# Open and closed tank for the heating system: volume calculation, DIY repair

## Recommendation points

No liquid-type heating system will work without an expansion tank. Correctly calculating the volume and determining its type is one of the primary tasks when designing heating for a private house, and today we will cover this topic in more detail..

## Why do you need an expansion tank

As you know, liquids are very reluctant to contract, but with increasing temperature their internal energy state increases, the distance between molecules increases, which results in a very significant increase in volume..

For reference: when 50 liters of water is heated from 20 to 80 ° C, its volume will increase by almost one and a half liters. Not so much, at first glance, but think about whether the displacement of your heating system can increase by the same value, or, in other words, the total internal volume of pipes, radiators and the heat exchanger of the boiler..

In practice, such an increase in volume translates into a pressure of more than 200 atmospheres, which in turn depends on the ability of the “vessel” to stretch to the required size. But the specific pressure, in fact, is not so important, because the critical point for the entire system is determined by the strength of the weakest plumbing connection. And the tensile strength for most fittings and taps rarely exceeds 5-7 MPa, that is, a tightly sealed system is guaranteed to fail several hours after the boiler is turned on.

To compensate for the volumetric expansion in the system, an expansion vessel must be present. Its device can be either closed or open.

## Structural differences between tanks of different types

An open expansion vessel is a conventional container installed at the highest point of the heating circuit. Its volume is very small and is equal at least to the difference between the volume of cold and heated liquid..

An open tank is not entirely convenient: water evaporates from it and it needs to be refilled periodically. And since such expanders are rarely kept indoors, you have to periodically look into the attic. Now imagine what would happen if the water in the tank freezes and the system suddenly turns into closed due to an ice plug? But she is able to withstand dozens of atmospheres.

There is, however, another method for arranging an open tank: the fitting at the very top of the system is connected with a thin silicone hose to an open container standing on the floor. When expanding, excess water is squeezed out of the circuit, and when the volume decreases, it is squeezed back by atmospheric pressure. It is only important to wind a heavy nut to the end of the hose so that it is immersed in water all the time. The disadvantage of this method is that air cannot freely leave the system, but for this there are Mayevsky taps.

Closed tanks are designed in a completely different way. Inside the body there is an elastic vessel – a rubber bulb, while the body itself is completely sealed. Either the pear or the residual space has a direct connection to the heating circuit. When heated, water, as it should, will continue to expand, but now this difference in volumes will be taken over by air, which will also be compressed, but the pressure will not increase so much, only by 0.5-1 bar.

1 – valve for air pumping; 2 – air chamber; 3 – membrane with heat carrier; 4 – threaded branch pipe for connection to the heating system

But there is an important caveat here: a closed tank also makes the entire system closed. At the upper point of the open system, the pressure was equal to atmospheric, and at the lower point, a certain number of millimeters of the water column was added to this value, corresponding to the level difference between the points of the system. In a closed system, the pressure at the top point is zero, that is, it is not excessive, and because of this, troubles can arise..

## How to calculate tank parameters for your system

One could argue that with the presence of forced circulation, the pressure difference does not matter, but this is far from the case. The most obvious reason: if the air entering the upper level radiators is not under sufficient pressure, then it will not be able to leave them and follow to an even higher point in the system. In addition, heating equipment should be operated under conditions specified by the manufacturer, almost each of them sets the operating pressure of 1.3–2 bar.

It is very easy to calculate an open tank. We said that when heated to 60 ° C, another half is added to 50 liters of water, that is, for a 100-liter system, the increase will be equal to three liters. This is a completely linear relationship that is easy to use.

This is not the case with a closed tank. With excess pressure, the coefficient of expansion of liquids changes and this is especially important if not water is used as a heat carrier. Note that in normal operation, the tank should be half full..

You will get the volume of the expansion tank if you divide the product of the system displacement, the temperature difference and the coefficient of volumetric expansion of the coolant by the ratio of the maximum working pressure to the minimum, subtracted from one. The minimum pressure is the pressure at the center of the tank when the system is filled with no excess water. The maximum pressure is determined by the manufacturer of the heating equipment and, accordingly, by setting the relief valve.

Note that this example assumes that both the dump valve and the tank are installed at the same point in the system, the lowest point. For example, let’s take a system with a level difference of 4 meters and a displacement of 200 liters. First, we calculate the increase in volume:

• 200 l 0.00045 (80 ° C – 20 ° C) = 5.4 liters.

The minimum pressure for the middle of the tank will be 0.28 atm, if its center is one meter higher than the lower point of the system, we will take the maximum as 4.5 atm. Dividing the minimum pressure by the maximum, we get 0.06, and subtracting this ratio from unity, we get 0.94. Thus, the minimum volume of a closed tank will be just over 5 liters, and with a safety margin of 25% – 6.3 liters. Since we plan to keep the tank half full, its minimum volume should be 10-12 liters.