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Can You Oversize a Mini-Split Heat Pump?

Ductless Mini-split Heat Pump

Yesterday I did a consultation with a homeowner in Michigan.   He’s got a 60 year old house with a boiler and radiant heating system that has become a burden.  The pipes for hydronic distribution system just developed their seventh leak since he’s lived in the house.   To repair the leak would require tearing up the kitchen, so he’s looking for other solutions.

One possibility is going with all ductless mini-split heat pumps, and one HVAC contractor proposed 7 indoor units for his 2,100 square foot, 4 bedroom, house.  That’s one ductless unit for each bedroom plus three for the downstairs.  The mini-splits might be sized appropriately, but it would only be by accident.  The contractor didn’t do or even propose doing a load calculation.

Can you oversize a mini-split?

And that gets me to the real point of this article.  A lot of people seem to think that you don’t have to worry about oversizing a mini-split heat pump, whether ductless or ducted, because these systems have variable capacity.  The heating or cooling output drops when the load is lower, so it’s OK to put in a unit that’s too big, they argue.

But is it true?  No!  Here’s why.  Mini-splits do ramp down in capacity as the load changes, but they don’t go all the way to zero.  There’s a bottom they won’t go below.

Understanding turndown

For example, the Mitsubishi FS06 wall-mounted ductless unit has a rated cooling capacity of 6,000 BTU per hour and goes down to a minimum capacity of 1,700 BTU per hour.  That’s a turndown percentage of 28% (1,700 ÷ 6,000).  If you put that unit in a room with a load of 1,500 BTU per hour, it will bottom out nearly 100% of the time it’s running in cooling mode.

Sometimes you’ll see turndown ratio, which is the same concept as turndown percentage but upside down.  It’s the ratio of maximum to minimum capacity.  For the example here, it would be 6,000 ÷ 1,700 = 3.5.  So the turndown ratio is 3.5 to 1, sometimes written 3.5:1.

What happens at part-load?

One of the great advantages of using mini-splits is their variable capacity.  By oversizing them, you can lose that advantage completely.  That 6,000 BTU per hour mini-split in a room with 1,500 BTU per hour of cooling load doesn’t act at all like it has variable capacity because it’s always running at the lowest capacity except on the really extreme temperature days.

When the bottom end of the capacity range is at or above the heating or cooling load, there’s no benefit in part-load conditions.  As the heating or cooling load drops from the design load, you want a system whose capacity also drops.

That 6,000 BTU/hr system with a bottom end of 1,700 BTU/hr serving a load of 1,500 BTU/hr doesn’t vary at all in part-load conditions.

When the load is 1,500 BTU/hr (the design load), the system will give you 1,700 BTU/hr.  When the load drops to 1,000 BTU/hr, the system will give you 1,700 BTU/hr.  When the load is only 500 BTU/hr, the system will give you 1,700 BTU/hr.

Yes, oversizing is a problem

So, yes, you absolutely can oversize a mini-split.  And when you do, you end up with some of the same problems you get from oversizing conventional systems:  poor humidity control, short cycling, and wasted money.

When a contractor proposes 7 indoor ductless units for a 2,100 square foot house without doing a load calculation, there’s close to a 100% chance that the systems will be oversized.  But this problem happens with new homes that get load calculations, too.  You’ve got to look at the specifications for any mini-split heat pump and put in one that will take full advantage of the variable capacity.


Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia.  He has a doctorate in physics and is the author of a bestselling book on building science.  He also writes the Energy Vanguard Blog.  For more updates, you can subscribe to the Energy Vanguard newsletter and follow him on LinkedIn.


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This Post Has 67 Comments

  1. What about bathrooms, walk-in closets, and utility rooms? Do they get heating/cooling?

    1. Roy, we may or may not put supply vents in bathrooms and walk-in closets when we do a ducted design. It depends on the load. With ductless, a utility room may get a unit, but it depends on the load and the design of the house. But they all get heating and cooling one way or another.

      When I was growing up in Texas and Louisiana, I lived in some houses that didn’t originally have air conditioning. (My grandparents’ house in Leesville, in fact, didn’t have air conditioning until I was about ten years old.) In Florida, I lived in similar houses. They were all cooled with window units. My expectations for building enclosures, mechanical systems, and thermal comfort have certainly changed since then, but I remember being fine in those houses as long as the AC had been running long enough for the cooling to make it to the bedrooms.

      With better building enclosures, we can do that with ductless units, too. I still like ducted units for bedrooms, though.

