Mitsubishi hyperheat multi-splits and modulation
I’m thinking about having an MXZ-4C36NAHZ installed. The indoor units would be one (1) MVZ-A18AA7 ducted air handler and and two (2) MSZ-FH09NA wall units.
The 4C36NAHZ submittals say it can modulate down to 6,000 btu cooling and 7,200 btu heating.
I cannot find the minimum operating capacity of the MVZ-A18AA7, but the similar PVA-A18AA7 can modulate down to 7,000 btu cooling. The MSZ-FH09NA units can modulate down to 1,600 btu cooling.
I had previously assumed that, as long as the sum total of the indoor units operating capacities equaled or exceeded the minimum 6,000 of the outdoor compressor, that the indoor units could modulate freely. For example, I assumed that, if the air handler were turned on and operating at 7,000 btu, that each wall unit would have the capability of operating at its minimum 1,600 btu, and the total load on the compressor would be 7,000 + 1,600 + 1,600.
However, I recently read a post from Dana Dorsett, who seems to be an expert, that the Mitsubishi MXZ compressors do not modulate. The post is here: https://www.greenbuildingadvisor.com/community/forum/general-questions/105304/mitsubishi-pvapuz-central-heat-pump-or-mini-split.
Dana wrote: “MXZ compressors are not full-on VRF systems. They will support a number of different heads/cassette capacities, but as I understand it even those heads/cassettes that can modulate when married to a dedicated compressor do NOT modulate when on an MXZ compressor. That means the heads have to be reasonably sized to their room/zone loads to hit the efficiency numbers, and provide reasonable comfort.”
Is this accurate? If that is true, then rather than being able to operating at 7,000 + 1,600 + 1,600, my three indoor units would have to operating at 18,000 + 9,000 + 9,000??
Please tell me this is not correct. I was very excited about the possibility of 7,000 + 1,600 + 1,600 operating capacities but am far less excited if the MXZ cannot modulate.
Also, I don’t see how it is possible that it does not modulate. If the minimum cooling capacity is 6,000 btu, as the submittal claims, and it requires a minimum of two indoor units to be connected to it, Mitsubishi’s smallest indoor unit is 6,000 btu rated cooling capacity. If there is no modulation, then the lowest capacity the MXZ would ever operating at is 6,000 x 2 = 12,000. If that were the case, what is the point of a submittal stating 6,000 btu minimum operating capacity?
So surely the MXZ series can modulate in the 7,000 + 1,600 + 1,600 fashion described above?
I’m hoping somebody can tell me that I mis-read Dana Dorsett’s post quoted above, and that the MXZ series does indeed fully modulate within the ranges stated on the submittals!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
One follow up. In this thread, Dana seems to indicate that the MXZ series will modulate an indoor wall unit as long as the sum total of the indoor wall units is operating at the minimum 6,000 btu capacity of the MXZ:
https://www.greenbuildingadvisor.com/community/forum/mechanicals/98944/mitsubishi-minisplit-reconfiguration
So I still have hope that if I had an MXZ-4C36NAHZ combined with an MVZ-A18AA7 + two FH09NA, that the indoor units would be able to operate at 7,000 + 1,600 + 1,600.
Please tell me it's true!
(Also, if it is true, can anybody actually confirm the minimum operating capacity of the MVZ-A18AA7? The 7,000 figure I'm using is from the similar--but obviously not identical--PVA-A18AA7.)
I'm not really an "expert" on the inner workings of Mitsubishis- I read the same specs you do and infer what I can, which often times isn't much. I don't install them for a living but have specified both ducted and ductless mini-splits on several projects, only to discover that many of the contractors installing them know even less about their characteristic than I do, and have had to push back (or change contractors) to get the right equipment installed. If that makes me an expert we're all doomed! :-) I figure I'm only a third the way up the learning curve (at best- that might be a delusion), and only a expert in relative terms.
I haven't done the deep dive on the MVZ documentation, but my current impression is that they don't really modulate at all. The reason you can't find minimum capacity numbers is because they doesn't modulate- it would be all over the documentation if they did, since that would be a great marketing feature. Most ductless heads will modulate if on dedicated modulating compressors, but generally don't when on a multi-split. The multi-split adjusts it's output in steps based on which heads/cassettes are calling for refrigerant, but don't try to serve multiple modulating-masters with proportional outputs the way full-on VRV systems do. The MVZ air hander submittal sheets show only one set of capacities are only compatible with MXZ multi-split compressors, which don't fully modulate. The capacity at 47F is published, but the capacity at +17F or lower depends on the compressor it's married to. They have multiple speed settings, but the blower doesn't modulate, though it will change speeds when necessary for defrost cycles, etc..
Having a requirement for a minimum of two heads does not mean that both must operate at the same time. There is something about the MXZ designs that keeps running some refrigerant through cassettes that are not actively calling for refrigerant or blowing air whenever any one head on the system is active, a "feature" that I've only become aware of in the past few months.
The PVZ air handlers do modulate, but only on their own dedicated PUZ compressors (or maybe on a City-Multi VRV system.) The turn down ratios are much smaller (less than 3:1) than typical Mitsubishi mini-split turn down ratios, so sizing them optimally requires more careful load analysis than with mini-splits.
