GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Single Ducted Minisplit in Attic vs Heads in Each Room

Jonathan B | Posted in Energy Efficiency and Durability on

A question for the community: Is it better to have 4 mini-splits in bedrooms, or a single ducted mini split serving all the bedrooms from an unconditioned attic? The main goal is cooling.

We’re in California, Zone 3C, in a 1961 house with a ventilated, unconditioned attic. The second floor is 1200 sq ft with electric baseboard heaters and no AC. Based on a Manual J calculation from CoolCalc, 15000 btu is plenty of heating, and 18000 for cooling. However, there’s an open staircase to the first floor, and a lot of the cooling will flow downstairs.

Here are the pros and cons that I can think of:

In-room:
Pros:
* No duct loss
* Mini splits are in-room, not in unconditioned space
* Each room is its own zone
Cons:
* Oversized. Even 9000 btu is more than needed for a 150 ft2 enclosed bedroom. 4 rooms * 9000 btu = 36000 btu, which is about double what we need.
* No way to heat or cool the bathroom or tiny office if the mini splits are in the bedrooms
* Expense: 4 mini splits vs 1 ducted unit.

In-attic:
Pros:
* A single ducted mini split with registers can easily handle all of the rooms, including the bathroom and tiny office
* Less visible, since a register is much smaller than a wall or ceiling unit
Cons:
* Need to run ducts
* Duct loss and unconditioned space likely means a big loss of efficiency

Some ideas we had:
* Turn the attic into conditioned space. Unfortunately, there’s not really enough space to get in there and add more insulation, except maybe by blowing it in. The roof was recently replaced and we added solar panels, so we don’t want to add a layer of foam on top and redo it all.
* Put the indoor unit inside the conditioned space, like in a soffit or a closet, and run just the ducts through the unconditioned attic. The ducts would be R-6, most likely.
* Build soffits and run ducts inside the conditioned space. In practice, the only place we could do this would be down the length of the hallway.  We’d have to move the lighting and smoke detectors. We’d end up with a long hallway with a low ceiling. Might be ok?
* Add a window AC in a couple rooms. The windows are all horizontal sliders, so not a great match for a window AC.

Any other ideas? What solution would you choose?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.

Replies

  1. Jonathan B | | #1

    Here's another possible solution, from the archives:
    https://www.greenbuildingadvisor.com/question/retrofit-single-ductless-minisplit-to-cool-entire-upstairs

    The idea: put a single ductless mini split in the hallway and leave the bedroom doors open during the day

  2. Deleted | | #2

    Deleted

  3. Charlie Sullivan | | #3

    The option of just putting the heads in the hallways is particularly viable for a cooling dominated climate where the peak load is in the middle of the day when the doors are open.

    Another option is to add an insulated enclosure in the attic for the ducted units and ducts. I'm not sure what that needs to look like without more details of the layout. People build whole rooms to form conditioned space for full-size air handlers. This might be much smaller.

    Depending on the layout, you might do better with two units instead of one, to shorten the duct runs and enable more of it to be in closets more easily--oh I see the layout. That does seem to lend itself to two units, one at each end of the hall.

    One final option is to go with an air-to-water heat pump like Chiltrix, which allows very small "heads" in each room without the oversizing problem. To me, that's the nicest solution, but finding installers and designers with the right skills is hard.

  4. jrpritchard | | #4

    I would suggest using a ducted unit as opposed to multiple heads or cassettes. I have found that customers are more comfortable with registers as opposed to more frequent cycles and higher airflows. One thing most people dont realize is that each piece, indoor or outdoor, has at least two control boards plus their own fans, motors, wheels, metering devices. Having 4 indoor units, plus a single outdoor unit would result in at least 10 control boards, multiple temp controls, blowers, wheels, fans, 16 refrigeration connections, 4 condensate lines, four filters to change. The list goes on but the point is that with a lot of these systems there are a lot of things that can go wrong. One to one systems tend to be simpler, more reliable and easier for customers to take care of. Also, I agree with you that there is some loss of efficiency but we find a lot of multi zone systems that are not charged correctly which kills your efficiency from the start. Installing a one to one system make it much easier to charge them correctly which is critical for efficiency.

