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Looking for advice on mini-split placement in Passive House (PHIUS)

Mike AbiEzzi | Posted in Mechanicals on

TL;DR: In a passive house, will a mini-split effectively cool the area that’s in the opposite direction of where it’s blowing? (see attachment for layout, stairs open -treads on a single steel beam stringer)

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Hi again! (zone 5b, dry)

I’ve been mulling over a good location for a mini-split in our passive house and would love to get feedback from everyone here.

See the attached images of the floor plan. My primary use case for the mini-split is cooling. [Heating is also important but we’ll have towel heaters in the bathrooms for zoned heating (mini-split will still do most of the work setting temps during the day while doors are open) and we’ll also have a wood burning stove for those really cold nights (https://www.blazeking.com/products/wood-inserts/sirocco-25/).]

Here’s our cooling load requirements from our Wufi Passive analysis:

5,000 btu load with a target temp at 77º
13,000 btu load with a target temp at 72º

I’m undecided between the MSZ-FH09NA (range 1,700-12,000) and MSZ-FH12NA (range 2,500-13,300). Going up to the MSZ-FH15NA might be sized better, but it can’t module as low (range 6,450-19,000). I definitely want something that can modulate at low speeds so that temps for even mild cooling can be maintained with no short-cycling. Thoughts? Am I wrong?

Initially I’d like to try to get away with one one mini-split, but if necessary, I can install a second floor mini-split on the lower floor, possibly below the stairs or in-front of the kitchen island (which would probably be a more effective placement, but highly visible). I’d only do it if I’d need to supplement for cooling. Thought?

In terms of placement (see attachment, stairs open -treads on a single steel beam stringer), I’d like to place the mini-split where the blue arrow points with the unit facing west. I could have one vane set to blow horizontally towards the upper floor bedroom doors and the other vane set to blow down into the lower level towards the master bedroom door. Cooling bedrooms is the biggest priority. The location also seems to be the most aesthetically pleasing by being the least visible in line of site when walking around the home. My only concern is that I might not get enough cooling in the kitchen since the mini split wouldn’t be blowing in that direction, and cooking can generate a lot of heat (this is ultimately the main question/concern of the post). Thoughts? Other suggestions?

Some other notes that might be helpful:

– There will be almost no solar gain in the summer.
– I prefer a central wall unit over a central ducted horizontal minisplit with zones (via AirZone). Wall units are more efficient and can modulate lower, I don’t have to worry about dust accumulating in ducts, I also won’t have tons of vents everywhere, and I don’t have to worry about noise transfer between rooms through ducts. Each room will have some sort of sound proofing in the interior walls. The cooling season is relatively short (3 months), so I’m prioritizing sound isolation and simplicity over zoned cooling.
– If there was such a thing as ultra-low modulating wall units that can be right sized for our bedrooms, I’d put one in every room and have zoned cooling and heating while doors are closed, but that doesn’t exist. Maybe in 10 years.

P.S. This community has been pretty amazing! Congrats on your success Martin and thank you creating an online place for everyone to share with each other!

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Replies

  1. Jon R | | #1

    Dry climate, but I'd still take a look at sensible and latent components of your btu load (including mild weather) and compare it to what various mini-splits will remove.

    > and have zoned cooling and heating while doors are closed, but that doesn’t exist

    Agreed - it's either ducts or hydronics/fan coils.

  2. Trevor Lambert | | #2

    I'll be interested to see what the experts say, but my intuition is that the floor plan is not very conducive to a ductless minisplit. Too many rooms and too long a distance to the furthest areas.

  3. Expert Member
    Akos | | #3

    I have a similar setup at an apartment. It is much smaller though (~500sqft) with 1 bedroom on the 2nd floor.

    The setup works well for cooling the open main floor and the bedroom provided the bedroom door is left open. Once the door is closed, the bedroom gets uncomfortable within a couple of hours.

    The problem with bedrooms is that people make a fair bit of heat. Add in high insulation, the bedroom will get hot just from the people in it without active cooling.

    Nobody likes ducts or registers but there is no easy way to get decent comfort without it.

