What F75 actually means
Vaillant's own engineer documentation records F75 as "Fault: Pump/water shortage", with the cause given as "Insufficient pressure jump detected when starting the pump".
That wording is worth reading twice, because it is not the same as "low pressure". The boiler is not complaining that the gauge is low. It is complaining that when it switched the pump on, the water pressure sensor did not register the small rise in pressure that a pump pushing against a full circuit always produces. The boiler cannot tell the difference between a pump that did not push, a sensor that did not report, and a circuit with nothing in it to push. So it stops, and shows you one code for all three.
This matters for you because two of those three causes are boiler repairs, and the third is a leak that is quietly emptying your heating system into a floor, a wall or a ceiling void. Repeatedly resetting the code does not tell you which one you have. Nor does topping the pressure up.
Rule out the two non-leak causes first
Vaillant's fault table sends the engineer to check, in order: the water pressure sensor, the heating pump for blockage, the heating circuit for air and sufficient water volume, the adjustable bypass, and any external expansion vessel, which must be connected on the return. Those are all boiler-side checks, and they should all happen before anyone lifts a floorboard.
The water pressure sensor
The sensor is a small transducer on the hydraulic block. When it drifts or fails, the boiler sees a flat reading whatever the pump does, and throws F75 with a system that is perfectly full. The tell is that the pressure shown on the display is stable, credible and unchanged over days, and there is no wet anything, anywhere. A sensor is a part swap and takes an hour. It is not our work and we will say so.
The pump
Pumps seize after a summer standing still, and they clog with magnetite in an old system that has never had a filter. A pump that spins but cannot move sludge produces exactly the missing pressure jump the code describes. Symptoms worth noting before you call anyone: the boiler fires and the radiators stay cold, there is a low hum from the boiler rather than the usual circulation noise, or the fault appears on the first cold morning after the heating has been off all summer.
Air in the circuit
Air behaves like a shortage of water because air compresses and water does not. A system refilled after a radiator was changed, or one with a high towel rail that was never bled, can throw F75 with the pressure sitting perfectly in the band. Bleed every radiator and the towel rails, top back to the correct pressure, and see whether the fault returns.
How to tell a genuine water loss apart
Here is the test that separates the sensor and pump faults from the leak, and it costs nothing.
- Top the system to 1.2 bar cold using the filling loop, with the boiler off and cool.
- Write the reading and the date down. Not a mental note. On paper, or a photo of the gauge with the timestamp.
- Do not touch the filling loop for a week and read the gauge cold each morning, before the heating fires.
If the cold reading is still 1.2 bar seven days later and F75 keeps returning, the system is not losing water and this is a boiler repair. If the cold reading has fallen, the water is going somewhere, and every reset you have done has been putting fresh mains water into the pipework to replace it.
One pattern that looks like a leak and is not: pressure that climbs high when the heating runs and drops below normal when it cools. That is usually an expansion vessel that has lost its air charge, so the system has nowhere to put the expansion. Vaillant's maintenance schedule has the engineer measure the pre-charge at the vessel valve and top it up if it is below 0.75 bar. That is a recharge or a replacement vessel, not a detection job, and it should be ruled out before anyone opens a floor.
Where the water actually goes
A sealed heating circuit is closed. If it is losing water, there is a hole, and there are only so many places a hole can be. In rough order of how often we find them:
| Where | What it looks like |
|---|
| Radiator valve spindles and tail nuts | A white or green crust at the nut, or a faint rust bloom on the pipe below. Often dries between visits. |
| Towel rails | Weeps at the blanking plug or the element boss. Frequently missed because the water evaporates off a warm rail. |
| Pinhole in 15mm copper under screed | Nothing at all, for months. The screed absorbs it and the heating dries it. The only symptom is the gauge. |
| Underfloor heating circuit | A warm stripe or a warm patch underfoot away from a manifold, or one zone that never gets to temperature. |
| Inside the boiler case | Staining under the boiler, or corrosion on the hydraulic block. A Gas Safe engineer's job, not ours. |
| Pressure relief valve | The discharge pipe outside drips after the boiler fires. Not a leak in the pipework; a failed valve or a flat expansion vessel. |
The reason a slow loss deserves attention rather than a weekly top-up: each refill introduces oxygenated mains water into a circuit designed to hold the same deaerated water for years. That accelerates internal corrosion, which makes more pinholes, which means more topping up. Meanwhile the water that left is sitting in a joist end, a screed bed or a ceiling void.
What a detection visit does
The circuit is isolated and pressure-tested first, which confirms the loss is real and gives a rate. That single step regularly ends the job, because a circuit that holds pressure for an hour on a gauge is not leaking and the fault is at the boiler. Where the loss is real, the run is traced with thermal imaging, acoustic listening and moisture mapping, and on a drained circuit with tracer gas, which is non-invasive and finds what thermal cannot under thick screed.
The point is to open one place rather than search blind. Once the leak is located we repair the pipework and put the floor, plaster or ceiling back, which is the half of the job most detection firms leave with you. Our rate is £150 per hour, agreed before we attend, and if we do not find the leak you do not pay for the time we spent looking. The rate is fixed; the total depends on the hours, so we will tell you where we are as we go.
When not to call us
- The pressure holds steady cold for a week and F75 still returns. That is a sensor, pump or bypass fault inside the boiler. Call a Gas Safe registered engineer or Vaillant.
- The fault appeared immediately after a boiler service or a radiator swap. Air is the likely cause. Bleed, repressurise, retest.
- The boiler is under a Vaillant warranty. Use it. Opening the case yourself or letting anyone unregistered near it can void it.
- There is a visible drip at a radiator valve you can reach. Tighten the nut a quarter turn with two spanners and watch it. If it stops, you have fixed it.
Come to us when the pressure is genuinely falling and nothing is wet where you can see it. More on the pattern behind all of this on the boiler and heating pressure hub, and on the repair itself under boiler and heating leak repair.
