How to Add Water to a Hydronic Boiler System

Boilers operate as a closed-loop system, meaning they don't consume water like a standard faucet or shower. If your pressure gauge is dipping below 12 PSI, it means you have air pockets, a leak, or the expansion tank has lost its air charge, causing the system to lose efficiency or shut down entirely. Adding water is the immediate fix to get the radiators warm again, but it should be done carefully to avoid over-pressurizing the pipes. Done well, this process restores your heat without introducing excessive minerals or air into the system. You are looking for a steady climb on the pressure gauge; if the needle spikes quickly, you are inviting trouble. Keep a close watch on the gauge throughout the process, and remember that slow and steady is the golden rule for maintaining the right pressure in your home's boiler loop.

1. Read the pressure gauge. Look at the circular pressure gauge mounted on the boiler. A standard residential boiler should hold steady between 12 and 15 PSI when the system is cold.
2. Find the feed valve. Follow the small copper pipe running from your domestic water supply to the boiler. You will see an automatic feed valve with a lever or a manual ball valve bypass.
3. Open the fill valve. If you have an automatic valve, lift the fast-fill lever to its upright position. If you have a manual bypass valve, turn the handle slowly to allow water into the system.
4. Watch for 15 PSI. Watch the needle on the boiler gauge as water enters. Stop the flow immediately once the needle reaches 15 PSI.
5. Shut the valve tight. Flip the fast-fill lever back down or turn the manual valve to the fully closed position. Ensure there is no drip or sound of flow.
6. Bleed the air out. Adding fresh water often introduces air into the lines. Use a radiator key to open the bleed valve on your furthest or highest radiator until water flows out steadily, then close it.