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E26

Viessmann E26 on a Vitocal heat pump: outlet water overheating

Verified with sources1 sourceReviewed Oct 4, 2026
Technician coming? See the 3 steps they must show you
Air-to-water heat pump outdoor unit next to a house
Mueller felixCC BY-SA 4.0Image source

Risk level

Medium

Proceed carefully and stop if the check is unclear.

Urgency

Medium

Quick answer

On the Vitocal 100-AW AM2V, E26 is outlet water over heat on the outdoor unit. Viessmann links it mainly to low water flow. Check the error list, the visible valves and the emitters, and call the contractor if it repeats.

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. Do not vent the circuit yourself: the water can come out very hot.

Safe checks you can perform

  • Read the error list on the controller
  • Check that the emitters are open
  • Check the visible valves in the circuit
  • Check pump, venting and circuit filter

Leave these tasks to a qualified professional

  • Do not attempt this professional task: Check water flow before replacing the pump
  • Do not attempt this professional task: Secondary pump only after the calculation
  • Do not attempt this professional task: Reset after E44 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.

  1. Check water flow before replacing the pump

    What they doThe contractor checks pump, air, circuit resistance, valves and filter, in the order Viessmann gives.

    They must show youthe cleaned filter, the venting done and the pump running with no fault on its indicator.

    In writingthe confirmed cause, what was done and a test run without E26.

  2. Secondary pump only after the calculation

    What they doIf they propose adding a secondary pump, the contractor first calculates the hydraulic resistance of the circuit.

    They must show youthe resistance calculation using circuit length, elbows and valves.

    In writingthe calculation result, the pump chosen and the test after the work.

  3. Reset after E44 only once the cause is fixed

    What they doIf the unit has reached E44, 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 E26 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.

  1. Read the error list on the controller

    No special tools

    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 errors and warnings. Note the codes and confirm the notification as Viessmann describes.
    Result that matters:
    knowing whether only E26 appears or also E18, E25, E44 or another notice, such as IDU Pump or Low Pressure.
    If E44 appears:
    E26 has occurred three times and Viessmann requires a manual reset; call the contractor.
    If only E26 appears and it does not return after confirming:
    note the date and whether anything changed in the system.
    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

  2. Check that the emitters are open

    No special tools

    Who:
    you, without tools.
    How:
    check whether radiators, fan coils or underfloor zones have been closed, or whether zone thermostats have shut almost every circuit at the same time.
    Result that matters:
    Viessmann lists terminals not opened among the causes of E26.
    If almost all were closed:
    open them, note which ones and watch whether E26 returns.
    If they were open:
    rule them out and move on to the visible valves.
    Evidence to keep:
    a list of the emitters or zones that were closed.

    Evidence: Viessmann Vitocal 100-AW AM2V installation, service and operating instructions 6221389-11

  3. Check the visible valves in the circuit

    No special tools

    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 after building work or recent maintenance.
    Result that matters:
    Viessmann includes valves that should be open but are not in its low flow list.
    If you find a valve closed or half closed:
    open it, note which one and watch whether E26 returns.
    If they are all open:
    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

  4. Check pump, venting and circuit filter

    Tools required

    Who:
    a licensed professional contractor.
    How:
    check the pump and its indicator, open the high point vent until no more gas comes out, calculate circuit resistance and clean the water circuit filter.
    Result that matters:
    which cause on the Viessmann list is confirmed before parts are replaced.
    If air comes out or the filter was dirty:
    it is fixed by venting or cleaning and tested again.
    If circuit resistance is too high:
    Viessmann suggests adding a secondary pump.
    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. Outlet water can be very hot.

    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 E26
  • Viessmann heat pump E26
  • Viessmann Vitocal 100-AW E26
  • Vitocal agua de salida sobrecalentada
  • Vitocal E26
  • Vitocal E26 overheat
  • Vitocal E26 sobretemperatura
  • Vitocal outlet water over heat

What it means

Viessmann defines E26 on the outdoor unit of the Vitocal 100-AW AM2V, a North American model, as outlet water over heat. It lists three causes: insufficient water flow, improper selection and terminals not opened. For flow it gives five checks: pump running and its indicator, high point vent, circuit resistance, valves open and the circuit filter. If E26 occurs three times, the unit shows E44 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. Do not vent the circuit yourself: the water can come out very hot.

    Evidence: Viessmann Vitocal 100-AW AM2V installation, service and operating instructions 6221389-11

Symptoms and causes

Probable causes · in order

  • Insufficient water flowCommon

    Viessmann asks to check that the pump runs and its indicator light, to vent air at the high point and to review circuit resistance when flow is low.

    Evidence: Viessmann Vitocal 100-AW AM2V installation, service and operating instructions 6221389-11

  • Terminals in the system not openPossible

    Viessmann lists terminals not opened among the causes of E26: if emitters or their valves are closed, the hot water has nowhere to go.

    Evidence: Viessmann Vitocal 100-AW AM2V installation, service and operating instructions 6221389-11

  • Closed valves or dirty filterPossible

    The Viessmann list includes circuit valves that should be open and a water circuit filter blocked with impurities, which is cleaned.

    Evidence: Viessmann Vitocal 100-AW AM2V installation, service and operating instructions 6221389-11

  • Improper selectionLess common

    Viessmann mentions improper selection without further detail. If circuit resistance is too high, it suggests calculating it and adding a secondary pump.

    Evidence: Viessmann Vitocal 100-AW AM2V installation, service and operating instructions 6221389-11

Virtual technician

Does E26 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.

Start diagnosis

Sources and technical references

  1. 1

    Viessmann Vitocal 100-AW AM2V installation, service and operating instructions 6221389-11

    Viessmann · 2026 · manual

Sources: Viessmann Vitocal 100-AW AM2V installation, service and operating instructions 6221389-11reviewed 2026-10-04 · verified

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