why does the manometer show zero even though there is pressure in the system

A manometer is often the only indicator for the average boiler owner of whether the heating system is functioning correctly. A common and confusing situation is when the scale shows zero, even though you know that pressure definitely exists in the system. This anomaly is not just a technical curiosity, but a signal of a serious fault that requires immediate diagnosis and intervention by an experienced technician to prevent damage to the equipment or endanger the safety of the operation.

When pressure exists, but the manometer shows zero

The most common reason why a manometer shows zero in a functioning system is the mechanical jamming of the internal mechanism or a crack in the Bourdon tube, which transfers pressure to the needle. If water under pressure of 1.5 to 2 bar is present in the system, but the needle remains completely motionless at the zero mark, it means that the force of the pressure cannot overcome the friction in the gear train or the pressure is completely cut off from the measuring element. In such a case, the device is physically non-functional, and its replacement is the only possible repair, as recalibration on site is not possible.

Another technical reason may be clogging of the threaded connection or the inlet channel by deposits, which prevents the transfer of the pressure medium into the manometer. With long-term operation without maintenance, deposits of corrosion products form in these areas, effectively sealing off the passage for pressure. This condition is critical, as even if you see pressure at other locations, this particular manometer will mislead you and give a false sense of security. Therefore, it is important to regularly check not only the values, but also the physical condition of the entire measuring circuit.

Influence of temperature and thermal expansion

Temperature conditions have a direct impact on the accuracy of measurement, especially if you are using a cheaper model without temperature compensation or if the manometer is located close to a heat source. If the manometer is exposed to temperatures above 80 °C, it can lead to deformation of the spring or a reduction in the viscosity of the filling liquid, causing the needle to "drop" to zero even at high pressure. This phenomenon is common in older types of manometers that were not designed for modern condensing boilers with higher temperature requirements.

If you have an axial thermomanometer 0°C - 120°C 0 - 6 bar, 63 mm mounted on the boiler, you should know that the combination of a thermometer and a pressure gauge in one body increases the risk of mutual influence under extreme temperatures. The thermometer can cause local heating that affects the pressure gauge spring, or conversely, vibrations during heating can loosen the mechanism. For professional applications, it is often better to use a separate axial thermometer 0°C - 120°C, 100 mm that is separated from pressure measurement, thus minimizing the influence of temperature on the accuracy of the pressure gauge.

In practice, we have encountered a case where a homeowner had a pressure of 2.2 bar in the system, but the manometer showed zero. After disassembly, it was found that the manometer was located directly at the burner outlet, where the temperature exceeded 100 °C, causing permanent deformation of the Bourdon tube. The solution was to move the manometer to a cooler part of the system and replace it with an axial manometer 0 - 10 bar, 63 mm, which has higher resistance to thermal influences and is suitable for most domestic boilers.

Mechanical damage and wear

Long-term vibrations and impacts are one of the main causes of mechanical damage to the internal mechanism of the manometer, leading to the needle no longer responding to pressure and remaining at zero. In gas-powered boilers or during gas exchange in the boiler, frequent vibrations occur, which gradually loosen the bearings and gear train. If these parts become loose, the pressure can no longer move the needle, and the device loses its function.

• Disruption of the connection between the Bourdon tube and the gear mechanism.
• Breaking of the gear wheel due to impact loading during boiler startup.
• Detached needle that stops at zero due to loss of force connection.
• Permanent damage to the spring due to excessive pressure (exceeding the range).

In such cases, it is not possible to repair the manometer, only to replace it. It is advisable to choose a model with a more robust design, such as the axial manometer 0 - 6 bar, 63 mm, which is designed for normal operation in home heating systems and offers sufficient reserve for stable measurement. When selecting, always ensure that the manometer is protected from direct exposure to vibrations and impacts.

Influence of incorrect range and calibration

One of the common mistakes during installation is the use of a manometer with an incorrect range, which can lead to a false display of zero or inaccurate measurement at the operating point. If the pressure in the system is 1.5 bar and the manometer has a range of 0 - 4 bar, the measurement can still be accurate, but if the range is too large, for example 0 - 10 bar, the needle may only move in a small part of the scale, increasing the risk of error and in extreme cases, the needle may "hang" at zero due to friction.

