Many people who use conventional tank water heater know about the thermal expansion tank, as it the major safety component. So, when you upgrade to a tankless unit multiple doubts arises. So, read below conversation between Joseph and Home owner to know clearly before buying a thermal expansion tank for a tankless water heater.

Joseph: hmm, this is a thermal expansion tank, it is used to control the pressure inside the hot water tank when the water expands while heating.

Fig. 2 Delivered steam pressure is regulated by balancing forces acting directly on the valve itself: the downward force caused by compression of the adjustment spring against the upward force from the secondary pressure acting against the underside of a bellows or diaphragm.

Expansion tank for tankless water heaters are available in different sizes like 2 gallon and 5 gallon tanks you can choose them according to the heater size and capacity. Larger water heaters require bigger expansion tanks.

If you have any questions on expansion tanks or about water heaters don’t hesitate to ask joseph by leaving a comment below! Joseph is here to assist you and provide all the information you need. Your feedback is important to us, so please feel free to share if this information helped.

According to IPC a thermal expansion tank is not specifically needed for tankless water heaters. But if a tankless heater has a storage tank connected in it to store hot water for recirculation’s, creates pressure from thermal expansion. So, the increased pressure must be stabilized with it.

Calculate and display steam property values using pressure-based saturated steam tables, temperature-based saturated steam tables, and heating steam tables.

Joseph: When the hot water faucets are farther from the heater, as soon as we turn on the faucet water flows which turn on our tankless heater and heats water. But, it takes few seconds for the hot water to reach the faucet till then you get cold water. To avoid this and get instant hot water buffer tanks with recirculation systems are used where a small tank is placed near the faucets and hot water recirculates regularly. This type of system is called a closed system and as it has a buffer storage tank in it, we need to install an expansion tank in the system.

The way it works is quite simple, when the water is heated, it expands and presses against the elastic membrane, compressing the air or gas contained in the other part of the container. The air or gas thus absorbs this increase in pressure, ensuring that a stable pressure is always maintained in the system. This stable pressure is crucial for the proper functioning and longevity of the system. Without an expansion tank, this increase in volume can cause pressure to build up in the system, which can cause leaks and damage to the pipes and also to the heater.

In this system, initially hot water is sent to storage tanks and if there is no demand the hot water stays for long time in tanks and gets cooled down, these water again sent to the heater and gets heated with the help of pump. Heated water get back to the tanks. This process is called recirculation, if there is no demand in hot water and outlet taps are closed, pressure is created due to thermal expansion. So, expansion tank is needed in this system.

A common way of lowering pressure is by throttling down the size of the steam passageway. For the most basic pressure reduction, it is possible to simply use a conventional globe valve in a fixed partly-open position, or by inserting an orifice plate into the flow of steam. However, any fluctuation in flow rate would be accompanied by a corresponding fluctuation in pressure. To avoid such circumstances, pressure reducing valves (PRVs) can be used to provide precise control of downstream pressure. They automatically adjust the amount of valve opening to allow the pressure to remain unchanged even when the flow rate fluctuates.

PRVvalve

In this article let’s learn in detail about Tankless Water Heater Expansion Tank. In February 2024 when joseph went to install a tankless water heater in Colorado replacing with the traditional tank heater. So, while installing it the home owner got a doubt about the expansion tank and asked joseph, does a tankless water heater need an expansion tank?. Without any delay Joseph explained her what is an expansion tank, what are its functions, where and when it is needed and finally is it required in tankless setup.

In direct acting pressure reducing valves, the amount of valve opening is determined directly by the movement of the adjustment spring. If the spring is compressed, it creates an opening force on the valve which increases flow. As pressure builds downstream, equalizing occurs by feeding the downstream pressure to the underside of the adjustment spring (usually against a bellows or diaphragm) where its upward force counter-balances against the spring compression. Spring compressive force which opens the valve is limited to allow sufficient spring sensitivity to equalize with downstream pressure changes. The net result is simple pressure control through a valve orifice where high flow rates can cause pressure droop.

In a steam-using plant, steam is often generated at high pressures and reduced locally to provide heat for each steam user. This is usually done to minimize the diameter of steam distribution piping and enable more cost-efficient steam delivery.

