- Heat pumps
- Air-to-water heat pump
E25
Viessmann E25 on a Vitocal heat pump: inlet and outlet water gap
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
Medium
Quick answer
On the Vitocal 100-AW AM2V, E25 means the temperature difference between the outdoor unit inlet and outlet water has been exceeded. Viessmann links it to low water flow. Check the display and the visible valves and call the contractor if it returns.
Safety first
Stop and check this first
- Do not open the outdoor unit or touch the refrigerant circuit: the Vitocal 100-AW contains flammable R32 refrigerant and Viessmann warns that some boards stay live after the power is switched off and asks to wait at least 4 minutes before removing covers. Licensed professional contractor only.
Safe checks you can perform
- Read the error list on the controller
- Check the visible valves in the circuit
- Look at the outdoor temperature on the display
- Check pump, venting and circuit filter
Leave these tasks to a qualified professional
- Do not attempt this professional task: Check water flow with the Viessmann list
- Do not attempt this professional task: Reset after E42 only once the cause is fixed
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.
Check water flow with the Viessmann list
What they doThe contractor works through steps 1 to 5 of water flow protection: pump, air, circuit resistance, valves and filter.
They must show youthe cleaned filter, the venting done and the pump condition.
In writingthe cause found, what was corrected and a test run without E25.
Reset after E42 only once the cause is fixed
What they doIf the unit has reached E42, the contractor fixes the cause and resets the outdoor unit by switching its breaker off and back on.
They must show youthe corrected cause before the reset.
In writinghow many times E25 appeared, what was done and the reset date.
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.
Read the error list on the controller
- Who:
- you, on the indoor unit controller.
- How:
- if the home screen shows the error indication, press the navigation dial to see the list of detected errors and warnings. Note the codes and confirm the notification as Viessmann describes.
- Result that matters:
- knowing whether only E25 appears or also E18, E26, E42 or another notice, such as Low Pressure of the heating medium.
- If E42 appears:
- E25 has occurred three times and Viessmann requires a manual reset; call the contractor.
- If only E25 appears and it does not return after confirming:
- note the date and watch for repeats.
- Evidence to keep:
- a photo of the error list with the date and time.
Evidence: Viessmann Vitocal 100-AW AM2V installation, service and operating instructions 6221389-11
Check the visible valves in the circuit
- Who:
- you, without opening any unit.
- How:
- look at the visible valves on the heating circuit next to the indoor unit and at the manifolds, especially if someone has worked on the system recently.
- Result that matters:
- Viessmann lists valves that should be open but are not among the causes of low flow.
- If you find a valve closed or half closed:
- open it, note which one and watch whether E25 returns.
- If they are all open:
- rule them out; the rest of the list is for the contractor.
- Evidence to keep:
- a note or photo of the valve found and its position.
Evidence: Viessmann Vitocal 100-AW AM2V installation, service and operating instructions 6221389-11
Look at the outdoor temperature on the display
- Who:
- you, looking only.
- How:
- the controller home screen shows the outdoor temperature. Note it at the moment E25 appears.
- Result that matters:
- Viessmann asks to check whether ambient temperature is above 86 °F (30 °C) when flow is normal.
- If it is above 86 °F:
- tell the contractor, together with the time of the fault.
- If it is below:
- water flow is still the first thing the contractor should check.
- Evidence to keep:
- a photo of the display showing the outdoor temperature and the time.
Evidence: Viessmann Vitocal 100-AW AM2V installation, service and operating instructions 6221389-11
Check pump, venting and circuit filter
- Who:
- a licensed professional contractor.
- How:
- follow steps 1 to 5 of water flow protection: pump running and its indicator, high point vent, circuit resistance, valves and water circuit filter.
- Result that matters:
- which low flow cause is confirmed before any part is replaced.
- If air comes out or the filter is full of impurities:
- the cause is hydraulic and is fixed by venting or cleaning.
- If flow is normal:
- Viessmann asks to check ambient temperature and, if it persists, to hand it to a technician.
- Evidence to keep:
- a report of what was checked at each step.
Viessmann says installation, adjustment and maintenance must be done by a licensed professional contractor. Hot surfaces and fluids can cause burns.
Evidence: Viessmann Vitocal 100-AW AM2V installation, service and operating instructions 6221389-11
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
- Viessmann
- Product type
- Air-to-water heat pump
Known code variants
- Viessmann bomba de calor E25
- Viessmann heat pump E25
- Viessmann Vitocal 100-AW E25
- Vitocal diferencia de temperatura entrada salida
- Vitocal E25
- Vitocal E25 caudal de agua
- Vitocal E25 water flow
- Vitocal inlet outlet temperature difference
What it means
Viessmann defines E25 on the outdoor unit of the Vitocal 100-AW AM2V, a North American model, as an excessive difference between inlet and outlet water temperature. It gives one cause, insufficient water flow, and refers to steps 1 to 5 of water flow protection E18: pump, air, circuit resistance, valves and filter. It then asks to check whether ambient temperature is above 86 °F (30 °C). If E25 occurs three times, the unit shows E42 and needs a manual reset. The code on its own never proves that a part is faulty.
Warnings and stop conditions
- Higheditorial warning
Do not open the outdoor unit or touch the refrigerant circuit: the Vitocal 100-AW contains flammable R32 refrigerant and Viessmann warns that some boards stay live after the power is switched off and asks to wait at least 4 minutes before removing covers. Licensed professional contractor only.
Evidence: Viessmann Vitocal 100-AW AM2V installation, service and operating instructions 6221389-11
Symptoms and causes
Probable causes · in order
- Insufficient water flowCommon
This is the only cause Viessmann gives for E25: with low flow, water leaves the outdoor unit much hotter or colder than it enters.
Evidence: Viessmann Vitocal 100-AW AM2V installation, service and operating instructions 6221389-11
- Valves not open or dirty filterPossible
Viessmann refers to the water flow protection list, which includes circuit valves that should be open and a water circuit filter blocked with impurities.
Evidence: Viessmann Vitocal 100-AW AM2V installation, service and operating instructions 6221389-11
- Air in the circuit or pump not moving waterPossible
The same list includes air left in the circuit, vented at the high point, and a pump that is not running or shows a fault on its indicator light.
Evidence: Viessmann Vitocal 100-AW AM2V installation, service and operating instructions 6221389-11
- High ambient temperatureLess common
With flow confirmed as normal, Viessmann asks to check whether ambient temperature is above 86 °F (30 °C) before handing the case to a professional technician.
Evidence: Viessmann Vitocal 100-AW AM2V installation, service and operating instructions 6221389-11
Virtual technician
Does E25 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
Viessmann Vitocal 100-AW AM2V installation, service and operating instructions 6221389-11
Viessmann · 2026 · manual
Was this entry useful?