A water heater expansion tank, also called a thermal expansion tank, is a safety device designed to protect your household plumbing from thermal expansion. The risk of pressure damage from thermal expansion is seldom a concern for tankless water heaters, but if you own a tank-style heater, your plumbing system may be at risk.
When you consider that 50-gallons of cold water will, with the help of thermal expansion, become at least 52-gallons once it's heated, the additional 2-gallons of water will no longer fit in the water heater's tank. That's where a thermal expansion tank comes in.
A water heater expansion tank is simply a small tank that absorbs water into a bladder to reduce excess pressure in your water heater.
In the past, when water expanded it wasn't a problem, the extra few gallons would simply flow back into the city's water supply. But today, the expanded water has no place to go because current plumbing regulations prohibit it from entering the city's system where it could contaminate the public water supply.
With the required check valves and backflow preventers in place, your water pipes, water heater and even some appliances must deal with the increased pressure caused by thermal expansion. This causes unnecessary wear-and-tear which can result in damage to hot water-use appliances, including your water heater. It could even be the cause of a bursted pipe!
Watts DET-5 Water Heater Expansion Tank
The Watts DET-5 is the perfect size for 50-gallon water heaters.
How a Thermal Expansion Tank Works
Within the expansion tank is a pressurized air bladder which absorbs the additional water by expanding and contracting.
As the water in your water heater becomes hot, it expands and increases the pressure within the tank and plumbing system. However, instead of allowing the pressure to build, the excess water enters the expansion tank.
When a faucet within the house opens (or the water cools) the water within the thermal expansion tank is released back into your hot water system. An expansion tank only contains overflowing water. It does not store water on a permanent basis.
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Going without a water heater expansion tank is risky. Although, many water heaters don't have an expansion tank, we highly recommend adding one. It'll protect your plumbing, appliances and your water heater.
Overflowing hot water can cause the pressure in the water tank to rise to dangerous levels, which may result in the components breaking. As you might imagine, overflowing hot water can also leak out through the T&P Valve and cause extensive water damage.
Even when components don't break outright and no leaks develop, there can still be negative consequences for going without an expansion tank. The increased strain caused by an overly full tank of hot water can shorten the service life of the heater, not to mention that the components may wear out sooner than expected.
Installing an expansion tank to your hot water heater is not only a good idea, its also a requirement in most areas, and determining the right size for your system is important. Unfortunately, when it comes to thermal expansion tanks there isn't a one-size-fits all option.
How to Properly Size an Expansion Tank
Selecting the correct size is relatively easy. There are two primary factors that need to be taken into account:
- Water Heater Capacity - This information can be found on the factory label attached to your water heater.
- Household Water Pressure - The water pressure, measured in psi (pounds per square inch) within your home's plumbing can be measured with a gauge. Simply attach the gauge to a hose or faucet and turn on the water. It's a best practice to check your water pressure over a 24-hour period. However, another option is to contact your water company.
Keep in mind, you should install a pressure reducing valve if your water pressure is above 80 psi. The ideal water pressure is between 50 to 60 psi.
Water Pressure Gauge
This water pressure gauge comes with adapters that allow you to measure the water pressure in multiple locations.
General Guidelines for Sizing an Expansion Tank
This chart will help you properly size an expansion tank to your residential water heater. It assumes that the temperature is set to 150-degrees.
Water Heater Capacity
Supply Pressure (psi)
Expansion Tank Size
40 to 60-gallon
40 to 60-gallon
If your water heater falls outside of these static supply pressure readings or capacities, it's best to contact a professional. They can come out and make the necessary calculations and even install the expansion tank for you.
Amtrol Expansion Tank
Amtrol manufacturers an excellent 2-gallon water heater expansion tank.
What Happens if the Expansion Tank is the Wrong Size?
Purchasing the right sized thermal expansion tank is important, but if you're in doubt, it's best to error on the side of larger than smaller.
An expansion tank that is too large for your system will still be able to safely handle the extra water. However, a tank that's too small can trigger the temperature and pressure relief valve to open so the excess pressure is relieved.
If you have a bit of mechanical skill and enjoy taking on DIY tasks, you should be able to install an expansion tank yourself. Or, you can simply contact a professional to do the job for you.
Installing an Expansion Tank
- As described above, determine your household water pressure with a pressure gauge. If the pressure is above 80 psi, install a pressure reducing valve.
- Check the air pressure on the expansion tank with a tire gauge.
- Adjust the air pressure of the expansion tank to match the maximum household water pressure. Using a hand pump NOT an air compressor.
- Install the tank on the cold water line (see video for details).
- Open a faucet and allow it to run until you have a steady stream of water. This will remove any trapped air within the tank.
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Water heater expansion tanks need regular maintenance to ensure optimal operation. Most experts recommend that you or a licensed professional perform routine maintenance annually.
Why is Maintenance Needed for an Expansion Tank?
Expansion tank's are designed with an internal bladder that separates the air and water. Through a process called "diffusion" the bladder will leak air. The leak can often be at a rate of 1 psi a year, which is enough to make a difference after only 12 months.
If the internal bladder ruptures the expansion tank will fill with water and not properly drain. When this happens, you'll need to purchase another expansion tank since there's no way to repair the bladder. The average lifespan of a thermal expansion tank is 6-years, provided it was installed correctly.
Checking the Bladder
We'll give you an easier method, but if you call a professional to inspect your expansion tank, these are the steps he'll use:
- Turn off the supply of water to your house and relieve the pressure by opening a faucet.
- Remove the cap to the valve stem on expansion tank and attach a tire pressure gauge. The pressure should be above 75 psi.
- If there is no air pressure within the tank, the expansion tank has failed. You will need to replace it with a new one.
- If there is pressure inside the tank, you'll need to check the water pressure.
- The expansion tank and water pressure should be similar. If its not similar, you'll need to use a hand pump to add air to the expansion tank.
However, if you're looking for a quick and easy way to inspect your tank you can simply press the Schrader valve that's located on the outside of the tank.
If air comes hissing out when you press down on the valve, the bladder is in good working order. But if water comes dripping out instead, the bladder has most likely ruptured and you'll need to purchase a new one.
Checking the PSI
After checking that the bladder is in good repair, you'll want to check the tank's psi pressure. Your owner's manual will have the correct psi for your tank. You may want to write this number down on the tank so you don't need to look it up again in the future.
Take the reading with a tire pressure gauge. If the psi is too high, you can simply let some air out by pressing down on the valve until enough air has escaped. If the psi is too low, you'll need to add some air with a tire pump.
We highly recommend using a hand pump instead of an air compressor when adding air. An air compressor could easily rupture the bladder.
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Last update on 2022-09-23 / Affiliate links / Images from Amazon Product Advertising API