are connected in series (for a total of 80 gallons in the zone), I picked up a Watts PLT-12 at a local plumbing supply store for $50, which is about what they sell for on Amazon. You can also find them at Home Depot, Lowes, or your local hardware store. The original installing plumber had actually used the smaller tank size in this house, so I figured this was the right time to correct that mistake and go one size bigger. I’m not sure the smaller size contributed to the failure of the tank, but the price difference between the two sizes is minor, so go for the larger one if you can.

After bringing the new tank home, I first made sure all the proper valves were closed and pressure relieved from the system, following all the above steps required to test the expansion tank pressure — including turning the gas valves to PILOT and partially draining the tank.

Next, I removed one side of the support strap that was helping to keep the old tank in place (it was screwed into the ductwork overhead). Your unit may not have a support strap (it only needs one if the tank is installed horizontally).

Remove old support strap

Third, I placed a towel on top of the water heater, underneath where the expansion tank connected to the cold water line. Since the failed tank is likely full of water, be ready for it to spill.

Fourth, unscrew the tank from the threaded fitting in the line. You may be able to get enough leverage on the tank to unscrew it without any tools, but channel locks will come in handy if you can’t. Here’s the fitting with the tank removed:

Fitting where the expansion tank connects

Here’s a shot of my old Amtrol tank (blue) next to my new Watts tank (white). Both manufacturers make quality tanks right here in the USA. I simply picked the Watts tank because it happened to be in stock.

Old Amtrol tank next to the new Watts tank

As mentioned earlier, all new expansion tanks come from the factory with some amount of pressure in the diaphragm , meaning the diaphragm will be inflated enough so that reaches the opening of the tank. On a functioning tank with pressure in the diaphragm, you should be able to stick your finger through the top and touch it.

However, when I inspected my old tank, I couldn’t feel the rubber diaphragm through the opening. That means it had indeed failed, as I suspected. I stuck my phone’s camera into the hole to get a peek at the failed diaphragm deep inside the tank:

Failed diaphragm inside an expansion tank

Even though the new tank already had some amount of pre-charge from the factory, it’s still important to set the correct pre-charge pressure in the new tank before installing it and allowing water pressure to push against the rubber diaphragm. Again, even if your yoga ball was partially inflated when you bought it, you still wouldn’t try to inflate it all the way while you’re sitting on it, would you?

Any type of air compressor that you’d use to fill your car’s tires will work to set the proper pressure in your expansion tank. I used a next-door neighbor’s compressor (it was easiest to take my tank to him, rather than bring his compressor to me). First, I tested the initial pressure of the new tank, and it was exactly 20 psi, as stated on the box:

Watts expansion tank initial pressure was exactly 20 psi

It didn’t take long to add some air and take the pressure to 60 psi:

Adding air to the expansion tank with a compressor

Expansion tank pressure set to match system water pressure

Before installing the new tank, I added some Teflon tape to the male threads:

Add Teflon tape to male threads to ensure a good seal

Then I installed the new tank by slowly twisting it into place, and tightened it down with channel locks:

Tightening the new tank into place

Finally, I tested for leaks by slowly turning the cold water inlet valve back on:

Cold water inlet valve opened to test for leaks

No leaks! I secured the new tank with the existing metal strap that had been supporting the old tank. If the tank is installed vertically (either above or below a line), you don’t need any additional support. However, if installed horizontally, you’ll need support straps to help hold the weight of the tank when it’s partially full of water.

Securing the new tank

After making sure the tank was securely in place, I turned my gas valve back to ON, turned on my recirculating pump and opened the return line valve.

Now, my system is once again protected against potential damage from thermal expansion and water hammer. I only spent $50 on the new tank, $9 on the water pressure gauge (which I’ll reuse often), and used a tire gauge I already had. The entire process, including the testing, took less than 30 minutes. I promise it’s something you can do, even if you’re not particularly handy.

But this important piece of safety equipment in your house can’t work properly if it’s not set properly, and if it’s not tested regularly. I was surprised today when I discovered mine had failed, and glad that the fix was inexpensive and straightforward. I’ve now added “Check expansion tank pressure” to my home maintenance schedule and plan to do it every six months (it’s easy to do when you’re flushing your tank and checking your anode), and suggest you do the same.

As always, I welcome your comments and questions below!

Special thanks to BJ Plumbing Supply in Orem, UT for their expert advice on tackling this project, and for always providing the parts I need at a good price.