Pressure in the heating system in a private house

Recommendation points

• What is the pressure in the heating system for?
• Optimal pressure selection
• Pressure normalization
• Control method and diagnostics

At the design stage of the heating system of a private house, the pressure is negotiated within strict limits. In many areas of the system’s wiring, the project installs devices for its control and maintenance. We will tell you about what pressure should be and what any deviation from the norm means..

What is the pressure in the heating system for?

Only the excess pressure in the system, which exceeds the natural atmospheric pressure, is taken into account. It is he who is shown by manometers, and he needs to be controlled. Includes:

1. Static – the pressure of the liquid column, equal to the height of the heating circuit from the highest point to its base.
2. Dynamic – the pressure created by the pump, as well as during the convective movement of the liquid through pipes and channels.

However, it does not constantly and regularly changes during operation due to:

• thermal expansion of the coolant during heating;
• decrease in the volume of the coolant during cooling;
• linear expansion of pipes;
• the presence of air;
• locally, at points with a change in the channel cross-section, shut-off valves, a connection point for pipes with a different diameter.

In a normal state, the entire heating circuit is a balanced hydrodynamic system, in which a constant and uniform flow of the coolant is maintained and with effective heat transfer between the boiler and the air in the room, as the extreme points of the entire system. There are several boundary factors to consider:

1. When the pressure drops below atmospheric, the risk of boiling of the coolant increases at temperatures below 100 ° C. The risk of gas, water vapor entering the pipes and the formation of air locks that can block the flow of water increases.
2. With an increase, the heating efficiency increases. With increasing pressure, the hydrodynamic resistance of all circuit elements decreases, and the transient or turbulent movement of water is maintained.
3. With an excessive increase, the risk of breakage increases. If the permissible pressure for the weakest link in the circuit is exceeded, a leak or rupture may occur..

In a system with natural circulation, the pressure is only slightly higher than the static pressure and is formed only due to the height of the highest water level in the circuit.

In a system with forced circulation, the pressure is set by a number of regulating devices, and the performance of heating in the house depends on the correct choice of its value..

Optimal pressure selection

For natural circulation, the pressure is set by the position of the expansion vessel. It is installed at the highest point of the circuit and is needed to compensate for thermal expansion of water or to vent air. The tank setting and filling level sets the total pressure in the system. For every ten meters of height of the water column, the pressure at the lowest point increases by about 1 atm.

In practice, the expansion vessel is connected to the highest piping point directly above the boiler. A distributor, a collector, a large-diameter pipe running along the perimeter of the heated room with a constant slope is diverted from this point. It is advisable to raise the tank above the distributor by another 5–7 meters so that in any part of the circuit where the coolant circulation is maintained, excessive pressure is created. This will increase the heating efficiency..

With forced circulation, the entire circuit is sealed, and the pressure is set initially when filling with a coolant, it is regulated using a membrane-type expansion tank.

The pressure in the circuit takes on a minimum value in a cold state and a maximum value when the coolant is heated to operating temperature. The nominal working pressure is calculated for a specific temperature of the heating medium.

The nominal overpressure is selected in such a way that with any natural change during heating or cooling of the heat carrier, pipes, heat exchanger and radiators, the actual value is:

• did not fall below zero, that is, less than atmospheric;
• did not exceed the permissible threshold for the “weakest” link in the circuit.

In practice, the permissible range of values ​​turns out to be wide, therefore, one should start from the upper threshold, not forgetting that when the pressure in the heating system increases, its efficiency increases.

When determining the “weakest” link, device or wiring element, it should be borne in mind that the allowable pressure depends on the temperature. For example, with an increase in temperature for polymer pipes, the permissible maximum operating pressure is greatly underestimated, at which their trouble-free operation is guaranteed..

Information on permissible operating conditions should be taken from the technical documentation for the equipment and materials from which the heating system is mounted. Given the high degree of standardization, it is safe to say that a closed heating system will be configured in the range from 1.5 to 3-4 atmospheres. Expansion tanks, safety groups, boilers and circulating pumps are most often designed and manufactured to work in this range..

Pressure normalization

To maintain a constant pressure and compensate for the thermal expansion of the heating medium and structural elements, an expansion vessel is used.

When the coolant, heating up, increases its volume, the excess enters it. As soon as the temperature drops, the coolant is compressed, the liquid from the expansion tank flows back into the circuit, maintaining the working volume of the liquid.

For open heating, the expansion tank is a sufficient device to compensate for thermal expansion, and at the same time to exhaust air.

In closed, sealed heating systems, you will need:

1. Diaphragm expansion tank.
2. Air vent.
3. Safety valve.

The tank is a sealed container, inside which the volume is divided into two parts using an elastic membrane. On the one hand, there is access to the coolant through the connection, on the other, there is an air chamber in which excess pressure is created, like in car chambers in the wheels. Excess coolant during expansion enters the tank, taking the membrane towards the air chamber.

To determine the limit values ​​and solve problems with overpressure or the formation of gas pockets, a safety group is used with an included pressure gauge, automatic air vent and safety valve. The valve is triggered when the set maximum pressure in the heating circuit is exceeded and discharges part of the heat carrier.

Control method and diagnostics

Manometers are used for control. These can be sensors with a digital or analog output for connection to a microcontroller, or classic models with a dial and an arrow..

Due to the presence of dynamic pressure, the head created by the pump, as well as the different resistances of the wiring elements, the pressure in the circuit is not constant at different points. It is important to know the values:

1. Before and after the boiler.
2. At the inlet and outlet of the circulation pump (each, if there are several).
3. Similarly on both sides of the coarse filter.
4. In the expansion tank.

Given the serial connection of all these elements, only two or three pressure gauges are required to get a complete picture of the state of the system.

1 – boiler; 2 – safety group with an expansion tank; 3 – heating radiators; 4 – coarse filter; 5 – circulation pump; 6 – manometers

The measuring range and scale of the pressure gauge should correspond to possible changes in pressure in the system, but without excessive margin, so as not to lose accuracy. Seeing, for example, the drop in pressure after the coarse filter by only 0.2-0.3 bar, one can judge that it is time to clean it.

Changes in pressure in the circuit as a whole or in a separate section give a clear unambiguous signal of a breakdown or other problem that requires an immediate solution. An accurate diagnosis can be performed by a specialist, however, based on the information specified in the instructions for the boiler or circulation pump, and the values ​​of the pressure gauges, you can independently find out the reason why the heating system loses its efficiency and the batteries began to heat the room worse.