Vaillant F.821 fault codewhat it means and how to fix it
Sensor fault on the immersion heater flow temperature. Vaillant give a disconnected or short-circuited sensor, or both flow sensors failed, and the fix is a sensor or a cable harness inside the indoor unit.
Take care
Get an F-Gas certified heat pump engineer to look at your Vaillant F.821
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.821 means
The immersion heater in this code is not an element in your cylinder. It is Vaillant’s electric back-up heater, built into the uniTOWER plus or hydraulic station, which tops up the heat pump in cold weather, carries the anti-legionella cycle and takes over if the heat pump fails. F.821 is the sensor that measures the water temperature leaving that heater reporting nonsense.
Vaillant’s appendix gives the causes as the sensor not being connected or its input having short-circuited, or both of the flow temperature sensors in the heat pump being defective, with the remedies of checking and if necessary replacing the sensor, or replacing the wiring harness. Why the sensor matters is visible elsewhere in the same appendix: status S.351 shuts the heat pump down if the flow temperature at the immersion heater exceeds 75 °C, and fault F.1100 describes the safety cut-out tripping if the heater runs above 89 °C. The flow sensor is the first line of that protection, so without a trustworthy reading the electronics cannot safely run the back-up heater at all.
The consequence for you is indirect. The heat pump itself may carry on producing heat, but the back-up is unavailable, and Vaillant note in the owner instructions that when the back-up heater is out of action the anti-legionella function is not always guaranteed. In a cold snap you may also find the house does not quite reach temperature, because the top-up that normally covers the coldest days is off. It is not an owner repair — the sensor sits inside the indoor unit beside mains wiring — but it is worth booking promptly rather than living with, for both of those reasons.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Sensor disconnected or input short-circuited | Vaillant’s published cause. A plug disturbed during servicing or a sensor lead chafed against the casing. | Common |
| Both flow temperature sensors defective | Vaillant’s second cause, remedied by replacing the wiring harness. Points at the wiring loom rather than two separate failures. | Occasional |
| Sensor drifted out of range | Not in the two-line table, but the practical version of a defective sensor: the electronics see a reading the sensor cannot legitimately produce and flag it. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£90–£250
Time on site
10 minutes
An installer visit is typically £90 to £180. The sensor itself is inexpensive; a replacement cable harness is more labour than parts.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Vaillant fault codes
- Sterilisation cycleUsually simple to fixVaillant call it the anti-legionella function: the cylinder is driven above 60 °C on set days, usually at 4 a.m., by the electric back-up heater. It looks like a fault and is not one.Related: hot water and cylinder
- F.1120Get it checkedPhase failure on the electric back-up heater: its internal circuit breaker has opened. Vaillant say the competent person resets it, and warn the anti-legionella function is not guaranteed meanwhile.Related: hot water and cylinder
- F.1100Get it checkedThe safety cut-out on the electric back-up heater has opened. Vaillant name low flow, air, a heater run on an unfilled circuit, a trip above 89 °C that needs a new fuse, and external heat.Related: hot water and cylinder
- 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: same brand
- 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
Own a Vaillant heat pump? Save it and its codes come first next time.
Related guides
Common questions about Vaillant F.821
Is the immersion heater in my cylinder faulty?
Different heater. Vaillant’s "immersion heater" in this code is the electric back-up heater inside the indoor unit, not an element in the cylinder. Some systems have both; F.821 concerns only the one in the heat pump.
Why does the house feel cooler in cold weather since the code appeared?
Because the back-up heater is what covers the coldest days, and without a valid flow temperature the electronics will not run it. The heat pump alone may fall a degree or two short until the sensor is replaced.
Does this affect the legionella cycle?
Yes. Vaillant publish that heat pump systems use the back-up heater for legionella protection and that the function is not always guaranteed when the heater is defective. That is the reason not to leave this code for months.
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.