ASTG18/22/24KMCA - Refrigerant Noise at Heating Start-Up

This article summarises the quality assurance investigation into refrigerant noise reported on ASTG18KMCA, ASTG22KMCA and ASTG24KMCA indoor units when heating operation starts.

Issue Summary
  • Models affected: ASTG18KMCA / ASTG22KMCA / ASTG24KMCA
  • Total reported cases: 8 units
  • ASTG18KMCA: 4 units
  • ASTG22KMCA: 1 unit
  • ASTG24KMCA: 3 units
  • Symptom: Refrigerant noise when heating operation starts
  • Cooling start-up: Noise not reported

Problem Description

Refrigerant noise was reported when heating operation started. The issue was not reported when cooling operation started.

The original report also noted that the issue could still occur after replacing both the indoor and outdoor units, indicating that the behaviour was related to system control rather than an isolated hardware defect.

Factory Replication Test

FGL carried out replication testing using both a sample unit supplied from Australia and factory units. The reported noise was reproduced on both.

Reported Occurrence Conditions

  • The noise occurs when heating operation starts.
  • The report references start-up after the compressor has been stopped for approximately 3 hours or more.
  • The EEV pulse control differs depending on whether compressor stop time is within 3 hours or 3 hours or more.
  • The report notes occurrence when outdoor temperature is 20°C or higher.
  • The report also states that when outdoor temperature is below 12°C, the issue does not occur.
  • A typical compressor stop period associated with the investigation was approximately 8 hours / overnight.
Source note: The original QA wording also says the issue was "easy to occur when outdoor temp is low", while the detailed replication conditions state occurrence at 20°C or higher and no occurrence below 12°C. This article preserves the detailed test conditions exactly as reported.

Factory Cause Assessment

The factory attributed the noise to the behaviour of R32 refrigerant flow during heating start-up.

  • When the system has been stopped for a long period, liquid refrigerant may remain in the indoor unit.
  • At heating start-up, the refrigerant may initially be in a mixed liquid and gas state.
  • When compressor frequency is approximately 66 rps or higher, refrigerant velocity is high.
  • The difference in flow velocity between liquid and gas can become significant.
  • This may create refrigerant turbulence immediately upstream of the EEV.
  • When the turbulent refrigerant enters the EEV, the resulting sound can transmit into the indoor space.
Factory finding: If the EEV is held more closed during heating start-up, the refrigerant noise does not occur. The factory assessment was that this allows the refrigerant condition to transition more readily from a liquid/gas mixture toward liquid flow before passing through the EEV.

Comparison with R410A Models

The original report states that the same basic control strategy had been used on R410A models, but the same refrigerant noise issue had not occurred.

Corrective Action

The corrective measure was a software change to the EEV control during heating start-up.

Key countermeasure: Heating start-up EEV control was revised to reduce the likelihood of refrigerant turbulence and the associated start-up noise.

Factory Implementation Schedule

ModelFactory Countermeasure Date
ASTG18KMCA28 July
ASTG24KMCA4 August
ASTG22KMCA8 September

Action for Existing Product

  • Already installed product: Address on a case-by-case basis if the noise occurs.
  • Stock product: No blanket corrective action was specified in the original report.
  • Manufactured product: Factory countermeasures were scheduled by model as listed above.

Service Guidance

If a customer reports refrigerant noise from an ASTG18KMCA, ASTG22KMCA or ASTG24KMCA at heating start-up:

  1. Confirm the exact indoor and outdoor unit model.
  2. Confirm the noise occurs specifically when heating operation starts.
  3. Record how long the compressor had been stopped before the noise occurred.
  4. Record the outdoor ambient temperature at the time of occurrence.
  5. Confirm whether the noise is limited to the initial heating start-up period.
  6. Where applicable, confirm whether the unit is an early-production unit that may pre-date the revised EEV software control.
  7. Escalate for the applicable software or product countermeasure if required.
Technical summary: The QA investigation identified the issue as a heating start-up refrigerant-flow noise associated with R32 and EEV control, rather than a fault that could reliably be corrected by replacing the indoor or outdoor unit.