Causes of pressure drop in the gas boiler

How to reduce the pressure in the boiler

There may also be a question of how to reset the excess pressure in the gas boiler, because the constant increase provokes emergency shutdowns, and this can lead to failure of the boiler. Pressure can be reduced by means of a Mayevsky cock. Water make-up tap or secondary heat exchanger must also be checked and repaired.

To prevent increase of pressure in the circuit can be installed brass safety bar 3. 4 bar. We recommend adding corrosion inhibitors to the heaters to remove clogged filters.

Examples of pressure drop in the heating system

Example one

Let’s say you come in between working on the pump, saw that the pressure has dropped. You bring it back up to the standard position. And you leave. Then the boiler has performed a cycle of one-time run, heated and cooled down, and you may not even know that at the time the safety valve was triggered. And without knowing it you have to re-pressurize, not understanding the real reason of what happened to the coolant and why the pressure is dropping so quickly.

Example two

For the second example, here is an even more specific case. There is a boiler of indirect heating, German company Vaillant, the coil installed in it in some undetected area has a leak. And the plumbing water area and the heating water area have merged. This leads to the fact that when the outboard pressure in the water system is high, the plumbing water enters the area of the heating water and the pressure in the system rises. And when the plumbing water pressure decreases, the pressure in the heating system drops along with it.

Unfortunately, the only way out of this situation is the complete replacement of the boiler indirect heating.

Example 3

causes, pressure, drop, boiler

For the third example, we will consider two-circuit heating boilers. One of the most frequent causes of why the pressure in the heating system falls, is the initial incorrect choice of boiler size. This is one of those cases where, instead of small heating radiators, which differ in the size of the stored coolant, installed pipes that use a much larger amount of water. And this fact is not taken into account in the initial consideration of the necessary area of the premises to be heated.

Consequently, in the calculations you make, exactly the same error will lead to a drop in pressure in the heating system.

Types, normative values and pressure control

Regardless of the number of circuits in the heat generator, there are several types of pressure in the heating system:

  • Artificial dynamic force in a closed system due to pump operation or expansion of heated water. The value is affected by the temperature of the coolant, the characteristics of the pumping unit and the tightness of the heat loop.
  • The natural static force due to the action of gravity on the coolant (1 m of heating circuit riser creates about 0.1 bar).
  • Real operating force, consisting of the sum of static and dynamic pressure. This is the characteristic which is determined by the measuring instruments. Normal operating pressure for a gas boiler is 1.5-2 bar.

On the manometer 2 bar is the working pressure in the boiler×683.jpg

  • The maximum possible force, the excess of which, even for a few seconds, causes depressurization of the heating circuit. Hydraulic shock leads to rupture of piping, batteries or heat exchanger in the heat generating unit.

Navien boiler units are equipped with built-in manometers. They are the ones that measure the working pressure value of the coolant. Specialists advise to equip the system with an additional control and measuring device. Usually perform the installation of security group one element of which is a pressure gauge.

The presence of an additional measuring device allows you to check the readings of the main pressure gauge. It can break, in spite of the factory version. Both dial and electronic pressure gauges are failing.

Navien wall mounted boiler unit with built-in pointer gauge and remote control

Why does the pressure drop in the boiler when you turn on the hot water to disassemble the causes

When in heating, the expansion tank is at the top and is in contact with air. this is an open system. This method has a number of disadvantages.

For example, slow heating, low efficiency, as well as the possibility of corrosion of pipes and equipment parts. To avoid this, use a closed heating system. Boiler. But sometimes there is a problem with it too, for example the pressure starts to drop.

There can be several possible causes of pressure drop. It is worth considering the most common ones.

In order to raise the pressure in the heating system, the first thing to do is to find out under what circumstances it happens. If it occurs when you open the hot water tap, all indications are that hot water is leaking from the heating circuit into the hot water supply circuit.

The presence of the problem in question can be determined by the fact that the water from the heating circuit has an unpleasant characteristic smell. The fault can be eliminated by closing the make-up valve. However, when a leak is detected, it may be that it needs to be replaced or the heat exchanger itself needs to be replaced.

Why does the pressure constantly drop? In addition to an open make-up valve, causes of pressure drops or spikes can also be such factors as:

  • Breakage or total failure of one or more elements;
  • A ruined three-way valve, broken coolant;
  • leaking pipes;
  • formation of airlocks in the heating circuit;
  • accidental increase in the area of contact of the circuits with cold and hot water;
  • Wrong pressure gauge, a special device that measures pressure in heating systems.

A leak in the heating system

Leaky heating circuits are a common cause of a sudden or permanent drop in pressure in the heating system. To find a leak, a visual inspection of all underfloor heating pipes and radiators should be done. If this does not help, try putting a cloth to the heating pipes and radiators.

