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Good HVAC Design Is 99% Invisible


Sometimes people are bothered by how things look.  They don’t want a return grille showing in that location or they’d rather have the supply registers low on the wall or they just can’t stomach the thought of a wall-mounted ductless mini-split anywhere.  We all have our preferences about how things look and that’s important.  We spend a lot of time in our homes and we want them to look nice.

Now, consider this.  After a few days, the exposed infrastructure in your home becomes invisible.  When is the last time you noticed those smoke detectors on the ceiling?  What you may find obtrusive at first melts away into the background of your consciousness after a relatively short time.

But how things look isn’t the end of the story.  Vision is only one of our senses after all.  When your air conditioner comes on, do you hear it?  Do you start to feel a little cooler shortly after your heating system goes off in the winter?  Does the humidity in your home stay too high unless you turn the thermostat low enough to turn your hands blue?  Do you feel a vibration when the system is on?

A lot goes into a heating, ventilating, and air conditioning (HVAC) design.  Yes, looks matter.  But so do comfort, noise, vibration, and more.  You’ve probably heard of indoor air quality, IAQ.  That’s only one factor in the broader field of indoor environmental quality, IEQ.  The amazing Robert Bean of Healthy Heating says IEQ is the sum of:

  • Indoor air quality
  • Indoor thermal quality
  • Indoor lighting quality
  • Indoor sound quality
  • Indoor odor quality
  • Indoor vibration quality

Good HVAC design addresses most of those.  (Lighting doesn’t really come under the purview of HVAC.)  The designer’s goal is to minimize the negative impacts to any of those various indoor qualities.  Success means the HVAC systems are 99% invisible. 


About the title

Yes, I’m a big fan of the 99% Invisible podcast, hosted by Roman Mars.  If you are, too, you probably know that the name of the show is taken from a Buckminster Fuller quote:  “Ninety-nine percent of who you are is invisible and untouchable.”  The idea is that most of the stuff in our lives is behind the scenes.  It’s a fantastic podcast, and you should go listen to it.


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Photo of living room by Ceylon Tea Trails from, used under a Creative Commons license.


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This Post Has 23 Comments

  1. I agree to some extent on the
    I agree to some extent on the “invisible” stuff on walls and ceilings, but a minisplit casstte on a wall will always be a wart in front of your nose, and no matter how much makeup you put on, you’ll always see the wart!!!

    1. Armando, you couldn’t have

      Armando, you couldn’t have said, “Yes, I’m an architect” any more directly.

    2. For maybe a thousand years
      For maybe a thousand years fireplaces and chimneys were the unhideable warts and sore thumbs of a prior era of home environmental management, but somehow over time architects found positive ways to work with them.

      1. This is one of the most
        This is one of the most astute observations I’ve seen regarding the mini-split head units. It should open up a huge number of design suggestions from inventive minds – like combining the head unit (up high) with a wall-mounted flat screen (below) where the wall panel area (and head unit) are painted a dark color to contrast with the rest of the room.

    3. I always see the huge return
      I always see the huge return grill on the wall of my living room, too! And I always hear it, throughout the house, when it runs and rattles. So it all depends on which wart you find least objectionable, perhaps.

      1. Leigha — First of all there
        Leigha — First of all there is no reason to have a return in your living room unless it can be totally closed off from the rest of the house. Second if it is noisy the return is probably too small for the airflow of the oversized HVAC equipment.

    4. Well, here in Europe where
      Well, here in Europe where wall mounted mini-splits are the norm, all the mainstream manufacturers have at least one, often several “design” oriented units which are available for a small premium. Mitsubishi, Daikin and Panasonic have some outstanding stylish units. The Daikin units I have installed are a real feature of each room and a talking point for first time visitors to the house. The motorised front panel which opens when the unit is switching on or off plus clever wide louvred flaps and horizontal airflow adjusters have an origami-like quality, and the plastic finish is very tactile. When I compare this to a dull off-white diffuser or, worse, a utilitarian plain grille which gets plonked randomly in most US houses, I don’t think we’re worse off. My unit in the kitchen/breakfast nook gets far more attention than any granite worktop would and the cost differential between than and a laminate counter almost paid for the whole-house installation I had done.

      But you do have a point when it comes to bad or ill-considered unit placing (pipe runs in plastic conduit on the walls as rear piping wasn’t an option being a pet hate of mine). And a cheap horrid budget indoor unit will always look bad and most likely be noisy and show it’s age after a few years.

      And ugly afterthought bump-outs for ducting make any space look compromised.

