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Community and Q&A

Adding Multiple Zones for One-to-One Minisplit System

maxwell_mcgee | Posted in Mechanicals on

I’m working with my mechanical designer now to begin to specify the HVAC system for our new build in Southern Ontario Climate Zone 5/6.

Right now the plan is to use ducted cold climate Mitsubishi minisplits — we’ll have 3 units, 1 per floor. Each indoor fan coil will be 1:1 paired with an outdoor unit. It’s a large house (7000+ conditioned sq ft) — we haven’t run the energy model yet, but given the size of the footprint, 3 systems doesn’t seem unreasonable/oversized to me (will confirm when we run the numbers).

My question — can we achieve any more granular zoning than this in our home? In particular, if I think about our upstairs level where the bedrooms are, there will be quite different heating/cooling loads on the South-facing bedrooms vs. the North facing ones. Not to mention differences in temperature preferences between the occupants of the rooms.

Similarly, on the main floor, there are pretty significant differences in the amount of glazing on the West vs. East side of the building.

What options are there for more zoning? I don’t want to move to a multi-split system as I understand there are a lot of comfort issues with those.

Mitsubishi used to offer their own zoning system, but a) it doesn’t work very well: https://www.greenbuildingadvisor.com/question/a-warning-and-a-question-about-mitsubishi-zuba-zoning and b) it’s apparently no longer supported if you’re using their built in heat-strip system for backup heat in extreme cold conditions.

What else can be done re: zoning? Are Smart Registers (e.g., Flair) a good idea and do they work? Other options?

Should I actively consider moving to more than 3 1:1 systems? Maybe 5 systems so long as each indoor/outdoor pair has a high turndown ratio?

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Replies

  1. Expert Member
    AKOS TOTH | | #1

    With a properly sized heat pump, I can't see needing backup strip heaters in southern Ontario. If you really must have one, you can put one into the main floor unit which can be left un-zoned.

    You can do zoning with ducted mini splits a couple of simple ways. The airzone setup is the best but does require careful setup as you can see from that thread.

    The simplest zoning is to install dampers driven directly by a local thermostat into areas you are looking to have some control. This does not tie into the air handle in any way. The key is to set it up so that the zone is over previsioned when the damper is open and adjust the closed position so it doesn't close all the way and deliver some amount of flow (say 50% of required). This way the zone damper is only modulating the flow instead of on/off control and most ducted units will have no problems modulating around that much flow change. This should only be done for smaller sections, so even if all zones are in closed position you get sufficient airflow through the air handler.

    The other option is to use a thermostat interface module and use an off the shelf zoning kit such as the Honeywell one. This is not perfect as with most thermostat modules you loose some level of modulation but with a bit of care configuring the setup, it can be made to work.

    Overall if you are looking for comfort, you best option is to improve your envelope. Simple things such as air sealing and making sure windows are properly shaded can make a big difference in comfort for not much extra cost. Aiming for above code R values also helps.

    1. maxwell_mcgee | | #3

      Thanks, Akos.

      We're going to be building to a pretty good house style standard. ACH of ~1.0, glazing R-values of R5 or more, walls of R30, etc. We'll pick our SHGCs on our windows appropriately too.

      My biggest concern is that my SO is a hot sleeper who needs the temperature ~3-5 degrees cooler in the bedroom than the rest of the house to get a decent night's sleep. If the issue were that we needed to over-heat a single room, that'd be super easy, as I'd get a small electric resistance space heater and be done with it. But for differentially cooling a space I'm a lot more worried that without zoning, there's not much else that can be done.

      I like your flow modulating damper idea a lot though. I imagine it shouldn't be excessively expensive to implement either.

      You mentioned Honeywell but are there any other thermostat / controls systems that can handle this type of setup?

  2. insaneirish | | #2

    This is primarily a heating/cooling load problem rather than a zoning problem, which reduces the situation to a standard duct design problem.

