Diagnostics and repair of the boiler pressure switch

1. The purpose of the pressure switch.

2. The principle of work.

3. Symptoms.

4. The reasons for the breakdown.

5. Diagnostics and repair of the boiler pressure switch.

The purpose of the pressostat.

A pressure switch, it is also a smoke pressure switch, a differential relay, a smoke sensor is a sensitive device designed to turn off the boiler in case of poor removal of spent fuel. It controls the operation of the fan and, with insufficient flow of polluted air in the chimney, sends a signal to the control board to turn off the gas burners. These sensors are equipped with gas water heaters with a forced smoke exhaust system.

Principle of operation.

The pressure switch of a gas boiler consists of a housing with nozzles, a microswitch for 2 positions, a movable membrane with a pin that divides the internal space of the housing into 2 chambers.

When the exhaust fan does not work, contacts 1, 2 are in the closed position (contact 2 is not connected to the control board). When the fan starts up, a vacuum occurs in chamber 2, which is transmitted from the rotation of the turbine through the venturi, the condensate collector tube and pipe 2, due to which the membrane with the pin is attracted and closes contacts 1, 3 (contacts 1, 2 are open). This signal, about the normal operation of the smoke exhauster, is fed to the control board (contacts 1, 3 are connected to the control module), which ensures the operation of the boiler according to the specified algorithm.


If the smoke detector breaks down, the following symptoms appear:

1. In the differential relay, when the fan is turned on, there is no click of the micro switch.

2. The boiler does not work, and gives approximately the following error: "The air blower is off, the fan is working."

The reasons for the breakdown.

The following causes may cause a breakdown or malfunction of the relay:

1. Oxidation of contacts. At the beginning of the fan, a click of the microswitch is heard, but the circuit does not close due to the oxidized film on its conductive parts.

2. Wear of the membrane material. The deterioration of the technical properties of the moving body of the smoke sensor contributes to the disruption of its operation.

3. Clogging, damage, or the formation of a water seal in the tube with the condensate collector. When cracks, ruptures form on this element, or when a tube becomes clogged or when water is filled with water, the sensor responds inadequately to changes.

4. Reduced fan performance. The forces of attraction arising from the discharge in the turbine chamber are insufficient to move the membrane to the microswitch.

Diagnostics and repair of the boiler pressure switch.

To identify a failed node, you can do the following:

- turn off and disconnect the boiler;

- open the front of the case;

- interconnect contact 1, 3;

- reset the error and start the boiler.

If the boiler starts to work, then the breakdown is in the pressure switch. For its diagnosis and repair, it is necessary to examine the condition of the following elements:

1. Condensate tubing. It is inspected for the presence of thermal wear and the filling of the internal cavity with dirt and condensate. The tube is placed in an environment of elevated temperatures. From thermal radiation, the tube material loses its physical properties or becomes covered by through cracks. Also, its channels can be clogged with dirt or filled with water, which is formed due to sudden changes in temperature, between the environment and hot exhaust gases. If defective areas are found, the tube must be replaced with a new one, or try to repair the worn product yourself.

For example, deep cracks on a cylindrical surface can be insulated with a heat-resistant material (electrical tape, aluminum tape).

2. The inner chamber of the smoke sensor. When moisture gets inside the case, the contacts of the microswitch begin to oxidize intensely. Therefore, if condensate is constantly present in the relay, especially this process is pronounced at low temperatures, it is advisable to transfer the pressure switch of the gas boiler to another place, away from the cold wall of the supply duct.

3. Electrical contacts. Connect the multimeter to pins 1 and 3 in the diode test mode.

Using your mouth, create a vacuum in the smoke detector chamber No. 2. If the tester protects or shows a short circuit, it means that the conductive parts of the relay are not oxidized, if one is displayed, then they are malfunctioning. Many models of smoke pressure switches are enclosed in one-piece housing, so it will be problematic to inspect the condition of the contacts. Since the halves of the plastic case are interconnected by soldering, therefore, for its separation, it is necessary to carefully clean the melted places. After opening, clean the conductive parts, inspect the membrane for defects and, in the absence of significant damage, assemble the repaired device in the reverse order. If you decide to replace it, then you need to choose a similar model.

4. Fan power. If the smoke exhauster power deteriorates due to a decrease in the mains voltage, severe turbine fan contamination or bearing wear, the instrument membrane will not be fully attracted to the microswitch. Although there are adjustable screws on the relay with which you can adjust the diaphragm response range, it’s better not to touch them, since you can reset the factory settings and then set the desired position for a long time. Therefore, carefully inspect the condition of the listed elements, possibly a breakdown lies in the boiler exhaust fan. You can find out which components may be faulty in it from the article: “Diagnostics and repair of the boiler fan”.