Cleaning the vacuum and draught switch of an ABC Combo boiler

What is the vacuum or draught switch used for?

The vacuum or draught switch of an ABC Combo wood and pellet boiler monitors whether sufficient negative pressure and flue draught are present during operation. It works with the flue gas path and flue gas fan to help verify that combustion gases are discharged safely.

If the vacuum hose, connection nipple or draught switch becomes contaminated, the pressure reading may be affected. The boiler may then receive an incorrect signal, start unreliably or stop during operation. Regular inspection and cleaning support stable performance.

Why is cleaning the draught switch important?

Dust, ash and fine combustion particles can build up inside the vacuum hose, connections or pressure measuring points. Even a small blockage may prevent the pressure difference from reaching the switch correctly.

A clean draught switch and clear vacuum connection support reliable flue gas monitoring. Proper maintenance helps extend boiler service life and supports safe flue discharge and optimal operation.

When should the vacuum or draught switch be checked?

Inspection is recommended during periodic maintenance and when the boiler does not start correctly, stops unexpectedly or shows a message related to draught or negative pressure. Such a message does not automatically mean the switch is defective; contamination or another flue gas issue may produce the same symptoms.

• The boiler does not start or interrupts the start-up phase.
• The burner stops unexpectedly during operation.
• The vacuum hose contains dust, ash or condensation.
• The hose connection or measuring nipple is partly blocked.
• The flue gas fan runs, but the switch does not respond correctly.
• There are signs of insufficient draught or contamination in the flue gas path.

How do you begin the work safely?

Switch off the ABC Combo boiler fully according to the manufacturer’s instructions. Allow the shutdown cycle to finish and wait until the boiler, flue gas components and burner area have cooled completely. Disconnect the electrical supply before opening or removing technical components.

The draught switch is connected to electrical wiring and a pressure sensing circuit. Removal, electrical testing or replacement must therefore be carried out by a qualified technician. Never work on the boiler while it is energised.

Which parts around the draught switch should be checked?

Do not inspect only the switch itself. Check the complete connection between the flue gas path and pressure switch. A blocked hose or nipple can cause the same symptoms as a defective switch.

• Check the vacuum or pressure hose for blockage, cracks and kinks.
• Inspect connection nipples and measuring points for ash and dust.
• Look for condensation or moisture inside the hose.
• Check electrical plugs and connections for secure contact.
• Inspect the flue gas fan and flue gas path for contamination.
• Check that the hose is fitted to the correct connection.

How do you clean the vacuum hose and connections?

Carefully disconnect the vacuum hose after the boiler has cooled and the power has been isolated. Inspect the hose for ash, dust, condensation, cracks or deformation. Clean it only with a suitable method that does not damage or stretch the inner surface.

Also inspect the connection nipples on the switch and flue pressure measuring point. Carefully remove loose contamination without damaging or further blocking the opening. Do not use aggressive cleaning agents or blow debris towards sensitive components.

Can you blow into the draught switch?

A draught switch contains a sensitive diaphragm and switching mechanism. Blowing hard, using compressed air or opening the component mechanically may damage it. Focus on cleaning the hose, nipples and external connections according to the technical documentation.

If the switch is internally contaminated, damaged or unreliable, it must be tested and, where necessary, replaced by a qualified technician. Do not force the switching mechanism.

Which other causes can affect flue draught?

A vacuum measurement issue is not always caused by the draught switch. Dirty flue gas passages, a blocked outlet, a poorly operating flue gas fan or insufficient combustion air can also affect the measured negative pressure.

• Dirty heat exchangers or flue gas passages.
• Ash or soot inside the flue connection.
• A blocked or incorrectly designed flue outlet.
• A dirty or defective flue gas fan.
• Insufficient combustion air in the boiler room.
• Leaking doors, seals or inspection covers.

Which checks are required after cleaning?

Reconnect the vacuum hose to the correct ports and check that it is not kinked, loose or damaged. Also verify that the electrical plugs are secure and all removed covers have been installed correctly.

Then test the complete start-up cycle of the ABC Combo boiler. Check the flue gas fan, response of the draught switch, ignition and any messages on the control panel. Technical functional measurements must be carried out by a qualified technician.

How does regular maintenance extend service life?

Keeping the combustion chamber, heat exchangers, flue gas passages, flue gas fan and pressure sensing connection clean helps the flue gas system operate more reliably. The fan encounters less resistance and the controller receives a more accurate pressure signal.

Cleaning the vacuum and draught switch should therefore be combined with a wider boiler inspection. Cleaning only the switch is insufficient if the flue gas path or combustion air supply is dirty or blocked.

Watch the instruction video

In this video, we show step by step how to inspect and clean the vacuum and draught switch of an ABC Combo boiler. Use the video as a practical addition to the manual and technical documentation for the specific unit.