    1. qofmiwok | | #7

      Can you explain this further? Why 16 refrigeration connections for 4 indoor units? Why 10 control boards?

      1. jrpritchard | | #9

        Each indoor head is connected to the outside unit with a line set which had two copper lines. So each head has two connections at the head and two connections at the outdoor unit. These are usually flare joints which are prone to leaking if not done correctly. 4 indoor heads x 4 connections per head. Most of these units modulate based on temperature and pressure and it takes a lot of controls and sensors to do that. To accomplish that most have at least two control boards per indoor unit and at least another two controls for the outside unit. There are a lot of successful installations with multiple indoor heads but more often than not I have found that the ‘head in every bedroom’ systems do not meet peoples expectations for several reasons

        1. Expert Member
          Akos | | #10

          I can attest to the compounded reliability problem here. I had to replace a blower motor in one head already (squeaky bearing) and I can hear another one is on the way. Having also cleaned three blower wheels last year, I can say it is you want to avoid.

          Stick to slim ducted unit with an easily accessible over sized filter.

          If you want a bit of zoning, what you can do over provision that branch with say 120% of the flow required. Install a motorized damper+thermostat on the branch and set the damper stop so it allows about 80% of the flow when closed. All modern inverter units will easily modulate around that much flow/pressure change without any fancy zoning controls.

          Ductwork in the attic is no ideal, however in a milder climate is not as bad as in place like Texas. As long as you air seal well and insulate all your runs, the efficiency hit will not be all that much.

          1. Jonathan B | | #11

            Thanks, @Akos and @jrpritchard.

            I'm looking into how big an opening I'd need in the ceiling to fit a ducted unit up into the attic. Ceiling joists are 24" on center. Might be able to add an access panel and just barely fit a 24k btu ducted unit.

            Or a 12k btu is small enough to be pretty simple, and it could handle the bedrooms plus bathroom, and then a 12k btu high wall or recessed unit for the family room.

  5. Jonathan B | | #5

    Thanks for the tip about Chiltrix, @charlie_sullivan! We're planning to replace our gas water heater with a heat pump water heater soon. We were thinking of a Rheem, but seems like Chiltrix could be a great solution for both hot water and HVAC. Let's see if we can find a local installer.

  6. HoverDA | | #6

    I live in the similar climate zone and my solution is to install a good 1 to 1 small mini split for each bedroom. You could consider Gree Sapphire 9,000 btu or Gree Livo Gen3 12,000 btu (or Boreal Brisa 12,000 btu, which is a rebrand of Gree Live Gen3).

    Gree Sapphire's cooling capacity ranges from 1535 - 12966, heating capacity ranges from 2388 - 13648.
    Gree Livo Gen3's cooling capacity ranges from 1024 - 12624, heating capacity ranges from 2388 - 13648.

    Both of them can provide very low minimum cooling capacity. When you first turn on the mini split, it will run at full speed to cool down the room quickly and runs at minimum capacity to maintain the room temperature. If the mini split's minimum cooling capacity is still larger than the cooling load on some not-so-hot days, you can just leave the door open.

    1. qofmiwok | | #15

      What did you do in the bathrooms then?

  7. qofmiwok | | #8

    I'm interested in the answers as well. Especially because I keep hearing about how mini-splits modulate better than multi-splits.

    I am only putting splits in the upstairs; the downstairs is radiant. So I have 3 rooms and 2 baths. The ceilings are 9' and my architect suggested dropping the closets to 8' and putting cassettes over the tops of the closets pointing to each room. I'm not exactly sure how this will work yet, but one closet adjoins 1 room and 1 bath, and the other closet adjoins 2 rooms and 1 bath.