  2. Allison, I also grew up without air conditioning and I am not willing to return to those days. The last I saw from the census, 94% of new houses have central air. I would not own a house that has a bathroom with a shower but no forced-air heating or cooling. I don’t run my bathroom exhaust fan when it is very hot or very cold outside because I want air circulation during those periods to either remove the moisture with my AC unit in the summer, or distribute it to the rest of the house when it is dry in the winter. To do this I keep a temperature and humidity sensor in my bathroom to tell me when to turn on the exhaust fan during and after a shower. This the main reason, after cost, that I don’t like ductless units.

  3. I really don’t see how you could size mini splits for both heating and cooling in Michigan. It’s a compromise here in the inland South where you either go oversize on cooling or add supplemental heating. Personally I have units slightly undersized for cooling which are great in the cooling months but do require about 50% of their nameplates in supplemental heat during the winter. When I replace them I’m going with the same sizing but am going to use hyperheats to minimize expensive supplemental heat. Turndown will also be a consideration because even though I’m undersized for cooling humidity control isn’t that great in the spring.

    1. Matt, it’s not just a mini-split issue. It’s a heat pump issue. When you use one piece of equipment for both heating and cooling, optimizing the system can be tricky. But two things help quite a bit.

      First, some mini-splits can provide full heating capacity down to single digits Fahrenheit and still provide a lot of heat below zero.

      Second, the better the building enclosure is, the closer the heating and cooling loads are to each other. That means more insulation, less air leakage, less thermal bridging, better windows.

      1. I live in south Texas and want to go minisplit. My house is small at slightly over 1600 square feet. I want separate zones for the beds and the largest is like 14×15 and the smallest 10x 12. I have an open floor plan in the rest of the house except for a door for the laundry room and of course for the garage which will end up being finished off as a family room. I figure I am going to have to use to systems. I think 6k would be plenty for the bedrooms and the laundry rooms. I am having trouble finding that size with a system. Any ideas how to do this without oversized it. Besides the fact that these systems I like the fact that the parts are cheaper to replace. I had a big system 17 sear and it lasted one year after its warranty expired(11years). Any suggestions?

        1. @Toni, Mitsubishi makes a high-seer 6k ductless mini-split (MSZ-FS06NA & MUZ-FS06NA), also marketed by Trane and American Standard. The smallest ducted mini’s are 9k. You’ll need to get a room-by-room load calc to know what sizes would work best.

          1. The problem I had with the Mitsu 6k’s is that they don’t modulate very low, and with my super insulated house would still short cycle. A Mitsu sales manager told me they were coming out with a 4k but I haven’t seen it yet. Thus I ended up going with Daikin VRV Life where each unit output can drop very low if needed.

  4. Hey Dr. Bailes, In discussion at a building scientist convention you’ve attended, on HVAC contractor presented a study of the challenges they faced incorporating a multi head system with a single exterior unit, ultimately having to scrap the strategy and move to individual systems because of significant balancing issues. Perhaps you could explain their challenges better than I have.

    I continue to enjoy and employ your building science discussions–Thanks,
    Arthur Beeken

    1. Hey Arthur, yes, one-to-one mini-splits are usually better. That unit I mentioned in the article has a 6k indoor unit properly matched with a 6k outdoor unit can go down to 1.7k. If you put that same indoor unit onto a multi-split system with an outdoor unit that bottoms out at 6k, now the indoor unit has no ability to modulate downward when it’s the only indoor unit being used. So you’ve got to look at the system and how it’s going to be used to see how much modulation it will actually have.

    2. @Arthur, I routinely steer clients away from multi-splits for exactly the reason Allison described in his reply. Moreover, you can’t just assume multiple single splits necessarily cost more… Depending on the particulars — especially the # of zones and how the zone sizes break against equipment capacities — it’s not unusual to see single-split proposals come in lower than multi-splits. Keep in mind that all-ductless systems (i.e., head in every room) are almost never a good option. Individual rooms in homes built to today’s energy codes simply don’t have enough load for that to work!

  5. Absolutely I have seen the results of oversized minisplits – 85% RH with mold growing on wooden furniture.

    The turndown calculation / considerations get even more tricky when we deal with a single outdoor unit feeding multiple indoor heads.

    Another issue is that minisplit heads tend to run their fans at times when the compressor is inactive – I gather that is done so the mini senses room conditions more accurately, but anytime air blows across a room-temperature refrigerant coil, some condensate will be re-evaporated, aggravating humidity control.

