Help Sizing A Multi-Zone Mini Split System - Minimum Capacity?
We're considering a multi-zone mini split system for an older home (built late 70s) in Massachusetts. With the inverter technology, my understanding is that the compressor can ramp up and down btw its rated minimum and maximum capacities. What about the indoor units?
For example, consider Mitsubishi - if we install a 3.5 ton outdoor unit paired with several indoor units, what happens if/when those indoor units are operating solo (for whatever reason) and they are below the outdoor unit's minimum capacity (9K indoor on a 12K outdoor which seems quite common for M's outdoor units).?
The way I understand is that is that indoor unit will not be realizing the benefits of inverter technology if it's ever running solo. I imagine that the indoor unit will only be able to output 9K of BTUs, what happens to the difference from 12K minimum? I think that I've seen mention how the remaining BTUs will get sent to the other zones so they might overheat/cool even though they're not calling for heat/cool?
Comments
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Your math is fuzzy. You first mentioned a 3.5 ton outdoor unit (42K), then give an example of a 12K outdoor with a 9K indoor, which isn't a thing as far as I know.
Multiple indoor heads are matched to the outdoor unit by the manufacturer. Using your 3.5 ton, and 4 indoor heads of 12+12+9+9, a 9 can certainly run by itself and still "realize the benefits". The indoor units have thermistors to sense air and coil temperature. With manufacturers like Daikin and Fujitsu, put the fan on auto, and it will ramp, and correspond to the load. The refrigerant can be diverted to other heads on low loads, but its nothing that will be noticed in those spaces. All heads should always be on, anyway. Set the temperature, and leave it.
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Sorry, when I mentioned 12K outdoor, I neglected to state minimum capacity there.
Basically, I'm just concerned about having too many units that are below the outdoor unit's minimum capacity (i.e, 9K indoor on some Multi-zone outdoor unit that has a minimum capacity of 12K, as an example). Trying to determine if that's something that I should worry about. I've seen people comment that it is but since the vendor lists them as valid, I wonder if it is OK.
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Don't worry so much about the manufacturer if you stick with the major brands. Worry about the installer.
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I wish it were that simple, I've had some rotten luck with installers that presented well and then unfortunately, not so much. That's why I feel like I need to double check and understand these things like this.
So, in any case, is it a good idea to install indoor units that are below the minimum capacity of the outdoor unit?
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the manufacturer specifies what size and how many indoor unit units per outdoor unit.
if you’re doing a whole house go with two or three complete systems. If or when something goes wrong you’re not dead in the water.
who’s going to install these?
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So it sounds like you're saying that if Mitsubishi says that you can install a unit of a certain size (even if it's below the minimum capacity) that it'll work fine. But just so I understand, that unit will run at 100% of its rating and the outdoor unit will run at its minimum capacity as long as it's the only one calling, right?
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Typically modulating heat pumps can go down to about 25% of capacity. So a 12K minimum output from a 3.5 ton unit sounds about right.
You're absolutely right that if you have a 9K head in this system, and it is the only head calling, something has to give. Different manufacturers have slightly different approaches, but that head is going to be full on, regardless of the actual load, and the extra capacity is somehow going to be shared among the other heads that aren't calling.
The practical limit to how many heads you can have on one compressor is when this behavior starts creating unacceptable comfort.
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Thanks for confirming that… so should this be avoided if possible? Or am I making a bigger issue of it than it truly is?
Something else I am wondering - how much would this affect CoP and how much potential for short-cycling? An earlier installer proposed units that were were already larger than their heat loads (for example, a 7K for a 3.5K BTU load, a 12K for a 4.3K BTU load). Seems to me like that smallest one is pretty oversized and really below the 12K minimum capacity.
Those are probably not a good approach, right?
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NO ONE makes a system that matches your Heat load / loss under all conditions. This is where the installing contractor is the most important.
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If your trying to go ALL Electric then multiple systems are a must. Too many computer controls and so few trained techs that understand them.
In Mass I strongly recommend keeping the Steam / Hot Water as a back up and for those cold windy nights.0 -
We have hot water baseboard as the existing heat and will be keeping that as a backup.
OK, so I guess it's just something that's just the way it is and while not ideal, just something that we all have to live with, right?
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Again
Who’s installing these?
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Of all the things to be concerned about with a multi head ductless system, that is way down at the bottom of the list. Its not even a concern. It just is what it is.
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OK, this is seriously odd… I posted earlier that we don't have an installer yet, still getting proposals, and that Premier Energy Solutions was the only one so far with Mitsubishi… yet, the post doesn't seem to appear above… weird.
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why do you keep posting a link.
who else have you con?0 -
What? "Who else have you con?" - what does that even mean?
You asked who was installing it so I told you… I guess I'm not supposed to do that?
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Who else have you contacted?
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There is a couple of limits with these. The outdoor unit still has a minimum capacity, generally that minimum capacity goes up as more indoor heads are added as well. Especially on these many small indoor head systems, the min modulation tends to be a above the load of a single head so the unit will have to run refrigerant through zones not on. This also means that the indoor heads never modulate, they are either fully on or off. This tends to create issues with humidity removal and comfort.
Unfortunately this info is not well documented and you won't know how it will run until installed.
On heating it gets even worse as the unit needs to flow refrigerant through all the heads. This tends to tank the COP of the unit when only a few heads are calling for heat. It also means that zones not on will also get heat which tends to create overheating issues.
Overall you want to stay away from multi zone units.
If you do go with multizone, the heads need to be sized very closely to load. There are very few bedrooms out there with above 3000BTU load, so this won't work for most houses as even the smallest head is about 2x or 3x oversized. The best setup for a multizone is a small number of large zones. Something like a pair of ducted air handlers feeding a bunch of rooms works pretty well.
It would really be great if these worked as there are no easy options for places with a lot of rooms.
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This is exactly the sort of thing that I am concerned about. The manufacturer says that a given outdoor unit can support up to 8 and so an installer just thinks that they can add a unit to every room. All this without an consideration for how each individual unit will work solo.
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if there not a match trouble codes go up.
all the equipment communicates with each other.0 -
Mitubishi has an app note specifically to address this issue but only nerd would read that.
If you do go with a multi, make sure to ask wired or wifi remotes for it. This at least allows you to program in fan off in cooling and heating which fixes some of the comfort issues.
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