The PSI for an expansion tank typically starts with a pre-charge of 40 PSI, but this pressure may need adjustment based on your home's water system.
Expansion tanks play a crucial role in plumbing systems, particularly those with closed loops like water heaters. They accommodate the thermal expansion of water as it heats up, preventing excessive pressure buildup that could damage pipes or fixtures.
Understanding Expansion Tank Pressure
According to standard practices, and as highlighted in the provided information:
- Expansion tanks are pre-charged from the factory with a 40 PSI air charge. This is the default pressure before installation or connection to the water system.
- However, the tank's air pressure must be adjusted if the inlet water pressure of your system is higher than this initial 40 PSI.
- The adjustment involves setting the tank's air pressure to match the system's inlet water pressure. Specifically, this matching pressure should be based on the highest recorded inlet water pressure in a 24-hour period of regulated water pressure.
- Crucially, the adjusted air pressure must not be higher than 80 PSI. This serves as a maximum limit to protect the tank and system components.
Key Pressure Values
Here's a quick look at the critical pressure points for an expansion tank:
| Pressure Type | Typical Value(s) | Notes |
|---|---|---|
| Factory Pre-charge | 40 PSI | The default setting out of the box. |
| Adjusted Pressure | Matches inlet water PSI | Required if system pressure > 40 PSI; based on highest 24hr reading. |
| Maximum Allowed Level | 80 PSI | The absolute highest the tank's air pressure should be set to. |
Why Adjustment is Necessary
Setting the tank's air pressure correctly is vital for its proper function. If the tank's air pressure is set too low relative to the system's water pressure, the tank will fill with water too quickly, potentially becoming waterlogged and unable to effectively absorb expansion. If set too high (above system pressure), water won't enter the tank until a very high pressure is reached, defeating its purpose.
Adjusting the tank's pressure to match the system's static (non-flowing) pressure ensures the tank is ready to accept the expanding water as soon as the system pressure slightly exceeds the pre-charge.
Practical Steps
To ensure your expansion tank is set correctly:
- Check your home's water pressure: Use a water pressure gauge on an outdoor spigot or laundry hookup. It's best to do this when no water is running in the house to get the static pressure. As the reference suggests, monitoring the highest pressure over 24 hours is ideal for precision.
- Compare to the tank's pre-charge: If your water pressure is 40 PSI or less, the tank's factory setting is likely sufficient.
- Adjust if needed: If your water pressure is higher than 40 PSI (but not exceeding 80 PSI), you'll need to adjust the air pressure in the expansion tank to match your system's pressure before installing or while the system is depressurized (no water pressure on the tank side). You'll need an air pump and a pressure gauge (like a tire pressure gauge) to do this via the Schrader valve on the tank.
- Stay within limits: Never set the tank's air pressure higher than your system's pressure or above the 80 PSI maximum.
Maintaining the correct PSI in your expansion tank is a key part of ensuring the longevity and reliability of your plumbing system, especially for water heating appliances.