    My goto unit for many single head projects is a Mitsubishi mini with a particularly wide operating range: 1500 – 12000 Btuh cooling. For some reason that unit’s lowest output is well below other lower capacity systems – 8:1 turndown!

    1. Curt, yeah, the GL12 is definitely a great option. The 6k and 9k units have the same turndown so that’s one case where you can put in a higher capacity without worrying…unless you use it in a multi-split system.

      I have two SEZ ducted units in my house and even with the blower running while the compressor’s off, our humidity stayed in less than 60% RH last summer. Sizing the system below the MJ cooling load helped with that, I’m sure, but we also aren’t as humid as Florida.

      1. Actually, a quick browse of the submittals sheets for MSZ-GL09NA-U1_MUZ-GL09NA shows minimum cooling of 3600 Btuh vs 1500 for the “12s”, so we propose only the “12” even if loads are lower to get that turndown.

        The SUZ / SEZ combos are our go to for compact ducted apps – most often they go into “mother-in-law” or guest cottages – dwelling units separate from main homes in our custom home niche.

        I’m not real jazzed by any system that runs the fan independently from the compressor…our humidity is brutal. In the 20-odd years I’ve lived near Jax I’ve watched the mid summer dewpoint creep up from 71-73 to 75-77, and that has made a huge difference in how we do business.

        I used to regard separate dehumidifiers as a costly bandaid for a failed envelope…now we propose them routinely.

        Each summer starting in late June I closely monitor National Weather Service temperature forecasts – I pay particular attention to the predicted overnight low temperature since in our area the dewpoint closely tracks the overnight low – in other words, our RH hits 95-100% for several predawn hours many summer mornings.

        Whenever overnight low temp forecast meets or exceeds 75*F, I issue an all-staff warning to get ready for odd moisture complaints – wet registers, ceiling stains, growth on walls high in bathrooms, etc. Staff are amazed at how accurately I predict that call volume…physics is what it is.

        I also ask our field staff to suss out client cooling setpoints – day and night. To improve accuracy (i.e truthfulness) I train a “cop 101” (law enforcement) interview technique designed to elicit the actual cooling setpoints routinely used in the home.

        This is crucial information needed to shield us and our builder partners from liability related to building science moisture failures – stated another way, misuse of a thermostat can total a home. Clients that cool much below 75*F at night (we’ve seen cases of cooling setpoints of 64*F) are at much greater risk of uncontrolled moisture failures.

        A simple maxim I use to explain the idea is that if the cooling setpoint is much below the outdoor dewpoint / overnight low temperature, probability of a building science moisture fail is greatly increased.

        1. You just blew my mind. I’ve been searching and searching for something better than the Mitsu 9kBTU because it doesn’t modulate low enough. And the 12 goes lower? Sure enough, I checked the datasheets and the 12 goes to 2000 heating and 1500 cooling, white the 12kBTU is 4500 and 3600 . Are we sure this is real and not just marketing? Have you observed the 12’s modulating lower?

          1. Ah upon further review, the MSZ-GL12NA & MUZ-GL12NA modules much lower than the MSZ-FH12NA & MUZ-FH12NA. What’s the difference between those models?

          2. The FH is the “hyper heat” series, which can operate at temps as low as -13F and has significantly higher heat output in the coldest range than the GL series.

            Although the FH series has higher cooling efficiency (26 SEER for the FH vs 23 SEER for the GL), heating efficiency falls off sharply below freezing, presumably traded off to get higher capacity. And the FH12 only modulates down to 2500 in cooling (vs 1500 for the GL12), and 3,700 in heating (vs 2000 for the GL12).

            All of this information is provided on the submittals.

          3. I’ve just discovered the newer FS models (replacing the FH I’m told?). They aren’t in a lot of the Mitsu catalogs yet. But they work in even colder temps, and the 6 & 9 modulate down to 1600.

    2. Curt, I am guessing that the Mitsubishi indoor unit supplies about 400 cfm at full capacity (12,000 Btu/hr = 1 ton and typically 400 cfm/ton). To me, 400 cfm seems like a lot of airflow from one supply, which is one of the complaints I typically hear about ductless indoor units. At minimum capacity (12,000/8=1500 Btu/hr), the indoor airflow should probably be 50 cfm (400/8) to keep giving good latent capacity. If it is controlling airflow rate in that manner, can you even feel 50 cfm from the unit, let alone get good air distribution in the room? Is there movable supply register on that unit that increases the supply air velocity at low airflow rates?

