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Standard HVAC vs. minisplits?

Steve Stemper | Posted in Mechanicals on

Is it possible to install a regular ducted HVAC system that is comparable (as efficient) to a ductless mini split system?

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Replies

  1. Charlie Sullivan | | #1

    Yes, it is possible. Minisplits almost always have the best efficient variable speed motors and compressors. Those are rarer in larger equipment, but they exist. For example, the Carrier Infinity Greenspeed heat pump rated at 13 HSPF.

    But there are perhaps more ways to screw up a conventional system installation--you need to design the duct system without excessive pressure drop to maintain fan efficiency, seal it well, and keep it inside the conditioned envelope.

    And you really want low energy consumption, not just an efficient HVAC system, the envelope is more important than that choice of HVAC system. And once you have an excellent envelope, the smallest regular system is likely to be bigger than you need.

    Another option is an air-to-water heat pump. Here's a good overview. I'm personally more enthusiastic about them than this overview is. https://www.greenbuildingadvisor.com/articles/dept/musings/air-water-heat-pumps

  2. GBA Editor
    Martin Holladay | | #2

    Steve,
    I'm not as convinced as Charlie that it's possible for a conventional ducted HVAC system to be as efficient as a ductless minisplit.

    Any duct system, even a perfectly designed and installed duct system, introduces static pressure that makes a fan work harder than it otherwise would. This fan energy is a significant component of the energy used by a conventional forced-air system or a ductless minisplit, so static pressure matters.

    A ductless minsiplit is an elegant, well-engineered device. Once you hook up any kind of duct system to these basic components, efficiency drops.

  3. Charlie Sullivan | | #3

    Note that I only said it was possible, not that it was likely to be successful! And I probably should have said you'd need to design the duct system for less static pressure drop than standard good practice, not just to avoid excess pressure.

  4. Steve Stemper | | #4

    Thank you both for your comments. I will have to study it further.

  5. Expert Member
    Dana Dorsett | | #5

    HSPF/SEER bench tests are one thing, in-situ operating reality is another.

    Operating in a modulating mode over most of the season is everything in terms of squeaking the nameplate efficiency out of either a mini-split or a modulating ducted system.

    The turn-down ratio of a GreenSpeed is only 2.5:1, (at +47F outdoors the bench tested output at minimum speed is fully 40% of it's output at maximum speed) which means properly sizing of the equipment to the load is a bit tough in many or most climates. Average seasonal loads in many climates are much lower than 40% of the 99% or 1% loads. In my area (US climate zone 5A) a GreenSpeed sized EXACTLY to the calculated load at the 99% outside design temp will spend most of the season cycling on/off rather than modulating, but it wouldn't be too bad. If oversized a bit for extra margin, it would be cycling on/off rather than modulating, even at the average min-winter load. To get the modulating efficiency out of them it's common to size it to the load at the ~95th percentile temperature bin, but include resistance heaters to cover the shortfall, which turns out to use less electricity than letting the thing cycle on/off all season long, but still undercuts it's as-used efficiency.

    Most decent min-splits have turn-down ratios of 4:1 or greater (The Mitsubishi MSZ/MUZ-FH09NA has a best-in-industry turn down ratio of about 11:1 !). Sizing them correctly for the loads is still critical for getting the efficiency out of them, but it's a bit easier to find a good fit.

    HSPF numbers presume entering air temperatures of no more than 70F at the indoor coils, which is not always going to be the in-situ case with mini-split heads mounted high on the wall, so the numbers may be overstated somewhat for wall-coils. With floor mounted coiles (or return ducts) drawing air near the floor the as-used efficiency may be somewhat higher than in an HSPF test, particularly for optimally sized modulating heat pumps with wide turn down ratios.

    While it's possible to get nameplate efficiency out of a modulating ducted heat pump, I'd hazard that 95 out of 100 real-world installations are too oversized to come anywhere near those levels.

  6. Charlie Sullivan | | #6

    Dana, thanks for adding the information that the turn-down ratio is only 2.5:1. That reduces my already low optimism considerably.

