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

Welcome to our 5-minute guide to diagnosing a faulty boiler air pressure switch. Here, we’ll explain what an air pressure switch is, what causes it to malfunction, and what symptoms you may observe if the air pressure switch isn’t working properly.

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 the fan expels the waste gases through the flue before allowing the boiler to fire up. 

The fan creates a draft which allows harmful gases to be released from the boiler via the flue. And for your boiler to fire up, the air pressure switch needs first to recognise that the fan is running. Essentially, the switch gives 1 of 2 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 that simple to diagnose.

For instance, the air pressure switch might not allow the boiler to fire for a legitimate reason, such as:

  • A Blocked Flue
  • Faulty Fan
  • Broken PCB

A rarer example might be the pilot light getting “blown out” during high winds. If this happens, it’s more likely a fan issue caused by a low (and incorrect) speed setting. Since 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.

As you can see, this is definitely not an air pressure switch problem.

That said, on other occasions the air pressure switch will get stuck open or closed, leading to a false signal to the circuit board and an eventual boiler lockout.

Problem 1: Wiring Faults

The switch itself might be in perfect working order. But it’s an electrical component, and if the wiring has degraded, or hasn’t been wired correctly, the pressure switch may develop problems or cease working altogether.

Problem 2: Moisture

Moisture can work its way into the electrical components, including the air pressure switch in a boiler. Older boilers’ electrical parts typically aren’t well-sealed, 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 air pressure switch to stop the boiler from firing up. This includes kinked and broken hoses. A Gas Safe engineer will be able to identify this problem and fix it without much trouble.

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Symptoms of a Faulty Boiler Air Pressure Switch

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

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

#1 – Boiler Cycles On and Off Randomly

If the air pressure sensor has wiring issues, its communication with the PCB (Printed Circuit Board) may be intermittent. This fault will cause the boiler 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 allow the boiler to fire up.

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

#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 an Air Pressure Switch on a Boiler

There’s no “set price” when it comes to the cost of replacing an air pressure switch on a boiler. So, we’ll give you some estimates, but these figures are just that — educated guesses based on historical averages. Also, keep in mind that labour prices differ throughout the UK, and your boiler brand will be a significant factor in the cost of the air pressure switch.

Air Pressure Switch Cost — Parts

Expect the air pressure sensor itself to cost £50, whilst a complete unit with hoses can set you back as much as £100.

Air Pressure Switch Cost — Labour

Installation labour should cost around £75-150. These figures will vary depending on the part of the country you are in and if you need a full unit fitted.

Total Cost of an Air Pressure Switch 

Typically, air pressure switch replacement cost will hover around £175-250. Again, variables such as the region, boiler brand, and scope of work may affect this estimate.

What’s Next?

Thanks for reading our 5-minute guide to boiler air pressure switch problems. Let us know if we can answer any further questions you may have by leaving a comment below.

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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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