Multi System Not Working Individually (Heads OK Together)

Summary

On multi-split systems, where indoor units operate correctly when multiple heads are on, but fail when run individually, the most common cause is incorrect wiring or incorrect pipe connections between indoor and outdoor units.

This guide provides safe diagnostic steps, pump-down guidance, and best practice commissioning checks.

Safety & Qualification

• All refrigerant work must be carried out by a qualified, licensed HVAC technician.
• Systems use R32 refrigerant (flammable) – follow safe handling and recovery procedures.
• Use proper recovery equipment if refrigerant needs to be removed.

Typical Symptoms

• System works when 2 or more indoor units are running
• Individual indoor units:
  • Do not cool/heat
  • Blow incorrect air temperature
• No obvious fault codes (or inconsistent behaviour)

Most Likely Cause

In most cases:

• Interconnection wiring (indoor ↔ outdoor) is crossed

The installation manual reinforces:

• Each indoor unit must match the correct pipe connection AND electrical terminal (A-A, B-B, etc.)

First Checks (Before Pump Down)

1. Run system in Cooling Mode only
2. Turn ON one indoor unit at a time
3. Observe:
   • Any other indoor coil becoming cold or icing → indicates cross connection

This works because:

• In Cooling mode, only the active indoors EXV opens
• Other units should remain inactive

Why Heating Mode Can Mislead

• In Heating mode:
  • All EXVs open partially (~50 steps)
  • Refrigerant flows through all indoor units
• This can make the system appear “OK” even when incorrectly connected

Recommended Fix Approach (Best Practice)

Step 1 – Check Wiring First (Preferred)

• Confirm:
  • Terminal matching (A to A, B to B, etc.)
  • No swapped communication/wiring

This is usually faster and avoids unnecessary refrigerant work.

Step 2 – Only If Required: Identify Pipework (Nitrogen Test)

If pipework is suspected:

• Isolate system
• Use nitrogen to confirm correct pipe pairing between indoor and outdoor units

Pump Down Procedure (If required)

Option 1 – System Operational

1. Run system in Cooling Mode
2. Turn ON the indoor unit(s) to be isolated
3. Connect gauges
4. Close liquid line valve
5. Monitor suction pressure:
   • Should drop toward 0 (vacuum)
6. At ~0:
   • Close suction valve

This isolates refrigerant in the outdoor unit
Leaves pipework and indoor unit clear

Option 2 – System Not Running

• Use a recovery unit
• Fully recover refrigerant into a cylinder
• Weigh and record charge

Recommissioning / Recharge

• System must be:
  • Pressure tested
  • Vacuumed to 500 microns and held
• Recharge:
  • By measured weight
  • Based on manufacturer data

Additional Checks

Before finalising:

• Are there any fault codes present?
• Is the correct refrigerant in the system?
• Has pipe connection order been followed correctly?
• Have indoor units been tested individually in cooling mode?

Key Takeaways (Technician Quick Reference)

Installer Note

This solution should be used alongside the official installation manual for the model installed:
Install Manual ODU - Braemar - NCHVB-02-A