  6. It can be pretty hard to actually find the turndown ratio or lowest cooling BTUs, I’ve found. A lot of product information only puts out max cooling capacity on the easily available spec sheets and manuals. One has to dig into the engineering manuals and even then it can be tricky, I’ve had to call the manufacturer directly. It’s also going to change whether it’s multi zone or single zone. I think I had a mitsubishi rep one time tell me that the “rule of thumb” for multi zones is that the lowest capacity is about 30% of max capacity on their m-series multizone, but I also had different people from mitsubishi tell me different things about that.

    1. Yes, it’s telling that their system design software will show “as implemented” maximum capacities but says nothing about turn-down ratios.

      I can also see that it’s legitimately hard to know this stuff. For example, you’d have a different minimum for a variety of scenarios with other units on the refrigerant loop. I mean if one unit was on, but all others were off, compared to all others being on and seeking a balanced amount of heat/cool, vs the very difficult circumstance of one unit at set point, but all others calling for max capacity. That last scenario is (if I understand correctly) why one gets the “temperature drift” discussed here: (which in my experience is just as problematic in hot climates as in cold climates).

      So, really, they need to run a simulation in conjunction with an EnergyPlus type building model. But, you know, that’s what computers are for 🙂

    2. Leigha, you didn’t mention which brand you’re having trouble with, but Mitsubishi usually has the minimum cooling capacity on the submittal. You have to look for the submittal that includes both the indoor and outdoor unit, like the ones here:

      And yeah, as James said, with multi-splits you have to consider other factors.

  7. Also worth considering is the cost to clean the dust out of 7 units. Rarely discussed, but it’s on the order of $200-$400 a unit and needed something like every 1-3 years. It’s also very messy and disruptive. And then there’s the every 2 week cleaning of the filters, 2 per unit, so that’s 14 … and the instructions say to clean them and then let them dry. Every 2 weeks …

    Can we please, pretty please, have some tiny ducted units with proper media filters on the return vent. Like capacities from 500 btu/h to 3,000 btu/h. Something small enough to fit between 16in on center joists and serviced from below, or wall units serviced from in front (for the often discussed but rarely seen closet install). Oh, and then we’d need something like a 8C24 outdoor compressor (8 heads but adding up to 24k btu/h), not the nonsense of 4C36 …

    Hmmm, sort of makes small radiant ceiling panels combined with central dehumidification seem the right approach. Just one blower motor and one drain …

    1. “Can we please, pretty please, have some tiny ducted units with proper media filters on the return vent.”

      Hear, hear!

      Mitsubishi is working on lower capacity units, but I haven’t heard anything about proper filters.

      1. And why are there no splits that can go in a crawl with registers coming up through the floor? That would make them so much simpler to install. Is there a technical reason this won’t work? They have units that sit on the floor, in the attic, in the walls, on the walls, but nothing below the floor.

        1. We’ve done a dedicated ducted unit in a crawl space. Washington State Energy Code doesn’t allow this to be used for an energy credit as it does with mechanicals in a conditioned attic.

        2. @Cindi, I’m struggling to see how one might design a ductless unit for the floor. In order for the return and supply to be in the same unit, the head can’t be perfectly flush with the floor. And it’s always a bad idea to locate a return inlet in the floor… think about the amount of dust that would get sucked in. And lastly, to get decent coverage, it would need to be near the center of the roof (like a ceiling cassette). That would just be weird. OTOH, I can almost imagine how one might design a long’ish baseboard unit…

          1. Hmmm, good point. Never thought about how that’s a big reason ducted forced air system ducts get so dirty.
            If not for the dirt though, couldn’t you have a number of feeds on the floor but one large return like they do with forced air? And just duct the return ?

          2. Cindi wrote:
            > If not for the dirt though, couldn’t you have a number of feeds on the floor but one large return like they do with forced air? And just duct the return ?

            Well sure you can do that… with a ducted mini. But ducted mini’s don’t have great turndown.

    2. I’ve done small ducted minisplits with a regular filter. Just have to design for appropriate static pressure. Not 1/4 ton though! 1 ton

  8. Allison, if you were doing the same thing as your Michigan client to your house in Atlanta, what would you choose, multi-split or mini-splits? Would the extra cost of the mini-splits ever be recovered by their superior efficiency compared to a multi-split?

