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Green Building Curmudgeon

Multifamily HVAC Woes

Ductless minisplits are a more efficient alternative to the oversized heat pumps commonly found in multifamily buildings

Tightly sealed, well insulated buildings can be conditioned well with a single ductless mini-split head, using small exhaust fans to transfer air into bedrooms as needed.
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Sure seems like a lot of ductwork for a 700 square foot apartment. Tightly sealed, well insulated buildings can be conditioned well with a single ductless mini-split head, using small exhaust fans to transfer air into bedrooms as needed. This 12 kBtu head conditions the 1300 square foot first floor very evenly. On a 15 degree day there was only a 2 degree temperature difference across the house.

We work on thousands of multifamily housing units every year providing certification, consulting, and testing services for affordable and market rate developers. Our influence tends to be somewhat limited as we do not have a financial stake in the projects, and we are only one of many project team members. The majority of our work is certification under the National Green Building Standard, EarthCraft, Enterprise Green Communities, LEED, and Green Globes.

Over the years we have worked steadily to improve the quality of design and construction on our clients’ projects. Some projects are receptive to our suggestions and invite us to participate in the design process. This allows us to help identify potential pitfalls and opportunities for improvement. In others, our suggestions are mostly ignored, and our work is limited to inspecting and providing certification. In that role, however, we still have some influence on the project, making sure insulation and air sealing are installed correctly, specifications are followed, and when testing is included in the work scope, that envelope, duct leakage, and ventilation flows meet targets.

While this does help improve the overall quality of construction, we see far too many missed opportunities where better decisions early on would have improved a building’s performance with little or no increase in cost. This is no more true than in the choice of HVAC equipment for these buildings.

King sized HVAC equipment, uncomfortable homes

The biggest challenge we find with multifamily projects is improving HVAC designs and equipment. Particularly in the Southeast where almost all apartments are conditioned with split system heat pumps. This equipment is typically located in a small mechanical closet with a lowboy electric tank water heater beneath it. The primary problem with these installations is the equipment sizing.

The smallest standard heat pumps available are a minimum of 18,000 Btuh, or 1.5 tons. These are typically 14 or 14.5 SEER, 8.2 HSPF, with single speed fans. Not terrible performance; Not much of a stretch over the building code either. The problem is that most of the units these heat pumps are installed in have loads in the range of 6,000 to 9,000 Btuh, providing far more capacity than required.

In warm and mixed-humid climates where air conditioning is used a large part of the year, this oversizing can create comfort issues as systems short cycle, limiting dehumidification. Occupants feeling clammy and uncomfortable will then set their thermostats to very low temperatures in order to lower the humidity. In particularly humid climates, some developers install in-wall dehumidifiers that help. However, they create a fair amount of heat that must be offset by additional cooling.

Lower capacity HVAC equipment is available with mini-split technology, both ducted and ductless, but adoption is still very limited in our southeastern market due to cost constraints (real or imagined) and fear of unfamiliar technology.

Ducts are overrated

Based on my personal experience with ductless mini-splits, I have talked clients ears off about the many benefits. Unfortunately, I’ve had no success getting anyone to adopt this technology in their projects. Tightly sealed, well insulated buildings can be conditioned well with a single ductless mini-split head, using small exhaust fans to transfer air into bedrooms as needed–my own house works this way.

Even when using a standard split system heat pump, it is very possible to reduce the duct runs, eliminating supplies at the perimeter walls and windows in favor of high sidewall registers facing towards the exterior. Compact designs can reduce ducts by 60% or more, lowering installation cost, with fewer opportunities for duct leakage.

On one recent project, during our initial review very early in the design process, the client and architect were receptive to alternative HVAC designs, including compact duct systems. Sadly, this information was not shared with the mechanical engineer who proceeded to prepare a standard layout with perhaps more ductwork than I had ever seen in a small apartment. Obviously cautious and risk averse, during a follow up call, the engineer stated that he designed what he was asked to design, the work was done, there would be no changes. This was followed by silence from the architect. The client was unfortunately unable to join the call and, as the project is already being priced by contractors, I have low expectations of anything changing.

