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Hitachi Yutaki M & Yutaki S fault 12

Indoor water outlet thermistor abnormally (THMwo)

Heat pump

What it means

Indoor origin alarm indicating abnormality of water outlet thermistor THMwo.

Likely causes

  1. Loose, disconnected, broken or short-circuited connector
  2. Pump not operating within operating range
  3. Blocked or restricted water circuit
  4. Air trapped in water pipes
  5. Clogged water strainer

Where to start

  1. Check connector and wiring of THMwo
  2. Check that the water pump of the space heating circuit works within the pump operating range
  3. Verify the water flow is over the pump's minimum threshold for the unit size

The full answer, free

  • The readings and component tests from the manual
  • How to put it right, step by step
  • The manual page for this fault (page 334), 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 Hitachi manual for the Yutaki M & Yutaki S. Check the model on the data plate before you order parts.

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