There is a lot more control hardware & software required to run a full-on VRV multi-zone system, which is why step modulating multi-splits that don't modulate the heads have crummier efficiency spec, even when using the same heads that have the modulating smarts on-board to work with a dedicated single-zone compressor.
Ok, thanks for the response. If your understanding is accurate, that's pretty disappointing. Really makes the whole concept of a multi-split drastically less appealing. Arguably makes them less effective than even a run of the mill two-stage standard ducted heat pump.
Consistent with Dana's understanding, I note that for the indoor units like the MSZ-FH06NA, Mitsubishi provides submittals for both single-split and multi-split configurations. Here's the multi-split submittal: http://meus1.mylinkdrive.com/files/MSZ-FH06NA_For_MXZ_MULTI-ZONE_SYSTEMS_ProductDataSheet.pdf. It does not give a minimum capacity, nor does it give a range. Just gives one rated capacity for cooling and heating at 47 F.
I had always assumed that the multi-split submittals were just lacking in detail compared to the single-split configuration, which does provide minimums and ranges. But I suppose the lack of information is consistent with Dana's understanding that the multi-splits don't really modulate, and hence, wouldn't have the minimums and the ranges of the single-splits.
I may try to contact Mitsubishi and get some confirmation of this because I don't want to believe that the multi-splits have such poor modulating capability.
Oh well. I suppose I may create a separate thread seeking feedback on whether the first floor of my house is a candidate for multiple single-zone units.
Thanks for the feedback, Dana.
Dana, for retrofitting w/mini splits, are there models that DO modulate down to achieve the desired efficiency? Or is every minisplit install bound by these same constraints? (I agree this diminishes the appeal of investing in a whole house or multi-zone mini split setup! Seems to me like forced oversizing...) Thank you.
Daikin sells full-on VRV multi-zone systems targeted at the commercial (not residential) market but I think the smallest is 3 tons (?). Haven't really researched it. The Mitsubishi City-Multi systems are full VRV.
As I dig deeper into the Daikin VRV line-up, I found this which was just introduced earlier this year for the residential market:
http://www.daikinac.com/content/assets/DOC/Product%20Brochures/CB-VRVLIFE-12-17.pdf
Looks very similar to the VRV IV-S that is targeted at commercial, except the LIFE has a 2-ton unit and allows back-up/alternative heat-sources to be integrated into the system.
Jonathan, any idea how far down the Daikin can modulate? And what the minimum modulation is on the indoor units? I didn't see that in the brochure.
I'm also concerned that it would be difficult to find a competent installer for something like that...
Aun,
The submittal sheets for Daikin show a compressor operating range of 14% - 100%.
Your concern is justified, as even a great system can perform poorly if incorrectly installed. I reached out to my local Daikin dealer, who I also buy a lot of my plumbing supplies from, and he stopped selling Daikin and is now selling York. He conferenced me in with the York sales person who confirmed that they have a mini-VRF offering. York is owned by Johnson Controls who owns 60% of Hitachi and the York VRF are rebranded Hitachi units. They have a 3, 4 and 5-ton offering and the compressor operating range is 10% - 100%. Nominal heating capacity on the 3-ton unit is 40,000 btu's so it should modulate down to 4,000 btu's. The COP they are showing for this unit in a non-ducted setup is over 5 and still a high 3.9 in a ducted setup which are easily the highest I have seen. Oddly enough, the COP boxes on the Daikin submittals for the VRF Life units are blank.
You can find the York submittal sheets in the attached link under the product catalog section.
http://www.york.com/for-your-workplace/ductless-and-vrf-systems/variable-refrigerant-flow-systems/york-mini-vrf-outdoor-unit-208-230v-single-phase-heat-pump
Using the heating output and heating input provided in the Daikin submittals for the 3-ton unit, I calculate a COP of 3.5. They don't specify, but I assume that would be the COP at 47F for non-ducted.
I just got off the phone with mitsubishi tech support, and they verified that while the fan speed only moderates between 3 different levels, the cooling/heating capability of an MVZ air handler is attached to the modulating capability of the outdoor compressor. So while the fan on the air handler might be set to always run on the high setting (maybe to overcome static pressure in a poorly ducted system), the air handler could still vary anywhere between 6,000 and 36,000 btu's (if that is what the compressor is capable of)
Which means that while a 3 ton PVA/PUZ unit can only modulate down to 18,000 btu's, a 3 ton MXZ/MVZ combo can modulate down to 6,000 btu's. And according to this link, 1:1 pairing of larger MXZ/MVZ units is possible.
http://meus1.mylinkdrive.com/sfiles/Application_Note_1029-MVZ_MXZ_configurations_v1.3.pdf
It looks like only 3 ton and larger can be set up 1:1, and only if connected through a branch box, not directly connected to a port, and there is no AHRI testing of that configuration. But it's good to know that the MVZs can be set up on their own. (That was news to me! Thanks!!) I'm left wondering why the 1:1 configurations via the branch box isn't AHRI tested & listed?
My more recent understanding was that multi-splits don't modulate continuously, but only in larger steps comparable to the maximum capacity of a half-ton head (or maybe in quarter-ton steps, half ton minimum) which means only larger heads/cassettes) would have a shot at modulating with load (and only at larger loads.)
"I figure I'm only a third the way up the learning curve (at best- that might be a delusion), and only a expert in relative terms."
-Dana if this is true, then there's very little hope for any of us.