If you find a puddle under one of the radiators, you should most likely replace that heating element. If the depressurization occurred at the junction of pipes or other elements of the system, to fix the problem it is worth using sealant or specially designed for this purpose tape.


By the way! Leaks lead to a drop in the quantity (volume) of the heat carrier in the general heating system (falling pressure) that can be corrected by caulking the hole (leakage) and adding water.

Cracks in the heat exchanger

Cracks do not always start leaking immediately. Sometimes they become the reason that the boiler does not hold pressure. Basically it happens because of depressurization of the system and the formation of air bubbles inside.

To diagnose this problem, you need to inspect the batteries. if they are too warm at low boiler output, look for cracks. To do this you will have to disassemble the boiler, starting with the front cover of the body of the device. To eliminate the problem, you need to perform deflating, that is, the release of air masses from the heating system.

By the way! Specialists believe that the reasons of choking of the heating systems are often not cracks, but the use of aluminum batteries.

The problem in the expansion tank

Two-circuit boiler in the heating system of a private house or apartment has in its design two tanks, called expansion tanks.

Their nipples may be broken, as evidenced by the hissing sound of air coming out, and when you press the nipple begins to drip water.

You can also try tapping on the metal body: if the sound is humming, the expansion tank most likely needs to be replaced. Also from a long time of use periodically heating boilers have burst membranes. Replacing the broken part will solve the problem.

An additional indication of a malfunction in the expansion tank will be the frequent need to add coolant to the system. Variation in the measurements of pressure gauges located in different parts of the system, should also be alert. In this case the wrong pressure in the system or the situation when it disappears or jumps constantly, can lead to large steam formation and the consequence is the triggering of the emergency valve.

Trapped air

Among the causes of pressure drop in the gas boiler also stand out the appearance of plugs from the accumulated air. The cause of such plugs is depressurization, which in turn is caused by improper operation of the device and the suction of air masses inside.

How to make a diagnosis? You should simply touch the radiators in different places. Some sections of the battery will become colder. this is an indication of the presence of air in the circuit. Also the presence of plugs from the air confirms gurgling, gurgling water in the pipes, rust (corrosion lesions).

To expel the undesirable air from the heating batteries and increase the pressure in the heating system can restart the boiler. This will allow the heating pipes to fill up with water again. from the bottom to the top.

Problem with the bleed valve

The drain cock serves to reduce excessive pressure in the system. If the faucet is out of order and starts to play, it may cause low pressure that needs to be corrected. This special reset valve is normally turned on when the fluid temperature reaches 85-95 degrees on the Celsius scale. In this case, about four percent of the expanded water is released.

Faulty pressure gauge to measure the pressure in the system

This device is necessary to measure the pressure in the boiler, as well as in the entire heating system. You can only have it inspected by a professional to make sure it is in working order. Diagnosis can only be made by them, and it is necessarily necessary to carry out the removal of the device from the boiler. Also do not forget that the measuring device must undergo obligatory calibration.

Safety valve malfunction

Often the pressure in the gas boiler spikes or drops due to a faulty valve that protects the device and its users from failure. It is the valve (if it is in good working order) that regulates the pressure in the system if it becomes excessive. If this part has been recently replaced, the failure will indicate a faulty choice of valve assembly.

A large amount of debris, scale and other unwanted substances in the area of the moving part of the valve will indicate its malfunction. Most often this valve is even difficult to turn. in this case you should try to perform cleaning, because this usually increases the pressure to normal. After removing such a “clog” you should observe the subsequent operation of the boiler for another one or two hours using a pressure gauge.

Why the pressure in the system increases?

Excessive overpressure in the boiler can be caused by two things:

Water is an excellent solvent, and it contains not only mineral salts, but also gases, including nitrogen, oxygen and carbon dioxide. Their solubility decreases as the temperature rises. Gases escape from the liquid in the form of bubbles and enter the system. Most of them are removed through the automatic air vent, but sometimes they pass into the radiators. Cork of the air disrupts coolant circulation, which is accompanied by an increase in pressure in the boiler. One of the most common causes of “choke” is too high a system filling rate.

To detect an air lock, just compare the temperature at the top and bottom parts of the heater. You do not need a thermometer for this, the difference will be felt by hand. To remove air corks from the radiators are installed on them Maevsky cocks or air vents of other designs. After several heating and cooling cycles the air evolution will stop.

When you turn on hot water, the pressure in the boiler drops: causes

The gas heating unit with a closed circuit is often used for the arrangement of the heating of the dwelling, hot water supply. The coolant constantly moves through the pipes of this design. In its operation, it is often a problem that when you turn on the hot water pressure in the boiler drops. To get rid of it, it is important to understand the functioning of the heating system, to study the possible breakdowns, as well as the causes of their occurrence.

