Mini-Split HVAC Systems – Heating and Cooling On Demand

Mini Split Heat Pump Comfort HVACPay-As-You Go Heating and Cooling

It’s very simple. With ducted or ductless mini-split systems, you only pay for and use as much or as little heating and cooling as you need at any given time in any given room? How does it work?

In technical terms, there are three primary components to making this possible.

1. An inverter-driven scroll compressor inside the outdoor unit (a.k.a. condenser, compressor) that adjusts its rotation speed, and the electrical draw, to precisely match the load requirements within each zone of the house or building.

2. Quiet, low-wattage fans in the “mini” air handlers and outdoor units modulate their speed to match the need of individual zones (air handler) and whole system demand (outdoor unit)

3. Dual electronic linear expansion valves (LEV) that adjust the amount refrigerant being delivered to all air handlers, and within each air handler, based on what each zone or all zones are calling for.

In practical terms, it’s similar to a pay-as-you-go cell phone, where the user only pays for the minutes used. No more, no less. . With this technology, the users heat and/or cool as much or as little as they want, and only pay for what they use.

Let’s Break it Down

The inverter-driven scroll compressor in the outdoor unit adjusts the amount of electricity, and the linear expansion valves adjust the amount of refrigerant, the hot and cold liquid that is used to condition the air being supplied to each zone, to deliver exactly the amount of heating and cooling needed at any given time in any given room and/or zone. No more, no less. The system only uses the energy it needs to do this, whether its a little or a lot.

Mini Split Heat Pump Outdoor Unit HVAC Design

Typical Outdoor Unit

The mini air handlers (a.k.a. indoor fan coils), which range in capacity from 6,000 btu/h to 24,000 btu/h for residential, and up to 96,000 btu/h for commercial applications, are selected through a process that is based on the heat loss and gain calculations, a.k.a. load calculations.  How many air handlers, how much air needed, and how that air is delivered to each room or zone, are the remaining steps in the HVAC Design process.

Outdoor units can have as few as one (1), and as many as eight (8) air handlers in residential applications, and up to sixty-four (64) units in commercial. This is what gives these systems their name “mini-split” heat pumps, since conventional split systems have one outdoor unit and one indoor unit to serve individual or multiple zones. The “mini” refers to the multiple air handlers that are not only smaller in capacity, but also in physical size.

Mini Split Heat Pump Wall Mounted Unit   Ducted mini-split fan coil in attic before insulation

  IMAGE LEFT – Typical Wall-Mounted Ductless Air Handler

 IMAGE RIGHT – Typical Concealed Ducted Air Handler

The air handlers can have ductwork and be concealed in a ceiling, attic or floor cavity to serve up to three or four rooms. OR, they can deliver the air without ducts with wall-mounted or recessed (in wall/ceiling) ductless units. Either way, these air handlers deliver air with a fan that is generally quieter than a human whisper.

They just make sense

Everything about these systems are variable, efficient, quiet, great for good indoor air quality, and extremely effective. What more would you want in an HVAC system?

Contrary to what you might be thinking, or know, these systems can and DO work in all climate zones. They’ve been used in Europe, Asia, Canada, and South America for decades, and have proven themselves to be reliable. HVAC companies rave about how few call backs and/or repairs that there are with these systems.

Mini Split Heat Pump Ducted Concealed

Typical Ducted Unit

Generally, they can be as much as 30%-50% more efficient than conventional systems. Though they’re rated efficiency (at full capacity) ranges from 14 to 20 SEER, they rarely ever run at full capacity. This makes their real world efficiency seem higher. In fact, an alternate efficiency rating, called Integrated Energy Efficiency Ratio (IEER), is used to more accurately factor the variability of the equipment in to the rating.

A final technical note: Conventional outdoor units (heat pump or air conditioner) draw about 25-40 amps of electricity when in use.

The maximum amp draw on mostresidential mini-split outdoor units is 18-20 amps. Again, because of the variability, and the fact that the system is never on full capacity, the amp draw is usually much lower…maybe around 10 amps or lower….

Mini Split Heat Pumps Energy Savings Graphic

To the left is a graph showing just one of the many ways that the mini-split systems are better at minimizing energy use.

If you need more information about these systems, or are considering them for your new or existing home, don’t hesitate to contact us. We have experience with these and many other types of systems, and can design a system that suites your needs…precisely.

 

written by Chris Laumer-Giddens, Architect, HVAC Designer, Residential Builder, HERS Rater, Building Science Professional, EarthCraft TA

 

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24 Responses so far.

  1. John Poole says:

    Chris,

    Thanks much for posting this. All useful information, and I really like the idea of dividing capacity and matching it to local demands only. I just have a few questions:

    1) Are mini-splits ever combined with heat pumps? (Somehow I recall hearing that they sometimes are, or can be, but I suspect I might be mistaken).

    2) Do those SEER numbers you’ve cited take source efficiency into account? (I am ignorant of how they are calculated, and was just wondering if they did).

