17 Facts On Does Boiler Pressure Increase: Why, How, When

Boilers are normally used to add heat in the system and convert the phase of working fluid. Lets examine how and why boiler pressure increases.

Boiler pressure is the pressure at which the boiler is designed to work. The pressure inside the boiler is not constant. It changes during the phase transformation process.

We shall discuss more about when , how and why boiler pressure increases and reasons behind its increase/decrease in this article.

What is boiler pressure?

Boiler is a mechanical device used for conversion of liquid water to gaseous steam. This comes with a lot of pressure generation. Now let us focus on what is boiler pressure.

Boiler pressure is the pressure inside the boiler whose value depends on the working state of the boiler whether ON/OFF.

Why does boiler pressure increase?

The value of boiler pressure can go up as well as come down depending upon the events happening inside the boiler. Now we shall see why does boiler pressure increases.

  • Faulty valve– When a valve is faulty, it is safe to conclude that the valve is allowing more water than needed which means the pressure is more than anticipated.
  • Conversion of water to steam- The phase transformation comes with a lot of pressure generation. This happens due to the fact that water expands to become steam.
  • Defective gauge reading– Sometimes the boiler will be working at nominal pressures. But the gauges that read pressure will show wrong readings. This is because the gauge is defected.
  • Leaks– When there is a leak in the boiler, the pressure will keep decreasing because the gases are being expelled out of the system.
  • Addition of heat– When there is excess addition of heat than required, then the boiler pressure might go above the nominal value.

How does boiler pressure increase?

The boiler pressure is a key parameter kept in mind while designing a boiler. Let us see the circumstances under which the boiler pressure is increased.

Boiler pressure increases due to addition of heat to the system. The heat generates steam from water and increases the pressure of the boiler.

Due to the expansion of working fluid, pressure is exerted on the walls of boiler. This will eventually increase the boiler pressure. Because of this, boilers are designed to work in such a wide pressure range.  

When does boiler pressure increase?

Maximum value of boiler pressure is reached when the working fluid is entirely converted into steam. Let us check when does the boiler pressure increases.

The boiler pressure increases when there is steam being generated inside the boiler. The steam generation process exerts pressure on the walls of boiler because steam has lower density as compared to liquid. So it expands and exerts pressure on the container walls.

What happens if boiler pressure is too high?

Anything in excess is dangerous and so is the value of boiler pressure. Let us see what happens when the boiler pressure is too high.

The boiler will start venting from the loose ends in order to decrease the pressure, if the pressure is too high and the boiler valves too tight then there is a chance that the boiler will explode. This happens because the force exerted by the gases inside the boiler exceed the safe limit set as per the design of boiler.

Does boiler pressure increase when heating is on?

It is natural for boiler pressure to rise up and come down as per the working conditions. Let us discuss what happens to the boiler when heating kept on.

The trapped gases keep on expanding as a result of which pressure is exerted on the walls of boiler vessel. Kinetic energy of the gas molecules will rise up as heating will take place. This energy is then converted in the form of pressure.

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Image: Fire tube boiler

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What should boiler pressure be when heating is off?

The pressure is affected due to heating and cooling of boiler. The exact value of boiler pressure is discussed in the below section.

When there is no heating, no expansion takes place due to which the pressure comes down to 1 bar. The boiler pressure decreases as the heating stops. As it cools down the pressure goes to 1.5 bar.

How to increase pressure in boiler?

The units of pressure is measured in bars. We shall discuss different boiler types and see how boiler pressure is increased in those types-

  • Ideal– To increase boiler pressure in ideal boilers, we eject air in the system. This is done by rotating the blue valves placed beneath the boiler. The boiler pressure gauge should show a value between 1-2 bars.
  • Worcester– In Worcester boilers, a white coloured key is given to repressurise the system. The key has an arrow marked to show the direction of its head. The key must be inserted inside the hole and the white nut beside this valve must be turn loose. After doing this procedure the boiler pressure gauge goes between 1-2 bars. When it reaches the desired pressure we must lock the valve with the key.
  • Valliant– Firstly the boiler is turned off. Then the isolation valve is turned anti clockwise until its line matches the line on the pipe. Then finally the handle Is rotated anti clockwise, as we do this the pressure inside the boiler starts increasing.
  • Baxi– Baxi boiler also uses similar procedures as mentioned for the above boilers. First the boiler is switched off. A silver cap is identified and then rotated anti clockwise. Then we start fitting the filling loop, it is held mid way between the silver caps. Once it is placed in the mid way then it must be tightened with wing nuts. Finally a tap on the left hand side is rotated. Now the pressure of the Baxi boiler will start increasing.
  • Combi– Combi boilers should  have a pressure range generally between 1-1.5 bars. The pressure inside the boiler is increased using a valve situated below the boiler. The valve must be closed as the boiler pressure reaches a value between 1-1.5 bar.