  8. michaelp_391 | | #12

    The advice typically given by the experts on GBA in these scenarios is to do a ducted air handler vs individual in-room units, as the output of even the smallest in-room heads will vastly outstrip the need for bedrooms and other small rooms.

    Carter Scott was doing the single wall-mounted unit upstairs in Massachusetts. This option is very attractive for its simplicity. People have concerns about what happens when you close bedroom doors.

    If you can put insulation on the attic floor, keeping the air handler on the second floor in a closet or something for access, then putting the ducts in the attic and burying them in fluffy insulation is a viable option, and the specs for this in terms of total amounts of insulation has been written up by Joe L and others.

    ductsinside.org is a good resource for strategies to keep ducts inside the conditioned envelope.

  9. Lukas Peter | | #13

    Facing a similar situation in my CZ3 (California CZ8) home. The loads in our bedrooms are so low that I can't justify individual heads.
    Experimenting with window AC in bedrooms taught me that open doors are not sufficient for distribution.
    I talked to a home performance contractor who recommends the ducted mini split path with a single piece of low static equipment and ductwork per Manual D located in the attic.
    They'll air seal the ceiling first, install R8 insulated ducts low on top of the ceiling framing, seal the ductwork, test to confirm minimum leakage and insulate the HVAC equipment. Finally they'll bury the ducts under R38 blown insulation.
    You could install a ducted mini split air hander vertically in one of your closets, draw the return from the hallway and burry well sealed supply ducts under insulation in the attic. Spray foam the ducts if condensation is a concern in your coastal climate.
    An alternative solution for our climate zone (my preference right now) is to spray foam the underside of the roof deck with low density foam to bring the attic inside the thermal envelope.

  10. Max E | | #14

    essential craftsman just posted a ducted install yesterday here

    https://youtu.be/hQmAgZc5Tak?t=242

    you may find some nuggets here. for example I really like the supply plenum he used . it looks extremely simple to construct and it has 8 outlets! I think the farthest outlets will have the least flow so those could be directed to the areas that need less cfm

  11. Jon R | | #16

    Note that spray foaming ducts in an attic takes far less spray foam to achieve the same heat loss as spray foaming the roof. The latter typically requires much thicker foam (or foam + fluffy) and it needs to be applied over a much larger area.

    Poorly done attic ducts vs a well done conditioned attic is not the right comparison.

  12. James Howison | | #17

    Avoid in the in-bedroom units. They will be noisy. You will forget to clean all the filters (every 2 weeks, more if you have pets ...). They will need to be cleaned (every 3 years?) with chemicals sprayed in them, plastic tubes and buckets trying to control the overspray. That will cost $200-$400 a unit ...

    They will turn on and off because they can't modulate as low as needed (modulating and minimum capacities on multi-splits is a shrug from engineers). Think that you are saying "double the needed capacity" but that's more like quad or more in the non peak periods (ie most of the time).

    Burying ducts works surprisingly well, see Building Corp reports. https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-094-no-sweat

  13. Jonathan B | | #18

    Thanks, everyone! This has been super helpful.

    What I'm leaning towards now:
    * put a single heat pump air handler in one of the closets
    * put the air return in the hallway
    * run ducts in the attic
    * bury the ducts in insulation
    * put a ceiling register in each bedroom plus the bathroom

    For the air handler, I'm leaning towards a conventional unit rather than a ducted mini split. The regular single zone heat pump air handlers fits better in a closet, since they're shaped more like a normal furnace. About 21" square at the base, and 5 or 6' tall. That works well in a standard 24" depth closet.

    Not sure if the air handler will be noisy to have in a bedroom closet. We could potentially wall it in with some insulation for soundproofing.

    The only other downside is losing a bit of closet space. A ducted mini split indoor unit could fit in the attic -- though we'd have to cut a bigger opening in the ceiling to get it up there. That would make it a little harder to service or replace in the future. We have fairly generous closet space, especially for an older house, so giving up 2 ft isn't so bad.

  14. Expert Member
    Dana Dorsett | | #19

    >"4 mini-splits in bedrooms, or a single ducted mini split serving all the bedrooms from an unconditioned attic?"