“Value” engineering is part of the problem

Some of our clients use ductless equipment on projects, but it’s typically through the wall package terminal heat pumps (PTHP), the type of equipment used in many hotel rooms. So far most are unwilling to look at more efficient ductless alternatives. One project in the design phase, consisting primarily of studio apartments, is currently considering ductless mini-splits. While we are hoping that this will actually come to pass, I am not holding my breath.

I find it interesting that with all the apartments being built that none of the major equipment manufacturers have come up with smaller heat pumps. What is keeping them from producing 1 ton or smaller systems? Mini-split systems are some of the only equipment available below 1.5 tons. These systems tend to both be significantly more efficient and more expensive. Even when they are considered, they’re frequently value engineered out of the project in an effort to control costs. While we have low expectations, we hope that we will someday see improved HVAC systems on more of our projects and will continue to keep up the good fight until it happens.

-Carl Seville is a green builder, educator, and consultant on sustainability to the residential construction industry. After a 25-year career in the remodeling industry, he and a partner founded a company, SK Collaborative. Photos courtesy of the author.

25 Comments

  1. nfpagroup | | #1

    Thanks for sharing this awesome blog.

  2. John Clark | | #2

    I've spent some time in the 10-15 yr old new condos built in the downtown Atlanta area. All of them were uncomfortable during the summer and winter months because the balconies were not thermally broken from the interior and the floor-to-ceiling windows were leaky and/or poorly insulated. A truly miserable experience when you get more than a couple of people in there.

  3. Russell Miller | | #3

    That fan from hallway to the bedroom works, huh...??!!

    Ive had many tell me it WOULD NOT

    1. User avater
      Dana Dorsett | | #4

      The small fan works fine for ventilation (and thus latent-cooling), but for sensible cooling and heating, not so much.

      1. Richard Klancer | | #15

        Dana, I'm curious why that is?

        Is it that the rate of humidity transfer effected by the small fan is typically big relative to the humidity transfer from the environment to a warm+humid room, but the rate of *heat* transfer due to the fan is typically small relative to the heat transfer into the room from the environment?

        Alternatively, is it that humans "notice" small changes in humidity more quickly than small changes in temperature?

        1. User avater
          Dana Dorsett | | #16

          The thermal mass of air by volume is only about 0.018 BTU per cubic foot per degree F. So even at a 5F difference (80F in the remote room, 75F in the room with the ductless head) a 100 cfm (=6000 cubic foot per hour) fan is only delivering...

          ...5F x 6000 cf/hr x 0.018= 540 BTU/hr

          That's roughly the sensible cooling load of two sleeping adult humans, or one conscious sitting adult working on a computer. So any additional heat gain ( sun through windows, etc) is piling on top of that.

          But a continuous 100 cfm of dry air exchange keeps up with the latent loads of a 150-200 square foot room quite easily.

          1. User avater
            Carl Seville | | #20

            Dana - your calculations align pretty well with how my house works. There is minimal heating or cooling required due to the high performance envelope - R22 cavity plus R3 continuous wall insulation, R 50 attic insulation, and a total of about 400 CFM50 envelope leakage for the 2600+ SF house. The rooms that are supplied by the fan have East, North, and South facing windows. The south windows have a 3' overhang, and the East have partial shading from large trees, so the radiant heat gain is modest, and it rarely gets so hot or cold outside that there will be much heat transfer through the walls and windows.

          2. Richard Klancer | | #21

            Thanks, Dana; I wasn't sure how to start doing back-of-the-envelope computations here.

            I started imagining a closed system of equal sized rooms where one room was held at constant temperature and humidity, but in that case the heat capacity of air factors out (only the temperature and humidity difference, and the flow rate relative to the room volumes, matters). In that system, the humidity difference would halve in the same time as the temperature difference. Which made it fairly clear, intuitively, that the heat transfer from the environment or point sources must be "bigger" than the humidity transfer, but late at night I didn't have time to work out the computation.