    1. Tim, I do have mini-splits in my house. One is a ducted multi-split system with a 24k outdoor unit (MXZ-3C24NAHZ2) with 9k (SEZ-KD09NA4) and 18k (SEZ-KD18NA4) indoor units. I went with the multi-split only because Mitsubishi wasn’t making one-to-one systems with Hyper-Heat that could be used with these air handlers. I’ve thought about possibly changing out the outdoor unit with two smaller ones, but the system has worked well. I attribute that to my undersizing it. Here’s the article I wrote about it:

      My Undersized Ducted Mini-Split Heat Pump

      Efficiency isn’t main reason to go with one-to-one units. Comfort and having a system that can take full advantage of its capacity range are.

  9. I am a bit confused, why would you not use a buffer tank and use that to pump fluid to the heat adsorption unit? Hope the new book covers some hydronics and dew-point control!

  10. Dr. Bailes,
    Isn’t 6,000 BTUs with a minimum of 1,700 a turn down ratio of 71.66%, that is 6,000-1,700 = 4,300 turned down BTUs, then 4,300/6,000?

    It is true that the same mini split can run as low as 28% of maximum capacity or 1,700/6,000.

    1. Mark, I’ve always seen turndown ratio defined as the ratio of the maximum to minimum capacity and stated as something like 4:1 (4 to 1). I prefer to do the reciprocal of that ratio because the meaning of 3.53:1 doesn’t seem as obvious to me as 28% of maximum capacity.

  11. With the absolutely insane housing boom in E. Tennessee, I cannot find a single HVAC contractor willing to do anything outside the “norm”. They all want to put in two huge central units for a 2 story 3000 sf house and call it a day. They will not consider mini-splits or even doing a manual J calculation. Everyone I’ve talked to (licensed pros) tells me they “know what they’re doing” and if I don’t like it, they’re too busy to deal with me. So as the homeowner/remodeler of this 100+ year old house, I’m feeling like I might have to go the DIY Mr. Cool way. Am I crazy? Are there any resources that a mere mortal can used plan out what size/how many units to install given a certain floorplan/room size?

  12. You are in a tough spot. You can DIY your own Man J via HVACCALC dot com – $49 for a temporary use license. You’ll need accurate details as to your home’s envelope components and infiltration rate (blower door test)

    It is a very difficult time to ask for anything outside any contractor’s comfort zone. I’m delaying our own replacement home until sanity returns.

    My clients have not had great experience with sourcing HVAC systems via the internet – quality is uneven at best and support is a nightmare. There are some national forums that might help you find a “woke” local contractor.

    1. Thanks Curt. You’re spot on. I may have to wait. Woke is dirty word around here. They’d install coal burning stoves if they could. But your comment also made me think of reaching out to Ingram’s (equipment wholesaler) to see if it’s a service they can perform if I buy the parts from them.

  13. Also beware of possible loss of manufacturer warranty coverage on systems bought online. It is unlikely an internet wholesaler can / will offer installation services since that often requires a state or county trade license.

  14. why not consider using an air to water heat pump in conjunction with the boiler and install a solution that also uses the water for cooling. Id think that a modernized radiant system would be hard to beat in Michigan.
    Fwiw we have a ducted heat pump (climate zone 4) and the biggest mistake I think we made was relying on the heat pump to remove humidity. like Central air I find myself running the ac just to remove humidity. I think that we will end up adding a dedicated dehumidifier but keeping our radiant system and high ceilings.

    1. @andre wrote:
      > I find myself running the ac just to remove humidity

      A dedicated dehumidifier may not be the best solution. It’s always best to determine the cause before implementing a solution. For example, if you have a ventilation system, it may be hyperactive. Or the spot exhaust for a high use shower may be compromised. Or the air handler may simply be set too high. These things are easy to fix.

      In any case, you can increase latent capacity by reducing the blower speed — either all the time, or on-demand (requires a thermostat RH display and with DH terminals, a couple of spare wires in the t’stat cable and a 24VAC SPDT or DPDT relay). This would allow you to set the t’stat to reduce the blower when indoor RH exceeds, say, 55%. That way you can benefit from maximum sensible capacity and efficiency when indoor RH is not high. This work should be done by a qualified technician because if the airflow is lower than about 285 CFM per ton, your evaporator coil could freeze up. OTOH, if you have a ‘communicating’ AHU, it likely already has this feature built in, but requires a compatible t’stat.

