Mini Splits, Single vs Multi Zone Condensers Sanity Check

[Bob Beasley]

I've got plenty of out door space for multiple condensers as well as room in my panel for a dedicated 240 circuit for each. Piping runs are also not a consideration with my install.

It seems there might be a penalty for using a Multi Zone condenser? Lets say, for this example, I've got 12K and a 9k air handlers hooked up to a 2 zone condenser. The 12k is modulated down to 4k but the 9k is calling for all 9k. How does the Multi Zone condenser handle this?

Same situation as above but with separate condensers. Would one solution be more efficient than the other?

Let's assume both solutions are within the min modulation range of all units.

Oh, I live in the tropics of South Louisiana so I pretty much stay focused on the cooling side of things.

Many Thanks!

 

[Dana]

The control algorithms used by mini-split vendors are proprietary- they don't tell you exactly how they work, or how they prioritize the split in refrigerant volumes between heads. The heads sense the load by the incoming air temperature, and modulate up/down with the load. They work most efficiently when cruising along a part load, and least efficiently at full-on maximum.

If the 12K head is modulated down to 4K, it's keeping up with the load, and the temperature in the room is fairly steady, it's doing the right thing. If the 9K head is calling for the full 9K and running continuously and not over-cooling the room, that too is doing the right thing (but the 9K head might be undersized for that zone if it's not quite keeping up.) Sizing the head correctly for the zone is also important for efficiency- too large or too small is a problem.

If they are cycling on/off rather than modulating it's an efficiency problem. The limitations of multi-splits is usually the minimum modulated output of the compressor, which is often much larger than with individual mini-splits, which causes the compressor to cycle on/off when the total load is low, which cuts into efficiency. When deciding whether to go with individual mini-splits rather than a multi-split it's important to look at the minimum modulated output numbers on their submittal sheets (lower is better, as a rule.) A pair of mini-splits with a combined minimum-modulated output substantially higher than the minimum modulated output of the 2 zone compressor won't necessarily be more efficient.

Of the name-brand mini-splits I like the Mitsubishi FH09 NA, since it can deliver over a 12,000 BTU/hr of cooling, but can dial back to 1600BTU/hr. Even a pair of those would have much lower minimum modulated output than anybody's 1.5 ton or 2 ton multi-split. Of the second tier vendors, LG's Art Cool Premier series can modulate down to about 1000 BTU/hr, whereas their multi-splits won't run anywhere near that low.

 

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[Bob Beasley]

The control algorithms used by mini-split vendors are proprietary- they don't tell you exactly how they work, or how they prioritize the split in refrigerant volumes between heads. The heads sense the load by the incoming air temperature, and modulate up/down with the load. They work most efficiently when cruising along a part load, and least efficiently at full-on maximum.

If the 12K head is modulated down to 4K, it's keeping up with the load, and the temperature in the room is fairly steady, it's doing the right thing. If the 9K head is calling for the full 9K and running continuously and not over-cooling the room, that too is doing the right thing (but the 9K head might be undersized for that zone if it's not quite keeping up.) Sizing the head correctly for the zone is also important for efficiency- too large or too small is a problem.

If they are cycling on/off rather than modulating it's an efficiency problem. The limitations of multi-splits is usually the minimum modulated output of the compressor, which is often much larger than with individual mini-splits, which causes the compressor to cycle on/off when the total load is low, which cuts into efficiency. When deciding whether to go with individual mini-splits rather than a multi-split it's important to look at the minimum modulated output numbers on their submittal sheets (lower is better, as a rule.) A pair of mini-splits with a combined minimum-modulated output substantially higher than the minimum modulated output of the 2 zone compressor won't necessarily be more efficient.

Of the name-brand mini-splits I like the Mitsubishi FH09 NA, since it can deliver over a 12,000 BTU/hr of cooling, but can dial back to 1600BTU/hr. Even a pair of those would have much lower minimum modulated output than anybody's 1.5 ton or 2 ton multi-split. Of the second tier vendors, LG's Art Cool Premier series can modulate down to about 1000 BTU/hr, whereas their multi-splits won't run anywhere near that low.

Would you rather have a 12K unit, keeping the temp steady, running wide open or a 18K unit keeping the temp steady while modulated back to 12K? Assuming the 18K unit stays above it's min modulated output most of the time. Thanks!

 

[Dana]

I'd rather do the load math carefully and size accordingly.

When the instantaneous load is 12K a 1.5 tonner modulating at 12K would usually be considerably more efficient than a 3/4 tonner capable of 12K running at it's maximum, but you really have to pay attention to the average loads comparing against the minimum modulated output to keep it from running into cycling. The 1% design condition is only met or exceeded 87-88 hours per year, and running at really great efficiency for 1% of the time isn't worth trading for compromised efficiency at the average load.

The turn-down ratios vary (a lot!) from model to model, vendor to vendor, and it's a moving target (they're getting better every year), but as a general rule of thumb (with many exceptions), when a mini-split at maximum speed is more than ~1.5x the 1% design load it begins to lose efficiency to cycling on/off at lower loads.

 

[Bob Beasley]

I'd rather do the load math carefully and size accordingly.

When the instantaneous load is 12K a 1.5 tonner modulating at 12K would usually be considerably more efficient than a 3/4 tonner capable of 12K running at it's maximum, but you really have to pay attention to the average loads comparing against the minimum modulated output to keep it from running into cycling. The 1% design condition is only met or exceeded 87-88 hours per year, and running at really great efficiency for 1% of the time isn't worth trading for compromised efficiency at the average load.

The turn-down ratios vary (a lot!) from model to model, vendor to vendor, and it's a moving target (they're getting better every year), but as a general rule of thumb (with many exceptions), when a mini-split at maximum speed is more than ~1.5x the 1% design load it begins to lose efficiency to cycling on/off at lower loads.

I really appreciate your thorough replies!

I live in a all cypress Acadian Cottage built in 1896, it was 800 Sq Ft. We added an architecturally correct, but well insulated, 700 Sq. Ft. addition. The original cottage and addition are open to each other, it looks like it was made this way.

A Mitsu installer came out and measured, then sent it all to Mitsu to size. His recommendations are a 12k, 9k,12k for the downstairs and another 12k for the 400 sq. ft. loft, included in the original 800 sq ft cottage. He told me this was his best guess but due to the unusual nature of my house it was just an educated guess.

I'm going to install a unit myself and start with the new room that was spec'd for a 12K unit. We currently have a 12K window unit in there and while it struggles, with outside temps above 92 degrees, it does an okay job.

I'm looking forward to the install. I'm not in a hurry and I'm not afraid to make mistakes, even ones that cost. My idea is to tip toe in and see how it goes.