            Your response reminds me that you can reconcile things by reasoning about "sensible" and "latent" loads that are expressed in a common unit (BTU/hr).

            ...Making me a less-green green builder, so to speak.

  4. User avater
    Jon R | | #5

    Common assumptions about sizing, modulation and latent performance are often wrong.

    See Figure 7 here showing that sizing of 10x current load on cycling conventional systems can have little effect on latent removal. See Table 7 here showing that much higher % loads on mini-splits can provide almost no latent removal. Few mini-splits maintain proper CFM/ton when modulating. We have seen data where a cycling over-sized mini-split has better latent removal than a perfectly sized one.

    Always install a dehumidifier - even with perfectly sized AC systems. Consider Daiken Quaternity (although I'm not convinced it is cost effective).

    1. User avater
      Carl Seville | | #10

      John - I have had mixed experience with dehumidifiers. I installed one in my house after construction was complete to see if it would help in the bridge seasons. Since I didn't have a traditional duct system, I installed a supply and return in my downstairs hallway. I was not aware of the amount of heat they put out - in my case the supply air was coming out at about 100 degrees, overheating the hallway and requiring extra cooling to even things out. It would have worked better in a ducted system where the heat would have dissipated, but regardless it would require recooling using both the dehumidifier and the AC. There are some split dehumidifiers that reject the heat to the exterior which is helpful but they are quite expensive. I believe they work in standard ducted systems, probably better in larger homes more so than smaller ones. I ultimately decided that if my RH is too high during moderate temperatures, I will just turn the AC a little lower. Luckily I had a great place for the dehumifier after I removed it - it is now in the conditioned crawlspace of my 1925 era guest house/office.

      1. User avater
        Jon R | | #13

        Agreed, do account for the sensible load (perhaps 2 kBtu/hr) that the dehumidifier will produce.

  5. Steve Black | | #6

    Any thought on why a ductless mini-split isn’t available with a geothermal heat exchanger. Seems like most everything would be the same except the air source is replaced with a water heat exchange.

    1. User avater
      Dana Dorsett | | #8

      >"Any thought on why a ductless mini-split isn’t available with a geothermal heat exchanger. "

      Huh?

      High wall coils (what many people think of as a minisplit) are available for use with hydronic geothermal systems for both heating & cooling. See Figure 2:

      https://www.supplyht.com/articles/99139-combining-a-geothermal-heat-pump-with-a-boiler

      Many people object to the high-wall coil "minisplit" look, but hydronic floor mounted fan coils & consoles are also available that work just fine with a hydronic geothermal heat source.

      But geothermal heat pumps also suffer from being way oversized for the low-loads of individual units in multi-family buildings.

      Check out the picture of the Multi-Aqua high wall fan coils use in this (arguably botched) geothermal system:

      https://www.greenbuildingadvisor.com/question/geothermal-nightmare

      https://s3.amazonaws.com/greenbuildingadvisor.s3.tauntoncloud.com/app/uploads/2018/08/08101414/Geo10w-300x300.jpg

      or from the manufacturer's site:

      https://www.multiaqua.com/index.php/water-fan-coils/mhqww-1-hi-wall-220

      Finding geothermal components small enough to be appropriate for low-loads would still be a problem.

      1. User avater
        Jon R | | #11

        Chiltrix sell fan coils down to 3,379 Btu/hr. Between the inverter drive (on their ASHP compressor) modulating down to 1/4 ton and possibly a buffer tank, compressor sizing is a non-issue.

        1. User avater
          Dana Dorsett | | #17

          >"... compressor sizing is a non-issue."

