Vaillant F.788 fault codewhat it means and how to fix it
The building circuit pump has detected a fault of its own and locked itself off. Vaillant’s first remedy is a thirty-second power-off, and the causes include dry running and a blocked filter.
Take care
What F.788 means
Modern heat pump circulation pumps are electronically controlled, and F.788 is the pump’s own electronics reporting trouble. Vaillant’s appendix describes it precisely: the electronics system of the high efficiency pump has detected a fault — they give dry running, blockage, overvoltage and undervoltage as examples — and has switched off and locked the pump. The heat pump does not give up straight away. The status code that precedes the fault, S.277, explains that when the pump is inactive the heat pump switches off for ten minutes and restarts, and only after three unsuccessful restarts does it stop and display F.788. So by the time you see it, the pump has refused three times.
Vaillant’s remedy list starts with something an owner can legitimately do once: switch the heat pump off for at least 30 seconds, with no current, which clears the lock in the pump electronics. The rest of the list is the installer’s — check the quality of the plug contact on the PCB, check that the pump functions, purge the building circuit, and check that the dirt filter is permeable.
Vaillant UK’s owner page adds the electrical connection and wiring harness to the pump, a faulty pump, air in the building circuit and a blocked filter or Y strainer. Dry running is the cause that catches newly filled or recently drained systems: a pump that starts with air in it instead of water overheats its own bearings and locks out to protect itself, and the fix is a purge rather than a pump.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Pump ran dry or with air in it | Vaillant’s first example. Common after a drain-down, a refill or radiator work, when air is sitting in the pump housing. | Common |
| Filter or strainer blocked | Named by Vaillant in both documents. A pump working against a blocked filter trips its own protection. | Common |
| Supply voltage outside range | Vaillant give overvoltage and undervoltage as triggers. A poor supply or a loose connection at the PCB plug produces a pump fault that is really an electrical one. | Occasional |
| Pump failing | The component end. A pump that locks repeatedly with a clean filter and a purged circuit is wearing out. | Occasional |
What to try yourself
Switch the heat pump off at its isolator for at least 30 seconds, then back on. This is Vaillant’s own first remedy and it clears the lock in the pump electronics.
Do this once. If F.788 comes back, the pump is locking for a reason and repeating the power cycle only postpones finding it.Read the system pressure and think about recent work. A drain-down or radiator job in the last few weeks makes air the likely cause.
Ask your installer when the filter was last cleaned. A pump straining against a blocked filter is Vaillant’s other named cause.
If the code returns, book the installer and tell them the pump has locked out. Purging, checking the PCB plug and testing the pump are their steps.
Get an F-Gas certified heat pump engineer to look at your Vaillant F.788
We do not pass enquiries to a repair firm, so there is no form here. When you book an F-Gas certified heat pump engineer, tell them the make, the model and the exact code or message on the display, and ask whether they carry parts for this make — that is what turns one visit into a repair rather than a quote.
What it costs to fix
Typical repair cost
£90–£450
Time on site
15 minutes
Nothing if the power cycle clears it. An installer visit to purge and clean the filter is typically £90 to £180; a replacement building circuit pump is £250 to £450 fitted.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Central heating circulating pump
CH-PUMP-15-60£90–£180Plumbing merchant
Price checked 28 September 2026
Magnetic system filter, 22mm
MAG-22£65–£120Plumbing merchant
Price checked 28 September 2026
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Related Vaillant fault codes
- F.22Get it checkedWater pressure too low. Vaillant publish exact thresholds and two instructions: the indoor unit manual says inform your competent person; Vaillant UK’s fault page lets a competent owner top up to 1 to 1.5 bar.Related: water flow
- F.532Get it checkedBuilding circuit flow rate too low, on the earlier aroTHERM monobloc. Vaillant list pressure, blockages, a dirty filter, balancing and air as causes, and send this one to your installer first rather than to Vaillant.Related: water flow
- F.820Get it checkedConnection fault on the building circuit pump: the pump is not reporting any signal back to the heat pump. Vaillant’s remedy is the cable first and the pump second.Related: water flow
- Vapour from the outdoor unitUsually simple to fixA cloud rising off the outdoor unit is the defrost cycle. Vaillant’s own wording is that you do not have to do anything.Related: same brand
- Running all the timeUsually simple to fixA heat pump is meant to run long and low. The alarm signal is not the running time, it is the bill — and turning the thermostat up will not warm the house faster.Related: same brand
- F.511Get it checkedThe sensoCOMFORT controller has lost contact with the heat pump control module. Vaillant give two causes, a defective cable or a plug connection, and the house may stay warm while it shows.Related: same brand
Own a Vaillant heat pump? Save it and its codes come first next time.
Related guides
Common questions about Vaillant F.788
Is it safe to switch the heat pump off to reset the pump?
Yes, once, and it is Vaillant’s own published first remedy: switch off for at least 30 seconds with no current. Use the isolator or breaker, because the unit has no on-off switch. What is not sensible is doing it repeatedly for a code that keeps returning.
What is the difference between F.788 and F.820?
F.788 is the pump reporting a fault and locking itself out — it is alive and talking. F.820 is the heat pump receiving no signal from the pump at all, which points at the cable or a dead pump. Different diagnosis, different first check.
How much is a new pump?
Around £250 to £450 fitted for the building circuit pump on a Vaillant indoor unit, but most F.788 faults do not end in a new pump. A purge and a filter clean clear the majority.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
- Manufacturer support portal
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.