While it is possible to maintain a constant pressure by using the combination of an actuated control valve, a pressure sensor, and a controller, a pressure reducing valve offers the advantage of being able to control pressure through fully-automatic self-contained operation, requiring no type of external power source. It can offer the further advantage of extremely rapid response action by immediately sensing and adjusting based on the downstream pressure.

Regulatingvalve

Some tankless water heater systems require storage tanks for recirculation of hot water to decrease the delay in providing hot water at any outlet tap if they are at longer distance from the heater or when to provide large amounts of hot water due to the high demand.

An expansion tank is an essential component in maintaining a stable water pressure in a water heating system. It is a metal container with an elongated or cylindrical shape that is airtight sealed to prevent any air or water leakage. The interior of the container is divided into two parts by means of an elastic waterproof membrane. One part is in direct contact with the water in the system, while the other part is filled with air or some type of gas, usually nitrogen.

Homeowner: Ok, I got it. but as you said ours is an open system, so what is a closed system and do we need an expansion tank in closed system?

From the above characteristics, it can be seen that the function and applications of non-piloted direct acting PRVs differ substantially from those of pilot-operated PRVs.

Note: The pressure relief valve on a water heater cannot be used as the device to control the pressure caused by thermal expansion. The Uniform Plumbing Code requires thermal expansion tanks if there is a check valve, pressure regulator, or backflow preventer installed in the main supply line, regardless of the type of water heater.

Waterpressureregulator

In pressure reducing valves, the mechanism that automatically adjusts the downstream pressure typically uses the balance of forces between the steam pressure and an adjustment spring. At present, this is a universal concept on almost all manufactured pressure reducing valves. However, there are two different ways in which this mechanism is implemented to control the amount of valve opening:

In pilot-operated pressure reducing valves, a pilot valve is used to load a piston or diaphragm that increases the downward force used to open a larger main valve. This enables larger flow capacity with a lower pressure offset (droop). The opening and closing of the pilot valve is controlled by the balance of force between the adjustment spring and the secondary pressure in the same manner that a direct-acting valve operates. However, in a pilot-operated PRV, this opening and closing of the pilot valve purposely delivers pressure to the main valve piston or diaphragm. This pilot flow pressure then causes a downward force that is amplified by the area of the piston or diaphragm to enable opening of a much larger main valve, which in turn provides the ability for very high flow rates.

Pressureregulator

Now lets see few scenarios of tankless water heater systems, so you can understand where and where not thermal extension tanks are needed.

Expansion tanks are advised to fix on the cold water line near to the heater, you can place on cold water line anywhere but without any values in between heater and expansion tank.

Fig. 3 Delivered steam pressure is regulated in a similar manner as direct acting PRVs, however indirectly via a pilot valve. As the pilot valve is actuated, it provides the opening flow to the larger main valve which can supply significantly higher flow rates than the pilot itself. The pilot valve is then equalized by the secondary pressure, which correspondingly controls the opening flow to the main valve.

Watervalve

When the hot water faucet is closed, this system becomes closed. But as the water flow stopped the heater stops heating the water, thus no pressure is formed. Therefore expansion tank is not required in this system.

Because the downward force is amplified through the use of a piston or diaphragm, a small change in the opening on the pilot valve can result in a large change in flow and downstream pressure through the main valve. As a result, there is little change needed in adjustment spring force on the pilot to accomplish quick response over a wide range of steam flow rates. Quick response and tight delivered pressure control represent the main advantages of this type of valve over the direct-acting type.

There are two answers YES and Also NO. It depends on which type of tankless water heating system you are using. To understand clearly we will explain few scenarios where expansion tanks are needed and where they are not needed. Before that read the regulation on Thermal expansion control given in 2012 International Plumbing Code (IPC) in the below image.

Joseph: No, we don’t need it in our tankless setup, this tankless unit doesn’t has any storage tank and it heats water only when there is a demand of hot water. As this is an open system the pressure generated with thermal expansion goes out through the hot water faucet itself.

In this scenario, the water is heated only when the hot water faucet is open. When the hot water faucet is open, cold water flows through the heater gets heated, and exits through the faucet. The thermal expansion of the water occurs, but as the system is open, the pressure is released directly through the hot water outlet.