What should be the pressure in the heating system of a private house

Recommendation points

• What is excess pressure for?
• Rules for drawing closed contours
• Selection and installation of an expansion tank
• Automatic feeding of the system
• Setting up and troubleshooting

Novice specialists in the design and installation of hydraulic heating systems ask themselves: why is it so important to maintain excess pressure in the circuit, and in what ways this can be achieved. We invite you to find out the answers to popular questions on this topic in our review on closed heating systems.

What is excess pressure for?

Imagine a heating system with an open expansion tank. In its upper part, the coolant is under a pressure of about 100 kPa or 10.2 m of water column. Unlike the upper filling limit, which is under normal atmospheric pressure, in the lower part the coolant is compressed even more, the difference is just equal to the height difference and is expressed in meters of water column. Changes in atmospheric pressure and altitude, as well as in the density of the coolant, can make their own adjustments. However, in practice, such small errors are neglected..

In normal operation of such a system, a significant amount of water is heated inside the boiler heat exchanger. Heating is accompanied by expansion, due to which a portion of water with a higher temperature exerts additional pressure on the adjacent volume of the coolant. In this case, a liquid with a lower density rushes upward: the pressure force of the heated coolant is summed up with the pressure of the atmosphere and the internal pressure of the system, which allows the coolant to be pushed along a closed loop. The higher the speed of movement of the coolant, the more pronounced the convection effect and vice versa..

What happens in the so-called closed heating systems? In them, the coolant is isolated from the atmosphere, as a result of which the pressure is practically zero at the upper filling limit, and at the lowest point it is equal to the actual height of the liquid column. The increase in pressure due to the expansion of water does not always allow to overcome the hydrodynamic resistance of the system, because when expanding, water is forced in both directions, its expansion downward is not compensated by additional pressure.

An increase in volume during heating is characteristic of any heat carrier. In a closed heating system, this definitely leads to an increase in pressure.

It is possible to launch such a system only if a large amount of coolant is quickly heated, which is impossible in modern heating systems characterized by a small displacement and nominal pipe passage. Thus, excess pressure in the system serves as a kind of replacement for atmospheric pressure, greatly facilitating natural convection and the operation of forced circulation devices..

Of course, the pressure in the heating system does not have to be equivalent at different points. The highest rates are recorded in the heat exchanger and on the boiler return pipe, a slightly lower pressure can be recorded at the inlet of the circulation pump and the smallest – at the most remote and highest section of the supply line. One of the main tasks of a heating engineer is to provide excess pressure inside the heating system and monitor, so that in its various nodes a difference is observed that fits into the established norms.

Rules for drawing closed contours

For open-type hydraulic systems, the issue of pressure regulation is irrelevant: there simply are no adequate ways to do it. In turn, closed heating systems can be adjusted more flexibly, including with respect to the pressure of the coolant. However, first you need to provide the system with measuring devices – manometers, which are installed through three-way valves at the following points:

• in a safety group collector;
• on branching and collecting collectors;
• directly next to the expansion tank;
• on mixing and consumable devices;
• at the outlet of circulation pumps;
• at the mud filter (to control clogging).

Not every position is absolutely required, much depends on the power, complexity and degree of automation of the system. Quite often, the piping of the boiler room is arranged in such a way that the parts important from the point of view of control converge in one unit, where the measuring device is installed. So, one pressure gauge at the pump inlet can also serve to monitor the condition of the filter..

Why do you need to track pressure at different points? The reason is simple: pressure in the heating system is a collective term, which in itself can only indicate the tightness of the system. The concept of a worker includes static pressure, formed by the action of gravity on the coolant, and dynamic – oscillations that accompany the change in the operating modes of the system and appear in areas with different hydraulic resistance. So, the pressure can change significantly when:

• heating the coolant;
• violation of circulation;
• inclusion of make-up;
• clogging of pipelines;
• the appearance of air congestion.

It is the installation of control pressure gauges at different points of the circuit that allows you to quickly and accurately determine the cause of failures and begin to eliminate them. However, before considering this issue, you should study: what devices exist to maintain the working pressure at the desired level.

Selection and installation of an expansion tank

The main way to maintain a stable pressure in the heating system is to install an expansion tank. This is a sealed, non-flowing vessel, inside which a pear is installed, filled with air under a certain pressure. Since air can be compressed, this damper helps to smooth out dynamic pressure fluctuations..

