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Diagnosing A Faulty Boiler Air Pressure Switch [And What It Costs To Fix]

Welcome to our 5-minute to diagnosing a faulty boiler air pressure switch when a boiler keeps gaining pressure.

We’ve split our guide into sections:

  • What is An Air Pressure Switch?
  • Reasons A Boiler’s Air Pressure Switch Will Fail
  • Symptoms Of A Faulty Boiler Air Pressure Switch
  • Cost Of Replacement

Before going any further, it’s important to understand this is a job for a Gas Safe engineer. It is not safe (or legal) to open up a boiler and work on parts within the combustion chamber, without Gas Safe accreditation.

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What Is An Air Pressure Switch?

The air pressure switch in a boiler ensures that waste gases are being expelled, before allowing to fire up. 

The fan creates a draft which allows harmful gases to be released from the boiler via the flue.

For your boiler to fire up, the pressure switch needs to have recognised that the fan is running first.

The switch basically gives one of two signals to your circuit board:

  • Yes, the fan is operational – boiler is safe to ignite
  • No, the fan is not operational – boiler is not safe to ignite

Reasons A Boiler’s Air Pressure Switch Will Fail

Given there’s more than one part to the air pressure switch puzzle, it’s not simple to diagnose.

For instance, the air pressure switch might not be allowing the boiler to fire, but it could be just doing its job, rather than being faulty. On other occasions, it will get stuck open or closed.

It’s important to understand that any air flow issue will cause a air pressure switch to not allow the boiler to fire. But, that doesn’t mean the switch itself is to blame.

For instance, any of the following could cause airflow not within working range, so a boiler won’t fire:

  • A Blocked Flue
  • Faulty Fan
  • Broken PCB

A typical example might be a boiler whereby the pilot light is “blown out” during high winds.

It’s very rare for wind to actually blow out the pilot light.

If it does, it’s more likely a fan issue, and most likely, a low (and incorrect) speed setting.

The fan must expel flue gases. If it’s operating at a low speed, it won’t counteract the tiny back-draught in the flue.

The air pressure switch notices the pressure differential isn’t great enough, and shuts off the boiler. This is a fan problem, not an air pressure switch problem.

Problem 1: Wiring Faults

The switch itself might be in perfect working order. But, it’s an electrical component.

If the wiring has degraded, or hasn’t been wired correctly, this will cause the pressure switch to develop problems, and potentially not work at all.

Problem 2: Moisture

Moisture can work its way into the electrical components, including the boiler’s pressure switch.

New boilers have much better sealed electrical units than older ones. So, if you have an older boiler, there’s a chance condensation has worked its way into the switch or rubber hoses, and caused the fault.

Problem 3: Broken Or Kinked Hoses

Anything that messes with air flow and pressure, will cause the switch to tell the boiler it’s not safe to fire up. That includes kinked and broken hoses.

Symptoms Of A Faulty Boiler Air Pressure Switch

These are a few symptoms of a faulty air pressure switch in a boiler.

However, similar symptoms can occur when other parts are faulty, so it’s important to run a diagnostic. Otherwise, you’ll be replacing expensive boiler parts, only to find the same problem still occurs.

#1 – Boiler Cycles On And Off Randomly

If the air pressure sensor has wiring issues, it’s likely its communication with the PCB will be intermittent. This will cause it to cycle on and off sporadically.

#2 – Fan Works, But Boiler Won’t Fire Up

If the fan is definitely running, in theory the air pressure switch should communicate with the PCB and fire up.

But, that’s only the case if there isn’t another air flow issue. For instance, a working fan, but a blocked flue would cause the pressure switch to remain in the closed position.

#3 – Boiler Displaying Specific Fault Codes

Some boiler manufacturers have specific fault codes for an air pressure switch failure.

As an example, the Ariston E34 fault code signifies that the problem is the pressure sensor.

Refer to your owner’s manual and find out if the code relates to an air fault.

Cost To Replace A Boiler’s Air Pressure Switch

There’s no “set price” when it comes to the cost of replacing a boiler’s air pressure switch.

The cost of replacement mentioned below is an estimate. Prices will differ for labour throughout the UK, and the specific boiler you have will determine the exact price of parts.

Cost Of Parts

Expect the air pressure sensor itself to cost £50. A complete unit with hoses can be up to £100.

Cost Of Labour

Expect labour to cost £75-150 depending on the part of the country you are in, and if you need a full unit fitted.

Total Cost

The total cost of a replacement switch should be in the region of £175-250.

What’s Next?

Need help diagnosing and fixing your heating or prefer a replacement boiler? Get your price online in less than 90 seconds here.

Thanks for reading our 5-minute guide to boiler air pressure switches.

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  1. Hiya I have a heatline s24. Pressure switch keeps coming on and off so hot water is doing the same. Flue is clear. Had new pressure switch fitted and still doing the same. The fan is working but do they get slower with age and do you think this is a option. Any thoughts would be a great help . Thanks john.

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