Fixing a Boiler Expansion Tank with Excess Water

double boiler water heater
  • 1-2 hours
  • Intermediate
  • 0-50
What You'll Need
Garden hose
Wrench set

Water boiler heat systems typically include an expansion tank. While the water in a boiler is definitely above the boiling point, the water does not turn into steam because of the expansion tank. The expansion tank, along with a pressure valve, allows the water to be kept under pressure to maintain the boiler’s water level at half full. If you notice leaking around the expansion tank you’ll need to do some repairs.

Step 1 – Trouble Shoot the Problem

Begin by tapping on the expansion tank lightly just a little higher than halfway up the tank. Determine if the tank sounds hollow or full. Carefully feel the bottom portion of the tank. The expansion tank should be warm to the touch at the bottom of the tank and cooler at the top of the tank. If the tank feels hot on the top as well as the bottom chances are the tank has accumulated excess water and it must be drained. Check the relief valve as this may be the source of your leak.

Step 2 – Prepare the Tank

Turn off all power to the tank and boiler. Make sure the shutoff valve is closed. Allow both the boiler and the expansion tanks to cool completely before attempting any repairs. The boiler system is comprised of many different pipes and valves. These pipes carry hot water. Be aware of the pipes around the area you will be working and take caution not to get burned by bumping into one of these pipes. You should wait a minimum of four hours for a small tank to cool. Turn off the main water supply at its source.

Step 3 – Drain the Expansion Tank

Generally, an expansion tank has its own drain valve. Attach the garden hose to the drain valve. Run the hose to a floor drain, outside or into a large bucket. Depending on the size of your expansion tank you may need to drain three to five gallons of water from the tank. In the case where the expansion tank does not have a drain valve you will need to place the bucket directly beneath the drain. It is unlikely you will be able to open the drain valve by hand. The best way to open the valve is with a wrench, if that fails cautiously use the pliers.

Step 4 – Refill the Expansion Tank

Remove the hose and replace the drain valve. The valves should be returned to the desired setting before you turn on the water and the power to the boiler. Then turn on water and power to the expansion tank. The normal activity of the boiler system will fill the expansion tank correctly.

Once the tanks have filled correctly with water and the boiler has had time to heat the water. The air pressure gauge should measure 12 psi.