1 – the state of the heating system before filling with the coolant; 2 – normal operating mode of the heating system; 3 – excess pressure as a result of overheating of the coolant

For the expansion tank to work properly, its volume must be high enough. You should also take into account the working design and maximum pressure in the system, which is determined by the boiler’s passport data. Thus, the specified minimum of 0.3–0.5 bar is required for the stable operation of the heating unit, while a limit of the order of 1.2–1.5 bar limits the destructive effect of pressure on the heat exchanger. The indicated values ​​are taken from the example of most domestic boilers with a capacity of up to 15 kW, for more efficient boilers higher values ​​apply. It is possible to determine the operating pressure of the system during the hydraulic calculation, the desired value depends on the hydrodynamic resistance of pipes, heating devices and fittings.

The volume of the tank is determined by the displacement of the system, the thermal expansion of the coolant and the ultimate pressure. As a first approximation, the capacity of the tank should be about 0.07–0.1 times the volume of the coolant. A more accurate value is found by the formula (Vt *?) / K, that is, the product of the total volume of the coolant by the coefficient of its expansion, divided by the operating coefficient of the tank. The latter is determined by the difference between the limiting and working pressure, divided by the limiting pressure, to which the unit is added: (P max – P slave) / (P max + 1). In this case, the working pressure is measured at the top point of the system (on the safety group) and should be close to atmospheric at a water column height of up to 4-5 m.

There is no practical difference in the installation location of the closed expansion tank. To ensure efficient operation of the device under various modes of use of the heating network, the tank is installed in the zone of the highest static pressure, that is, at the lowest point of the system, in a place convenient for maintenance. The tank cuts through a tee into the main return pipeline, the connection is made through a shut-off ball valve.

Automatic feeding of the system

The second unit, which ensures the maintenance of excess pressure in the system, is an automatic make-up device. Of course, it is possible to pump water into the system manually, but this is inconvenient for large volumes of leakage. For example, if there are many fittings in the system or there are gaps through which microscopic doses of the coolant regularly seep. Also, automatic make-up is almost indispensable for closed systems with a special coolant – without a pressure pump, it is simply impossible to provide a sufficiently high pressure.

The first type of automatic make-up devices works on the principle of a compressor automation group. The high and low pressure switches turn on and off the make-up if the pressure in the system is below or, respectively, above the set threshold. Such devices are the simplest and cheapest, but they have the main drawback – they do not take into account the temperature of the liquid and the degree of its expansion..

For example, during system operation, the pressure drops 20-30% below the operating pressure, but at the same time does not reach the minimum threshold to which the relay is set. This is not surprising, because the relay is calibrated in a cold state of the system. Another special case: when the relay is triggered, the make-up is turned on, adding a portion of cold, that is, not yet expanded liquid to the system. If the expansion tank has insufficient capacity, as a result, the expansion of the coolant will trigger the safety valve, part of the coolant will be released, the pressure will drop again, the make-up will turn on again and then in a circle.

The described nuance is important for heating systems that contain over 300 liters of water. It is optimal in such cases to use digital make-up dispensers, which are supplied with most advanced boiler technology. The controller will make the necessary corrections and add a strictly defined amount of coolant to the system, taking into account its temperature and ability to expand. Like conventional mechanical make-up valves, it is better to connect the electronic dispenser to the supply line immediately after inserting the bypass tube into it in order to avoid the temperature shock of the heat exchanger. It is recommended to install a mud or cartridge filter on the inlet pipe for the coolant, the injection unit is connected through a ball valve.

Setting up and troubleshooting

Maintaining the pressure in the heating system is impossible without observing the rules for filling it. This must be done with a minimum pressure and with open valves to bleed air in the radiator network. The underfloor heating loops are filled alternately, otherwise, due to the difference in length, air will certainly be displaced into longer coils. After the system is filled, it is pressurized with double working pressure and the readings of the manometers are monitored for a certain time. Usually, the pressure of the water supply system is sufficient for pressure testing; otherwise, you have to use a manual plunger hydraulic pump. After checking, the pressure is reduced to a minimum, the system heats up to the maximum operating temperature, upon completion of the heating of the entire volume of the coolant, the pressure is measured: it must be less than the limit by 20-30%.

Decreasing pressure over time is common in fresh water systems. Dissolved oxygen is released from it, respectively, over time, the total volume of the coolant decreases. You just need to periodically recharge the system until the effect disappears by itself. An increase in pressure is a clear sign of incorrect calculation of the expansion tank, its volume needs to be increased. Small drops in the range of 10-15% of the working pressure are considered quite normal, this is due to the linear expansion of the pipes. If the pressure surges during heating and cooling of the system go beyond 30% of the nominal, this indicates either damage to the membrane in the tank, or the presence of air locks in the system.