    In roughly 99% of the cases the head-per-bedroom approach becomes LUDRICROUSLY oversized, way too oversized for efficiency, and if tied to a single compressor can't really modulate low enough for comfort.

    A right-sized ducted cassette with tight better-than-code insulated ducts works. Be sure to provide adequate return paths for all rooms with supply registers or you'll end up using "the great outdoors" as part of the return path.

    >"For the air handler, I'm leaning towards a conventional unit rather than a ducted mini split. The regular single zone heat pump air handlers fits better in a closet, since they're shaped more like a normal furnace. "

    What is the total cooling load? It looks like maybe 1000 square feet, which for most homes would be less than a ton of cooling (even with the parasitics of ducts in unconditioned attics. Most conventional models don't go that low, and oversizing by more than 1.5x will reduce comfort efficiency.

    The Fujitsu xxRLFCD series can be mounted vertically just like an up-flow furnace, easy to fit into a closet. If building a "utility closet" for a the -9RLFCD or -12RFLCD can usually be designed to eat up less than 10 square feet of floor area. Even the 3/4 ton -9RLF can deliver over a ton of cooling at your likely 1% outside design temperature:

    https://ashp.neep.org/#!/product/25310

  15. Jonathan B | | #20

    @DanaDorsett, it's around 1200 ft2. According to CoolCalc, it comes out to 17,510 btuh for cooling, with a design temperature of 78F. Lots of south and southwest windows, only 6" of ceiling insulation.

  16. Expert Member
    Akos | | #21

    Having previously lived with an air handler in bedroom closet, a couple of pointers.

    Make sure to size your ducts for lower velocity (600fpm or less) for reduced air flow noise.

    Get an air handler with an ECM motor. Standards motors have a hum and you'll hear it, since it is low frequency it is next to impossible to sound proof.

    If possible get a modulating unit. Something that can run continuously and match cooling load is much better. Air handlers cycling on-off can wake you in the middle of the night.

    Even if more cost, I would spend the extra money on the ducted mini split such as Dana suggested. There are other ones (ie Midea mid static units) that can also be mounted vertically. These are all modulating units and are way quieter than any standard air handler I've heard.

    Lot of them have a built in condensate pump but can also be plumed for gravity drain. Unless impossible, go for gravity drain. Condensate pumps are quiet but still noticeable.

  17. jrpritchard | | #22

    The Fujitsu low static units fit great in closets and if the ducts are designed properly can move plenty of air. The 18k model can be mounted directly to a wall with some isolators and would only be about 8” deep by maybe 30” wide. These slim ducts mounted a couple feet off the ground give a lot of options for returns especially if you are pulling central return from the hallway. Mitsubishi makes a nice upright air handler that would give you the mini split modulating benefits with the conventional footprint you are looking for. With the weight of them they need a prefab base or a well built return air base to pull return air into the bottom of the unit. Good luck with the project

  18. Jonathan B | | #23

    Thanks, @akos [email protected] I'll take a look at both Fujitsu and Mitsubishi and see if I can find a good installer.

  19. James Howison | | #24

    If you are at all thinking about the horizontal ducted units still also consider that they need service space beneath them. I believe that the Mitsu ones need like 20-24"? I think the standard practice is either to mount them that high above the joists in attics, but I've never understood how that would work with buried ducts ... so I think they get mounted on threaded rods, can be disconnected from the ducting "bell" and raised up sufficiently high (assuming enough head room). Either way, how does one get to the filter box to replace it? One would be swimming through insulation?

    I do think an upflow unit in a closet is a good plan, filters are accessible, service is possible ducts stay under the insulation. Is the return path going to be done with door undercuts? Jumper ducts? Consider that even if bedroom doors are usually open there will be times one wants to sleep with them closed, such as having young children with early bedtimes (and cooling is key at night, especially as built up attic/wall heat permeates)

  20. Jonathan B | | #25

    @jameshowison, good points about servicing and air return path.

