Sizing an expansion tank is crucial for managing the pressure increase that occurs when water heats up in a closed-loop system, like a water heater or hydronic heating system. Proper sizing prevents excessive pressure buildup, which could damage pipes, fixtures, or the system itself.
According to the reference provided, one method for sizing an expansion tank, specifically a Therm-X-Trol volume (Tv), involves the following equation:
Tv = Design Pressure Factor x expanded water
Let's break down this formula and the concepts behind it.
Understanding the Sizing Formula
The formula Tv = Design Pressure Factor x expanded water highlights the key elements needed to determine the required volume of an expansion tank (Tv).
- Tv: This variable represents the Total Volume of the expansion tank required, typically measured in gallons. This is the value you are solving for.
- expanded water: This refers to the volume of water that expands when heated from its initial temperature to the maximum design temperature of the system. Water expands significantly as it heats up, and this increased volume needs a place to go within a closed system.
- Design Pressure Factor: This factor accounts for the system's operating pressures. It is derived from the system's minimum fill pressure (when the system is cold) and the maximum allowed working pressure. The tank needs to be sized to accommodate the expanded water volume within this pressure range.
What the Variables Represent
To use the formula effectively, you need to determine the values for 'Design Pressure Factor' and 'expanded water'.
Expanded Water Volume
The volume of expanded water depends on:
- The total volume of water in the system (e.g., the capacity of the water heater or the entire hydronic system).
- The temperature change the water undergoes (from its cold fill temperature to the maximum operating temperature). Water density decreases as temperature increases, causing expansion.
Tables or calculations based on water's coefficient of thermal expansion are typically used to find this volume. For example, if a system holds 100 gallons and the water heats significantly, the expanded volume might be a few gallons.
Design Pressure Factor
The Design Pressure Factor is influenced by:
- System Fill Pressure: The pressure of the system when it is cold (e.g., static pressure from the plumbing or the initial pressure of the system).
- Maximum Allowable System Pressure: The highest pressure the system components (pipes, fittings, heater, etc.) are designed to withstand and operate within safely.
The factor ensures the tank can absorb the expanded water volume without exceeding the maximum pressure limit while still maintaining adequate pressure when the system is cold. This factor is often found using charts or formulas provided by tank manufacturers, based on the ratio of these minimum and maximum pressures.
Example Scenario from the Reference
The reference provides a scenario to illustrate the application of this formula, although it doesn't include the values for the factors needed to complete the calculation:
- System Type: Water Heater
- Water Heater Volume: 240 gallons
- Aquastat (Temperature) Setting: 150˚F
- Maximum Pressure Requirement: 125 psi
In this example, you would first determine the 'expanded water' volume based on the 240 gallons heated up to 150°F. Then, you would find the appropriate 'Design Pressure Factor' based on the system's fill pressure (not explicitly stated in the example but necessary for a real calculation) and the 125 psi maximum pressure limit. Finally, you would multiply these two values to find the required tank volume (Tv).
Steps for Sizing (General Approach)
Based on the formula structure, sizing typically involves:
- Determine the total volume of water in the system.
- Determine the maximum operating temperature.
- Calculate or look up the 'expanded water' volume based on steps 1 and 2.
- Determine the system's minimum fill pressure.
- Determine the system's maximum allowable operating pressure.
- Calculate or look up the 'Design Pressure Factor' based on steps 4 and 5.
- Multiply the 'expanded water' volume by the 'Design Pressure Factor' using the formula Tv = Design Pressure Factor x expanded water to get the required tank size (Tv).
Note: Specific charts and calculators are usually provided by expansion tank manufacturers or found in plumbing/HVAC code books and engineering handbooks to determine the 'expanded water' volume and the 'Design Pressure Factor' accurately for various system conditions.
Why Proper Sizing Matters
- Safety: Prevents dangerously high pressures that could cause leaks or bursts.
- System Longevity: Reduces stress on pipes, fittings, valves, and the water heater or boiler.
- Efficiency: Helps maintain stable system pressure.
- Prevents Relief Valve Discharge: Stops the pressure relief valve from opening repeatedly, which can be wasteful and potentially cause water damage.
In summary, sizing an expansion tank using the provided formula involves calculating the volume of water expansion due to heating and applying a factor based on the system's pressure limits.
