As a general rule, tankless water heaters don’t require expansion tanks. These devices are only necessary with traditional water heaters.
A tank water heater holds water that increases in pressure as it expands, making some type of pressure relief necessary. In comparison, a tankless system doesn’t need the same pressure relief because it doesn’t store water.
However, you should always check out your local plumbing code to find out what your area’s requirements are.
TL;DR: Do You Need an Expansion Tank With a Tankless Water Heater?
- Most tankless systems don’t need expansion tanks.
- However, some setups require one. Closed systems need an expansion tank to handle high incoming water pressure.
- Check your building code to determine the local rules in your area.
- Talk to your plumber to find out your unique requirements.
What Is an Expansion Tank and Why Does It Matter?
An expansion tank is designed to absorb thermal pressure. Water expands when it is heated, so traditional water heaters require expansion tanks to accommodate the pressure. If the expansion tank didn’t exist, then thermal expansion would lead to equipment failures and safety hazards.
Most homes have closed systems, so water can’t flow back into the city’s pipes. Instead, pressure continues to increase until it is released.
How Tankless Water Heaters Work and Why They’re Different
A tankless water heater is unique because it provides on-demand heating without the need for a storage tank. Instead, it heats water as the water is needed. There is a significantly lower risk of pressure-related damage because no thermal expansion occurs.
When a Tankless Water Heater Needs an Expansion Tank
There are a few instances where it makes sense to add an expansion tank to your tankless water heater.
1. Closed Loop Plumbing System
In a closed loop plumbing system, pressure is trapped and reduced through pressure-reducing valves, an expansion tank, and backflow preventers. Often, city codes require expansion tanks for this type of system.
2. Recirculation Pump Systems
Any hybrid system or tankless unit that has a recirculation pump will need an expansion tank. The setup uses a miniature tank for instant water access, which is why it needs an expansion tank.
3. High Incoming Water Pressure
If your city’s water pressure is over 80 psi, it can end up stressing components in your home’s system. As a result, you will need an expansion tank to regulate the amount of pressure.
4. Local Plumbing Code Requirements
On top of the previous reasons, you may need an expansion tank if your city requires it. To stay compliant, always talk to your city’s plumbing department about local codes.
Do You Need an Expansion Tank If You Have a PRV or Backflow Valve?
Yes. Because these features create a closed system, added pressure can build up. You need an expansion tank to lower the pressure level and prevent system failure.
Risks of Not Installing an Expansion Tank If You Need One
If you make the mistake of not getting an expansion tank, you may face a number of potential issues.
- Damaged pipes
- Potential penalties for code violations
- Failed inspections
- Burst pipes
- Voided warranties
- Heater failure
Benefits of Using an Expansion Tank with Tankless Water Heaters
By installing an expansion tank with your tankless water heater, you can enjoy a few important benefits.
- Protect your plumbing system from damage
- Extend your water heater’s lifespan
- Ensure your home’s code compliance
- Prevent water hammer
How To Tell If You Need an Expansion Tank
To see if you need an expansion tank or not, ask yourself the following questions.
- Do you have a PRV or check valve?
- Are you using a recirculation pump or a hybrid system?
- Is your city water pressure above 80 psi?
- Are you required to comply with local plumbing code?
If you responded affirmatively to any of the preceding questions, you need to get an expansion tank.
How To Install an Expansion Tank on a Tankless Water Heater
An expansion tank is installed on your cold water line between the water heater inlet and the shut-off valve. The size depends on the water temperature and the incoming water pressure. To ensure your family’s safety, you should always get a professional plumber to handle the installation.
Expansion Tank Alternatives
If you don’t want to have an expansion tank, you may be able to get thermal expansion relief valves or water hammer arrestors. A thermal expansion relief valve is an inexpensive solution, but it can drip and waste water.
Meanwhile, water hammer arrestors can help with pressure-related problems. Unfortunately, they only work for surges in pressure and not gradual increases.
Frequently Asked Questions (FAQs)
Over the years, we’ve fielded a few frequently asked questions.
Does a tankless water heater need an expansion tank in a closed system?
In a closed system, an expansion tank is necessary to prevent damage.
Is an expansion tank required by my plumbing code?
Many municipalities require expansion tanks, especially if you have backflow preventers or PRVs.
Where should an expansion tank be installed on a tankless system?
It should be installed on the cold water supply line.
How long does an expansion tank last?
While you should perform periodic pressure checks, these tanks generally last 5 to 10 years.
Final Takeaways
While tankless heaters don’t always need expansion tanks, there are instances where you may require one. When you have a recirculation pump, backflow preventers, or high water pressure, these tanks are necessary. You can reach out to our licensed plumbers to determine if your home requires an expansion tank.
Need Help? Let a Pro Handle It
Our team of punctual plumbers can install expansion tanks and tankless water heaters in Clearwater and the surrounding areas. For more information, reach out to our Florida-based team today.