    If the proportionality of load swings wildly between seasons (e.g. in winter, room A has a 1,000 BTU/h heating load and room B has a 2,000 BTU/h heating load, but in summer, room A has a 2,000 BTU/h cooling load and room B has a 1,000 BTU/h cooling load), you should probably look to optimize the glazing in each room appropriately, but I wouldn't see this as a reason to try to zone the rooms differently.

    I would probably never try to damper zone a single family residence unless it's commercial sized. Too much complexity for not enough benefit.

    Three cheers for this comment from Akos: "Overall if you are looking for comfort, you best option is to improve your envelope. Simple things such as air sealing and making sure windows are properly shaded can make a big difference in comfort for not much extra cost."

  3. greenright | | #4

    I know you stated you dont want a multi split, but bedrooms can be handled by a multi. In my experience full VRF setups work well with multi as their turn down is very good in the 3-5 ton range. I have installed both Mitsubishi PUMY and Fujitsu Airstage VRF in very large houses (10-15k sqft) with great success.

    1. Expert Member
      AKOS TOTH | | #8

      Would you have some more information about these systems?

      I know all of these rely on a branch box for connecting the indoor units. How low can the indoor units modulate? Is there any cross talk between zones? A bedroom in a well sealed house is under 2000BTU heat load so it is still well bellow the smallest head most manufacturers sell.

  4. walta100 | | #5

    Given the high quality of the building envelope you have described I think 3 zones is enough assuming the rooms all have similar percentages of glass to walls.

    You do have a room by room manual J?
    look at the heat load for the north and south rooms for heating I am guess they will very similar by the square foot of exterior wall.

    My guess is to get enough heating capacity your units will be more than 4 times larger than needed for the cooling loads. If you feel you want more zones

    I think you would be better off installing with 6 tiny systems than 3 systems with 2 zones each.

    If you have a good envelope the temperatures in the rooms in the envelope will all be very similar, zoning is necessary when rooms have very different loads than other like when one room is all glass.

    Walta

  5. johngfc | | #6

    Given the (large) size of the house, expensive HVAC setup, and differential heating demands, would this be a situation to consider a residential VRF system?

    1. maxwell_mcgee | | #7

      Thanks for mentioning this, and please pardon my ignorance -- but what exactly is a VRF?

      In my naive mind, I thought a mini split system is capable of Varying its Refrigerant Flow which is why you have these turndown ratios in the spec sheets.

      So how is VRF different? I've never really understood.

      Thanks

      1. jameshowison | | #9

        I think in this context johngfc is refering to a ducted system with a central blower. ie something like a Mitsubishi "Multi Position Air Handler" https://mylinkdrive.com/USA/M_Series/R410A_Systems-1/Multi_Position_Air_Handler-4

        Just a different size/form factor for VRF than the ducted minisplits that Akos is talking about. The damper stuff would be identical. Main difference with the multi-position units is that you can service them from the side (like a traditional single stage AC/Furnace) rather than from below like the ducted minisplits.

  6. johngfc | | #10

    Best for someone with more expertise and (any!) experience to respond, but in the meantime: I wasn't referring to a central blower system, but a more efficient system for moving the refrigerant around. Here's a site with a simple explanation: https://www.ferguson.com/content/trade-talk/tricks-of-the-trade/what-is-a-vrf-system
    My understanding (which is limited) is that VRF systems are very common in commercial applications where building size is larger and heating zones more heterogeneous. They typically are not competitive for residences because most the compressors (heat pumps) are larger and the initial cost is higher. For appropriate situations (size, complexity, climate, building specifications), the advantages can be: (1) more energy efficient; (2) very high degree of modulation, (3) can have many independently operating zones (e.g. bedroom much cooler than adjacent room), (4) can heat and cool at the same time, (5) it's easier and less expensive (?) to move the heat/cool around via small refrigerant pipes than much larger (and noisier) ducting.

    I'm sure there are more considerations. For a 7,000 sf house you're well out of my league and into options not really practical for smaller residences. Again - worth consulting a pro about this. I'm sure I recall an blog on GBA in the past couple years about an architectural/building firm constructing their new HQ and installing a VRF system - can't find the link - who remembers this?

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