I see that as you add more heads the SEER decreases. Is this decrease in efficiency only when all heads or operating or just because all three are plumbed together? Second question, I see that in some cases with multiple head systems the total BTU of the heads can exceed the rating of the main unit. Why is this? Thanks

The efficiency of multi-splits is lower than single-zone mini-splits primarily because the individual heads don't modulate (much) with the load. The compressor modulates up/down in steps based on the capacity of the heads that are currently calling for refrigerant. Single zone mini-splits modulate over a wide range, and there is a "sweet spot" where it's efficiency is maximized, which doesn't coincide with the nominal output of the head when married to a multi-split.

In many cases it's allowable to install more head capacity than compressor capacity, but when all heads are calling at once the total capacity is limited by the capacity of the compressor.

When specifying multi-splits it's important to keep the heads' oversize factors for their loads roughly proportional to each other. If one large zone's head is nominally right-sized, and the heads in the bedrooms are oversized by 2-3x just the bypass refrigerant from the big zone running through those low-load room at high duty cycle can overheat/overcool those rooms even when the low load room heads are "off".

This is the other argument against the "head in every room" approach- not only is it more expensive up front and less efficient it's often lower comfort. Splitting the output of mini-duct cassettes between a few low-load rooms is the better solution.

Thanks!

It appears to be cheaper and more efficient to use individual outdoor/indoor units than one outdoor unit with multiple cassettes.

Posted By littleboss on 17 Feb 2019 05:39 PM
It appears to be cheaper and more efficient to use individual outdoor/indoor units than one outdoor unit with multiple cassettes.

That is generally true. The multi-split option is often considered "worth it" when the number of heads/cassettes gets high enough that the visual clutter of all those compressor units (they're only "condensers" during the cooling season) outside becomes overwhelming. For new construction there's never really a need for that, even building at IRC code-minimums, even less so when going better than code and taking site factors & orientation to the sun into consideration during the planning phase.

This discussion has changed my plans for how I replace carbon fuel heat. I have a single 15K for much of one zone that I am pleased with both the performance and efficiency of. I had been thinking of a second zone solution with an 18K and two heads, but am now planning to go with a single 12K, pending contractor agreement for my old construction home. If I ever do the 3rd zone (a guest area seldom used), I plan the same. Great info!