Gauge for gas device×800.jpg

Causes of increased pressure in the case of boiler damage

If there are cracks in the connecting diaphragm, the expansion vessel will not be able to smooth out the water hammer.

Coolant flows into the tank without obstruction, the pressure decreases. The pressure gauge reading will increase and the joints will leak.The fault is eliminated by replacing the diaphragm or the expansion tank.

The pressure in the heating system increases if one of the components of the line is broken:

  • The value rises when the make-up valve is open. There is a contrast between the low pressure in the unit and the high pressure in the plumbing. Water is added to the heating main and the boiler pressure rises as a result. When the head decreases, the coolant cuts off seepage into the boiler, the values on the pressure gauge decrease. To prevent such surges, the upstream shut-off valves must be closed and leaking valves replaced in good time.
  • An increase of 1 atmosphere can be caused by a leakage of the heat exchanger. Leaks on the body occur as a result of wear and tear during prolonged use of the equipment, as a result of a factory defect, hydraulic shocks. If the heat exchanger is broken, the water is constantly flowing into the circuit. The fluid evaporates at the cracking points during heating, so it is difficult to visually trace the failure that causes the pressure to rise. The problem can only be solved by replacing this circuit.
  • The pressure in the heating system rises when the three-way valve malfunctions or is worn. Mixing of the heating medium from the system and the cistern. All sorts of dirt often collects on the valve, so the unit should be periodically cleaned. To reduce the risk of falling debris from the water mains into the heating network and subsequent valve failure, it is recommended to install a mechanical filter.
  • When the pressure in the heating system jumps for no apparent reason, you should check the pressure gauge. Defective meter must be replaced.

The secondary heat exchanger is a second circuit for water heating and hot water supply. Failure of this circuit results in fistulas. Warm and cold water mix, the fluid flows into the heating system causing a water hammer.

The fault can be eliminated by replacing the heat exchanger or soldering the leakage. Brazing is resorted to in situations where there is a single fistula of small size.

Replacement of the heat exchanger is performed in this order:

  • close the boiler feed tap;
  • drain the coolant;
  • Remove the panel and unscrew the screws securing the heat exchanger;
  • remove the defective part;
  • Replace the gaskets at the attachment points of the unit;
  • connect and bolt the new part.

Filling is performed through the make-up valve in a thin stream until the pressure gauge readings rise to 1.2-1.5 bar. After supplying water to the boiler must be sure to check the quality of connections for leaks, then install the cover in place and turn on the equipment. To turn on the boiler without water is categorically impossible!

What is the problem with pressure decrease?

There are many reasons why such a malfunction may occur. Among the most probable are problems with the tank, sometimes various damages to the heat exchanger. However, in most cases, the cause of pressure drop in the gas boiler is caused by a faulty make-up valve. It may not be tightly closed, or it may be faulty at all. Even minor mechanical damage to the valve can cause a pressure drop. Otherwise it is called a connected valve or fill tap. It is mainly installed on combination or two-circuit boilers. The fact that the water supply system and the heating circuit communicates through an unclosed or faulty filler tap, and this should not happen. The consequence of this is pressure equalization, as the two systems are connected to each other. Usually inside the plumbing, the internal pressure is much higher than what it should be in the heating system. Therefore, this pressure quickly dissipates through the hole, and the result is that the boiler and the water supply system will essentially be subject to the same pressure.

In this case, the boiler continues to work, but the pressure remains the same, because the water, expanding due to heating, flows into the water pipe. As soon as we turn on the hot water, the pressure is immediately reduced. That is, because of the unclosed faucet, the boiler and the plumbing are essentially communicating vessels, trying to balance each other out. This explains the pungent unpleasant smell, because the water, before flowing out through the faucet, passes through the plumbing, into which the already used water drains. But as soon as you turn off the hot water, the pressure in the whole system comes back to normal. It returns to the condition it is in when cold water enters the house. Only the pressure in the boiler remains low, because the water fills the boiler through the same make-up valve.

It follows that this problem causes the tank to be constantly filled and emptied. It’s very bad for the whole system, and there are two reasons for that:

  • Oxygen that tap water brings with it. It oxidizes all the pipes, causing them to malfunction. Corrosion of the metal has a very negative effect on it, which can eventually lead to serious problems. Corroded pipes deteriorate and water begins to flow through them after a while. Repairs can be very expensive because all the equipment has to be replaced. Constant flow of fresh water reduces the service life of the equipment by a factor of five to ten.
  • Salts in large quantities contained in the same water. They lead to the formation of scale, and this, too, is not good for the boiler. It will be almost impossible to clean this scale out of the pipes without damaging them. Leading manufacturers of heating equipment strongly recommend that the make-up should not be more than the volume of the boiler tripled.
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