How to Recharge Your Hot Water Boiler Expansion Tank

Son watching father check hot water tank with energy application on digital tablet
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Your hot water expansion tank is an important part of maintaining a comfortable, safe and efficient home. That’s why it’s important to keep your boiler in tip-top shape. Here’s a quick look at how to recharge your boiler.

Boiler Tank Functionality

Your expansion tank provides an air cushion for the water as it expands during heating cycles. Without the air cushion, the interior pressure of the tank would surpass the release setting of the pressure release valve (PRV) and cause low pressure throughout the entire system.

Identifying Your Boiler Tank

Suspended horizontal tanks are found in attics or suspended between joists in your basement. Newer systems include a diaphragm expansion tank near the boiler itself. The age of your boiler will determine how you recharge your system.

Diagnosing Boiler Problems

Boiler problems differ depending on the age and make of the unit. Here’s a look at the tell-tale signs of trouble with each type of boiler:

  • Suspended boilers. PRV leaks, high pressure and overheating.
  • Diaphragm boilers. PRV leaks, and tank overheating.

Recharging Suspended Boiler Tanks

The source of PRV leaks, high pressure and overheating is usually too much water and too little air in your expansion tank. Here’s how to fix it:

  1. Feel the bottom of the tank. The bottom of your tank should be warmer than the top. If not, your tank is too full of water and needs to be drained.
  2. Turn off the boiler. Close the water shut-off valve and let the tank and piping cool.
  1. Drain your tank. Attach a hose to your combination valve and drain three gallons of water. Shut off the boiler and drain the entire tank if you have an older tank without a valve.
  2. Open the water supply. Switch on the water supply valves and let the system refill.
  3. Turn the boiler on. Turn your boiler back on. Your system should automatically recharge. Let your boiler tank run for one to two hours before checking on the system. 

    Recharging Diaphragm Boiler Tanks

    The source of PRV leaks and overheating is usually too much water and too little air in your expansion tank. Here’s how to fix it:

    1. Turn off the boiler. Close the water shut-off valve and let the system cool.
    2. Check water-side pressure. Your pressure should be at zero pounds-per-square-inch (PSI) in order to get a clear reading. Attach a tire gauge to the recharge valve at the bottom of the tank to check the pressure.
    3. Add more air if needed. Use a bicycle pump to fill the valve to the manufacturer recommended PSI. 
    4. Refill the system. Open the water-supply valves and let the system fill back up.
    5. Turn the boiler on. Turn your boiler back on. Your system should automatically recharge. Let your boiler tank run for one or two hours before checking on the system. 

    Before you begin, be sure your diaphragm-style tank is on the supply side of your system. Diaphragm tanks installed in joists can malfunction and require expensive repairs.