Expansion Tank Size Calculator
Use this calculator to determine the appropriate size for an expansion tank in a closed-loop hydronic heating system or a domestic hot water system. An expansion tank is crucial for accommodating the thermal expansion of water, preventing excessive pressure buildup, and protecting your plumbing system.
Required Expansion Tank Volume:
Understanding Expansion Tanks
An expansion tank is a vital component in any closed-loop hot water system, whether it's a domestic hot water heater or a hydronic heating system. Its primary function is to absorb the increased volume of water that occurs when water is heated. Unlike air, water is virtually incompressible. When heated, water expands, and if there's no space for this expansion, the pressure within the system will rise rapidly. This can lead to the activation of the pressure relief valve, which discharges water, or worse, damage to pipes, fittings, and appliances.
How Expansion Tanks Work
A typical diaphragm-type expansion tank consists of a steel shell divided into two sections by a flexible rubber diaphragm or bladder. One side contains pressurized air (the "pre-charge"), and the other side is connected to the system's water. As the water in the system heats up and expands, it pushes against the diaphragm, compressing the air on the other side. This compression allows the expanded water to enter the tank, preventing excessive pressure buildup in the main system. When the water cools and contracts, the compressed air pushes the water back into the system.
Why Correct Sizing Matters
- Too Small: If the expansion tank is undersized, it won't have enough capacity to accommodate the full expansion of the water. This will cause the system pressure to rise above the relief valve setting, leading to frequent discharge of water and potential water waste. It also puts unnecessary stress on system components.
- Too Large: While an oversized tank won't harm the system, it represents an unnecessary upfront cost and takes up more space than required.
Factors Influencing Tank Size
The correct expansion tank size depends on several key factors:
- Total System Volume: This includes the volume of the water heater, boiler, radiators, baseboard heaters, and all associated piping. The more water in the system, the more it will expand.
- Minimum Operating Temperature: The temperature of the water when the system is at its coolest (e.g., incoming cold water temperature or cold fill temperature for a heating system).
- Maximum Operating Temperature: The highest temperature the water will reach under normal operation. The greater the temperature difference, the more the water will expand.
- Maximum Operating Pressure: This is typically the setting of the system's pressure relief valve. The tank must prevent the pressure from exceeding this limit.
- Tank Pre-charge Pressure: This is the pressure of the air inside the expansion tank when it's empty of water. For optimal performance, the pre-charge pressure should be set equal to the minimum static pressure of the system when it's cold.
Installation and Maintenance Tips
- Pre-charge Adjustment: Always check and adjust the expansion tank's pre-charge pressure before installation, ensuring it matches the system's minimum static pressure. This is done with an air pump and a pressure gauge.
- Location: Install the expansion tank on the cold water supply line to a water heater, or on the suction side of the circulating pump in a heating system.
- Regular Checks: Periodically check the tank's pre-charge pressure and ensure the diaphragm is intact. A "waterlogged" tank (full of water on the air side) indicates a failed diaphragm and needs replacement.
Example Calculations:
Example 1: Residential Hot Water Heater
Let's calculate the expansion tank size for a typical residential hot water heater setup:
- Total System Volume: 55 gallons (e.g., 50-gallon water heater + 5 gallons for piping)
- Minimum Operating Temperature: 50°F (incoming cold water)
- Maximum Operating Temperature: 140°F (thermostat setting)
- Maximum Operating Pressure: 80 psi (relief valve setting)
- Tank Pre-charge Pressure: 40 psi (typical static water pressure in the home)
Using the calculator, the required expansion tank volume would be approximately 2.40 gallons. This would typically mean selecting a 2.5 or 3-gallon expansion tank (e.g., an Amtrol #60, which has a nominal volume of 4.4 gallons, would provide a good safety margin).
Example 2: Closed-Loop Hydronic Heating System
Consider a hydronic heating system with radiators:
- Total System Volume: 100 gallons (boiler, radiators, and piping)
- Minimum Operating Temperature: 60°F (cold fill temperature)
- Maximum Operating Temperature: 180°F (boiler operating temperature)
- Maximum Operating Pressure: 30 psi (relief valve setting)
- Tank Pre-charge Pressure: 12 psi (typical static fill pressure for a two-story home)
Using the calculator, the required expansion tank volume would be approximately 8.67 gallons. This would suggest a tank like an Amtrol #90 (8.6 gallons nominal volume) or a slightly larger one for a safety margin.