Vaillant Protective zonea plain-English guide
aroTHERM plus runs on propane. Vaillant define a zone around the outdoor unit where you may not add sockets, lights, vents or openings — and the duty is yours.
Stop — this needs an engineer
Get an F-Gas certified heat pump engineer to look at your Vaillant heat pump
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 this means
This is not a fault code. It is an obligation attached to owning the appliance, and almost nobody is told about it at handover. The aroTHERM plus is charged with R290, which is propane, and Vaillant do not soften the language: risk of death caused by fire or explosion if there is a leak in the refrigerant circuit. The product contains the combustible refrigerant R290, and in the event of a leak, escaping refrigerant may mix with air to form a flammable atmosphere.
What makes this different from ordinary refrigerant caution is the physics. Propane is denser than air, so a leak does not disperse upwards — it sinks and collects at ground level, and it flows into anything below it.
Vaillant therefore define a protective zone around the outdoor unit, and their requirements for it are specific: there must be no windows, doors, light shafts, cellar entrances, escape hatches, flat-roof windows or ventilation openings within it, and no ignition sources either, which they list as plug sockets, light switches, lamps, electrical switches or other permanent ignition sources. The zone must not extend to a neighbouring building or a public area. And the duty is placed on the owner directly: in the protective zone, you are not permitted to make any structural alterations which infringe the stated protective zone.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| An outside socket or light fitted near the unit | The commonest way the zone gets breached after handover. Both are ignition sources and both are ordinary jobs an electrician would do without knowing. | Common |
| A shed, decking box or store built beside the unit | Creates a low enclosure exactly where a heavier-than-air gas would collect. | Common |
| A new vent, cat flap, trickle vent or airbrick | Any opening into the building within the zone gives leaked refrigerant a route indoors. | Occasional |
| A light shaft, cellar entrance or sunken recess within the zone | Light shafts and cellar entrances are on Vaillant’s prohibited list, and they warn that refrigerant must not collect in grooves. Open gullies and downpipes are the exception: Vaillant state that within the protective zone they do not pose any safety risk, and they are where the condensate is meant to go. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£0
Nothing to comply with. What it costs is checking before you have work done near the unit, rather than after.
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
- 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
- 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
- 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: same brand
- 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: same brand
Own a Vaillant heat pump? Save it and its codes come first next time.
Related guides
Common questions
How big is the protective zone?
It depends on how the unit is installed, and Vaillant publish the dimensions in the operating instructions for each case. For a plain ground installation with nothing behind it the zone is a 1000 mm clearance around the product. In front of a building wall it becomes a rectangle 2100 mm deep by 3100 mm wide; in a building corner, on a raised plinth, wall-mounted or on a flat roof the drawing changes again. The same figures apply to the 35/6 to 75/6 and the 105/6 and 125/6. Ask your installer which drawing applies and for the dimensions in writing — you cannot comply with a boundary you have not been told.
Does this apply to my heat pump?
Only if it is charged with R290 propane, which on Vaillant means the aroTHERM plus range. The earlier aroTHERM and the aroTHERM split use R410A, which is not flammable and carries no protective zone. The refrigerant type is on the data plate on the outdoor unit.
Does the engineer need F-Gas certification for an R290 unit?
Not for the refrigerant. R290 is propane, not a fluorinated gas, so the F-Gas regulations that govern R32 and R410A work do not apply to it. What Vaillant require is that anything needing the unit opened is done by a competent person who knows the particular properties and risks of R290, and that work on the refrigerant circuit is done with specific expert refrigeration knowledge in compliance with local law, including the handling of flammable refrigerants. In practice that means asking whether the engineer is trained on A3 flammable refrigerants rather than whether they hold an F-Gas card.
Can I smell a propane leak?
Refrigerant-grade propane may not be odorised the way domestic bottled gas is, so do not rely on smell as your warning. If you suspect a leak, keep ignition sources away, ventilate at ground level, and call your installer rather than investigating.
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.