Do Tankless Water Heaters Need Expansion Tanks? Everything Explained

When you’re considering installing a tankless water heater, you might wonder if you need an expansion tank to go with it. This is a common question that homeowners ask, and the answer affects both your wallet and your plumbing system’s safety.

Most tankless water heaters do not need an expansion tank because they heat water on demand without storing it, which prevents the pressure buildup that typically requires an expansion tank. However, there are specific situations where you might still need one, depending on your plumbing setup and local building codes.

Understanding when an expansion tank is necessary can save you money and prevent costly plumbing problems down the road. We’ll walk you through everything you need to know about expansion tanks and tankless water heaters, including the exceptions to the general rule and how to determine what your specific situation requires.

Key Takeaways

• Tankless water heaters typically don’t need expansion tanks because they heat water on demand without storage
• Expansion tanks are required when tankless units work with storage tanks or recirculation systems in closed plumbing systems
• Proper installation and maintenance of expansion tanks when needed prevents pressure damage and extends your system’s life

Do Tankless Water Heaters Need an Expansion Tank?

Most tankless water heaters don’t need expansion tanks because they heat water on demand rather than storing it. The key difference lies in how thermal expansion affects closed plumbing systems and what manufacturers actually recommend for different setups.

How Thermal Expansion Works in Plumbing Systems

When we heat water, it expands and creates pressure in our plumbing pipes. In closed systems with check valves or backflow preventers, this pressure has nowhere to go.

Traditional water heaters store hot water in tanks. As the stored water heats up, it expands constantly. This creates ongoing pressure that needs an expansion tank to absorb the extra volume.

Closed vs. Open Systems:

• Closed systems trap pressure because backflow devices block water from returning to the main line
• Open systems allow pressure to release back through the water supply

Water pressure problems happen when expansion has nowhere to go. High pressure can damage pipe joints, valves, and the water heater itself.

The expansion tank acts like a pressure relief system. It has an air chamber that compresses when extra water enters, keeping pressure at safe levels.

Differences Between Tankless and Traditional Water Heaters

Tankless water heaters work differently than tank systems. They only heat water when we turn on a hot water tap.

When we open a faucet, the tankless unit starts heating. When we close the tap, heating stops immediately. This creates an open system while water flows.

Key Differences:

No stored water means no constant thermal expansion. The system stays open when water flows and closes when flow stops.

Exceptions exist when tankless units work with storage tanks or recirculation systems. These setups can create the same pressure issues as traditional water heaters.

Manufacturer Recommendations on Expansion Tanks

Most manufacturers don’t require expansion tanks for standard tankless installations. Their manuals typically state expansion tanks are unnecessary when no storage device is present.

We should always check local building codes first. Some areas require expansion tanks regardless of water heater type if the plumbing system is closed.

Common manufacturer guidelines:

• Standard tankless setup: No expansion tank needed
• Tankless with storage tank: Expansion tank required
• Recirculation systems: Usually need expansion tank

Some manufacturers suggest expansion tanks as optional protection. This helps extend system life and prevents potential pressure damage.

Installation manuals provide specific requirements for each model. We recommend following both manufacturer instructions and local plumbing codes for the safest setup.

When Is an Expansion Tank Required for Tankless Water Heaters?

Most tankless water heaters don’t need expansion tanks because they heat water on demand without storing it. However, specific situations like closed water systems with backflow prevention devices or local code requirements can make expansion tanks necessary.

Closed Water Systems and Backflow Prevention

A closed water system prevents heated water from flowing back into the main water supply. This happens when your home has a check valve, backflow preventer, or pressure reducing valve installed.

These devices protect your drinking water from contamination. But they also trap expanded water inside your plumbing system.

When water heats up, it expands by about 2-4%. In an open system, this extra water flows back to the street main. In a closed system, the expanded water has nowhere to go.

This creates pressure buildup throughout your pipes. The pressure can damage fixtures, joints, and even your tankless water heater.

Signs of a closed system include:

• Check valve on main water line
• Backflow prevention device
• Pressure reducing valve
• Water meter with built-in check valve

If you have any of these devices, you likely need an expansion tank even with a tankless unit.

Local Plumbing Codes and Requirements

Many local codes require expansion tanks for all water heater installations, regardless of type. These rules vary by city, county, and state.

Some areas mandate expansion tanks only for traditional tank heaters. Others require them for any water heating system in a closed loop.

Common code requirements:

• Expansion tanks required for all closed systems
• Mandatory for homes built after certain dates
• Required when installing backflow prevention
• Specific sizing requirements based on system volume

Contact your local building department before installation. They can tell you exactly what your area requires.

Some manufacturers also specify expansion tank requirements in their installation manuals. Check your tankless unit’s documentation for specific guidance.