      In short, it’s all just a question of what folks are used to, with getting used to the inevitable compromises for any air handling solution being a big part of that. And markets, along with tastes, do change

  2. Allison, I realize that the
    Allison, I realize that the capacity of the outdoor unit can be a limiting item on the sizing of an HVAC system. But when the indoor unit is variable speed – not fixed – as in with mini-splits and a growing number of forced air systems – does the fact that it is variable speed help to “adjust” for sizing errors? I am particularly speaking of very high performance homes where we might see one ton of HVAC for every 2k floor sqft or so. So if for example we have a 1,200 sqft very high performance home – would a 2 ton variable speed system be expected to perform much better than a 2 ton traditional forced air system (digital – either on or off – not variable)?

    Have a happy fourth!

    1. Charles, that’s a great

      Charles, that’s a great question.  My answer is that sizing errors are still sizing errors.  Even with variable capacity equipment, you want to size it correctly because you still want to be able to meet the low end of part-load conditions.  Yes, a system with variable capacity does cover you more in part-load conditions but you want as much coverage as possible.

    2. Variable speed indoor units
      Variable speed indoor units work well when coupled with two stage or better outdoor units. Today’s high end systems from almost all manufacturers utilize variable capacity outdoor units. It is then possible to build in moderate excess capacity and still operate appropriately during low load conditions. Some variable capacity heat pumps can ramp up past 100% to handle very low ambient conditions without requiring backup heat. Add in modulating zoning dampers and controls (Trane and Carrier) and you can approach museum quality control. Some might include a whole house dehumidifier for shoulder seasons when outside humidity is high but temperatures are mild, but a well sealed enclosure may not need the extra equipment.

  3. “Good HVAC design addresses
    “Good HVAC design addresses all of those”

    How does good HVAC design address Indoor Lighting Quality?

    1. Curt, yeah, you’re right.  On

      Curt, yeah, you’re right.  On my second reading, I had the same thought but didn’t change it.  I’ve revised it now.

  4. Ah yes Variable Compressor
    Ah yes Variable Compressor Heat Pumps — the savior of all HVAC problems. Folks — VCHP has nice theoretical advantages. Tested in real houses they do not live up to their theory YET.
    Their efficiency ratings are unrealistically high, their controls are problematic and they are oversold.
    The pimple on the wall will not do a good job of cooling or heating the next room. I suggest you read about closely monitored installations in:
    If you are going to delve into mini-splits do it with your eyes open at least read the executive summary of that report.
    That said, I love DUCTED mini-splits for a number of reasons: they come in small sizes, they use SHORT DUCTS (they have to), they can achieve good distribution. Forget the hype about their efficiency — look at the advantages of size and low duct losses.

    On another note — the less there is to hide the better it is: SHORT DUCTS, SMALL UNITS

  5. I agree with everything that
    I agree with everything that John said, but I will add a couple of more things just to stir the pot a bit. Let’s imagine that you have a choice between a 2-ton or 3-ton unit with the same SEER rating. Let’s also assume that they have the same efficiency (EER) at their maximum capacity (95 F outdoor temperature). The efficiency of both units will go up as the capacity decreases. So all else being equal, the 3-ton system operating at 2/3 capacity should have a higher efficiency than the 2-ton system at full capacity. This would indicate that you would use less energy with the larger system. However, there is a catch. No system can modulate down to zero capacity. So at lower loads, the larger system will begin to cycle before the smaller system, thus you lose the benefits of modulation sooner. So there is a trade-off between high, medium, and low load operation that depends on system sizing. Another issue is that if you like to change your setpoints, either for comfort or energy savings, larger systems will recover faster, and this can impact energy savings, especially for heat pumps in heating.

    1. Good Point RoyC. BUT in my
      Good Point RoyC. BUT in my climate (the rot belt), it is imperative that I know the SHR at those part load conditions. And when you ask many manufacturers what those numbers are, you get nothing but crickets. Who cares about efficiency when there is water dripping off the indoor unit because the latent removal capacity is nearly nonexistent. (Yes I have seen this several times with mini-splits.) Unlike sensible loads, indoor latent loads do not follow the same linear relationship. People still breathe, cook, take showers, wash clothes, mop floors nearly as much at 83 degrees as they do at 88 degrees.

      1. In the early days of multi
        In the early days of multi-capacity systems, some manufacturers did not adjust the indoor airflow rate in response to capacity changes, so latent capacity was greatly reduced at part load. I believe that all of the manufacturers do adjust indoor airflow rate to maintain latent capacity and some even provide manual user inputs or humidity sensor options that will reduce indoor airflow rate further when needed for additional latent capacity.

        Manufacturers do not provide much performance information beyond what is required because the vast majority of their customers (dealers) do not want it.

        1. Roy C. Just because some
          Roy C. Just because some important group of entities do not want something does not mean they are against it being available. That said providing the information so people can start to become more educated seems positive to me.

  6. For each of the line items of
    For each of the line items of good HVAC design, can we tag articles that help address these issues given most people inherit the design and structures? Indoor sound quality is a bane of my existence, and I would like learn of ways to improve. Thanks for the article!

