Please also read this page: General information regarding air pressure switches
What is an Boiler Air Pressure Switch?
A Boiler Air Pressure Switch ensures that the waste gases from the boiler are expelled properly.
It is a crucial part of the fire up sequence for all boilers and is a very important area to check when a boiler fails in the boiler start up sequence.
It’s very important to note that if the FAN is not working correctly or begins to fail, the differential pressure will be affected, this pressure will be picked up by the Air Pressure Switch hoses connected to the Air Pressure Switch. The switch will then (normally) activate a NC position (Normally Closed). The Main Gas Valve will close, which will shut down the boiler. Therefore if the fan is not working correctly, the system will not ignite via the air pressure switch and the following tests are valid.
This happens before the boiler ignites and fires up. Referring to the sequence of boiler operation page you’ll find that the boiler, once it has been activated and the circulator pump runs, will activate the fan. When the fan runs the air pressure switch is an activated.
Therefore the pump is first activated from the circuit board and the fan runs. The system then does an air pressure switch check. If the air pressure switch fails then the system will not ignite. (See general information regarding air pressure switches for reasons why the air pressure switch may not be working). If all is ok, the ignition continues and the fan energises.
An Air Pressure Switch can be faulty by being ‘stuck’ open or closed, which would not necessarily stop the boiler from starting. ANY problem with air flow and exhausting fumes within the boiler can cause the boiler to not start correctly. Check the Boiler Fault Codes page for information on boiler fault error codes.
The boiler fan must expel flue gases. If the fan is running at a slow speed, it won’t react to the back-pressure created in the flue system. The boiler air pressure switch recognises the pressure difference, and closes down the boiler. In this case the fault is with the boiler fan, an is not an air pressure switch issue.
The fan will create a draught which allows the harmful gases to be released from the boiler via the flue. In order for the boiler to fire up the air pressure switch needs to have recognised that the fan is actually running. The air pressure switch will indicate two signals to the circuit board:
- ON if the fan is operational therefore the boiler is safe to ignite or
- OFF the fan is not operational and therefore the boiler is not safe to ignite.
Summary from the ‘sequence of boiler operation’ that affects the APS
Boiler Switches on…
- Pump first from circuit board (check power to the pump) – fan runs…
- System does Air pressure switch check (APS)
There can be 3 electrical tabs on the APS
- C (common)
- NO (normally open)
- NC (normally closed)
These days, there are normally just two. C and NC (normally closed).
The pressure switch MUST CLOSE in order to prove that exhausts are being vented.
C and NC check, if there’s no continuity – It’s faulty and the boiler will not ignite.
- If all ok – Ignition continues – The Fan energises
In order to test the switch you will need to use a multi-meter. We need to measure the continuity of the switch when it opens and closes.
The boiler air pressure switch needs to ‘close’ when the diaphragm is pulled in, which we do by removing the pressure switch wires, turning on the boiler and using the multi-meter to check for resistance (put the multi-meter on continuity to have a tone emitted from the multi-meter to help test).
To test a two wire air pressure switch (no demand)
NOTE: This test checks the actual boiler air pressure switch mechanism and not the electrics outside the switch. For this reason we are checking the pressure is ‘switching’ the resistance on the switch between the 2 terminals of the APS via the pressure. The pressure tube is still connected but not the electric terminals to the switch.
- Electrically isolate boiler / system controls
- Disconnect both wires from the switch terminals
- Ensure both wires are resting safely and not touching any other component or earth – a well insulated heat mat is good for this
- Set multi-meter on ohms scale for resistance readings, across switch terminals (black test lead to C and red test lead to NC)
TEST
- Open circuit (infinity) across both terminals = ok (switch in rest / no air flow position)
To test a two wire air pressure switch (on a demand)
240 V LIVE TEST
- Ensure both wires are still resting safely and not touching any other component or earth
- Restore power and create a demand for heat
- Set multi-meter on ohms scale for resistance readings, across switch terminals (black test lead to C and red test lead to NC)
- With fan running (at high speed) check continuity/resistance readings across both switch terminals.
Note: Some appliances will give you very little time to check a proved air flow – ensure the fan is at high speed for this check.
TEST
- Continuity = ok (switch in proved air flow position)
- Closed circuit of less than 1 ohm = ok (switch in proved air flow position)
Should an open circuit (infinity) reading be present, it does not prove a fault with the switch itself, but could be fan, venturi, blockages, hose problems etc needs – further investigation. See also: Air Pressure Switches (General) page
Electrically isolate boiler – Replace both wires back onto their respective terminals – Restore power once more
To test a three wire air pressure switch (no demand)
- Electrically isolate boiler / system controls
- Disconnect all three wires from the switch terminals (note positions and colours respectively)
- Ensure all three wires are resting safely and not touching any other component or earth (a well insulated heat mat is good for this)
- Join safely both the C and NC wires together (two spade connectors joined together are good for this)
- Set multimeter on ohms scale for resistance readings, across switch terminals (black test lead to C and red test lead to NC)
TEST
- Closed circuit of less than 1 ohm = ok (switch in rest / no air flow position)
- Continuity = ok (switch in rest / no air flow position.
To test a three wire air pressure switch (on a demand)
240 V LIVE TEST
- Ensure all wires are still resting safely and not touching any other component or earth
- With the C and NC wires still touching (simulating rest / no air flow position), the fan should run at its normal high speed
- Restore power and create a demand for heat
- Set multi-meter on ohms scale for resistance readings, across switch terminals (black test lead to C and red test lead to NO)
- With fan running (at high speed) check continuity/resistance readings across both switch terminals.
Note: Some appliances will give you very little time to check a proved air flow – ensure the fan is at high speed for this check.
TEST
- Continuity = ok (switch in proved air flow position)
- Closed circuit of less than 1 ohm = ok (switch in proved air flow position)
Notes
Should an open circuit (infinity) reading be present, it does not prove a fault with the air pressure switch itself, but could be fan, venturi, blockages, hose problems etc. (needs further investigation). Electrically isolate boiler replace both wires back onto their respective terminals and restore power once more.
Where an air pressure switch is not present, purposely designed thermistors are used to measure the differential temperatures of the flue/air ducts, which are then transmitted to the printed circuit board for correct operation.
Some boilers use a low DC voltage, whereas others use a 240V AC voltage. (Remember – the input will appear as an output, unless a fault is present).
Some air pressure switches use a push (positive) pressure on the diaphragm to make the micro switch, whereas others will use a pull (negative) pressure on the diaphragm to make the micro switch. The operation depends on the position of the return force (spring or gravity) when the fan is at rest.
A continuity check of the wiring harness at the printed circuit board end will ascertain correct operation of switch and the integrity of wires also, where it is difficult to gain access.
Other resources:
- Air pressure switches general information