- Heat pumps
- Air-to-water heat pump
936
Ariston 936 on a Nimbus heat pump: underfloor safety thermostat
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
Medium
Quick answer
On an Ariston Nimbus with underfloor heating, 936 means the floor safety thermostat has opened, usually because the flow water got too hot or flow was too low. Heating and hot water stop. Record the message, check the zone settings and call your installer.
Safety first
Stop and check this first
- Ariston states that if the floor thermostat is wired to STT and trips, the circulation pump loses power and frost protection is not guaranteed; in freezing weather, get the installer out quickly.
- Do not cut power to the heat pump when temperatures can drop below zero: Ariston forbids it because the system could freeze.
Safe checks you can perform
- Note the message and when it appeared
- Review the underfloor zone settings
- Check that the manifold valves are open
Leave these tasks to a qualified professional
- Do not attempt this professional task: Measure underfloor flow and delivery temperature
- Do not attempt this professional task: Inspect and reset the safety thermostat
What a technician must be able to justify
Technician visit
These are the steps they must carry out and show you. Tick them as they work: if steps are skipped, do not accept a part swap.
These are not instructions for opening or handling the equipment yourself. They are the checks that should support a technical conclusion.
Measure underfloor flow and delivery temperature
What they doThe installer checks the underfloor circuit flow and the actual delivery temperature in each mode, including the change over from hot water, and corrects the control settings.
They must show youthe measured delivery temperatures and the underfloor flow.
In writingwhat caused the overheating and which settings were changed.
Inspect and reset the safety thermostat
What they doOnce the cause is fixed, the technician checks the connection on IN-AUX2 STE or STT, verifies parameter 1.1.4 and resets the manual reset thermostat.
They must show youthe terminal used, the thermostat condition and the value of parameter 1.1.4.
In writingwhether the thermostat was replaced, why, and which terminal it is wired to.
Before accepting a part or repair
Nothing has been shown yet. Do not accept a part swap based on the code alone.
Follow these steps
Note: Follow the documented order. An error code identifies the affected system, but it does not prove by itself which part has failed.
Note the message and when it appeared
- Who:
- you.
- How:
- in the SENSYS HD pop up, write down the code, description and device. Think about whether it came at the end of a hot water cycle or after a programme change, since Ariston flags the switch from hot water as a sensitive moment.
- Result that matters:
- a clue about what the heat pump was doing when the thermostat tripped.
- If it appeared right after heating domestic hot water:
- tell the installer, as it points at the floor flow temperature.
- If it appeared during normal heating:
- low underfloor flow becomes the stronger suspect.
- Evidence to keep:
- photo of the screen showing 936 and the rough time it appeared.
Evidence: Ariston NIMBUS S NET, manual de uso para el usuario (ES/PT) · Ariston Nimbus Plus S NET R32 technical instructions for installation and maintenance (UK)
Review the underfloor zone settings
- Who:
- you, only looking through the user menu.
- How:
- open the settings for the underfloor zone and write down the set temperature and the curve in use. Ask the household whether anyone changed them recently. Do not alter technical parameters behind the access code.
- Result that matters:
- knowing whether the floor setpoint was raised above its usual value.
- If someone raised the setpoint:
- put back the earlier value you had noted and mention it to the technician.
- If the settings are unchanged:
- the installer needs to measure the real flow and temperature.
- Evidence to keep:
- photo of the zone settings screen.
Evidence: Ariston NIMBUS S NET, manual de uso para el usuario (ES/PT)
Check that the manifold valves are open
- Who:
- you, without opening covers or touching the safety thermostat.
- How:
- open the manifold cupboard door if it is accessible and look at the visible valves and heads. Notice whether loops that used to be open are now shut. Do not remove heads or actuators.
- Result that matters:
- ruling out the underfloor system running with most loops closed.
- If some valves are closed because you shut them:
- return them to how they were and ask the installer to reset the thermostat.
- If everything looks open:
- the flow or wiring problem is for the technician.
- Evidence to keep:
- photo of the manifold showing the valve positions.
Evidence: Ariston Nimbus Plus S NET R32 technical instructions for installation and maintenance (UK)
Still not solved
If the error persists, stop here. See what the technician must do and show you.
Where this code applies
A code is meaningful only inside the right product and version context.
Primary scope
- System
- Heat pumps
- Brand
- Ariston
- Product type
- Air-to-water heat pump
Known code variants
- aerotermia Ariston 936
- Ariston 936
- Ariston heat pump 936
- Ariston Nimbus error 936
- E936
- Error 936
- error termostato suelo Ariston
- floor thermostat 1 error
- Nimbus 936
- Nimbus Pocket 936
- Nimbus suelo radiante 936
- Nimbus underfloor heating 936
- termostato de seguridad suelo abierto
- under-floor safety thermostat open
What it means
The Ariston Nimbus Plus S NET R32 installation manual (UK, 2023) explains that if the delivery temperature to an underfloor system is too high, the system stops in hot water and heating or cooling modes and reports 936, Under-floor safety thermostat open. It restarts only when the manual reset thermostat closes again. The error table asks for the underfloor flow to be checked, plus the thermostat connection on IN-AUX2 STE or STT. The code alone does not mean the thermostat is faulty; the reason the water overheated or stopped moving has to be found.
Warnings and stop conditions
- Highsource verified
Ariston states that if the floor thermostat is wired to STT and trips, the circulation pump loses power and frost protection is not guaranteed; in freezing weather, get the installer out quickly.
Evidence: Ariston Nimbus Plus S NET R32 technical instructions for installation and maintenance (UK)
- Mediumsource verified
Do not cut power to the heat pump when temperatures can drop below zero: Ariston forbids it because the system could freeze.
Evidence: Ariston Nimbus Plus S NET R32 technical instructions for installation and maintenance (UK)
Symptoms and causes
Probable causes · in order
- Flow water too hot for the floorCommon
Ariston stresses that in every mode, especially when switching back from hot water, the floor inlet temperature must stay within allowed limits; going over trips the safety thermostat.
Evidence: Ariston Nimbus Plus S NET R32 technical instructions for installation and maintenance (UK)
- Low flow in the underfloor circuitCommon
The 936 resolution starts with checking the flow of the underfloor system; with loops shut or weak circulation the water overheats.
Evidence: Ariston Nimbus Plus S NET R32 technical instructions for installation and maintenance (UK)
- Thermostat wiring or missing jumperPossible
Where no floor thermostat is fitted, Ariston requires an electrical jumper on IN-AUX2 STE or STT; a loose terminal or missing jumper gives 936 without any real overheating.
Evidence: Ariston Nimbus Plus S NET R32 technical instructions for installation and maintenance (UK)
Virtual technician
Does 936 still appear after these checks?
Answer a few questions to narrow down the likely cause and the safest next action. Guidance only - it never replaces a professional.
Sources and technical references
- 1
Ariston NIMBUS POCKET M NET R32, instrucciones técnicas de instalación y mantenimiento
Ariston · 2022 · manual
- 2
Ariston NIMBUS S NET, manual de uso para el usuario (ES/PT)
Ariston · 2022 · manual
- 3
Ariston Nimbus Plus S NET R32 technical instructions for installation and maintenance (UK)
Ariston · 2023 · manual
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