    For servicing, agree that an upflow unit in the closet will be easier.

    For the return path, I was hoping that the gap under the doors is already sufficient, or that they could be trimmed just a bit. For 18k btu, it's around 675 cfm max, spread over about 6 ducts. Doors are 32" wide, so shouldn't need to have too much of a gap.

  21. Jonathan B | | #26

    A wrinkle to the story... I had a more knowledgeable HVAC company come and take a look. They measured, and our attic is only 19" high at the peak. Said yes, they can put an upflow unit in a closet, and run trunk and branch ductwork down the center. But then the ducts would be very close to the top of the roof, and it's not possible to bury them in insulation. They don't recommend running ductwork in our attic, especially given how hot it will get in the summer. Also, the ducts would fill up what little space is big enough for a person to fit up there, so it would become completely inaccessible.

    They recommend going with 4 of the Mitsubishi ceiling units (the ones that fit between 16" joists), if we want to keep the walls clear, or high-wall units otherwise. Said that even though the smallest capacity ones are way oversized for the bedrooms, they can dial back enough to be comfortable.

    Any reaction to that advice? I'll probably get another contractor to come to the house.

  22. Jonathan B | | #27

    Update: I found a highly recommended Fujitsu installer who can do an upflow unit in the closet and run ducts through the attic and bury them under insulation. There's an extra charge for having to work in the extremely tight attic space, and I believe that's well justified.

    I'll have to figure out whether transfer grilles are needed in a couple of rooms. Door undercuts should be sufficient in most.

    Thanks for recommending the Fujitsu!

  23. Jon R | | #28

    You will be averaging about 110 CFM per room. How much airflow do you expect under a door?

    If the door is going to be closed often, I'd limit room pressurization to 1 pascal. No more than ~30 CFM for a door undercut.

  24. Steve Knapp CZ 3A Georgia | | #29

    Allison Bailes seems to like this product for reducing the pressure difference: https://www.tamtech.com/product/perfect-balance-in-door-return-air-pathway/

    1. Jon R | | #30

      Except that their "eliminates pressure imbalances" is a lie and rating at 5 pascals is ridiculous - which doesn't reflect well on the company. De-rate to about 1/2 of their CFM rating.

  25. Jonathan B | | #31

    Iirc, Alison had an article suggesting that a door undercut plus air leakage around the overall door frame might get to that 110 cfm number in practice.

    Thanks for the link, @Steve Knapp. That should do the trick for the larger room or two.

    And agree, @Jon R, that 5 pascals is high. That's energy star for a high air flow room, but we'll try to keep everything well under 3 pa.

  26. Jonathan B | | #32

    And an update, after a few months:

    We installed a Fujitsu upflow air handler, 2 tons, in the closet, and ran separate flex ducts with dampers to all 6 of the supply registers. We put in a jump duct in one of the rooms because a door undercut wouldn't be sufficient.

    We have not buried the ducts yet. We're planning to remodel a bathroom, and might put in can lights. It was recommended that we finish that job so we can properly air seal and then blow in insulation. After burying the ducts, the attic will be mostly full, with just a narrow service space down the center to get from the ceiling hatch to the plenum above the air handler.

    Everything works great. It's plenty of air and has had no trouble keeping up even on the hottest days.

    When I did the Manual J on CoolCalc, it came out to 20,000 btu for cooling. With the flex duct in the unconditioned attic, and uncertainties about how well we'd be able to bury it, we decided to go with a 2 ton instead of 1.5 ton unit. But... after getting through summer heat waves where the AC never even turned on high, it sure does seem like 1.5 tons would have been sufficient, even before adding more insulation.

    One note about the Fujitsu: their wifi connector and FGLair app are not very reliable, and a pain to connect to a normal wifi network. We're considering switching to the IR remote and receiver, because then we can use a Cielo or Mysa smart thermostat to send the remote signals. If anyone has had experience with either of those, I'd love to hear what you think!

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |