HeatingForce is reader-supported. When you buy through links on our site, we may earn an affiliate commission. Learn more
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.
Sick of your old and unreliable boiler? Use this 90-second clickable form over at Heatable to get a fixed price on screen. Once you’ve selected the best boiler for you, it takes less than 24 hours to get an install. Prices are the most competitive in the industry, with Worcester Bosch boilers going for as low as £1,785 with flexible financing available.
Table of Contents
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.
What kind of fuel does your boiler use?
Grab your fixed price online by 3PM, and get next-day boiler installation.
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.
The total cost of a replacement switch should be in the region of £175-250.
Need help diagnosing and fixing your heating or prefer a replacement boiler? Get your price online in less than 90 seconds at Heatable.
Thanks for reading our 5-minute guide to boiler air pressure switches. Let us know if we can answer any further questions you may have.