    Your layout is fairly straight forward, you can run a small bulkhead in the hallway just above the doors and be able to feed all the rooms with minimal ducting. If you don't want to see the ducts, you can probably get away with bumping up that wall to 2x8 and running the ductwork inside it. Hard to tell on how your walls line up, but you might also be able to drop down a run to the main floor bedroom ceiling as well.

    For noise there are in line silencers the work extremely well. A lazy S bend in flex pipe also works but adds more restriction, so you have to really do the math to make sure it can work with a low static ducted unit

    P.S. The good thing with designing for low static units is the ductwork is low loss. This means that you can combine the supply ductwork with the ERV without fear of unbalancing it. It wold simplify the HVAC layout.

    1. Mike AbiEzzi | | #4

      Thanks for sharing your experience. It’s pretty difficult for me understand how quickly a room will warm up over night. One factor to consider is that we’ll have an ERV with summer bypass, and all but several weeks should have night time temps less than 68°. I would imagine that would counteract the body heat, but I’m not sure.

      If I go with a ducted system, the horizontal ducted units with high static pressure have limited range for turn-down, PEAD-A12AA7 (5,200 - 12,000). That’s the biggest downside regarding performance.

      I haven’t been able to find any conclusive evidence of being able to isolate noise transfer through ducts from one room to another. The silencers you’re talking, are they good at silencing room-to-room noise or HVAC noise?

      1. Jon R | | #5

        You can figure about .5 btu per degree F per sq ft of drywall. This doesn't go far in terms of preventing warm up on a warm night. On the other hand, cracking open a window on a 68F night will certainly cool down the room.

        With a 5F difference across it, an open door will move about 1728 btu/hr. Larger openings move much more.

      2. Expert Member
        Akos | | #6

        I'm with Jon on this. If the outdoor air is cold enough and not humid, open windows is the way to go.

        Provided you keep the doors open during the day, the wall mount should keep everything comfortable.

        As DIY hack, you can upsize your ERV supply ducting a bit and T into it something like this if you need more cooling down the road:

        https://climateright.com/climateright-5000-btu-a-c-heater-2.html#product_tabs_product-specs

        Your rooms will still need some auxiliary heat in your rooms. The 2nd floor minisplit will also do very little for heating the main floor. If your loads are low enough a couple of wall panel heaters and maybe a bit of resistance mat floor heat might cover your main floor.

        I think overall, it is best to design for a low static ducted unit for the whole house, these are almost as efficient as the wall mounts and paired with the right outdoor unit should have good turndown. This way all the areas can get the right amount of cooling/heating. You do need to design the ducting properly, with a simple layout this is not that hard and if you can share it with the ERV supply, would not add too much extra cost.

        The high static pressure units are typically sized for much larger load and like you mentioned don't have good turndown. They are only really worth it for retrofit install.

        Wall mounts still need to be cleaned, cleaning blowers/coil is a pain (4 years, I had to do mine once already). With a ducted unit if you install a well sealed large inlet filter, the duct will stay pristine for many years.

        The silencers I've used work extremely well (you are looking for ones with perforated metal mesh on the inside, looks like a car muffler), a larger unit I was using you can yell into one end and only a whisper comes out the other end.

        1. Mike AbiEzzi | | #7

          Thanks for the rely.

          Outdoor air can be 80 on the hottest nights.

          You’ve reply definitely set me on the trajectory of ducted. It’s great to know the silencers are that effective and that ducted units require less maintenance.

          I have a few follow-up questions:

          Is it all possible for one horizontal ducted unit with .2 instead of .6 static pressure to be ducted to all 5 areas? If so, then the total turndown sounds like it can be acceptable (SEZ-KD15NA4 - 3,800), otherwise two units would have a larger combined turndown, which wouldn't be better.

          Moving on to figuring out what my static pressure requirements are: https://www.greenbuildingadvisor.com/question/horizontal-ducted-minisplit-ducting (see comments)

          1. Expert Member
            Akos | | #9

            There are a number of slim duct units out there that do between 0.3" to 0.4" (ie ARU12RLF ). Those can normally feed a number of rooms without issues.

