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Mitsubishi Ecodan fault 20

Water discharges from pressure relief valve (sanitary circuit)

Heat pump

What it means

Water discharge from the sanitary circuit pressure relief valve due to faulty pressure reducing valve, damaged valve seat, reduced expansion vessel charge, or DHW tank backflow.

Likely causes

  1. DHW tank may have subjected to backflow
  2. Immersion heater degradation causing earth leakage, triggering circuit breaker (heater always OFF)
  3. Water circulation pump wear leading to pump failure

Where to start

  1. Check the pressure in DHW tank and compare to incoming mains pressure
  2. Check OUT1 (Water circulation pump 1, Space heating & DHW) at TBO.1 1-2, connector CNP1.
  3. Check OUT2 (Water circulation pump 2, Zone1) at TBO.1 3-4.

The full answer, free

  • 9 more checks in order
  • The readings and component tests from the manual
  • How to put it right, step by step
  • The manual page for this fault (page 56), right under the answer
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Safety: Isolate the outdoor unit and any backup heater, and give the inverter capacitors time to discharge before you open the electrics. Anything on the refrigerant circuit needs F-gas certification.

From the Mitsubishi manual for the Ecodan. Check the model on the data plate before you order parts.

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