For the correct selection of the range, it is essential to know the maximum operating pressure of your system and choose a manometer whose upper limit is approximately 30% higher. If you have selected an axial manometer 0 - 4 bar, 63 mm for a system with 1.5 bar pressure, you will have a very accurate measurement. Conversely, if you use a manometer with a range of 0 - 10 bar for the same system, you may lose accuracy and in the case of mechanical wear, the needle may stop at zero even when pressure exists.

Calibration of the pressure gauge is a process that should be carried out regularly, especially in professional heating systems. If the pressure gauge is not calibrated, it may indicate zero even when pressure is present, because the mechanism may have shifted. For professional applications, it is important to monitor the maintenance and calibration of pressure gauges to avoid dangerous situations and ensure accurate pressure measurement in the system.

Installation and placement errors

Installation errors, such as incorrect tightening of the connection or using an unsuitable sealing, can cause pressure leakage at the connection point, leading to the pressure gauge not registering any pressure and showing zero. If the connection is loose, pressure escapes and the pressure gauge cannot measure the actual pressure in the system. This is a common problem in DIY installations, where the correct sealing is forgotten or the connection is not tightened sufficiently.

Another common error is placing the pressure gauge in the wrong location, for example, on the boiler outlet where the pressure is unstable, or in a location where the pressure gauge is exposed to direct sunlight. The pressure gauge should be installed in a place where the pressure is stable and where it is protected from direct sunlight and extreme temperatures. Correct installation and placement of the pressure gauge on the boiler is key to accurate measurement and safe system operation.

For correct installation, it is important to follow the manufacturer's recommendations regarding the type and diameter of the connection. If you choose a pressure gauge with a 63 mm connection, make sure it is compatible with your boiler. The difference between an axial and a radial pressure gauge in heating is important when choosing the right type for your needs. An axial pressure gauge is suitable for space-restricted areas, while a radial pressure gauge is suitable for standard installations.

Diagnosis and problem solving

When you encounter a pressure gauge that shows zero, the first step is to check the pressure in the system using another verified source, such as a pressure gauge on the expansion tank or on the main pipe. If another source indicates pressure, the problem is clearly in the pressure gauge itself. In this case, it is necessary to remove the pressure gauge and check its condition, or replace it with a new one. Common pressure gauge faults and how to eliminate them are topics that should be taken seriously, as incorrect measurement can lead to dangerous situations.

The next step is to check the entire measuring circuit, including the connection, sealing, and Bourdon tube. If the pressure gauge is damaged, it must be replaced. When replacing it, it is important to choose the correct type and range of the pressure gauge to avoid similar problems in the future. Maintenance and calibration of pressure gauges in professional heating are essential for ensuring safe and efficient system operation.

Frequently asked questions

Why does the pressure gauge show zero after gas exchange in the boiler?

After gas exchange in the boiler, vibrations can loosen the pressure gauge mechanism, or pressure changes in the system can cause the pressure gauge to show zero. It is necessary to check the pressure gauge and replace it if necessary.

Is it possible to repair a pressure gauge that shows zero?

Usually not, because it is a mechanical failure that cannot be repaired on site. The pressure gauge must be replaced with a new one that will accurately measure the pressure in the system.

What to do if the pressure gauge shows zero, but the boiler is running?

Immediately check the pressure in the system in another way, for example on the expansion tank. If the pressure is normal, the pressure gauge is likely damaged and needs to be replaced. If the pressure is not normal, it is necessary to add water to the system.

In conclusion, a pressure gauge showing zero when pressure is present is always a signal of a serious fault that needs to be addressed immediately. Correct diagnosis, replacement of the damaged device, and regular maintenance are key to the safe and efficient operation of the heating system.

Do you have a question about this topic?

Not sure or dealing with a specific situation in your home? Write to us - we are happy to help.