Quick Answer:
If you have an expansion tank but the relief valve still drips or system pressure spikes during warm-up, the tank is often too small or has the wrong precharge. Check the heater’s water volume and the system’s cold static pressure, compare that to the expansion tank manufacturer charts, and confirm the tank isn’t isolated by a check valve or a closed shutoff. If the relief opens repeatedly, it’s a sign the tank can’t absorb the expanded water—think “Pressure relief opens after heating cycles.”
Why This Happens
- When water in a closed heating or water-heating system warms up it expands. The expansion tank provides a place for that extra volume to go so pressure stays within safe limits.
- An undersized tank or one with a lower-than-needed air precharge can’t accept enough expanded water. Pressure then spikes and the relief valve drips or opens.
- If a check valve or an isolation valve is blocking the tank connection, the tank is effectively removed from the system and can’t do its job.
- Incorrect precharge (air pressure inside the tank) reduces usable tank capacity. That leads to fast pressure rises and repeated relief-valve action—another common pattern of Thermal expansion causing relief valve drip.
Step-by-Step What to Do
Step 1 — Confirm the symptom
- Watch a heating cycle. Note if the temperature rise coincides with the relief valve dripping or the pressure gauge jumping.
- Record the pressure at cold and at the peak during heat-up (rough numbers are fine for diagnosis).
Step 2 — Find the heater’s water volume and cold static pressure
- Look in the heater/boiler manual or spec sheet for water volume (gallons or liters). If you don’t have the manual, the model number on the unit will usually let you find the spec sheet.
- Measure cold static pressure at the system fill connection or pressure gauge with the system cold and pumps off. Typical domestic cold fills are 12–25 psi (0.8–1.7 bar) depending on building height.
Step 3 — Compare to the expansion tank manufacturer chart
- Using the heater volume and cold fill pressure, check the tank manufacturer’s sizing chart to find the recommended tank size (often listed by model number and usable expansion volume).
- Match the system volume and cold-fill pressure to the chart; if the installed tank falls below the recommended usable volume, it’s undersized.
Step 4 — Check the precharge on the expansion tank
- Turn off system power and isolate the tank per the manufacturer’s instructions. With the system cold, use a simple tire-pressure gauge on the Schrader valve to read tank precharge.
- Set the precharge to match the cold static system pressure (for example, if cold pressure is 15 psi, set the tank to 15 psi). If the gauge reads water at the valve, the tank is waterlogged and needs service or replacement.
Step 5 — Confirm the tank is connected and not isolated
- Trace the piping from the tank. Make sure there’s no closed shutoff, isolation valve, or check valve that isolates the tank from the system. Remove or open any isolating valve so the tank sees system pressure.
Step 6 — Upsize if needed and retest
- If the relief still drips and pressure peaks remain above safe levels, install a larger tank per the manufacturer chart and confirm precharge is correct.
- After changes, run several heat cycles and watch the pressure gauge and relief valve for improvement.
What Not to Do
- Don’t install the smallest tank by default—undersizing leads to constant relief drips and pressure spikes.
- Don’t block or cap the tank connection to “stop the drip.” That removes the safety function and can damage the system.
- Don’t overinflate the tank air side above the cold static pressure—overcharging reduces usable expansion volume.
When to Call a Professional
- If you can’t find accurate heater volume specs or aren’t confident matching the manufacturer chart.
- If the tank is waterlogged, leaking, or if replacing it requires soldering or significant piping changes.
- If pressure problems continue after sizing and precharge are corrected, or if the system includes high-pressure boilers or complicated controls.
Safety Notes
- Work only on cold systems whenever possible. Hot water and steam can cause severe burns.
- Before loosening fittings or testing the tank air valve, relieve system pressure and shut electrical power to circulating pumps and boilers per manufacturer guidance.
- If you are unsure how to safely isolate components or if valves are stuck, stop and contact a qualified technician.
Common Homeowner Questions
- How do I know the tank is waterlogged? If the Schrader valve squirts water or the tank feels heavy and pressure changes don’t match, it’s waterlogged.
- Can I add air to the tank myself? Yes—if the system is cold and you match the precharge to the cold static pressure; do not overinflate.
- Will a bigger tank always fix the drip? Usually if the tank was undersized; confirm sizing with volume and pressure numbers first and ensure the tank isn’t isolated.
Related Articles
If you’re troubleshooting a similar symptom, these guides may help:
For the full directory, see Thermal Expansion Pressure Behavior.