    3) Can mini-splits be integrated with mechanical ventilation in some manner, or do these systems need to be independent of one another?

    That’s it! Nice post, and glad to see your blog taking off!

    – John

  2. Chris – As an HVAC contractor/installer for new homes as well as existing homes, I couldn’t agree with you more. We have seen mini splits on the rise in the last three years and they will continue to do so for reasons you point out in your article.
    SEER ratings are unmatched and the inverter technology can satify almost any requirement. We have used them to satify rooms above a garage to agility dog courses in large metal buildings when the total load capacities can change dramatically. All clients have rated them A+. In fact it is now possible to tie eight (8) indoor units to one outdoor and all have separate thermostats. Outstanding!
    Thanks Chris

    Joe

  3. Hey, John,

    1. Would this be for a retrofit application? I have not heard of that, nor would I recommend it myself, but that does NOT mean it’s not possible. I would suspect that the efficiencies would be lost because the inverter is where these systems get their efficiency from. Because these are either heat pump or air conditioners, it would probably be just as cost effective to go with a new mini-split heat pump.

    2. SEER does not take source efficiency in to account, neither does the better rating IEER (which takes in to account part load conditions that mini-splits are in most of the time). The ratings come from AHRI (ahri.org), and come from testing done in a controlled setting with fixed outdoor conditions, but source doesn’t come in to play.

    3. Ventilation can be tied in to the ducted units, only. But, I would recommend that the fresh air is brought in near one of the ductless units so that most of it has a chance to be conditioned and filtered before entering the room. There are other options, but this one would is best to prevent really cold or really hot air from dumping in to the house and causing extreme discomfort to the occupants. Imagine being directly under the outlet during a cycle…yikes! Strategy (and design) is the key.

    I hope this explains it, and it answers your questions. If not, PLEASE let me know.

    Thanks, John. It’s gonna take some work, but we’re up for it!

  4. Thanks, Joe!

    I don’t know of a system that out performs, or that can integrate so well in to any design as mini- or multi-splits.

    Not only can they go up to 8 indoor units, the capacity of those indoor units can add up to more than the capacity of the outdoor unit. As much as 130%, as long as there is enough diversity designed in to the system.

    The list goes on….

  5. Sunil Sood says:

    Please let me know where it is available in India.

  6. Joe Ross says:

    Thanks for the info, It will take me a little time to digest all of it but I enjoy the read and study. I am also furthering my study with other material. One cannot learn enough about heating and cooling, and insulating our home envelopes.

  7. Hey Chris (or anyone else): Great article. I have been thinking of mini-splits for retrofits but one concern I have is the aesthetics of the wall/ceiling mounted equipment. Can you point us in the direction of the systems that you recommend (manufacturers and models) so we can research? Thanks! Rich

  8. Rich – if you can send me your email address I can send you photographs of recent installations that American Residential Services, DC Metro has completed. I haven’t figured out how to post pictures here. The mini split that we use most often is the Mitsubishi. Others are good too and some of the decsision to be made is how much one wants to spend. On another note, I just visited a homeowner who has a mini split that he recently purchased on-line and it doesn’t work at all. We frown upon on-line sales when it comes to HVAC equipment becuase it does require professional installation in almost every case.

  9. Thanks, Rich!

    The aesthetic concern is one I run in to on a daily basis, and have been working with manufacturers to come up with better designs (gonna take awhile…). In the meantime, I have been designing and specifying the ducted units that you don’t have to see. The ductwork has a very small impact on the efficiency of the equipment, and they can be concealed in the dropped ceilings, attics and crawlspaces (ideally encapsulated), or wherever you have space with access to a pathway to send the ductwork.

    For manufacturers, I would first recommend Mitsubishi Electric (www.mehvac.com). The MR SLIM models are all for residential. Ducted units all start with either SEZ or PEAD. Any of the M- or P- Series will work for a home.
    Which specific model and size you need depends on how much heating and cooling load you have, and that can be determined with an load calculation (Manual J) and (Manual S).

  10. Hey Thanks for this; I’ve been considering this for my own home, and for a couple I am embarking on designing. Do you know of a few contractors in the Denver Area?

  11. Thanks, Tom.

    For Mitsubishi products, go to http://www.mehvac.com, click on homeowners, and then enter your zip code. It will give you a list of ‘Diamond Contractors’ in your area.
    What I normally do when I design/specify a system is to make a few calls to interview local contractors and then make recommendations. This gives the homeowner, architect or builder that assurance that it’s going to be installed correctly.
    What I find is that if you just call a contractor, the system can be oversized. This leads to comfort issues and increased energy use. Having the design done fine tunes the equipment and the install, and it also makes the contractors life much easier. It’s spelled out for them, and they can get it done faster.
    Hope this helps…

  12. David says:

    We’ve certainly used these Mitsubishi systems over the years for remodelling projects, garages, workshops, or guest houses and they seem to be working well. Our installations have been in coastal areas where humidity is always a concern (as it is in Atlanta). Typically our homes are designed with conventional whole house heat pump systems to assist in maintaining humidity control, and we encourage our customers to maintain their temperature and humidity in a consistent way. We think maintaining consistent temperatures and humidity is important to indoor air quality, minimizing movement in trim, wood floors, framing and other materials and generally improving the indoor environment. I would not encourage operating a system which is only turned on when needed for comfort in a particular area of the home (unless it is a workshop or garage).

