One indoor unit replaced: the outdoor unit set the limit

Another installer had already been to a Sengkang flat and left the replacement head sitting in its box. It fitted the wall and matched the room size perfectly well. What it could not do was talk to the outdoor unit already on the ledge.

By Team Snowflake | Reviewed 25 Jul 2026

Case summary

Midea Multi-split2 years oldHDBSengkang, Singapore

Concern: The replacement order assumed any head of the right capacity would run off an existing outdoor unit, which holds for a single split and breaks down on a shared one.
Found: The replacement head was not a matched indoor model for the installed outdoor unit. Its control terminals and valve settings were never going to pair with the existing system
Key check: Read the outdoor unit model plate and its list of matched indoor combinations before sourcing any replacement
Result: The matched head paired at first power-up with no fault shown at the controller. All four heads now run together on a single call, with the outdoor unit staging as designed. Pipework and drain were reused, so no chasing was opened in the bedroom wall and no making good was needed.

What we were told

One bedroom head on a shared system had stopped cooling and was written off as beyond economical repair. A replacement was ordered by capacity and wall space, then left uninstalled once it refused to pair on site. The other heads sharing that same outdoor unit carried on cooling as they always had.

What we checked

Heads running off one shared outdoor unit are not interchangeable spares, so the outdoor model plate came first. We listed the indoor combinations that unit is built to drive, then compared them against the box already on the floor. Capacity is only one of several things that has to line up.

The outdoor plate listed a defined set of matched indoor models, and the ordered head was not on it.
Control wiring on the ordered head used a different terminal arrangement from the three heads already connected.
Branch pipe sizes matched, which is exactly why the mismatch was invisible from the wall.
The existing pipe run and drain line to that bedroom were sound and correctly sized.

What we found

On a single split the outdoor and indoor units form a closed pair, and a like-for-like swap usually just works. A shared outdoor unit drives several heads through one control line and one set of branch valves, and it only manages heads it is built to recognise. This replacement was sourced on capacity, which happened to be the one specification that matched. Nothing at the wall or in the pipe sizes gave the problem away, so it only surfaced at the terminal block.

What fixed it

We sourced a matched head from the outdoor unit's own combination list and sent the ordered one back before it was ever fitted. The existing pipe run and drain line were reused, since both checked out sound and correctly sized. Flare joints at the head were cut back and remade rather than reconnected, because a flare that has been broken once rarely seals the same way twice.

Outcome

The matched head paired at first power-up with no fault shown at the controller. All four heads now run together on a single call, with the outdoor unit staging as designed. Pipework and drain were reused, so no chasing was opened in the bedroom wall and no making good was needed.

What this case teaches us

On a shared outdoor unit, the indoor heads are not interchangeable

Matching capacity gets you partway and no further. The outdoor unit also has to recognise the head.
The outdoor model plate lists the combinations it can drive. That list, not the room size, narrows the choice.
Confirm the pairing before the head is ordered, because discovering it on the wall costs a second visit.

Questions this case answers

Can I replace just one indoor unit on a System 4?

Yes, but the replacement has to be one the outdoor unit recognises. On this Sengkang flat a head was ordered on room size and wall dimensions, and it would not pair once it reached the wall. The outdoor model plate lists the indoor combinations that unit can drive. That list narrows the choice, not the room.

How do I know which indoor unit matches my existing outdoor unit?

Read the model plate on the outdoor unit and check its list of matched indoor combinations. Photograph it before ordering anything. Capacity alone gets you partway. The head ordered here matched on capacity and even on branch pipe size, which is exactly why nothing looked wrong until the control terminals were opened up.

Do I have to change the pipework when replacing one aircon head?

Usually no. The existing run and drain line to that bedroom were sound and correctly sized, so both were reused and no chasing was opened in the wall. Flare joints at the head were cut back and made fresh rather than reconnected, because a flare broken once rarely seals the same way twice.

What happens if the wrong indoor unit is ordered for a shared outdoor unit?

It reaches the wall and stops there. A single split forms a closed pair, so a like-for-like swap usually just works. A shared outdoor unit drives several heads through one control line and only manages the ones it is built to recognise. Discovering that on site means the head goes back and a second visit follows.

Ready to get started?

Tell us what’s going on. Symptoms, setup, photos, anything we should know. We’ll assess and come back with the right next step.

WhatsApp us