      1. David thanks for the reply. For my humidity issues I believe it is rising from the basement. It’s an old stone house where the grade was raised over the years (possibly when the street grade was raised) and the solution was to encase the stone foundation in concrete which I believe is trapping moisture against the stone walls. We also have a broken rat slab, that is next on the list to address along with removing the concrete and grading the yard away from the house…possibly adding a french drain on the high side of the house since we only control four feet there (city neighborhood). If that doesn’t work, dehumidifier it is. The home was built with a massive dehumidifier known as a kitchen hearth.
        I think in Michigan you will always be building a heating system first. The radiant cooling would be based on floor or ceiling panels (possible fan coil units) that are primarily used for heating. It seems to me that cooling needs I public areas would be less than bedrooms and that the cooling needs in Michigan would be modest. Here in Philadelphia we have the bedrooms cooled and nothing else, only on the hottest days does it get warm and even then a ceiling fan does the trick. Of course it’s not a ranch house so having the ducted split at the top of the third floor stairs is very effective. Our other options were to destroy the house for central air or put in a lot of minisplits that are oversized for the rooms they are in.

    2. @andre wrote:
      > Id think that a modernized radiant system would be hard to beat in Michigan.

      One problem you run into with radiant cooling in humid climates is there’s always a conflict between the ideal chilled water temperature for cooling and the ideal indoor humidity. You obviously must keep the water temperature above the indoor dew point, but that means more radiant panels (and higher chilled water velocity) to satisfy the sensible load if you run a normal 55F dew point (keep in mind you need a safety margin when setting the mixing temperature). Maintaining a 50F indoor dew point with a dedicated dehumidifier can get very expensive.

      Radiant cooling makes more sense in dry climates. But even then there’s the matter of first-cost. Any time you ask a mechanical contractor to go outside the norm, you’d better be ready to open your wallet wide. And that’s assuming you can even find someone qualified to do the work.

      I’ve had the opportunity to design several radiant cooling systems (mostly in dry climates) and in every single case, the client eventually ended up going with conventional ducted. Either we were unable to get anyone to quote, or the quotes came back way beyond reasonable.

      I recall one project in Grand Rapids when I collaborated with Robert Bean. The only mechanical bid that came back was 6 figures!!… way beyond what the client was willing to pay. I never knew the total project budget but this definitely wasn’t a multi-million dollar home.

  15. Andre – You forgot to add a Messana Air Treatment Unit (ATU) and some radiant panels to work with that Air to Water Heat-Pump. That ATU is a combination HRV, De-humidifier, and Make-Up Air Unit – that uses hot/cool water from the heat-pump.

    1. The whole concept of a youngish radiant system has become a burden is suspicious. The homeowner should reconsider who performs the maintenance on the system.

    2. I’ve heard from some very smart people that those are great. But I almost choked when I heard the price. $120k for 4000 sf.

  16. Dr Bailes

    what happens when the smallest equipment exceeds manual s sizing at full capacity
    so, it might comply when its modulating at lowest capacity
    where do you find efficient duct fittings like the ones in you house with the ducted SEZ units .
    they don’t look like fittings in manual d


  17. I might add to this dialogue that we have a ducted unit and 4 heads in our home. One outdoor unit is connected to a ducted and ductless indoor units and the other outdoor unit is connected to three ductless units. Factoring in the cost of breakers, refrigerant, exterior finishes costs, etc. are real world factors. Some of our ductless units no doubt aren’t ideal as far as turndown ratio goes. We just turn them off when there aren’t loading conditions needing them.

  18. When we are in Greece or other places outside the US it’s common practice to *oversize* minisplits. What we consider oversized is undersized there. They don’t use their units the same way we do. We keep our houses at a constant temperature here in the US. There they only turn them on when they are in the house and sometimes only at night so they can sleep. They have to oversize the units compared to what we are used to here because they need it to cool down quickly and a unit sized like we do here would never get the job done fast enough.
    They treat it like I treat the AC in my shop. I have to vastly oversize it because it is not maintained at a constant temp so it takes allot of BTUs to quickly get the heat out of the air in the building and then the items in it.

    One big missed application for minisplits is in home offices. Especially now with more people working from home. The better half spends many hours in hers at home and it can easily reach 90* in the summer. That is because the door has to be kept closed and it only gets cool air when the HVAC is on for the rest of the house. A high end PC running multipole monitors etc creates allot of heat.
    I would really like to put in a minisplit but unfortunately to properly size it no one makes anything that small. I would need roughly a 3Kbtu unit but the closest is twice that size.
    The only way to make it work fairly decently is to use a small minisplit and then an external thermostat and spread the cycle temps further apart to lengthen run times and cut down on short cycling even at it’s low end.
    There is a market for 3Kbtu minisplits but no one will make them because they will cost as much as a 6K. Or we need a way to get the computer heat directly out of the house.