Signs an Expansion Tank May Be Needed

Several symptoms indicate your tankless system might benefit from an expansion tank. These signs show pressure buildup in your plumbing.

Water hammer is a loud banging noise when you turn taps on or off. This happens when pressure waves move through your pipes.

Dripping from the temperature and pressure relief valve on your water heater shows excess pressure. This valve opens to prevent dangerous pressure buildup.

Other warning signs include:

• Reduced water flow from fixtures
• Frequent plumbing leaks at joints
• Premature failure of fixtures or appliances
• Fluctuating water pressure

If you notice these issues after installing a tankless unit, an expansion tank may solve the problem. The tank provides space for heated water to expand safely.

Potential Risks and Problems Without an Expansion Tank

When tankless systems operate without proper expansion tanks in certain setups, high water pressure can damage expensive components and create safety hazards. These problems become especially serious in closed plumbing systems where pressure has nowhere to escape.

High Water Pressure Issues

High incoming water pressure above 80 psi puts serious stress on our entire plumbing system. Without an expansion tank to absorb this pressure, we risk damaging pipes throughout our home.

Pressure buildup creates several problems:

• Burst pipes from excessive force
• Leaks at pipe joints and connections
• Water hammer that causes loud banging sounds
• Failed plumbing inspections

Closed plumbing systems make these issues worse. When we have backflow preventers or pressure reducing valves, water pressure gets trapped in our pipes with no way to escape.

The pressure relief valve on our tankless unit can only handle so much. It’s designed for emergency situations, not constant high pressure from thermal expansion.

Damage to Tankless Water Heater Components

Excess pressure damages the internal parts of our tankless water heater. These units have sensitive components that can fail when exposed to constant high pressure.

Common component damage includes:

• Cracked heat exchangers from pressure stress
• Failed internal sensors and controls
• Damaged water flow switches
• Shortened lifespan of heating elements

Our manufacturer warranty may become void if we don’t install required expansion tanks. Many plumbing codes require expansion tanks in closed systems for this exact reason.

Recirculation pump systems face additional risks. The mini storage tank in these hybrid setups needs pressure relief just like traditional tank water heaters.

How Expansion Tanks Work with Tankless Water Heaters

When expansion tanks are installed with tankless systems, they work alongside pressure relief valves to control water pressure and protect your plumbing. The tank absorbs excess pressure while valves provide backup safety protection.

Relationship with Pressure Relief Valves

Expansion tanks and pressure relief valves work as a team to protect your tankless water heater system. The expansion tank handles normal pressure changes, while the pressure relief valve acts as a backup safety device.

When water heats up in your system, it expands and creates pressure. The expansion tank absorbs this extra pressure by allowing water to flow into its chamber. This keeps pressure levels safe and steady.

The pressure relief valve only opens if pressure gets too high. This happens when the expansion tank fails or becomes full. When the valve opens, it releases water and pressure to prevent damage to your system.

We recommend checking both components regularly. The expansion tank should maintain proper air pressure, typically between 12-50 PSI. The pressure relief valve should be tested annually to make sure it opens and closes properly.

Interaction with Pressure Tanks

Expansion tanks and pressure tanks serve different roles in tankless water heater systems. An expansion tank controls thermal expansion, while a pressure tank maintains steady water pressure throughout your home.

Pressure tanks store water under pressure to reduce pump cycling and provide consistent flow. When used with tankless heaters, they can create a closed system that requires an expansion tank.

The expansion tank prevents pressure buildup that could damage the pressure tank or other system components. Without it, thermal expansion could cause the pressure tank to fail or your plumbing to leak.

Both tanks need proper sizing for your system. The pressure tank size depends on your water usage, while the expansion tank size relates to your water heater capacity and system pressure.

Installation Considerations for Expansion Tanks

Getting the right expansion tank size and installing it correctly are the most important steps for protecting your tankless water heater system. The tank must match your system’s capacity and be placed in the right location to work properly.

Choosing the Right Expansion Tank Size

We need to select an expansion tank that matches our tankless water heater’s capacity and our home’s water pressure. Most residential systems require a 2-gallon to 5-gallon expansion tank.

The tank size depends on three main factors:

• Water heater capacity (GPM rating)
• System water pressure (typically 40-80 PSI)
• Temperature rise (usually 100-120°F)

For most tankless units rated at 6-8 GPM, we recommend a 2-gallon expansion tank. Larger systems above 10 GPM typically need 4-5 gallon tanks.

We should check our tankless water heater manual for specific expansion tank requirements. The manufacturer often provides sizing charts based on system capacity.