          It's still $illy to have to install a buffer tank to make a smallest-in-class ground source heat pump work well with an apartment that has a way sub-1-ton load. With some GSHP lines you can't even get ECM drive motors for anything under 2 tons. I'm not aware of any 1-ton GSHP systems that modulate, but maybe there are some 2-stagers that small (?).

          Hydronic fan coils are the easy part. The "... geothermal components small enough to be appropriate for low-loads..." is still the issue. With a big enough buffer tank you could even make a 5 ton GSHP system work, but "working" is different from "appropriate".

  6. EcoInspector | | #7

    You are preaching to the choir Carl. Perhaps the article could have focused on finished basements: despite lower loads I rarely see HP appliance less that ton and a half

  7. User avater
    Carl Seville | | #9

    To Russell and Dana RE: using fan to move air. The picture above is the 2nd floor of my house. the 180 CFM exhaust fan is ducted into the 2 front bedrooms on a relay so it turns on when the mini split runs. I find that with the bedroom doors open it doesn't make much difference, but with the doors closed, the modest amount of air movement keeps the rooms comfortable. Whatever people may say, I can assure you that it serves a purpose and functions well.

    1. User avater
      Jon R | | #12

      At 3F delta-T, your 90 CFM will deliver ~300 Btu/hr + latent Btu cooling to each closed door bedroom. About right to cover a sleeping person, leaving nothing for sensible heat gain from outside.

      If someone doesn't care about noise privacy, there are some 10" through wall fans that move 380 CFM. That's a typically much more useful 1231 Btu/hr @ 3F.

      1. User avater
        Carl Seville | | #14

        There is virtually no gain or loss most of the year through the building envelope - house is exceptionally well air sealed and insulated in CZ3 which has a very low delta T most of the year. And it works in practice.

    2. Brendan Albano | | #18

      While the goal is to keep the rooms comfortable, and as you say, the fans are keeping the rooms comfortable, I think there's value to the pushback you're getting about WHY the fans are keeping the rooms comfortable.

      If the fans are keeping the rooms comfortable because of the air movement, great! But that also means that a ceiling fan would probably work similarly, which is a useful piece of information.

      It seems reasonable that the effect you're getting is partially due to the small amount of heat being moved around (as Dana and Jon have done the math for) and partially due to the increased comfort provided by air movement.

      If the air movement is providing a fair amount of the comfort, I wonder if it might be even more comfortable to have the transfer fans controlled by a switch in the bedroom (like a ceiling fan would be) rather than by a relay connected to the minisplit. That way, if the occupant wants to cool down, they can flip the switch, and if they're comfortable, no need to run the fan.

      1. User avater
        Carl Seville | | #19

        Brendan - I have an override on the fan that I can turn off if I want to, and do often in moderate weather and when we keep the bedroom doors open. I use ceiling fans regularly (and properly - only when someone is in the room), but they will not help make the rooms more comfortable if heating, cooling, or dehumidification is needed and the doors are closed. The clearest example of this was one day last summer I had the fan turned off when my wife was ironing (she loves to iron!) in one of the bedrooms with the door closed and it was at least 8-10 degrees warmer in the room. Turning on the fan to move a little cool air into the room solved the issued.

  8. Kye Ford | | #22

    I have thought of incorporating mini splits in our apartment designs.

    Does anyone have any experience with cleaning and maintenance in their apartments?

  9. Stuart Miller | | #23

    I want to love ductless minisplits, but I've been skeptical of cost and airflow.

    I just acquired a 4-plex with a variety of HVAC systems in it. One of the units needing complete renovation needs a complete HVAC overhaul and doesn't have central forced air. I intend to install a ductless minisplit system. However, it's in the lower level (walkout basement style) of a purpose-built masonry 4-plex, rather humid down there (MO climate).

    My concerns with minisplits have always been air flow and cost. I'm hoping the Mr. Cool or Pioneer brands (or similar cheaper brands) are going to be acceptable in quality; the primary brands talked about here and referred to by HVAC contractors are just stupidly expensive and, honestly, I think it is hype like is done on sites like this that has driven the market price up for those; it's, like I said, just plain stupid otherwise.

