Vaillant F75 fault code: why the pump check failed

What F75 means on a Vaillant boiler

Vaillant UK's fault code page words F75 as "The pressure change is too low when the heat pump is started up". The causes it lists on the same page include "Internal heating pump defective", so read "heat pump" there as the pump inside your boiler. F75 has nothing to do with air source or ground source heat pumps.

The code is about circulation. The boiler's pump pushes water round your radiators, and each time it starts, the boiler checks that the push registered on its water pressure sensor. F75 means that check failed, and the boiler will not heat until it passes. On a combi that means no hot water either.

Why topping up did not clear F75

F75 is not a low-pressure reading. It is a failed test: when the pump starts, the pressure at the sensor should jump, and Vaillant's installation manual describes F75 as that jump being too small. The gauge shows how much pressure is in the system, not whether the pump moved it.

Low water is only one way to fail the test. The other causes on Vaillant UK's fault code page are "Air in the product/heating system", "Water pressure sensor defective", "Internal heating pump defective" and "Open circuit in cable harness", and the installation manual adds a blocked pump. A pump that is not moving water, or a sensor that does not register the rise, produces the same code with a perfectly normal gauge reading.

So a top-up fixes F75 only when the pressure really was low. If yours was, and the code has gone, the next question is where the water went.

Our Vaillant F22 low water pressure guide covers pressure that keeps dropping. If your display has also shown F73, the Vaillant F73 fault code guide covers the sensor-signal side.

The only checks to make yourself

F75 is not one of the codes in the homeowner fault table of Vaillant's ecoTEC plus ..CS/1-5 operating manual (0020308119_03, appendix C). That table lists only F.022 and F.028, and says any other code needs action by a competent person. Vaillant UK's fault code page allows one exception: when the pressure is below 1 bar, topping up can be done without an engineer "if you are competent to do so".

1. Note the code, your boiler model from the data plate, and any recent work on your heating, such as new radiators.

2. Read the pressure gauge. If it shows below 1 bar, top up slowly through the filling loop to between 1 and 1.5 bar, the range on Vaillant UK's fault code page, then close the loop fully. On ecoTEC plus ..6/5-5 models the reading flashes until it passes 1.1 bar (installation manual 0020244997_02, section 7.9), so carry on to just above that. Not sure where the filling loop is? Our Vaillant boiler pressure page shows where to find it. If you have never used the filling loop, leave this step to the engineer.

3. If the gauge already read 1 bar or more, add no water. Vaillant UK's fault code page calls F75 above 1 bar "a case for a Vaillant engineer" and gives no advice for a reading of exactly 1 bar, so handle 1 bar the same way.

4. Reset once. On ecoTEC plus ..CS/1-5 models, operating manual 0020308119_03 (section 3.1) says to press and hold reset for more than three seconds; on other models follow your own manual.

5. If F75 comes back, leave it there. Do not keep resetting, and do not try to free, tap or bleed the pump. Book a Gas Safe registered engineer.

F22, F73, F74 and F75 compared

All four codes involve the water pressure sensor or the water in the system, which is why they get mixed up. The labels in the table are quoted from the fault table in Vaillant's ecoTEC plus installation manual (0020244997_02).

What the engineer will check

For F.75, Vaillant's installation manual (0020244997_02) gives engineers this measure: "Check: Water pressure sensor, heating pump (blockage), heating circuit (air, sufficient water volume), adjustable bypass, external expansion vessel (must be connected at the return). Activate check programme P.0." In plain words, none of it a job to try yourself:

• The water pressure sensor: does it register the rise when the pump starts?
• The heating pump: does it run, and is anything blocking it?
• The heating circuit: is there air in it, and enough water?
• The adjustable bypass valve in the boiler, and how it is set.
• Any external expansion vessel: the manual says it must be connected at the return.
• Systems with a low loss header, or heating pipes wider than 1 1/2 inch: the manual tells the engineer to replace a 3/4-inch seal in the heating flow with a screen and, if required, fit Vaillant's "F.75 service set".
• Then Vaillant's check programme P.0, run from the boiler's own controls.

F75 after new radiators or other heating work

Look at that list again. An external expansion vessel, a low loss header and larger pipework all change when a heating system is altered, and the bypass valve inside the boiler may need resetting to suit the new system. None of these is a part wearing out.

If F75 started after new radiators, underfloor heating, a low loss header or other pipework changes, say so when you book. It points the engineer at the system side first, and if the work was recent, the installer who did it is the first person to call.

Whatever the cause turns out to be, ask the engineer which of these checks failed before any part is replaced.

Staying safe, and who to call

F75 is a circulation fault, not a gas fault, and the boiler stops rather than run without the circulation it expects. Keep the casing on: Vaillant UK's fault code page warns that "opening the boiler casing can be dangerous". Do not touch the bypass, the expansion vessel or the pressure relief valve, and do not let water out of the system.

If you smell gas at any point, call the National Gas Emergency Service on 0800 111 999 before anything else, and do not reset the boiler.

For the repair, Vaillant's fault code page gives a number for booking a Vaillant engineer, or you can use any Gas Safe registered engineer. The Gas Safe Register website lets you confirm an engineer's registration before work starts.

Can a Smart Plan cover plan help with F75?

Not with an F75 you have today. Smart Plan is a service plan, not insurance, from UK Boiler Company Ltd, and it is for breakdowns that happen after you join. A fault that was already there before the plan started counts as a defect once it is identified, and the terms exclude repairing defects (Definition 14 and general exclusion 10.13).

For a future F75, a repair to the pump or the pressure sensor would fall under the boiler cover limit in clause 2.1(d): £500 when the boiler is under 7 years old and £200 when it is over 7 years old, inclusive of parts and labour (2.1(d)), for each 12-month period from the start of your agreement (Definition 12). Every repair is still subject to the exclusions in the terms. Several bear directly on F75: works on low loss headers or underfloor heating are excluded from boiler cover (2.2(c)), and so are works on equipment not installed to regulation or manufacturer instructions (10.8), works caused by sludge, scale deposits or corrosion (10.22), and works caused by newly installed equipment, which the terms say includes pipe leaks from newly sealing or pressurising a system (10.9).

A visit that only needs the system repressurising or the boiler resetting is excluded (10.2 and 10.1). In that case a £95 call-out fee may apply, only in the situations set out in the terms (9.2).

If an engineer finds the PCB or the heat exchanger has failed, the terms treat the boiler as beyond economic repair (2.1(c)). Clause 9.5 then sets out a contribution payment, the lower of the boiler's retail value including installation or your cover limit less any work already done, rather than a repair.

Let and rented homes are eligible, but the plan must be held by whoever owns the boiler, usually the landlord, because works on equipment you do not own are excluded (10.17). Call-outs run Monday to Friday, 08:00 to 18:00, unless it's an emergency breakdown.

To compare Vaillant's own repair routes with a cover plan, read Vaillant boiler cover and repair costs.

Talk to us about cover for next time

An F75 already on your display cannot be added to a new plan. If you want the next breakdown covered, request a free call back and we will explain how the plan works for a Vaillant boiler. Smart Plan is a service plan, not insurance.

Helpful links

• All Vaillant fault codes
• Vaillant F22 low water pressure
• Vaillant boiler pressure
• Vaillant F73 water pressure sensor fault
• Vaillant F74 pressure sensor fault
• Vaillant boiler cover and repair costs
• Boiler & central heating cover
• Vaillant's official boiler fault codes
• Vaillant ecoTEC plus operating instructions (0020308119_03, PDF)
• Check an engineer on the Gas Safe Register