            For figuring out if you can make it work, some rules of thumb:
            -25x24 inlet filter
            -7" duct to a larger bed 6" to smaller 4" to bath
            -10" to open concept living area, two smaller ducts are better if long room.

            Want to limit your bends to three (4 if rest simple) 90deg elbows, this includes the bend at boot end if any. Stay away from ugly fittings (above 30' eq length) and flex duct bends, even better avoid flex ducts.

            You can tie your ERV fresh air feed into the inlet of the unit as the mini split blower runs all the time. Separate stale air return ducting for the ERV is still needed.

          2. Jon R | | #10

            If you are going to mix ventilation air with heating/cooling air, make sure that you still get the recommended CFM of outside air (say 20 CFM/person) to a closed door bedroom. Ventilation and heating/cooling room-to-room proportions usually don't match.

          3. Expert Member
            Dana Dorsett | | #13

            Fujitsu's 1.5 ton 18RLFCD is more efficient, has a wider modulation range (3100 BTU/hr min) and has a more powerful blower than the SEZ-KD15NA4, making it a better choice here, given the size of the duct system this house calls for.

            http://meus1.mylinkdrive.com/files/SEZ-KD15NA4_SUZ-KA15NA_Submittal.pdf

            http://www.fujitsugeneral.com/us/resources/pdf/support/downloads/submittal-sheets/18RLFCD.pdf

            It can also be mounted vertically (unlike the KD15) , which is useful for setting it up with a common return and easy access for filter replacement & maintenance, etc.

  4. Jon R | | #8

    You could consider a hydronic heat pump such as Arctic or Chiltrix. Conceptually, they are similar to multi-splits - except that buffer tanks solve problems with short cycling and flow rates and there are no minimum flow/oil distribution issues.

    1. Mike AbiEzzi | | #16

      This is a great idea. I wanted to increase visibility, so I posted my reply below as a separate comment to this thread.

  5. Norman Farwell | | #11

    Mike, I've installed minisplits in a couple of super insulated houses and I believe they are pretty standard in this application. Using a single head or maybe two for the whole house can work because 1) the house is losing/gaining heat very slowly, so you don't need multiple zones to address hot/cold areas 2) the HRV/ERV ventilation system tends to even out any minimal temperature differences between rooms. So the whole house functions as a single zone and can be served by a single head. And the single heads give you the greatest possible turn down ratio, which is important here. The last house we did had heat load of about 4200 Btu/h, so even the smallest mitsubishi FH06 was oversize.

    One problem you run into is night time set back--if you want the temp to drop 10 degrees for healthy sleeping, that's hard to do in a super tight house.

    Also, it's worth considering interactions between appliances. For example a heat pump dryer can work well in the same space as a heat pump water heater since the heat from the dryer can be scavenged by the water heater. (In the winter the heat pump water heater is working as a second stage heat pump, in the summer you get free cooling.)

    BTW, I'd guess lots of folks here will encourage you to ditch the wood stove. They don't play well with low load buildings for a number of reasons. I'm surprised PHIUS would sign off on it actually.

    There was mention a few months ago here on GBA that Mitsubishi is collaborating with a couple of other companies to develop the obvious thing--an integrated heat pump-HRV for low load buildings. Not sure what the time frame for that is, though. Anyway, good luck with your project.

    1. Mike AbiEzzi | | #17

      Thanks for sharing your experience. The "night time set back" definitely wants me to go the ducted route. Dana suggested the Fujistsu 18RLFCD as a good option, and I agree if I want to go that route. I'm actually really excited about what Jon posted about Arctic Heat Pumps, so I'm going full force into researching that. See my comment below at the end of this thread for more info.

      Our house with have a heat load of 18,000 btus. If you look at the wood stove specs, it can be turned down to about 10,000 btus when on a slow burn (one load can burn for up to 25 hours). So in theory, it's sized ok for our loads. It would only be used on those occasional cold spells or really cloudy day when we don't get any solar heat gain. I also like the idea of having an old school backup if I ever need it. If we want to use it for ambiance, we could close the bedroom doors to keep them cool and we could crack open a window in the living room if the living room got too hot. I lived with a wood stove for the past decade, so it's hard for me to give it up :) It's definitely a splurge. Does any of that change your opinion? Or am I crazy? I'm always open to criticism; that's how I learn.

