Is an Expansion Tank Required for Water Heaters?

This guide delves into the necessity of expansion tanks for water heaters, covering the underlying physics, code requirements, troubleshooting, and best practices.

Is an Expansion Tank Required for Water Heaters

The Physics of Thermal Expansion

Water, like most substances, expands in volume when heated. For every 100°F (56°C) increase in temperature, water expands by approximately 4.2%.

Molecular Level of Thermal Expansion - Is an Expansion Tank Required for Water Heaters
The image illustrates the effect of temperature on water at both the macroscopic (visible to the naked eye) and molecular levels. It shows three states of water: heated, room temperature, and chilled.

In a closed system, such as a plumbing system connected to a municipal water supply with a backflow preventer or check valve, this expansion has nowhere to go. The pressure within the system will rise dramatically.  

  • Pressure Buildup: This pressure increase can exceed the pressure relief valve (PRV) setting on the water heater (typically 150 psi), causing it to discharge water. Repeated discharge can lead to premature valve failure, water damage, and wasted energy.
  • Hydraulic Shock/Water Hammer: Rapidly closing faucets or valves in a system without an expansion tank can exacerbate pressure spikes, leading to water hammer – a loud banging noise in the pipes caused by the momentum of the water being abruptly stopped. This can damage pipes, valves, and connections over time. 

Watch this YouTube video about “Thermal Expansion of Water Experiment” for simple demonstrations of how water volume increases with temperature. These visuals help understand the underlying physics.

Expansion tanks and hydraulic locks both play crucial roles in pressure management. Explore how hydraulic locks work to maintain stability and prevent system failures.

Why a Backflow Preventer Makes an Expansion Tank Essential

2021 International Plumbing Code (IPC) often require backflow preventers (check valves, pressure-reducing valves with check valves, etc.) on the main water supply line.

These devices prevent contaminated water from flowing back into the municipal water supply. However, they also create a closed system, trapping the expanding water within the house plumbing. This is the key reason why expansion tanks are now almost universally required.  

Local codes may have specific requirements regarding tank size and placement. Always consult your local plumbing code for definitive guidance.

How Expansion Tanks Work

An expansion tank is a steel tank with an internal air bladder or diaphragm. The bladder separates the incoming water from a pre-charged air cushion.

Bladder Tanks vs Diaphragm Tanks - Is an Expansion Tank Required for Water Heaters

As water heats and expands, it flows into the tank, compressing the air cushion. This compression absorbs the increased volume of water, preventing pressure buildup in the plumbing system.

Here’s how they function:

  1. The tank is initially pressurized with air to match the incoming water supply pressure.
  2. When water is heated, it expands and flows into the expansion tank.
  3. The expanding water compresses the air within the tank, effectively absorbing the excess pressure caused by the expansion.
  4. Once the water stops expanding, the compressed air exerts pressure, pushing the water back into the supply lines.
  5. As the water cools and contracts, the pressure decreases accordingly.

Also Read: Considering a new water heater? Explore different water heater types to find the right option and understand how expansion tanks fit into each setup.

Sizing an Expansion Tank

Proper sizing is crucial. An undersized tank won’t adequately absorb the expanding water, while an oversized tank is unnecessary and takes up more space. Factors influencing tank size include:

  • Water Heater Capacity: Larger water heaters require larger expansion tanks.
  • Water Temperature Rise: The greater the temperature difference between the incoming cold water and the heated water, the more expansion will occur.
  • Water Hardness: Hard water can cause mineral buildup inside the tank, reducing its effective capacity. In areas with very hard water, it is recommended to upsize the tank.  

Consulting a sizing chart provided by expansion tank manufacturers or using online calculators is the best way to determine the correct size.

Closed Compression Expansion Tanks and Diaphragm Expansion Tanks - Sizing an Expansion Tank

Source: https://www.engineeringtoolbox.com/expansion-tanks-d_885.html

Installation and Placement

Here’s a short guide summarizing the key points about installing an expansion tank:

Expansion Tank Installation Guide:

  • Location: Install the tank on the cold water supply line to the water heater. The closer to the heater, the better.
  • Orientation: Mount the tank vertically, upright, to ensure proper function.
  • Support: Provide adequate support to prevent stress on the pipes and the tank itself.
  • Shut-off Valve: Include a shut-off valve on the cold water supply line leading to the tank. This allows for easy isolation of the tank for maintenance or replacement.

By following these guidelines, you can ensure proper installation and optimal performance of your expansion tank.

Learn more about expansion tanks and how to install them by watching this video: Expansion Tank Installation on a Water Heater

Troubleshooting & Maintenance

This section outlines common issues and maintenance procedures for expansion tanks used in water heating systems. Proper maintenance can prevent costly repairs and ensure the longevity of your plumbing system.

Troubleshooting:

  • Water leaking from the PRV: This is a primary indicator of a missing, undersized, or failed expansion tank.
  • Waterlogged tank (no air cushion): This occurs when the bladder has failed, and the tank is full of water. The tank will feel heavy when tapped. Replace the tank.
  • Banging pipes (water hammer): While not always solely caused by a lack of an expansion tank, its absence can exacerbate this issue.

Maintenance:

  • Check the air charge: Periodically (annually is recommended), check the air pressure in the tank. This is done by shutting off the water supply to the tank, draining the water, and checking the pressure through the Schrader valve (like a tire valve) on the tank. The pressure should match the incoming water pressure. If the pressure is low, add air using a bicycle pump or air compressor.

To ensure the safety of your water heating system, read about the dos and don’ts of hot water heater safety to complement your understanding of expansion tanks.

In summary: Expansion tanks are a crucial component of modern plumbing systems, protecting against damage caused by thermal expansion of water. Proper sizing, installation, and maintenance are essential for optimal performance and longevity. Ignoring this crucial component can lead to costly repairs and water damage