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If you're seeing the F75 error code displayed on your Vaillant boiler, it's likely you're now without heating and hot water thanks to the boiler locking out.
This a safety precaution to ensure the boiler isn't operating in an unsafe manner, or, continued operation doesn't damage internal components.
But what does the F75 fault code mean?
When you call for hot water and heating, your boiler's brain (the printed circuit board) expects to receive a signal from the pressure sensor, showing that pressure is rising slightly.
The F75 fault code shows because the boiler isn't detecting a rise in pressure when you fire up your boiler.
This error is common in a range of combi, system and regular boilers including:
As Vaillant owns the Glow-worm boiler brand, they also use the same F75 fault code. Expect to see this issue if you own the likes of the Glow-worm Energy, as well as most of their modern combi, system and regular boilers.
There are multiple culprits that could be causing the F75 error to be displayed on Vaillant and Glow-worm boilers.
The most common ways to clear the F75 fault code include replacing either the pressure sensor or pump.
Replacement costs will vary depending on the boiler model and labour rates in your local area.
However, we’d typically expect replacement costs in the region of:
If your boiler is under warranty, this will be fixed by the manufacturer for free on most occasions. If not, you’ll need to consider whether it’s worth investing cash fixing an old, inefficient and unreliable boiler.
To compare repair vs boiler replacement costs, jump over to our boiler quote form here. There, you’ll be able to get the likes of a 94% efficient (A-rated) Ideal Logic Plus with a 7-year warranty starting at just over £2,200.
Alternatively, jump over to our new boiler costs guide here.
All modern boilers use a pressure sensor. This confirms circulation, as well as letting the printed circuit board know if there is an issue leading to high pressure (this isn't healthy).
When a boiler starts, the pressure should rise thanks to the heat that begins to circulate. This is why when you check your boiler pressure, it will differ depending on whether it's hot or cold.
The issue on your Vaillant boiler causing the F75 fault code to be displayed could simply be a faulty sensor.
Pressure sensors can easily get blocked with heating sludge over time. This is a build-up of rust and limescale within your heating system.
Alongside this, the internal diaphragm can fail, or, wiring and connections become weak.
Put simply, your boiler could be operating perfectly. But if the pressure sensor gives a false reading (or none at all) to your boiler's printed circuit board, it will lock out and display the F75 fault code.
Pump faults are fairly common, as they deteriorate with age. And with extremely old Vaillant boilers, they use a sub-par pump. Newer models use a high quality Grundfos pump, whereas older ones use a Wilo pump that are much more unreliable.
If your pump isn't firing up when you're calling for hot water and central heating, there are a few potential culprits.
First, the rotor powering the pump could be seized. But, more typically, the pump is blocked with heating sludge.
When radiators and pipework are blocked with sludge, it's easy to get rid of with a chemical flush. Whilst this may work, generally heating sludge has already caused major damage.
If your pump seems to be working, yet the F75 error code is being displayed, another likely cause is an airlock.
Airlocks typically occur after heating maintenance. For instance, if you've topped up boiler pressure (by adding water) or drained down the system and refilled it, air has likely been introduced to the system.
Most modern heating pumps have a bleed valve for this reason. A Gas Safe engineer will be able to bleed the pump and reset your boiler to see if that fixes the issue.
If your boiler is suffering from a blockage or circulation issue you'd expect to see the following:
The simplest blockage comes via air. Above we've mentioned how this can build up in the pump, but it's actually more common to build up in radiators and pipework (radiators will be cold at the top).
You can fix this easily by using a radiator bleed key and slowly bleeding every radiator and towel rail in your property.
If this doesn't fix the issue, it's likely sludge is causing the issue. This could be blocking the heat exchanger, return pipework and even the isolation valves. In this case, the radiators would be cold at the bottom.
If the below sounds like your system, this issue is likely to be due to heating sludge and limescale build-up over years:
The F75 fault code isn't necessarily a "low pressure" warning. However, your central heating system will need a reasonable amount of pressure to begin with, in order for the pressure sensor to then notice a measurable increase on start-up.
Put simply, if your boiler sits below 1 bar, there's a chance that low pressure is causing the fault code to be displayed.
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Gas Safe engineers are the only people qualified to work on gas boilers. If a fix relates to working on your boiler, please contact a qualified professional.
The first step is to check system pressure. Typically, this should be anywhere from 1-1.5 bar when cold and up to 2 bar when hot.
If the system is below 1 bar, this may be causing the F75 fault code to be displayed; top up the boiler’s pressure.
Your engineer will need to bleed radiators and the pump. If excessive air is released, the system pressure may drop below 1 bar, and need to be topped if so.
The bleeding process will need to be repeated, as introducing water into the system, also introduces additional air.
Run a hot tap, or call for central heating via the thermostat.
Your engineer will be checking to see if the pump kicks into the action. For new (almost silent) pumps, leaning a screwdriver on the pump is a good method to feel vibrations.
If there's no action from the pump, it's time to use a multi-meter and check for current draw. Typically this should be 0.3-0.6A on modern heating pumps.
On the other hand if the pump is seized, rotating the spindle or tapping the housing can sometimes free it.
In some cases, the pump may have failed and a replacement will be needed.
If the F75 fault code is being displayed, there are ways to narrow it down to the pressure sensor.
Typically if the pressure sensor has failed you'll notice:
Check the pressure sensor with a multimeter and replace if necessary.
If your engineer suspects a blockage thanks to heating sludge they will:
Once done, they can reset the boiler which should clear the F75 fault code.
Prefer to get a new boiler rather than have your old one repaired?
Use our quote form for new boilers here and get boilers with 7-12 years warranty from £2,200.
Try our free quote tool. You'll have your personalised quote in under a minute.
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