Does increasing boiler pressure increase water pressure?

The entire effect depends on the type of boiler used. Different boilers behave differently in accordance with the water pressure. Let us discuss about the effect of increasing boiler pressure on water pressure.

Increasing boiler pressure does increase the water pressure upto some extent. Usually the pressure relief valves reduce the excess pressure generated due to the high pressure. In case one needs to increase water pressure, water booster pumps are available in the market that can be used.

Does a combi boiler increase water pressure?

A combi boiler pressure works on similar principles like other conventional boilers. We shall discuss whether a combi boiler increases water pressure.

The combi boiler increases water pressure upto some extent as it uses fuels like natural gas to heat up the water. Although the main limitation of combi boiler lies in the inability of the boiler to support an external water pump to further increase the pressure of water.

Does temperature affect water pressure?

When the boiler is set to heating, the water pressure starts increasing. Now let us read about the effect of temperature on water pressure.

The temperature does affect the water pressure. Due to the temperature rise, expansion of water takes place. This is the main reason behind the increase in temperature. Hence we can say that temperature affects the water pressure.

Does boiler pressure increase with temperature?

Higher temperature causes the water to expand. Note that the boiler is sealed. Let us see if boiler pressure increases or not.

The boiler pressure increases with temperature. However the boiler pressure will come to its normal condition when the boiler is cooling. The boilers are designed in such a way that they can handle the increasing pressure.

Does outside temperature affect boiler pressure?

The boiler pressure depends on the usage of the boilers in accordance with the weather conditions. Let us see if it depends on the temperature alone.

If the temperatures outside are low then the pressure inside the boiler will increase. If the boiler is used excessively during summers then there are high chances that its pressure will increase drastically during the winters. When the outside temperatures start getting low the pressure inside starts increasing rapidly.

How long does it take to increase the boiler pressure?

The boiler pressure does not increase in one moment. It takes a certain amount of time to reach the desired value. Let us see how long does it take.

To reach the desirable boiler pressure, it takes around 10 seconds for the water to increase the pressure inside the boiler.

It also depends on how low the pressure had been dropped. If the pressure is too low then it can take up to 10 seconds and if the boiler takes more time than that then there is probably a leak in the system which needs to be catered.

Does boiler pressure fluctuate?

It is completely normal for a boiler pressure to change its value but let us see if it is normal for the boiler pressure to fluctuate.

The boiler pressure ranges from 1 bar to 2 bars. The boiler pressure will rise when the heating is taking place. Similarly the boiler pressure will go down if the boiler is being cooled down after its application.

Why does boiler pressure fluctuate?

The boiler undergoes an intense heat transfer process. The boiler experiences tremendous amount of net heat transfer. Let us see if boiler pressure fluctuates or not.

Due to this, the boiler temperature goes up and down drastically. When the boiler temperature is high, the pressure inside the boiler also increases due to expansion of working fluid inside it. When the boiler cools down the temperature goes down and hence the pressure.

Is it normal for boiler pressure to fluctuate?

The boiler pressure is an important factor used for designing of boiler vessels. Let us see if it is normal for boiler pressure to fluctuate.

It is completely normal for the boiler pressure to fluctuate. As we have already discussed about the reasons behind such fluctuation in the above section.

One should not call the maintenance engineer directly if the pressure suddenly starts fluctuating. It is indeed normal for the boiler pressure to fluctuate.

How much should boiler pressure fluctuate?

The value of pressure fluctuates between its maxima and minima. Let us see the exact values that are given in the section below.

The boiler pressure can fluctuate between 1 bar and 2 bars in normal working conditions. This pressure difference is kept in mind while designing a boiler.

The pressures vary due to the heating and cooling of the boiler. When the temperatures are high the pressure may rise up to 2 bars and when the boiler cools down the pressure goes down to 1 bar.

Does combi boiler pressure fluctuate?

Combi boiler pressure acts like other conventional boilers. Let us see if the combi boiler pressure fluctuates like conventional boilers or not.

It is normal for a combi boiler to have pressure fluctuations. This is because the boiler uses fuel to heat the water inside boiler like other conventional boilers.

When the temperature is high the pressure will go up and vice versa will happen when the temperature is low or the boiler is allowed to cool. The temperatures go high when heating is taking place. After the application is performed, the boiler cools off during which the pressure goes down.


In this article we read about boiler pressure fluctuations, reasons behind those fluctuations and the methods of repressurizing a boiler after it has cooled off. There is a simple principle that boilers follow that is its pressure increases when it is functioning and when it is allowed to cool off, the boiler will depressurize.