    Question: Can these systems been integrated with conditioned outdoor fresh air.

  13. Gary Baggett says:

    Have you ever tied in an ERV or HRV with a mini split ducted system?

  14. David:

    Thanks for the great comment. There are a lot of garages, mother-in-law apartments, and workshops with mini-splits in them. It seems to be the one application that people associate most with these systems.
    The beauty of the mini-split VRF system is its ability to balance comfort through controlling humidity, temperature and air quality throughout a home or any type of building. The indoor fan coils act as completely separate systems in the sense that it doesn’t matter what’s happening anywhere else in the house, even though they might be connected to the same outdoor unit.
    This technology works great for any location, and are known for their de-humidification ability. Well spotted!

    As to your question: Yes, they can be integrated with outdoor fresh air. There are several ways to handle this.
    In most cases, I recommend an ERV or HRV, depending on the location and needs for humidity control. Most of the time, its an ERV. (Other parts of the country, an HRV might make sense)
    With ductless systems, the fresh air can be brought in anywhere in the house, but ideally near one of the fan coils to be able to filter and condition it.
    With ducted systems, it’s best to connect the ERV to the return plenum (as recommended by manuf.) so that it can be conditioned and/or filtered before it enters the home. ERV’s and HRVs also recover what would normally be lost energy from the house by exchanging that hot/cold energy through it’s core.

    One other good option that is just as effective, but that doesn’t have the ability to recover energy is the to use an Air Cycler. It’s a motorized control damper that brings in fresh air at pre-set intervals. It can be tied in to bath fans, like an ERV, and will just dump fresh air in to a strategic spot in the house. These are known for wasting energy when they’re connected to a conventional system because if the house is calling for fresh air, and the Air Cycler is kicks in, it also turns on the central fan to blow the fresh air in to the house. With mini-splits, the fan is always running (at a very very low amperage), so it’s not “extra” energy needed to bring in 50-80 cfm (average for most homes) of fresh air.

  15. Thanks, Gary. Check out my response to David, and let me know if that makes sense. I install ERV’s as often as the budget will allow (somewhat pricey). The Air Cycler is a GREAT more cost effective (initial) solution that, in my opinion, only works effectively with ducted systems.

  16. Gary Baggett says:

    Yeah, it makes perfect sense. I have a 2 story commercial space I am building into a recording studio in Long Island, New York. It has an old gas fired, atmospherically vented furnace that I want to get rid of. I’s like to salvage and seal the duct system, and use it with a new mini split along with an ERV. Fun, fun!

  17. Gerrie West says:

    Great article, thanks! I have a two-year old Mitsubishi ductless mini-split installed in my 500 square foot one-room double height Guest House. You mentioned that these units are “whisper quiet”- admittedly I am extremely sensitive to noise, but I find the constant hum of the indoor unit to be a deal-breaker for use in a full time residence. I’ve had my installer check the unit several times, and he says it is normal that when the power is on, the unit is on, and that the noise level is typical of these units.

    The Guest House has temporarily become my office, and with hardwood floors, wood paneled walls, and few fabric surfaces, it is a very “hard” space, acoustically. I’m sure the addition of absorptive surfaces would help, but I find myself turning the unit off, just to avoid the constant noise, then on again to modulate the temperature. It is also hard to resist turning the unit off at the end of the work day, with thoughts of the wastefulness of the fan running all night.

    I’d like to use my own experience with this to encourage my clients to use mini-splits when appropriate, but I don’t want the noise factor to be a discouragement. Will you comment on the proper operation of these units, and on my perception of noise pollution vs. efficiency?

    Thank you

  18. [...] conditioned air it was pure triumph when Chris handed me the remote for our new wall mounted ductless mini-split [...]

  19. Thanks. That is a really useful article.

  20. Choosing an efficient heating or cooling system for your house will help you maintain healthy indoor temperatures, reduce greenhouse gas emissions, and wield your power bill.

  21. Gerrie West says:

    Hi Chris,
    I commented a year ago on your excellent article, and wonder if you’d reply to my concerns?

  22. Dale Turner says:

    I am building duplex apartments, two story. Do they make a ducted system that will heat 1600 sq ft. kitchen living 1/2 bath downstairs, Three bedrooms two baths upstairs. That would be 5 to 7 zones with concealed units in the floor joist between the two floors, ducting to the appropriate rooms.

  23. […] are available for single- and multi-family homes, as well as commercial buildings of any size. Here’s a general overview of what the systems are and how they work, in case you’re not familiar. Compared to […]

  24. […] not designed to fully accommodate any type of air conditioning systems, other than, perhaps, a system without duct work. In the case of this home, the owners “do not want to see or hear” any part of the […]