      1. Jon R | | #18

        If you have a hydronic system, then a wood boiler + buffer tank (or concrete floors with tubing) is an option.

  6. Trevor Lambert | | #12

    I think the lower temperature for sleeping is a preference, not a health issue. I've never seen anyone claim health effects, and not everyone likes the lower temperature sleeping. I can tolerate a modest drop, but only because I can compensate easily with covers. It's hardly a want. A 10F drop is also pretty extreme. 58-60F is downright frigid.

    There already are two ERV heat pump combos on the market. Both are very expensive, and both provide pretty minimal positive heat gain when compared to a high quality ERV or HRV. Both have write ups on here, the Minotair and the CERV.

    1. Mike AbiEzzi | | #27

      If I can get the temp down to 68°, that’s good enough where I won’t lose sleep.

  7. Mike AbiEzzi | | #14

    Dana, that's an impressive range with the Fujistsu 18RLFCD. Thanks for sharing. Interesting that it can be mounted vertically.

  8. Mike AbiEzzi | | #15

    Jon,

    The Artic Heat Pump systems are really cool, especially since I was going to get a heat pump water heater with an outdoor compressor anyways. I can have zoned space cooling/heating without ducts and domestic hot water in one system (2-in-1). We'll also have a hot tub, which it says it can heat as well (3-in-1 !!).

    I'm currently looking into radiant ceiling panels instead of the mounted radiators for space cooling/heating. Ceiling panels won't interfere with our southern exposure radiant solar heat gain + concrete floors for thermal mass. It seems to be pretty affordable too since our loads are so low in a passive house, each panel only needs one 8x4 ceiling panel. Currently looking at https://radiantcooling.com/messana-radiant-cooling-products/ray-magic-radiant-panel/

    This checks all the boxes:
    - silent operation
    - doesn't allow sound transmission from one room to another
    - invisible
    - efficient
    - zoned with as many zones as I would like.
    - low maintenance (no ducts, units, or filters to clean)
    - handles very low loads (need to understand this some more)

    Bonus:
    - Same system provides domestic hot water and could heat a hot tub

  9. Expert Member
    Peter Engle | | #19

    What will you use for dehumidification? Many PHs run at high humidity, even too high humidity. Radiant panels operating below the dewpoint of the interior air will have condensation dripping from the panel.

    Running a separate dehumidifier does have an energy penalty in summer as its waste heat warms the space, but in winter the waste heat is a bonus. You do need to budget space for it though. There are some good reasons for separating HVAC and dehumidification. It's done all the time in commercial buildings, but it's not the cheapest option for residential.

    1. Mike AbiEzzi | | #20

      Messana's solution is to use smart controls that operate the radiant cooling panels and a HRV/Dehumidifier combination. The controls monitor the due point so that the environment is never put into a state that creates condensation.

      "The secret to #RadiantCooling is in the controls.
      Messana developed a unique control technology over 15-year experience.
      Dehumidification and proper ventilation are key to increase performance of the cooling system, but the condensation prevention is managed by the controls.
      A well designed radiant ceiling can extract up to 35btu/sf!"
      https://radiantcooling.com/the-secret-to-radiantcooling-is-in-the-controls-messana-developed-a-unique-control-technology-over/

      I'm just learning about it now, so there's still lots of research to do. I assume since colorado is dry, I'm in the best position to be in.

      Looks like they have several passive homes listed on their site.
      https://radiantcooling.com/radiant_cooling_projects/

      1. Expert Member
        Akos | | #21

        I lived in a place with radiant cooled floors in Germany. The one issue I had is that although it did cool the air, it didn't take any humidity out, it felt clammy. It reminded me of an unconditioned Canadian basement in the summer time.

        If you can control the interior RH properly, it will definitely work.

        Won't be cheap though, any time you say hydronic around here, add 20k to the price tag. Never mind reversible chiller...

