The joy of being a Heating Engineer

In this article on component testing, we look at the air-pressure switch, how it operates and how it functions as a safety device.

An air-pressure switch (APS) is a safety device. Its function is to prove the flue is clear and the fan is running at the correct speed. The correct volume of air needed for complete combustion may then be drawn into the appliance and the products of combustion expelled from the flue terminal safely.

Are all APSs the same?

The most common types of APS use a diaphragm/electrical switch, and this is the type covered in this article. Alternative types of airflow-proving devices are air-flow sensors and air-pressure sensors.

The term ‘air-pressure switch’ is the common name used by both manufacturers and operatives to describe a diaphragm/electrical switch. Some manufacturers have alternative names, such as ‘fume sensor’ (Ariston) and ‘pressure differential switch’ (Vokera).

Note: The name may change, but the operation remains the same – it’s just a switch!

How it works

A modern APS has two functions: to move to an ‘at rest/no air flow’ position or to an ‘operated/proved air’ position. When the fan operates at its nominal high speed, air movement creates a difference in pressure on the top and bottom of the diaphragm; this, in turn, pushes the diaphragm and an attached pin and lever. When the diaphragm has fully reached its proved air-flow position the pin and lever operate a microswitch. When the fan has stopped, a force (usually a spring or gravity) returns the diaphragm to its ‘at rest/no air flow’ position.

Note: If the APS rests in the ‘at rest/no air’ position and moves to the ‘operated/ proved air’position when the fan runs, there is no fault with the APS.

How does the fan operation create the difference in pressure?

To transfer the movement of air generated by the fan into the switch, most manufacturers use silicon rubber tubes. These tubes usually connect the APS with the fan and/or combustion chamber. To create a pressure differential, the tubes will be connected to, or have fitted inside them, a venturi. A venturi is a restriction that increases the air pressure when air is forced through it.

How can the microswitch be tested for correct operation?

Two-wire switch

An APS connected by two wires will rest with the microswitch in the ‘open/no air-flow’ position. When the boiler fan runs at its nominal high speed, the switch operates and changes the microswitch to the ‘closed/proved air flow’ position.

To test the two-wire switch:

• Turn the power off to the boiler and confirm safe isolation.
• Disconnect both wires from the switch and place in a safe position. Ensure the wires are not touching any other component or earth (this simulates the switch in a ‘no air flow’ position). The use of an insulating mat is recommended.
• Carry out a ‘no air flow’ resistance check on the switch. This requires a multimeter set to the Ohms scale. Expect open circuit across the two terminals (‘open circuit’ indicates the APS is in the correct rest position).
• Turn the power back on and create a demand for heat.
• With the fan running at high speed, carry out a continuity check across the two terminals of the APS.
  Note: Some appliances will give you only a short period of time to prove air flow – ensure the fan is at high speed when carrying out this check.
• You should expect a closed circuit reading, i.e. approximately 1 Ohm resistance. This means the switch has operated and has moved to the ‘proved air flow’ position.
  Note: An open circuit reading will need further investigation, but it does not prove that the APS itself is faulty.
• Turn off the power, confirm safe isolation, and replace the wires onto the correct terminals.

Three-wire switch

An APS connected by three wires will rest with the microswitch giving continuity between the common (C) and normally closed (NC) (‘no air flow’ position).

When the boiler fan runs at its nominal high speed, the microswitch operates and should provide a reading of continuity between the common (C) and normally open (NO) terminals (‘proved air’ position).

To test the three-wire switch:

To safely confirm if the switch is operating correctly the following method may be used:

• Turn off the power to the appliance and confirm safe isolation.
• Note the position (and colours) of the wires and the terminals to which they are connected, i.e. common (C), normally closed (NC), and normally open (NO).
• Disconnect all three wires from the APS terminals and safely join the common (C) and normally closed (NC) wires together. Place all wires in a safe position ensuring the wires are not touching any other component or earth (this simulates the switch in the ‘no air flow’ position). The use of an insulating mat is recommended.
• Carry out a ‘no air flow’ resistance check across the common (C) and normally closed (NC) terminals of the switch. (A reading of continuity, i.e. less than 1 Ohm, proves the switch is in the ‘no air flow’ position.)
• Restore power to the appliance and create a demand for heat.
• With the common (C) and normally closed (NC) wires joined simulating the switch in the ‘no air’ position, the fan should run at its nominal high speed. With the fan running at high speed, carry out a continuity test across the common (C) and
  normally open (NO) terminals (some appliances will give only a short period of time to prove air flow – ensure the fan is at high speed when carrying out this check). A closed circuit reading means the switch has operated and moved to the ‘proved air flow’ position.
• Turn off the power, confirm safe isolation, and replace the wires onto the correct terminals.
  Note: An open circuit reading will need further investigation, but it does not prove the APS is faulty.

