The operation of water heat supply networks is characterized by two main parameters - temperature and heat carrier flow. But there is a third value, which often attracts the attention of residents of apartment buildings and private houses - pressure in the heating system. The main question is what it should be for the normal functioning of all heating appliances - radiators, underfloor heating, and so on. Since there is no single answer, we decided to clarify the essence of the problem in the framework of this publication.
Related Information
First of all, we suggest considering why it is necessary to create excess pressure (above atmospheric pressure) in pipelines and how it is measured. Let's start from the end: the magnitude of the water pressure in a closed heating system is usually displayed in the following units:
- 1 Bar = 10 m water;
- 1 MPa equals 10 Bar or 100 m of water. st .;
- 1 kgf / cm² - the same as 1 technical atmosphere (Atm.) = 0.98 Bar.
For reference. A kilogram-force per cm² is a dimension often used in Soviet times. At the moment, it is customary to measure pressure in more convenient metric units - MPa or Bar.
Next, imagine a three-story cottage with a ceiling height of 3 m, which must be heated in the winter. To do this, batteries are installed on both floors connected to a common riser going from the boiler, as shown in the diagram. The real pressure in the resulting closed heating system will consist of three components:
- A column of water in a pipeline presses with a force equal to its height. In our example, it is 6 m or 0.6 Bar (0.06 MPa).
- The pressure generated by the circulation pump. It makes the coolant move at the right speed and overcome the resistance of three forces: gravity, fluid friction against the pipe walls and obstacles in the form of fittings and fittings (contractions, tees, turns and the like).
- Additional pressure arising from thermal expansion of the fluid. Practice shows that cold water with a temperature of 10 ° C after heating to 100 ° C adds about 5% of the original volume.
Note. The static pressure of the liquid column varies depending on the measurement location. With the pump turned off, the pressure gauge at the bottom of the system will show the maximum value - 0.6 Bar, and at the top - zero.
A very important point. In order to supply the required amount of heat to the premises, it is necessary to provide the required water temperature and its consumption - two main parameters of the water heating operation. The pressure arising in this case is only a consequence of the operation of the system, and not the reason. Theoretically, it can be anything, just to withstand radiators and a boiler plant.
Hence the concept of what is the working pressure in the heating system: this is the maximum permissible value prescribed in the technical documentation of the equipment - boiler or batteries. Regulatory documents require that in private homes it does not exceed 0.3 MPa, although some cheap units are not able to withstand 0.2 MPa.
Why raise the pressure
The pressure in the supply line is higher than in the return line. This difference characterizes the heating efficiency as follows:
- A small difference between the supply and return makes it clear that the coolant successfully overcomes all the resistance and gives the calculated amount of energy to the rooms.
- An increased pressure drop indicates increased site resistance, reduced flow rates and excessive cooling. That is, there is insufficient water consumption and heat transfer to the rooms.
For reference.According to the regulations, the optimum pressure difference in the supply and return pipelines must lie within 0.05–0.1 Bar, with a maximum of 0.2 Bar. If the readings of 2 pressure gauges installed on the line differ more, then the system is not designed correctly or needs to be repaired (flushed).
To avoid a high drop on long branches of heat supply with a large number of batteries equipped with thermostatic valves, an automatic flow regulator is installed at the beginning of the line, as shown in the diagram.
So, overpressure in a closed heating network is created for such reasons:
- to ensure forced movement of the coolant at the desired speed and flow rate;
- to monitor the state of the system by a manometer and to energize or repair it in time;
- the coolant under pressure heats up faster, and in case of emergency overheating boils at a higher temperature.
We are interested in the item on the second list - pressure gauge readings as a characteristic of serviceability and operability of the heating system. They are the ones who are interested in homeowners and apartment owners engaged in self-service of home communications and equipment.
