To repressurize an expansion tank that has lost air pressure, you add air to the tank's valve using either an air compressor pump or a manual tire pump. A manual tire pump is often preferred for this task.
Why Repressurize an Expansion Tank?
Expansion tanks play a critical role in plumbing and heating systems (like hydronic heating). They absorb the expansion of water as it heats up, preventing excessive pressure buildup in the system. They contain a diaphragm that separates the system water from an air or nitrogen charge. Over time, the air pressure on the dry side of this diaphragm can decrease.
Reasons for pressure loss include:
- Air leaks: Air can slowly leak from the tank's Schrader valve (similar to a tire valve) or through the diaphragm itself if it's compromised.
- Diaphragm issues: While a diaphragm failure usually means water is present on the air side (indicating a need for replacement), a slight leak can also cause gradual pressure loss.
If the pressure loss is due to a leak from the valve, repressurizing can often fix the issue.
How to Add Air to Your Expansion Tank
The process involves using a pump to inject air back into the tank through the Schrader valve, typically located at the bottom or top of the tank, often hidden under a plastic cap.
Tools Needed
Based on typical recommendations, the tools are:
- Tire pressure gauge: To check the current pressure.
- Air Pump:
- An air compressor pump
- A manual tire pump (like a bicycle pump)
Important Note: According to best practices, a manual tire pump is preferable because it is less likely to rupture the tank's bladder due to high pressure. Air compressors can build pressure very quickly, making it easy to over-pressurize the tank and damage the internal diaphragm.
Steps for Repressurizing
Here are the general steps, though specifics can vary slightly depending on your system:
- Depressurize the System (Optional but Recommended): Shut off the water supply and drain a small amount of water from a nearby drain valve or faucet to reduce system pressure. For heating systems, shut off the boiler and allow it to cool. This allows you to set the tank's air pressure to the minimum system pressure (usually the system's fill pressure when cold).
- Check Current Pressure: Remove the cap from the Schrader valve on the expansion tank and use a tire pressure gauge to read the existing air pressure.
- Add Air: Connect your pump (manual or compressor) to the valve. Slowly add air. If using a compressor, add air in short bursts and check the pressure frequently. If using a manual pump, pump steadily and check pressure often.
- Reach Target Pressure: Continue adding air until you reach the recommended pressure. This is usually specified by the system manufacturer or the tank manufacturer. For closed hydronic systems, it's typically the system's cold fill pressure. For domestic hot water tanks, it might be set slightly below the home's water pressure.
- Verify and Recap: Once the correct pressure is reached, disconnect the pump and replace the valve cap.
- Repressurize the System: If you drained system pressure in step 1, repressurize your system according to its specific instructions (e.g., opening fill valve for a hydronic system or turning the main water supply back on).
- Monitor: Observe the system and tank pressure over the following days or weeks to ensure the air pressure holds. If the tank loses pressure again quickly after adding air, it may indicate a significant leak or a failed diaphragm requiring tank replacement.
Adding air correctly helps ensure the expansion tank can properly absorb pressure fluctuations, protecting your plumbing or heating system components.
