Expansion tank for home heating system

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When building a liquid heating system, you will surely face the need to choose an expansion tank. What it is for and what is its role in the heating system of the house, we will discuss in this article. We will also give a calculation with which you can select the required tank for your system..

Expansion tank for home heating system

For heating, which uses a liquid as a heat carrier, its ability to expand with temperature changes is critical. In a sealed closed circuit, the thermal expansion of water can multiply the pressure, which threatens to break through pipes, radiators and breakdown of other elements. Therefore, an expansion tank that can compensate for thermal expansion or reduce its effect to nothing becomes a mandatory and integral structural element of heating..

What is an expansion tank for?

The water in the heating circuit constantly changes its temperature. In the boiler, it heats up, in the radiators it gives off part of the heat. However, thermal expansion does not affect the total volume, which fluctuates around a certain average value. Much larger volume changes occur when the boiler settings are changed, the target temperature changes and, of course, at the time of the system start-up in the fall and shutdown in the spring..

Expansion tank for home heating system

At the moment of starting the heating in the house, the coolant heats up by about 60-75 degrees and significantly increases in volume, while the volume of pipes, boiler and radiators is not changed, and the water has nowhere to go. An expansion tank is just needed to take in excess water in its volume so that when the temperature decreases, return the water back to the pipes. That is, it compensates for the lack of system volume.

What designs are used for this

Structurally, the expansion tank is a volumetric reservoir that is able to receive the required volume of water and, if necessary, return.

There are two options for implementation:

  • open;
  • closed or membrane.

In the first case, the heating system in which such a tank is installed is called open. The tank is not sealed, it has an outlet to the external environment. In this regard, it is installed at the highest point of the heating circuit so that liquid under the influence of gravity does not overfill it and does not pour over the edge..

Expansion tank for home heating system

Initially, when filling the system, cold water is poured through a tank or a special pipe near the boiler so that it fills all radiators, pipes, and the boiler. The reservoir itself is filled only to a third of its volume. After starting the boiler, the water heats up and expands, the excess volume enters the expansion tank. The main task of the correct choice of volume is to ensure that at maximum heating of the coolant, the displaced volume fits in it with a margin.

The main advantages of an open design:

  • ease of implementation. It can be a cube welded from sheet metal or any tank that is suitable for its performance;
  • the ability to control the level of the coolant in the system, supplement it or take it out as needed. In the event of a sharp overheating of the coolant, there will be no risks or restrictions, water will simply spill over the edge, or rather, into a specially provided drain pipe;
  • excess air and gases are removed through the tank, which can create a gas pocket in the radiators.

Expansion tank for home heating system


  • the tank itself is the main reason for the evaporation of the coolant; it cannot be closed tightly;
  • installation at the highest point is not always available, or it is simply not effective.

A closed, sealed expansion vessel can be installed anywhere in the heating circuit. Most often, they choose a position near the circulation pump or the direct return flow to the boiler, through which the already cooling coolant flows..

Expansion tank for home heating system

Structurally, it is made in the form of a sealed container of sufficient volume. Inside it is a rubber elastic membrane that divides the space inside the tank into two chambers that do not communicate directly with each other.

When an excess volume of water is formed in the pipes, it enters the tank and begins to press on the membrane. The second chamber contains only air, which, unlike water, is capable of being compressed, moreover, many times. The excess water volume is able to enter the expansion tank entirely and remain in it until the water temperature in the system drops again. As soon as this has happened, the pressure created by the air on the membrane pushes the water back, making up for the lack.

Expansion tank for home heating system

Advantages of a membrane tank:

  • aesthetic appearance;
  • installed almost anywhere in the system;
  • does not require maintenance, except for the replacement of the membrane, if provided by the instructions;
  • tight and durable.


  • high price;
  • difficulty in choosing.

Choosing the best option

The main choice concerns the design of the expansion tank. The open version is suitable for natural circulation heating systems, when it is possible to place the expansion tank in the attic and provide direct access to it. This is a simple and reliable option, which, however, requires more attention during operation..

A diaphragm expansion tank will cost more, but it will save you from a number of problems. There is no need to constantly check the fluid level, refill or worry about overflow. However, for this, one should initially be extremely careful about his choice. Diaphragm tank ideal for forced circulation heating.

Expansion tank for home heating system

Next, it is important to determine the useful volume of the tank, the minimum volume required to compensate for the thermal expansion of water in the system. The calculation is performed using the formula:

  • ?V = ?? tV0

where? V is the calculated increase in the volume of water in the system, V0 – volume of pipes, radiators and boiler,? – coefficient of volumetric expansion of water or used antifreeze,? T – change in water temperature from initial to maximum operating temperature. V0 – in this formula, determines the volume of the coolant in the system at the initial temperature. An important clarification, because the value obtained is only a useful volume. In addition to pipes, radiators and the boiler heat exchanger, all other elements must be taken into account, up to the circulation pump used..

Expansion tank for home heating system

For an open expansion tank, it is sufficient to increase the obtained value by about a third or 40% to obtain the required volume. The stock will allow you to take into account the filling of the tank itself initially and the stock in case the temperature rises or the formation of gas pockets in the radiators.

For a membrane, the calculation becomes more complicated. The operating pressures in heating are taken into account. The tank is initially adjusted in a chamber with air to a specific pressure:

  • ?V = ?? tV0 / (1 – (1 + Pmin) / (1 + Pmax))

where Pmin/ Pmax – the ratio of the minimum initial pressure in atmospheres required in the air chamber of the tank to the maximum pressure possible during operation. The resulting value is rounded up. The tank can be selected from those available on the market, but the volume must be greater than the number calculated by the formula. It is easier and more reliable to stop at the previous value and, knowing in advance the working pressure in the system, check the recommendations in the instructions.

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