Vaillant F.1100 fault codewhat it means and how to fix it
The 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.
Take care
Get an F-Gas certified heat pump engineer to look at your Vaillant F.1100
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.1100 means
Every electric heater that sits in a water circuit has a last-resort thermal cut-out, and F.1100 is Vaillant’s telling you it has opened on the back-up heater inside the indoor unit. Their appendix lists why it opens: insufficient volume flow or air in the building circuit, so the water past the element is not carrying its heat away; the heater being operated when the building circuit is not filled; operation at flow temperatures above 89 °C, which trips the safety fuse of the cut-out and requires that fuse to be replaced; and feed-in of external heat into the building circuit, such as a stove or solar thermal coil pushing the temperature up from outside.
The remedies follow the causes — check the building circuit pump is circulating, open stopcocks if they are closed, replace the safety cut-out, reduce or stop the external heat, and check that the dirt filter is permeable. Notice that three of the four causes are flow problems, which puts F.1100 in the same family as F.532 and F.788 even though the code points at the heater. The heater is simply the component that gets hot first when the water stops moving.
The consequence is the one described under F.821: the heat pump may keep running but the back-up heater is out of action, so the coldest days lose their top-up and Vaillant’s anti-legionella cycle, which relies on the back-up heater, is not guaranteed. The 89 °C trip with its replaceable fuse is the version to be most careful with, because a cut-out that has tripped on genuine overheating is doing its job, and Vaillant’s remedy is a new fuse fitted by the engineer after the cause is found, not a reset.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Insufficient flow or air past the heater | Vaillant’s first cause. The element heats water that is not moving, and the cut-out opens before the water boils. | Common |
| Dirt filter restricting the circuit | Vaillant’s remedy list ends with checking the filter is permeable. A blocked filter starves the heater of flow exactly as it starves the heat pump. | Common |
| Heater run on an unfilled circuit | Vaillant’s second cause, seen after drain-downs and refills when the back-up heater is called before the circuit is full. | Occasional |
| External heat entering the circuit | A solid-fuel stove or solar thermal system feeding heat into the same pipework can push the flow past the cut-out’s limit with the heater doing nothing wrong. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£90–£300
Time on site
15 minutes
An installer visit is typically £90 to £180. A replacement safety cut-out or fuse is a modest part; a filter clean and purge usually fit inside the same visit.
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.
Magnetic system filter, 22mm
MAG-22£65–£120Plumbing merchant
Price checked 28 September 2026
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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.821Get it checkedSensor 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.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.1100
Is the heat pump still working?
Usually, yes. F.1100 takes the electric back-up heater out of service and the heat pump can carry on without it. What you lose is the top-up on the coldest days and the anti-legionella cycle, which Vaillant say is not guaranteed while the back-up heater is out of action.
Why does a heater fault list flow problems as causes?
Because an element in still water overheats. Three of Vaillant’s four causes are the water not moving — low flow, air, an unfilled circuit — and the heater is just where the temperature shows first. Expect the engineer to look at the filter and the pump as well as the heater.
What is the 89 °C fuse Vaillant mention?
The safety cut-out has a thermal fuse that blows if the heater runs above 89 °C. Unlike a trip, a blown fuse does not reset; Vaillant say it has to be replaced. That is deliberate, so that a serious overheat cannot be cleared without someone looking at why it happened.
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.