Vaillant F.820 fault codewhat it means and how to fix it
Connection 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.
Take care
Get an F-Gas certified heat pump engineer to look at your Vaillant F.820
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 F.820 means
F.820 is one of the shortest entries in Vaillant’s appendix and the shortness is informative. The meaning is "Connection fault: Building circuit pump", the cause is that the pump does not report any signal back to the heat pump, and the remedy is two lines: check the cable to the pump for defects and replace it if required, then replace the pump. It is worth setting beside F.788, because owners meet the two together and they are not the same thing.
F.788 is the pump’s own electronics detecting a problem — dry running, a blockage, bad voltage — and locking the pump out; the pump is alive and saying so. F.820 is silence. The heat pump expects a feedback signal from the high-efficiency pump, receives nothing, and cannot tell whether the pump is running, stalled or unplugged. That is why the cable comes first in Vaillant’s remedy: a signal wire that has been disturbed, chafed or pulled at its connector produces F.820 with a perfectly healthy pump behind it, and it tends to appear after work in the cupboard, after the front casing has been off for a service, or when a connector was never fully home from the start.
When the cable checks out, the pump’s control electronics have failed and the pump is replaced as a unit. There is no owner remedy here beyond the observations that shorten the visit — whether you can hear the pump, whether the code appeared after a visit, and whether the heating is still doing anything — and the fault does stop the heat pump, because it will not run without confirmation that water is moving.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Signal cable to the pump defective or disturbed | Vaillant’s first remedy is to check this cable and replace it if required. A connector pulled during a service or other work is the everyday version. | Common |
| Pump control electronics failed | Vaillant’s second remedy is to replace the pump. A high-efficiency pump whose electronics have died gives no feedback even though its motor may still be intact. | Occasional |
| Connector never fully seated at installation | Shows up as F.820 on a fairly new system, often intermittently at first as the plug works looser with heat cycling. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£90–£450
Time on site
10 minutes
An installer visit is typically £90 to £180 and a cable or connector repair usually fits inside it. 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
Own this appliance and had this code? . Nothing is published until a person has read it.
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.788Get it checkedThe 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.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.820
Is this the same as F.788?
No. F.788 is the pump telling the heat pump it has a problem and locking itself out. F.820 is the heat pump hearing nothing from the pump at all. F.788 often clears with a purge or a filter clean; F.820 is a cable or the pump itself.
Will the heating work while F.820 is showing?
The heat pump will not run, because it needs the pump’s feedback to know water is moving before it starts the compressor. Depending on how your system was configured the electric back-up heater may take over, which keeps the house warm at several times the running cost.
Could a power cycle fix it like F.788?
Vaillant do not list one for F.820, and the logic is different: F.788 is a lock in the pump electronics that a power-off releases, whereas F.820 is a missing signal, which a restart does not create. If it clears briefly after a restart and returns, that points at a loose connector rather than a fix.
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
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.