  7. I don’t understand the
    I don’t understand the fascination with mini-splits. Its as if they are THE answer to all life’s problems. Apparently they will cure comfort issues, many forms of cancer, improve your marriage, increase tolerance to rap music, invite world peace and bring consensus to the ASHRAE 62.2 committee. Is there a latent and looming epidemic of “duct-phobia?? I am just not getting it.

    1. Ductless mini-splits have a
      Ductless mini-splits have a place in mechanical design but more often than not, they’re mis-used due to a lack of understanding of their limitations. Limitations that include…

      a) The minimum capacity of ductless heads is way oversized for most rooms in a home that meets current codes. On a 1:1 system, the min. capacity is typically several to many times more than all but the largest rooms, even worse on multi-splits.

      b) One head for multiple rooms or entire floor MIGHT be acceptable in a way-beyond-code home with an open floor plan in a cold (read non-cooling) climate, assuming occupants don’t mind wide temperature swings (for example, bedrooms 4 or 5 degrees cooler than main area), BUT you can’t get away with this in a cooling climate.

      The following limitations apply to ducted mini-splits as well..

      c) There’s a reason why multi-splits have significantly lower SEER ratings than 1:1 mini-splits, just as there’s a reason why larger systems have lower SEER ratings than 9k, 12k, and 15k systems (same is true for non mini-split systems). Likewise, ducted mini’s have lower ratings than ductless for same model series since it takes more fan power to push the air through ducts.

      d) Ducted mini’s have wider application than ductless (see a & b above). I’ve used them in a number of projects over the years. But as John P said, they have limited blower power compared to non-mini-split air handlers. Fujitsu and Daikin Skyair have significantly more available static than the Mitsubishi ducted ceiling head (SEZ). Daikin’s smallest ducted heads (less than 18k) units have barely enough static for the filter. A good designer can work within these limitations and select the most appropriate brand/model for the application. However…

      e) Ducted mini’s are a poor choice for homes that require multiple zones on a single ducted head ( see (a) above, which applies equally to ducted mini’s). Unfortunately, you can’t (easily) do damper-based zone control with ducted mini’s due to proprietary controls. After-market zone control interfaces are available but you give much of what makes a variable system tick. In particular, the system can no longer track the delta-T in the zone(s). Daikin introduced an integrated zone control system for it’s larger (18k and higher) systems but I’m highly skeptical that it manages airflow properly when less than all zones are calling (see (f), below). I would be happy to be proven wrong if someone ever does a deep-dive research study on this system.

      f) As Danny alluded to, the most serious downside with mini-splits in general is lack of latent capacity, depending on the specific model and size. The indoor unit (blower) doesn’t necessary (or ever) ramp down in concert with the outdoor unit, or the indoor unit that matches up with a particular outdoor unit is actually a re-labeled larger unit. Due to oversized indoor coil and blower, the latent capacity (ability to remove moisture) is reduced This is ESPECIALLY problematic with the smallest systems. Case in point, the Mitsubishi 30 SEER 6k system, which is the smallest 1:1 mini, and has the highest SEER of any air-to-air heat pump, has virtually NO latent capacity and not much at full load due to poor match-up between indoor and outdoor units. Ask Chris Laumer-Giddens how he knows this!

      Sorry for so many words, but I’ll leave on this note: Anyone considering mini-splits for whole-house HVAC better make SURE the designer understands *all* these limitations. And be prepared to be told why you’d be better off with another solution.

  8. And it is often useful to
    And it is often useful to look around at real world structures to see what not exactly visually attuned HVAC contractors (and, I suspect. even some architects) foist on buyers.

    For example, a quick look around design critiquing sites (McMansion Hell being a great place to see what *not* to!) shows a motley assortment of badly placed outdoor units, gruesome diffusers and just plain ugly baseboard heaters. This is one of the worst to recently feature.

    Even if you’re not a fan of mini-splits, good quality radiators and aesthetically pleasing diffusers made from better quality materials would have definitely helped the look of the rooms in this property. Oh, that, and not putting the outdoor unit on the front lawn by the entrance porch!

  9. My first such encounter (of
    My first such encounter (of which similar such occurred many more times over the years) was as a Junior Engineer working on a private set of offices for a then GM exec. Georgian design sensibilities I vaguely remember per the self-anointed genius involved in leading the design rabble.

    Anyway – it taught me about things to consider when HVAC was required for a 16th century English manor (pre Willis). A great exercise in restraint and careful selection regarding both noise and air throws / velocities figured prominently.

    HVAC etal can and should be invisible RE: practical sensibilities.

    That term practical can often be wrestled with via cost factors where resistance to logic is running rampant.

    Almost unseen is a good goal. Invisibility requires magic.

  10. I think that most people
    I think that most people would like their HVAC systems to remain largely invisible, however, I can’t help but think many of the people are also willing to sacrifice performance and efficiency for aesthetics. In my opinion, I would rather work around a better system than an “invisible” one.

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