Are ducted multisplits so bad?
In Martin’s great article “Getting the right minisplit” he goes into the drawbacks of multi-split units.
The article doesn’t explicitly say so, but it sounds like those drawbacks are associated with ductless units. In a ducted setup, do multi-splits have the same drawbacks?
I’m thinking of the situation of using a single multi-split paired with two air handlers in a ducted setup for a small two-story house. One air handler for upstairs, one for downstairs.
Since there are no “heads” that can be set to different temperatures, and since you shouldn’t use setbacks on an ASHP anyway, do the loading and efficiency concerns in Martin’s article still remain?
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I don't know, but can you explain the logic behind how the two indoor air handler coils are any different from two indoor 'heads'? To me, all the same concerns seem to exist.
I think the difference regards humidity. Ductless head units work best in arid climates, where the dehum duties are minimized. They don't accumulate mildew as much as they might in humid areas. Depending on how muggy your summers are, it may be better to go with an air handler since there is one central place for condensate to be drained and where cleaning takes place.
With ducted you can tie into existing ductwork, which is convenient if you don't have access to an attic or crawlspace for an all-new distribution design. The obvious drawback is that ductwork itself is bulky, sometimes noisy, and needs cleaning, not to mention how leaky ductwork can be, resulting in negative pressure on the home and energy inefficiency.
It’s not ductless vs ducted, it’s oversized vs right sized. If it’s right sized, you’ll be fine.
Ducted has a couple advantages,
-easier to right size with room combining.
-Better mold management (usually the fan is upstream of the coil, whereas ductless it's after).
-Easier serviciablity depending on location (square metal box is easier to work on than plastic that is clipped to together on your living room wall).
-better air distribution/comfort due to using diffusers (my ductless is fine until it's gets really cold (-15degC and below), and the house starts to feel cold in the corners due to less than ideal distribution).
-Better filtration options.
With a multisplit the modulation range is typically much narrower than a 1:1 so right sizing is even more important. My preference is to even slightly undersize heating in a heating dominant environment and have a secondary auxiliary heat source available to make up the difference (maybe heated bathroom floor/towel warmer, etc).
If I could do it again I would skip the ductless and go ducted, mine loads up with mildew far to quickly and starts to smell bad and is a pain to clean. (Mits FH09)
Scott and Josh,
You both make points why you think ducted is superior to ductless, but so far as I can tell none of them address the question of whether using a 'multi-split' setup with ducted has the same issues as with ductless. Again, I don't know the answer, but I have yet to hear a reason why the issue described in Martins article (and elsewhere) would not apply to any multi-split refrigerant system including ducted.
As far as dehumidifying capabilities of ducted units, what is it about them that allows for better humidity removal? Lower airflow over the coil?
In our ‘nearly passive house’ we worked with the HVAC designer and decided the best option was to use three air handlers each with their own outside unit to supply the seven zones of the house. They made the argument that this allows each of the outdoor units to run closer to their optimal, most efficient run point while also providing redundancy. I agreed and we are happy with the overall results and the amount of control we have in setting temperatures and maintaining comfort levels in all seasons. We did have to move one of the zones from one handler to another when we realized that the solar gains on the south side of the house was causing one of the units to run more often than the others which just emphasizes how important it is to get the load calculations right.
Just one other advantage of using ducted, btw, was the ability to add steam humidifiers to maintain the indoor RH during the (PA) winter.
Brian,
I think that a multisplit arrangement as you describe -- one outdoor unit connected to two air handlers -- will be subject to the problem I described (the problem that occurs with a single multisplit outdoor unit connected to several indoor heads).
As Dana Dorsett explained, "The multi-split compressors don’t modulate continuously with load, instead operating in steps based on the size of the zone calling for coolant. There is always a modest amount of ‘extra’ that gets distributed to the other heads, even if they’re ‘off,’ to avoid returning liquid coolant to the intake side of the compressor."
Thanks everyone.
I guess I had incorrectly interpreted "ductless" as equivalent to "spot heating" where there might be a lot of variations in set points and loads around the house based on room-to-room preferences.
For our ducted setup, I imagine we'd try to keep the whole house at about the same temperature, partly because the house is small and partly because I understand we shouldn't be setting back thermostats with heat pumps.
The multisplit issue sounds like it is a problem of unequal loads.
From what I've been told by HVAC installers, the downstairs system will do most of the work in the winter and the upstairs will do most of the work in the summer. So maybe there's no way to avoid the unequal loading issue.