Pre-charged expansion tanks come set to 12 PSI. We need to adjust this to match our home’s water pressure minus 10 PSI for proper operation.

Proper Placement and Installation Tips

We must install the expansion tank on the cold water line before it enters the tankless water heater. This location allows the tank to absorb pressure from the entire system.

The tank should be mounted vertically with the water connection at the bottom. This prevents air from mixing with water inside the tank.

Key installation requirements include:

• Install within 6 feet of the water heater
• Use a 3/4-inch tee fitting on the cold water line
• Add a shut-off valve between the tank and main line
• Install a pressure gauge to monitor system pressure

We recommend having a licensed plumber handle the installation. They ensure proper sizing, placement, and compliance with local codes. Most installations take 1-2 hours to complete.

Maintenance and Inspection of Expansion Tanks

Expansion tanks need regular checks to work properly and last longer. We should test the air pressure once a year and look for signs of damage or leaks.

Routine Checks and Best Practices

We need to check our expansion tank every 12 months to make sure it works right. The most important step is testing the air pressure with a tire gauge.

Annual Pressure Check:

• Turn off water to the tank
• Drain water from the system
• Remove the air valve cap
• Check pressure with a tire gauge
• Add air if pressure is too low

The air pressure should match what the maker says. Most tanks need 12 to 15 psi when there is no water pressure.

We should also look at the tank for rust, dents, or water stains. Check all pipe connections for small leaks. Wet spots around fittings mean we need to tighten or replace parts.

What to Look For:

• Rust or corrosion on the tank
• Water dripping from connections
• Dents or damage to the tank body
• Loose mounting brackets

Troubleshooting Common Issues

When our expansion tank stops working right, we usually see the same problems. Water coming out of the air valve means the inside bladder is broken.

Common Problems:

• No air pressure in the tank
• Water shoots out when we test the air valve
• Tank feels heavy even when drained
• Pressure relief valve opens too much

If water comes from the air valve, we need a new expansion tank. The rubber part inside has a hole and cannot be fixed.

Low air pressure is easy to fix. We just add more air with a pump until it reaches the right amount.

A tank that stays heavy after draining has a broken bladder. Water has filled the air side and the tank cannot do its job.

When the pressure relief valve opens a lot, our expansion tank might be too small or broken. We should call a plumber to check the whole system.

Frequently Asked Questions

Most tankless water heaters don’t need expansion tanks because they heat water on demand rather than storing it. However, closed plumbing systems may require expansion tanks to handle pressure changes and prevent damage.

Is an expansion tank necessary for a tankless water heater?

We typically don’t need expansion tanks for tankless water heaters. These units heat water on demand instead of storing heated water in a tank.

The main exception is closed plumbing systems. If our home has a closed system, we may need an expansion tank to manage thermal expansion.

Most tankless installations work fine without expansion tanks. We should check our specific plumbing setup to be sure.

What are the potential consequences of not having an expansion tank with a tankless water heater?

In closed systems without expansion tanks, we might experience pressure spikes. These sudden pressure increases can damage pipes and fixtures.

Water pressure fluctuations can reduce system performance. We may notice inconsistent water flow or temperature variations.

Excessive pressure can also damage the tankless unit itself. Relief valves might open frequently to release built-up pressure.

How does an expansion tank benefit a tankless water heater system?

Expansion tanks absorb pressure increases when water heats up. This protects our plumbing system from damage caused by thermal expansion.

We get more stable water pressure throughout our home. The tank helps maintain consistent flow rates at all fixtures.

Our tankless water heater operates more efficiently with stable system pressure. This can improve overall performance and reliability.

Can the installation of an expansion tank extend the lifespan of a tankless water heater?

Expansion tanks can reduce stress on tankless water heaters in closed systems. Lower pressure fluctuations mean less wear on internal components.

We protect our investment by preventing pressure-related damage. Consistent operating conditions help maintain optimal performance over time.

The tank reduces cycling of pressure relief valves. This prevents premature wear of safety components in our system.

What are the code requirements regarding expansion tanks for tankless water heaters?

Building codes don’t typically require expansion tanks for standalone tankless water heaters. Most jurisdictions recognize that these units don’t store heated water.

We should check our local plumbing codes for specific requirements. Some areas may have unique rules based on local conditions.

Closed systems might have different code requirements. We need to verify if our installation meets all applicable standards.

How do I determine the appropriate size of an expansion tank for my tankless water heater?

We calculate expansion tank size based on our plumbing system volume, not the tankless unit. The total water volume in pipes and fixtures determines tank size.

Most residential systems need tanks between 2 to 5 gallons. We consider factors like pipe diameter, total length, and system pressure.

A plumbing professional can help us size the tank correctly. They account for all system variables to ensure proper operation.