There's some debate about whether the best configuration of split systems for multiple rooms is to have multiple single splits or to use multi splits.

By Richard Keech  2018-06-18

[caption id="attachment_media-22" align="alignnone" width="3264"] Multiple single-split systems can be an eye sore[/caption]

Introduction

Increasingly split systems are used to heat and cool entire homes - not just single rooms.  The default way that split systems are deployed is what I call single splits (and sometimes referred to as unitary split systems).  These have one outdoor unit piped to one indoor unit.  And repeat that configuration for each conditioned room.

The alternative, available from many vendors, is a multi-split. This configuration involves more than one 'head' unit inside for each outside unit.  Often a whole home might be served with only one (larger) outdoor unit, and heads in all bedrooms and living spaces.

The single-split argument

Some would argue that having multiple single split installations is best because:

• multi-splits typically can't allow simultaneous operation of all the head units at their rated power, and when this is attempted the efficiency drops because the size of the outdoor unit (relative to all the indoor units) is just not big enough;
• typical installation arrangements of a multi-split would require longer lengths of pipe - increasing cost and reducing efficiency (all other things being equal); and
• the centralised configuration (ie based on a single outside unit) makes some failure modes more problematic, eg a refrigerant leak might be harder to troubleshoot, and the dependence upon a single outside unit presents a single point of failure. So it's possible for a single failure to take out all heating and cooling for the home.

The multi-split argument

There's an element of truth to all the above points.  However, on balance I think that multi-split systems are very often the best solution because:

•  Simpler overall, and greatly reduced visual impact on the outside of the house. Small- to medium-sized homes would usually only have to worry about fitting one unit outside;
• Greatly reduced standby power. Most split systems have a non-trivial standby power that is mostly associated with the outside unit. I measured mine at 15-18W continuous. (the US7 unit seems to be an outlier with very low standby power being reported). So for most situations, reducing the number of outside units can make a big difference.  Remember each ~11W of continuous standby power is 100kWh/annum wasted.
• It simplifies the electrics.  The mains electrical wiring is simplified (and cheaper) in proportion to the reduction in the the number of outside units;

It would typically be rare to be running all inside units flat out at the same time, so the issue with reduced efficiency in that case is perhaps less of an issue than some may imagine. And it can work the other way too, ie if you have only one head operating in a multi-split configuration (a common situation) then the single outdoor unit will provide (relatively) very good heat-exchanger area, so efficiency will be enhanced (compared to a typical single split with the same size head unit).

It's true that the pipe lengths will usually be longer for multi-splits, and sometimes this might be an issue. However this is not a big deal, and (at least partially) balanced by the simplified electrical wiring for a multi.

Because of the longer average pipe runs I'd recommend higher-than-usual level of pipe insulation (probably a good idea in any case).

It's true that a multi-split might have a possible single point of failure, ie there's a greater chance that a single failure will take our your entire home's heating and cooling. However, the better-known current systems on the market are mostly mature and reliable.

Having your cake and eating it too

Sometimes a mix of single- and multi-split systems might offer the best. For example a home might want the unique features and awesome efficiency of the Daikin US7. However the US7 doesn't come in a multi-split configuration, and it's expensive.  So for some a good compromise for a medium- to large-sized home might be:

• A single US7 unit serving the main living area where most of the heating and cooling work is done; and
• One multi-split installation serving all the other conditioned rooms.

Such an arrangement would be cheaper and visually simpler than having US7 units in every conditioned room.

Visual impact is the show stopper

I think for many people the prospect of perhaps five to seven outside units scattered around a house in an installation with multiple single splits is single the biggest problem.  In other words the potential visual impact of split systems on the outside of the home is a show stopper for many households.  A multi-split configuration makes it much easier to position the outside units discretely.

Conclusion

I've had, so far, six or seven years of reliable performance from my Daikin multi-splits. Multi-splits are very often going to strike the best balance for home heating and cooling. I have no hesitation recommending multi-splits when the situation arises.

Reference

1. Post here by Lee Booy which itemises efficiency of different single and multi-split units.  Includes a link to a Google Sheet.