I know that a hydronic system forms a closed loop. When the boiler is turned on, the water temperature will increase and expand. Since this system forms a closed loop, there is no volume for the hot water to expand. Therefore, the pressure inside the system will increase and explosion might occur if the pressure increases significantly. To maintain a constant pressure, an expansion vessel is installed to contain the increase in the water volume as it expands as it heats up.

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The expansion vessel is divided into two compartments separated by a diaphragm. There is water on one side and air on the other side. Initially the air pressure equals the system pressure. When the water heats up its pressure increases. Eventually, the pressure exceeds that of the air causing the diaphragm to bend back in the air space, compressing the air. Since the water is allowed to expand, this means the system pressure is maintained constant. On the other hand, the air pressure has been increased because its volume decreases. Now, more increase in the water temperature will increase its pressure. The hot water will not be able to push the diaphragm more unless the pressure water becomes greater than the air pressure, which means the system pressure is not maintained constant. Am I wrong?

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    $\begingroup$ The system pressure needs to be kept within an acceptable range, once the system pressure exceeds a set value a pressure relief valve vents the excess. $\endgroup$
    – Solar Mike
    Commented Nov 9, 2018 at 18:51

4 Answers 4


It is better to think of it in terms of volume than pressure. The key point is that air is compressible while water is not.

If you heat water is has to expand. If it is in a closed container then it can only do this by straining the container, this creates very large stresses which are the cause of the pressure increase. To put another way the pressure generated is a function of the stiffness of the container. In comparison the pressure generated by compressing air by a small proportion of its starting point is pretty small.

So its not so much about keeping the pressure in the system constant as giving the water somewhere to go as it expands which doesn't involve stretching the solid parts of the system.


The fundamentals are shown below.

sealed or open compartments

Assume air is an ideal gas and the process is isothermal. With a sealed compartment, the gas pressure will increase as the volume decreases (and visa-versa). With an open compartment, the gas pressure will remain constant.

The water pressure in the tank will be a sum of the pressure from the diaphragm and the gas pressure.


A device used to heat domestic potable water to about 120 $\circ$F (50 $\circ$C) is usually called a hot water heater. To heat water above that temperature the device used is called a boiler.

Hot water boilers are usually required to heat hot water systems and can operate from low pressures to 160 psig (1100 kPa) max.

Steam boilers provide steam to a steam system that is used for heating only.

Low pressure steam boiler or heating steam boiler can operate to a maximum pressure of 15 psig (103 kPa). High pressure steam boiler (as per ASME) operates at pressures over 15 psig (103 kPa).

High pressure and high temperature hot water boilers, the ones exceeding the above, are found in Manufacturing plants, hospitals etc. These types of boilers will need heavy duty non pressurized expansion tanks.


For boilers, you wouldn't use an expansion tank. You would use a steam drum. An expansion tank is for pumps that pulse (especially piston pumps). The difference is in the design:

Steam Drum

enter image description here (Source)

The liquid-steam interface is where expansion typically occurs. The water is given enough thermal energy to expand up the water tubes. However, the larger diameter down comer has different thermal heat transfer properties, and as such the water sinks inside of this tube.

Any expansion then is driven towards the condenser due to this expansion. The density differential between the heated steam and the cooled liquid is enough to overcome the pressure losses in the system and drive the steam forward.

Expansion Tank - Surge Suppressor - Pulsation Dampener

An expansion tank works how you describe, but they are mainly for pumps, such as diaphragm or piston pumps. enter image description here (Source).

The idea is to allow the pressure of the pump to stabilize against the compressed air, rather than deal with the jerky motion of the single piston compressing the liquid. They work sort of like a pass filter in an electrical cicuit - an extra large capacitor is not needed, because the extra surge pressure (or extra electrons) are depleted when the pump cavity is refilling (or when the electrical circuit is in the negative part of the wave).

  • $\begingroup$ He says hydronic system which implies a hot water boiler, not a steam boiler. So, I’m that case, an expansion tank would still be appropriate for the reasons given in some of the above answers. I think your answer has some good information in it though that I haven’t seen before. $\endgroup$
    – Secundus
    Commented Nov 18, 2018 at 1:50
  • $\begingroup$ Boiling water IS steam. You can't have a hot water boiler without steam. You can have a hot water heater, but in those cases the air side would be opened. (Flexible tubing handles this well) $\endgroup$
    – Mark
    Commented Nov 19, 2018 at 17:38
  • $\begingroup$ I understand boiling water is steam. However, hydronic systems for heating are quite common and use water in the 130ish to 180ish degrees F range. The equipment is still called a boiler (look at Bradford White and Laars for examples). Steam systems, obviously, are above those temperatures. The design of each of these system types is noticeably different, so I was simply saying that you can’t discount all of the hot water answers given above just because they are not steam and the OP was correct in his terminology regarding expansion tanks even when talking about boilers. $\endgroup$
    – Secundus
    Commented Dec 1, 2018 at 18:17

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