Does Tankless Water Heater Need Expansion Tank

You have been investigating tankless water heating systems because you have heard of the benefits of moving from tank system to a tankless one. From energy efficiency resulting in lower utility bills, to longer lifespans and space saving benefits, you are sold on getting  a system for your home. 

But you may be wandering if a tankless heater needs an expansion tank? Or in my case, I wondered “is it still a tankless system if it needs any sort of tank?”

This is a valid query because the type of water heater you have plays a role in whether you need an expansion tank. 

This post will explore this question in depth. 

Do Tankless Water Heaters Need Expansion Tanks

No. tankless water heaters do not need expansion tanks. Thermal expansion tanks are only required for systems in which large water volume is stored because the water expands and needs an outlet to relieve extra pressure.

Tankless units only heat up the exact amount of water that is required for your faucet, shower or other use. 

Expansion Tank Loop in Tank Water Heating System

Tank water heating systems have a tank with size range from 36 gallons up to 60 gallons depending on your needs. These tanks hold water and are constantly cycling on and off to keep the water in the heater hot in preparation for use.

The constant cycling of power to the storage tank water heater heats up the water, which in turn causes the water to expand. Due to the fixed size of water heating tanks, the extra volume needs a place to go to avoid leaks or failure of the valves on your water heater as well as other appliances in the home. The expansion tank on a water heater is designed to serve as a pressure relief valve for the main water tank.

Since tankless systems do not have a tank that stores water and only heats up just enough water for the specific use at the time of need, expansion tanks are not needed for tankless water heaters. 

Any potential pressure build up in a tankless water heater is relieved by the fact that water is flowing to its point of use (shower, faucet, dishwasher etc.)

Are Expansion Tanks Required by Code for Tankless Water Heaters?

No. Expansion tanks are not required by code for tankless water heating systems in any jurisdiction. Most plumbing codes require them for closed loop water heating systems. Tankless water heating systems are not closed loop systems because any pressure in the system is immediately relieve when the system is in use.

The California plumbing code  in Section 608.3 and the California Mechanical Code in Section 1005.0 requires that an expansion tank be installed in closed loop water heating systems for the purpose of relieving pressure build up. 

The International Code Council also provided updated guidance in 2018 stating that expansion tanks are required for storage or tank water heaters. 

The only time that an one may be required for a tankless unit is if it is connected in a closed loop to a storage tank system. 

How do Water Heater Expansion Tanks Work?

The cold water inlet from into your home from the city water supply is usually fitted with a backflow prevention device or check valve. This causes water pressure to build up when heated water begins to turn into gas. This excess water needs a place to go. That’s where the Water Heater Expansion Tank comes in. 

The Thermal Expansion tank is split into two halves by a rubber baffle. On the bottom of the water heater expansion tank is compressed air. On the top is where the hot water sits in its expanded state. As the water begins to cool, the air pressure at the bottom of the tank pushes the baffle back up to equalize the excess pressure. 

Expansion Tank Used with Tank Water Heating System

What Size Water Heater Expansion Tank Do I Need?

The size of the expansion tank is determined by the size of the tank on your hot water heater and the pressure of water in the your home.

In general, larger tanks with higher pressures will require larger expansion units.

You can easily find the size of your tank on the factory label on the tank itself. To determine the pressure of water in your home, you can use a water pressure gauge.

For standard water tanks up to 50 gallons in size, you can use a 2-gallon expansion tank provided the static supply pressure reading is below 60 psi (pounds per square inch). 

If you prefer, you can contact a local licensed plumber to help you determine what size tank you need. 

So Do You Need a Thermal Expansion Tank?

Tank-Style Water heaters require thermal expansion tanks because they are one piece of a closed water supply system. A closed-loop system typically has a backflow preventer or check valves that prevent the water from flowing back into your main water inlet as pressure increases as water is heated in the tank. 

The expansion tank helps relieve the pressure in the system.

Having said that, if you have a tankless heating unit, you do not require an expansion tank. Tankless systems heat only the amount of water required for your use, eliminating the need for a tank to store water. Because all of the heated water is utilized, there is less chance of pressure building up in the system.