The APS fails to operate. Is it faulty?

First, prove that the correct air-pressure differential is acting on the APS. It may be that a restriction or blockage in the appliance prevents the APS from operating. Some checks to consider are:

• blocked or obstructed venturi
• incorrectly fitted venturi
• loose or cracked silicone rubber tubes
• poorly installed or corroded flue seals/ducts
• blocked/restricted heat exchanger causing poor airflow
• slow-running fan
• partially choked fan blades
• incorrectly fitted or positioned air guides or baffles
• incorrect flue lengths
• incorrect or no restrictor rings in flue assembly.

When all of the above factors are deemed satisfactory, the problem may be a split or stretched diaphragm in the APS itself, which would then need to be replaced.

How does the APS fit into the sequence of operation?

The APS has only two functions. When the appliance has no demand the APS must move to the rest position (no air flow). When the fan is running at its nominal high speed the APS must move to its operated position (proved air flow).

No movement to the rest position (no air)

What happens if the APS has not moved to the rest position (no air) when a demand is placed on the boiler?

Two-wire switch

The PCB (printed circuit board) on most modern appliances will send a signal or a voltage to the APS when a demand is placed on the boiler. If the signal/voltage is returned to the PCB from a two-wire switch, the PCB will recognise the switch is not in the open circuit position (‘no air flow’ position).

Three-wire switch

With a three-wire switch, the signal or voltage sent from the PCB must be returned via the correct terminal or the PCB will recognise the APS is not in the ‘no air flow’ position.
Note: If the APS is not in its rest position before a demand for heat, most makes of boiler will not allow the fan to start.

No movement to ‘proved air’

What happens if the APS does not move to the ‘proved air’ position?

When the APS operates and moves to the ‘proved air flow’ position, this is the signal to the PCB that it is safe to start the ignition sequence.

If the APS does not move to the ‘proved air flow’ position, the ignition sequence will not commence. (This may result in lockout situations, or, alternatively, the fan may run at high speed for long periods.)

Some points to note:

• APSs only need 1 or 2 mbars of air pressure to move the diaphragm: do not blow into the switch because damage to the diaphragm may occur.
• Always ensure the silicon rubber tube connecting the APS and the fan venturi is as short as possible.
• NEVER attempt to adjust an APS – it is a non-adjustable safety device!
• A small number of boilers do not check the ‘no air flow’ position on two-wire switches. They will begin an ignition attempt, regardless of an APS being stuck in the ‘proved air flow’ position. This, combined with a seized or slow fan, can result in a main burner baffling out/smothering.
• A fine coating of debris/dust on a tube or venturi can restrict the air pressure sufficiently to make the APS not operate as designed.
• Some APSs ‘push’ (exert positive pressure) on the diaphragm to make the microswitch. Some ‘pull’ (exert negative pressure) on the diaphragm to make the microswitch. The operation depends on the position of the return force (spring or gravity) when the fan is at rest.
• It may be possible to attach a digital manometer to APS tubes to read the pressure exerted by the fan (the expected pressure differential required to operate the APS can often be found on a label attached to the APS).

Frequently Asked Questions

Q) What voltage is at the APS?

A) The voltage can range from 230V AC to a very low DC voltage. Remember, an APS is is a switch, so whatever voltage is put into the switch you should get out of it, providing it operates correctly. Following the tests described above will prove if an APS is operating correctly. You don’t need to know the voltage.

Q) I have checked for voltage at the common terminal of the APS and there is no voltage. Does this mean the PCB is faulty?

A) The APS can operate in two ways: voltage from the PCB can be sent to the common terminal of the APS; or, as is usual in later appliances, the voltage will go from the PCB to the normally closed terminal. With the APS in its rest position this will pass voltage to the common terminal and the fan. When the fan runs, the APS operates, and the common terminal will now have voltage fed from the normally open terminal to keep the fan running.