The pressure in the pipes of apartment buildings
From the contents of the previous sections, it becomes clear that the magnitude of the set in the central heating pipelines of high-rise buildings depends on the floor on which the apartment is located. The situation is as follows: if the residents of the first two floors can approximately orient themselves by the pressure gauge installed in the basement heating unit, then the real pressure in the remaining dwellings remains unknown, since it drops with each meter of water rise.
Note. In new buildings with apartment-wired heating from a common riser, where floor heating units are equipped, you can control the pressure of the coolant at the entrance to each apartment.
Moreover, knowing the magnitude of the head in a centralized network is not practical, since the owner cannot influence it. Although some argue as follows: if the pressure in the line has dropped, it means that less heat comes in, which is a mistake. A simple example: turn off the return line tap in the basement and you will see a jump in the gauge needle, but at the same time the movement of water will stop and the supply of thermal energy will stop.
Now specifically about the numbers. The diameters of the heat supply networks and the power of the pumps feeding from the boiler room are calculated in such a way as to ensure the rise of the required amount of coolant up to the last floor. This means that at the entrance to a multi-storey building, the working pressure in the heating system will be:
- in the old five-story buildings, where to this day pig-iron radiators are found, - no more than 7 Bar;
- in nine-story Soviet-built buildings, the minimum figure is 5 Bar, and the maximum depends on the proximity of the boiler room with pumps, but not higher than 10 Bar;
- in high-rise buildings - no more than 15 Bar.
For reference. At least once a year, pipelines and heaters must be tested under pressure, 25% more than the worker. But in real life, public utilities do not run the risk of checking house systems and confine themselves to testing external heat supply networks.
The information provided is only useful in terms of choosing new radiators and polymer pipes. It is clear that in buildings with high-rise buildings, cast-iron and steel panel batteries, designed for a maximum of 1 MPa, should not be mounted, which is described in detail in our selection guide and in the video from the expert:
Pressure indicators in a private house and the reasons for its fall
In closed heating systems of country houses and cottages, it is customary to withstand the following pressure values:
- immediately after filling the heating network with water and air exhaust, the pressure gauge should show 1 Bar;
- after warming up to operating temperature, the minimum pressure in the pipes is 1.5 Bar;
- during operation in different modes, indicators can vary within 1.5—2 Bar.
An important point. It was not in vain that we indicated what pressure should be provided for a cold heating system. The fact is that the vast majority of imported gas boilers equipped with modern automation are designed to start at a minimum pressure of 0.8-1 Bar and if it is not available, it simply will not turn on.
How to properly remove air from heating lines and create the required pressure value is described in a separate instruction. Here we will list the reasons why, after a successful commissioning, the pressure indicators can decrease, up to the automatic shutdown of the wall-mounted boiler:
- Residual air escapes from the piping network, underfloor heating and heating equipment channels. Water takes its place, which fixes the pressure gauge falling to 1-1.3 Bar.
- Due to the leakage of the spool, the air chamber of the expansion tank was empty. The membrane is pulled back and the container is filled with water. After heating, the pressure in the system jumps to critical, which is why the coolant is discharged through the safety valve and the pressure drops to a minimum again.
- The same, only after breaking through the membrane of the expansion tank.
- Small leaks at the joints of pipe fittings, fittings or pipes themselves as a result of damage. An example is the heating circuits of underfloor heating, where a leak can remain invisible for a long time.
- The coil of an indirect heating boiler or buffer tank has lost its leak. Then pressure surges depending on the operation of the water supply are observed: the taps are open - the pressure gauge readings fall, closed - they rise (the water supply presses through the crack of the heat exchanger).
Conclusion
As you can see, the importance of pressure in district heating networks is somewhat exaggerated. Even if the landlord is aware that he should have 0.7 MPa in the pipes, but this does not give him much. In addition to the correct selection of radiators and pipes for replacing highways.
In a private house, the picture is different: the pressure gauge, and even a puddle near the safety valve, serves as an indicator of minor or significant malfunctions. These things need to be monitored and reacted in time by recharging the system in order to raise the pressure to normal. Do not forget about the expansion tank - timely pump the air chamber and monitor the integrity of the membrane.