How to pressurize a water tank

A pressurized water tank uses compressed air to push water through your home's pipes. If your tank has lost pressure, you can restore it by draining the tank, refilling the air charge, and refilling it with water. The process takes about an hour and requires a few basic tools: a tire gauge, an air compressor, and a hose. Most homeowners can do this themselves, though if the tank is leaking or the valve is broken, you will need to call a plumber.

Water pressure tanks come in two main types: bladder tanks, which have an internal rubber bladder that separates air from water, and diaphragm tanks, which use a flexible disk instead. Both work the same way: air pushes down on water, forcing it out through your pipes. When the air pressure drops below the tank's minimum setting—usually around 28 to 30 pounds per square inch (PSI)—your water pressure falls and you may notice weak flow at your taps or the pump running constantly.

Key Takeaways

  • A water tank loses pressure when the air charge inside drops, which happens gradually over months or years as air dissolves into the water.
  • You can check the current air pressure with a tire gauge on the air valve at the top of the tank, which looks like a car tire valve.
  • Pressurizing the tank requires you to drain it completely, add air with a compressor until you reach the correct PSI, then refill it with water.
  • The correct air pressure is usually 2 PSI below your pump's cut-in pressure—check your pump's label or your manual to find this number.
  • If the tank continues to lose pressure within weeks, the bladder or diaphragm is likely torn and the tank will need to be replaced.

Check the current air pressure in your tank

Before you pressurize the tank, you need to know what pressure it currently holds. Locate the air valve at the top of your tank—it looks identical to a car tire valve and is usually a small brass fitting. Use a tire gauge (the kind you use for car tires works fine) and press it onto the valve stem. The gauge will show you the current PSI.

Write down this number. If it reads zero or very low, the tank has lost its air charge. If it reads higher than your pump's cut-in pressure (usually 30 to 40 PSI depending on your system), the tank is over-pressurized and you will need to bleed air out before refilling. Most tanks should sit at 2 PSI below your pump's cut-in pressure—so if your pump turns on at 30 PSI, your tank air pressure should be around 28 PSI.

Drain the tank completely

You must empty the tank before you add air. Turn off the pump at the breaker or switch. Open the lowest drain valve on your tank—this is usually a small ball valve at the bottom. Let all the water run out into a bucket or floor drain. This step is essential because pressurizing an already-full tank can damage the bladder or diaphragm inside.

Once the tank is empty, leave the drain valve open. You can also open a faucet in your house to release any remaining pressure in the lines. Wait a few minutes to make sure no more water drips from the drain valve, then close it.

Add air to the tank with a compressor

Connect an air compressor to the air valve on top of the tank. Use the same tire gauge to monitor the pressure as you add air. Pump air slowly and check the pressure every 10 to 15 seconds—it is straightforward to over-pressurize if you are not watching. Stop when you reach the target pressure, which is usually 2 PSI below your pump's cut-in setting.

If you do not know your pump's cut-in pressure, look at the pressure switch mounted on the pump or tank—it will have two numbers stamped on it, like "30/50" or "40/60". The first number is the cut-in pressure. Subtract 2 from that number to get your target air pressure. For example, if the switch reads 30/50, your target air pressure is 28 PSI.

Refill the tank with water

Once the air pressure is set, close the air valve and turn the pump back on at the breaker. The pump will run and fill the tank with water. You will hear the pump working and feel pressure building in the lines. This is normal. The pump will shut off automatically once the tank reaches the cut-out pressure (the higher number on your pressure switch).

Let the pump cycle on and off a few times to stabilize the system. Then turn on a faucet and check your water pressure. It should feel normal—steady flow without sputtering. If the pressure is still weak, the tank may have a larger problem and you should call a plumber.

Why water tanks lose pressure over time

Air naturally dissolves into water, especially when the water is under pressure. Over months and years, this dissolved air leaves the tank, and the air pressure inside drops. This is why even a tank that was pressurized correctly when installed will eventually need re-pressurizing. Most homeowners need to do this every 2 to 5 years, depending on water chemistry and how hard the pump works.

If your tank loses pressure much faster than that—within weeks or a few months—the bladder or diaphragm inside is likely torn. A small leak in the bladder allows air to escape into the water, and the tank will never hold pressure. In this case, the tank cannot be repaired and must be replaced. A plumber can confirm this by checking whether the air pressure drops again within a week of pressurizing.

When to call a plumber

Call a plumber if the air valve is stuck or will not accept air from the compressor. This usually means the valve is corroded or blocked and needs to be replaced. Do not force the compressor onto a stuck valve—you can damage it further.

Also call a plumber if you pressurize the tank correctly but water pressure does not improve, or if the tank loses pressure again within a few weeks. These signs point to a torn bladder, a leaking drain valve, or a problem with the pump itself. A plumber can diagnose which part is failing and replace it.

Frequently Asked Questions

What PSI should my water tank be at?

Your tank air pressure should be 2 PSI below your pump's cut-in pressure. Look at the pressure switch on your pump—it will show two numbers, like 30/50. The first number is cut-in. So if it says 30/50, your tank should be at 28 PSI. Check your pump manual if you cannot find the switch.

Can I pressurize the tank while it is full of water?

No. You must drain the tank completely before adding air. Pressurizing a full tank can rupture the bladder or diaphragm inside and ruin it. Always drain first, add air, then refill with water.

How often do I need to pressurize my tank?

Most tanks need pressurizing every 2 to 5 years as air naturally dissolves into the water. If you find yourself doing it more than once a year, the bladder is likely torn and the tank should be replaced.

What if my air compressor is not strong enough?

A standard household air compressor (usually 120 PSI maximum) is more than strong enough for a water tank, which typically needs 25 to 40 PSI. If your compressor will not build pressure, check that the hose is connected tightly and the valve is not stuck.

Can a torn bladder be patched?

No. The bladder is inside the tank and cannot be accessed or repaired. If it tears, the entire tank must be replaced. A plumber can confirm a torn bladder by checking whether the tank loses pressure within a week of pressurizing.