        1. Mike AbiEzzi | | #23

          I’m not too sure it will be more expensive. The home will require a little over a dozen 8x4 ceiling panels and it does double or even triple duty when comparing apples to apples. Water heaters with outdoor compressors and professionally installed ducted mini-splits aren't cheap. And if I can do the install myself given a design, that seems more possible than doing a minisplit install since I can’t do a vacuum test easily.

          1. Expert Member
            Akos | | #25

            Mike,

            If you DIY anything is possible. Component costs for most systems is small compared to the labor. Just keep in mind that somebody other than you has to be able to fix it down the road. I've done my fair share of hacking something on the cheap only having to re-do it with "proper" parts.

            The issues I see with the liquid systems is that there are not a lot out there. This means that any problems, you are pretty much on your own. From what I've seen here, the big problem is frost build up in the winter time and improper system design (short cycling, return water too hot) causing issues with the chiller.

            Most of the ones that do exist don't have low temperature vapor injection compressors, heat output and COP at low outdoor temp will suffer. I would check that you can get hot enough water out of the unit at low temp to put out useful heat.

            Either way, heating DHW with one is a no go. The best you can hope for is pre-heating incoming water or warming a pool / hot tub with one. Also when heating water, your cooling will be off, can be handles with buffer tanks, but again adding more complexity and cost.

            For DWH, your best bet is still a heat pump water heater, it will dehumidify a bit in the summer as a bonus. I still think you will need a separate dehumidifier to bring the interior RH down even in a dryer location.

    2. Deleted | | #28

      Deleted

  10. Josh Durston | | #22

    I have an BK Ashford... You'll love the Blaze King (if you install it correctly - 15' insulated chimney). Because of the very low min burn rate it is more sensitive to having a minimum chimney height to maintain some negative draft pressure, and it needs to be insulated since there isn't as much overheated flue gases to keep it hot and clean.

    It's one of the few wood stoves that can turn down to 5-10kbtu and burn low and slow for more than a day. The catalyst and thermostatic air control lets you run in dark mode where it's literal just burning smoke with the catalyst with no visible flames. Most stoves provide an overwhelming amount of heat early on which rapidly drops requiring a pulse and glide reload cycle, the blaze king gives a constant moderated output over pretty much the entire 20-30hr burn cycle. With the BK you just load it up and set the thermostat according to the conditions, and come back in 18-30hrs to put more wood in.

    1. Mike AbiEzzi | | #24

      Awesome! Thanks for suggesting the insulation. Makes sense. The chimney will be somewhere near 30’, so no issues there.

      I do hope that I can see some flames as a slow burn for ambiance over a few hours without overheating :)

      1. Josh Durston | | #26

        My Ashford weighs 400-500lbs (lots of thermal mass) so I will run it high enough to have flames for evening ambiance without overheating the place for 2-3hrs at a time. When no one is around I turn it down.

        I'll also time my reloads for when I'm around in the evening to enjoy the show. Best practice is to run on high for 20-30min after reload to drive the moisture out and get the chimney hot. Again the stove soaks up a lot of heat and releases it slowly so it doesn't immediately overheat if you turn it up for bit.

        hearth.com is full of Blaze King fanatics (including the VP of BK), they will give you all the help you can stand and then some. I would post your stove install configuration there before you proceed to get some feedback.

        I went with the BK because I live in a smallish 8' ceiling bungalow (heat loss is sub 30kbtu and will be 20kbtu once I insulate the basement walls fully), and was worried a regular secondary burn stove would bake me out of the house. I'm very happy with how it turned out. I run 23-24C in the living room and farthest bedrooms stay 20-21C, and kitchen around 22C. It has ruined me for winter heating, a furnace set to 20C just doesn't feel cozy anymore. I love coming home on a winter day to the radiant heat of a wood stove.

        1. Mike AbiEzzi | | #29

          Thanks for the tip on hearth.com. I know what you mean about wood stove heat. Our leaky condo has a wood stove and heats our place to 74° Or hotter if we’re particularly cold from a winter outing and the bedrooms stay cool for sleeping. And burning wood is beautiful. Only regret is that I didn’t know about BKs at the time and that would have been way better. It’s very cold when we have to reload in the morning.

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