    As for the air flow issue, I've tried to imagine a variety of scenarios for moving air, including the room-to-room exhaust as explained here, but even if it works, you've created noise (from the exhaust fan), a non-automatic ventilation system unless you really work to rig it based on sensors, and, most unavoidably, a noise penetration to other rooms that is likely to just be unacceptable. So IMO, room to room exhaust, without at least an insulated vent of some kind (you're back to ducts...) is a no-go.

    Which means you need more head units, just about one per room, really. And that complicates the minisplit scenario, because even the cheap brands go up pretty quickly as you add head units, and the minimum size also goes up.

    The poor man/easy way? Dedicated, cut-in window heat pumps, which can come with dehumidifiers. Maybe good enough for rental units. Louder, less efficient (though they can be decent) and even more unsightly, which is the sacrifice there. But no expensive central forced air and ducting, no expensive ductless mini-splits with multiple heads.

    If ductless mini-splits were competitive in price to put a semi-concealed, appropriately-sized head unit in every major room, that would be the solution. Not sure we're there yet.

    1. User avater
      Dana Dorsett | | #24

      When referring to the "...air flow issue..." are you really talking about heat/cool distribution? Moving around heat/cool with a bunch of fans doesn't really work well enough unless it's a high performance building envelope.

      What is the existing (if broken) HVAC solution for that masonry building walk-out basement unit?

      Are any of above-grade or below grade walls insulated? If so, how?

      Have you run a Manual-J on it, or an I=B=R heat load calc?

      How many square feet of floor area?

      What ZIP code (or 99% and 1% outside design temperatures, approximately.)

      Without at least some semi-real load numbers there's no way to optimize this.

      >"Which means you need more head units, just about one per room, really. "

      No, NOT really...

      At some point it has to be served up with a mini-duct cassette, not a head per room.

      >"If ductless mini-splits were competitive in price to put a semi-concealed, appropriately-sized head unit in every major room, that would be the solution. Not sure we're there yet."

      I'm positive that there will never be an "..appropriately-sized head unit in every major room..." approach that would work for an apartment- it doesn't even work for most single-family homes. The head per room approach ALWAYS oversizes the equipment way beyond what is optimal for efficiency (or comfort, if multi-split.)

      It's possible any 3/4 ton ductless mini-split would be oversized for the heating & cooling loads of a walk-out basement type apartment. Appropriately sized hydro-air coils or cheap fin tube baseboard appropriately sized for the heating loads running off the water heater, and/or through-the wall half ton PTHP for both heating & cooling (hotel-motel style) may be more appropriate, depending on where the numbers come in. The sensible cooling loads of most walk-out basements are almost vanishing small, but the latent loads are still real. If you're installing a 3/4 ton mini-split make sure it has a "dry" or "dehumidify" mode that is still operating at a temperature setpoint. It may still need a standalone dehumidifier during the stickiest summer weeks, but high SEER mini-split will just make it clammy-cool unless it has a dehumidification mode.

      So, run the load numbers, room by room, (you'll see what it means about sizing) , and see what it adds up to. Then make a sketch of the floor plan with the room load numbers penciled in and post it. There are some fairly inexpensive 3/4 ton and 1 ton mid-static mini-splits out there that might work, if you're up for running some duct too, eg:

      https://senville.com/concealed-duct/

  10. Kye Ford | | #25

    Again does anyone have any real world experience with the performance of mini splits in apartments. ? It is one thing to speak in hypotheticals about performance but I'd be interested to hear some actual experiences.

    I loathe cleaning my unit at home. If the air handler units are not cleaned at least yearly they get pretty nasty, and I sure this influences air flow rates. Does this become a yearly maintenance issue in apartments where you have to set up an inspection and cleaning. It seems like it could take up a lot of time if you have multiple apartments and could become costly if you are hiring out the service.

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