    1. FWIW, a Mitsu rep just told me they were planning to introduce a 4k in Q4.

      In general your comment is very interesting because I’ve been struggling with a solution for my low load upstairs, and it will be used just like you described. For cooling, probably very little and for short periods of time “to take the edge off” those few hot weeks each summer or when a lot of sun is coming in the windows. And in winter much of the heat will come from the downstairs. So it will probably be off a substantial amount of time. Maybe I shouldn’t be so worried about short cycling.

      But then I think I’m missing something in your comment because why wouldn’t your wife just use a 6k like they do in Greece? Full blast to get it where you need it, then turn it off?

      1. In her office it’s a constant load. Basically think of running a space heater in the room during the summer. Summer is usually around 180 days out of the year here in Texas. When it gets up to the 110* range outside here it’s hard to keep the house cool as it will be 90 by 9am and in the 90s till near midnight.

        I really wouldn’t mind it being a little big actually. IMO every house here should have at least 2 cooling systems and up north two heating systems. It’s not smart to rely on one system. When that system goes down and it will you will be happy you have an oversized backup. A friend that still does HVAC has lots of stories of trane units going down for a week while they wait for a replacement control board as they have to be programmed for their unit and shipped to them.
        I have a 2 ton window unit I use for the shop that in a pinch could be used for the house. I also have one of those inefficient portable units which right now is in use at my parents house as their unit is awaiting parts. Always have a backup plan.

        I may end up doing two minipslits in the future. One in her office and one in the kitchen to make cooking in the summer bearable. Our house system is sized to man J but the design temp they use here is much lower than what it should be according to my weather station data I have collected over the past 12 years. So we end up with undersized systems here.

        1. Got it. Yes, I admit I have trouble relating to climates where people have to run AC non-stop. I hate the feeling of it personally and thus don’t like being in those places. I have the distinct advantage that where I live it’s super dry, and it’s cold at night even when it’s 95 during the day.
          Agree backup heat / cooling sources (depending on climate) are a good idea but on the other hand it’s more things to buy, maintain, fix when they break. Depends on how insulated your home is to some degree, and how willing you are to adjust behavior for those worst case situations. Is it really so bad to plugging in an inefficient electric resistance heater for example, on the rare case you need to?

        2. Stoves make excellent backup heat (whether they be gas or wood). They can be used to heat a room or most of a house, very effective. I think I’d keep one if I lived in Texas after this winter where people had gas service but no electricity (at least if I had a gas stove). Gas service does go down but its extremely rare. More likely the equipment has an issue

  19. >”For example, the Mitsubishi FS06 wall-mounted ductless unit has a rated cooling capacity of 6,000 BTU per hour and goes down to a minimum capacity of 1,700 BTU per hour. That’s a turndown ratio of 28% (1,700 ÷ 6,000). If you put that unit in a room with a load of 1,500 BTU per hour, it will bottom out nearly 100% of the time it’s running in cooling mode.”

    It’s actually both better and worse than that. The rated 6000 BTU/hr isn’t it’s max capacity. When it has enough compressor behind it it’s good for 9000 BTU/hr running flat out at ARHI conditions. That’s a (9000/1700=) 5.3x turn down ratio, or (1700/9000=) 19% of max capacity.

    But when married to a multi-zone compressor it’s minimum output is constrained by the minimum level of the (much bigger) compressor, and the number & modulation levels of the other active zones. It will almost never be able to run as low as 1700 BTU/hr running on a multi-zone system.

    Even when running on a dedicated single zone compressor the sensible heat ratio when running at minimum speed is always wretchedly high for a humid climate due to the high cfm per ton, with the evaporator coil only a few degrees below the room temp.

  20. I almost switched to 3 individual splits rather than a multi. But then came this year, which is so hot I want AC for the first time ever. Currently looking at a 5 head Daikin VRV Life, basically 7+7+7+7+12 on a 35kBTU outdoor unit (they let you go to 150% on the heads). Their VRV tech seems slick. It modules to about 5kBTU, and I feel better about the modulation of 5k throughout the whole house than 2-4k per room when some of those rooms might only need 1k when not at peak load. Anyone have experience with those to confirm I’m on the right track?

    1. Fwiw since we had an existing heating system (Mid-Atlantic zone 4) we just put in cooling on the top two floors. Third floor (covers the front half of the house) is a 15k btu ducted split, back half is 9k ducted split, front bedroom is 6k btu. First floor doesn’t need any cooling except the kitchen rear addition which has a window unit for now. That’s a 1900 sq ft old, leaky house. I think if we’d put in the proposed first floor units it would have been complete overkill

  21. I have a Mitsubishi
    MUZ-HM24NA – outside
    MSZ-HM24NA – inside
    Units that I got at an auction I’m in San Antonio Texas and want to install in my 2 car garage
    Is the 24k way over kill I don’t really care about heating only the Cooling part.

  22. How do I tell if the contractor oversized our mini split? It’s a Single head in the bedroom that is 12×20 with a cathedral ceiling. We were away when it was installed and the head in the bedroom is huge! We have one in the living area that is much smaller and does it’s job. We are in the northwest lower peninsula of Michigan. We also use a wood stove.
    I don’t have the unit’s specs on me right now.

  23. This is about heat/cooling output and not physical size.  Although in general the bigger units have larger output.
    To know, you need to run a heat loss calculation. Use  Then compare that against the specs of the unit.  You should also ask the contractor for the Manual J that they ran when they selected the equipment.
    More here:
    Although I guess I’d ask myself what the action would be if you find out that the unit is oversized.  The contractor will just argue that it will “turn down” (ie a unit with a capacity of say, 24,000 can run as low as maybe 6,000. so if the load calc came in at 9,000 they’ll say well, you’ll have capacity in the bank for big events, and on the design days it’ll run just fine.  What they won’t discuss is all the days that are below maximum load where the unit will run at 6,000 and turn on and off, eliminating the advantages of variable refrigerant).

  24. I’m wanting to put a hvac system in my well insulated shop. It’s 38’wide, 60’ deep and the ceiling is 16’ high with no windows. There is also two 14’x14’ insulated doors. Would a mini spilt would in this or should I purchase a conventional system? What size?

    1. Lots of variables. To do it right and get pretty close find someone that can do a Manual N on the building. That’s the commercial one. It takes into account commercial style building methods operating the system only at times you choose higher heat loads from work inside and ventilation from dust collectors fume hoods etc. That said if you have it done by a local company they will have insight on IF the sizing it comes up actually works in your area and what their experience has been.

      But without being there… Some things to keep in mind. If it’s an insulated wood structure it will be far easier to cool than an insulated metal building. The conventional metal building construction puts in massive amounts of thermal bridging because the insulation is pinched between the skin and the steel framing. Spray foam is better only if it also covers all that interior steel in the same thickness as the skin.
      I am in Texas which has the worst overall weather in the country. Not a place you want to live if you enjoy being outdoors.. Now my 20x40x12 insulated steel building has 2.5 tons of AC. It’s a conventional 3″ vertical pipe welded to the slab plates and then a 2×2 welded frame. That is covered by standard metal building fiberglass with the white plastic on the inside. It will stay comfortable as long as you start the ac up early and the temps don’t get over 100* for very long.
      However in the heat of the summer you don’t have a chance. It doesn’t cool down at night and for me to use my shop in the summer when it’s 106* daily with the low at night of 80* and it’s still in the 90s after midnight… I would need somewhere around 6-8 tons of AC or more.
      Remember this is an intermittent use building with high external and high internal heat loads along with a huge amount of thermal mass in the building that has to be cooled. Houses only need tiny units to cool because they have little thermal mass and you are keeping it at a steady temperature. Shop and commercial buildings need far far larger units.

      If your building was here and was a metal building it would easily need 10+ tons.

      For unit types it depends on your needs and budget. Minisplits are nice but they are expensive(and efficient) and they have issues in shops because they have no real filtration on them to catch workshop dust. Basically a window unit filter. You can use conventional residential units as they are the cheapest and can be set up with good filters.
      There is also the case for package units which I like because they are entirely outside can be filtered and ducted well and they come in one box ready to run. No copper to run etc which is a cost and a liability. They are not as efficient but they are convenient easy to work on and replace. They are the quietest units as well because they are all outside.

      Just some info to get you started. Location type of construction type of use etc play a huge role as well as are you going to keep it at temp all the time or intermittently. Two identical buildings side by side could vary in their AC needs by 50% just due to their intended usages.

  25. There’s nowhere near enough info so far to size an HVAC system, but one big open room of that size might be efficiently and economically conditioned with a ductless mini head at each end, possibly also augmented by 2 or more ceiling fans.

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