    Chiltrix has about a 4.5:1 turndown ratio. Depending on what Steve's objectives are, that could be a good option, although the problem of finding a good designer and a good installer is in some ways harder for Chiltrix.

  7. Expert Member
    Dana Dorsett | | #7

    From their own marketing copy:

    "Adaptable speed means a furnace or heat pump** with Greenspeed intelligence can run at 40% capacity, 100% capacity—or anywhere in-between-to adapt to exactly what the home needs for heat, or in the case of the heat pump, for cooling, too. The benefit of that range is that the system is smart enough to pick the operating capacity at which it is most efficient. That means you get your precise, desired temperature and you can save money doing it."

    Click on the "GREENSPEED" tab.

    http://www.carrier.com/residential/en/us/innovation/

    So, if you size it so that 40- 45F your heat load is 40% of the Greenspeed's capacity, it'll do really well, as long as it has enough at max speed to cover the load at the 99% outside design temp. A code-min house has an outdoor heating/cooling balance point of 60F or lower these days. So, as long as your 99% design temp is not colder than 20-25F and it's sized to still be modulating at 40F+ it'll do great! It could be an effective solution in places like Seattle or Vancouver, despite excessive defrost cycling on many mid-winter days.

    But if your outside design temp is 0F, even if it's sized EXACTLY for the load at 0F the thing will be cycling on/off any time the outdoor temp is above 25F or so (=most of the season). If sized bigger than the load at 0F it'll be cycling on/off a lot even in mid-winter, which is not a great solution.

  8. Steve Stemper | | #8

    Dana, Im in zone 4 and Im planning on building a super insulated home, double stud walls, sealed crawl, energy heel trusses, with a air sealed envelop, incorporating mechanical ventilation. I read about how efficient mini splits are so I was considering using one or more to heat and cool my 1600 sq ft house. The layout is semi-open but not entirely so I was concerned about air movement in the bedrooms that's when someone mentioned using a high efficiency ducted heat pump instead of a mini split. So I hope that this extra info will make it clearer as to what I should do. Thank you

  9. GBA Editor
    Martin Holladay | | #9

    Steve,
    To design a heating system, start with a room-by-room heating load calculation. It's hard to give advice before you do that. If you do the calculations properly, you will probably be surprised how low your loads are.

  10. Steve Stemper | | #10

    Thank you Martin I will do that.

  11. Charlie Sullivan | | #11

    Steve, it sounds like Martin has you on the right track. One option you may end up wanting to consider is a ducted mini-split, which has a head with a few very short ducts to distibute the heat to a few rooms.

    Dana, do you understand what would limit the turndown ratio on a heat pump? Once you are using an inverter motor drive, the motor speed should be able to go down as low as you want. So I imagine the differences are either in the expansion valve capabilities or in the compressor characteristics.

  12. Richard McGrath | | #12

    Gentlemen ,

    Thought this may be the opportune time to show you the future . Refrigerant basically out of the home . Heating and cooling with water . No boiler , water heater will do . No complex controls . No short cycling of anything .

    Do remember , loads shown are at specific supply temps for both heating and cooling but utilizing varying temps based on actual building needs these can go lower than advertised . Please tell me what you think . These will be available rather soon .

    http://htproducts.com/fan-coil.html

  13. B V | | #13

    I'm new to this forum and not sure if it's okay to ask questions in the 'answer' section. I have a similar question to the OP but slightly different. If I should post my question separately, please let me know.

    Assuming all things are equal (correctly sized system, equally knowledgeable installer etc.), is a 'ducted' mini-split system more or less efficient than a regular HVAC forced air system? My main reason for going with a ductless system is because it can be all electric as opposed to HVAC which usually has gas. I think a heat pump based conventional HVAC system will also be electric I guess. What costs more (both initial capital and regular use), is easier to implement/install correctly and is less maintenance? Which is greener - if we go solar at some pointed. Home in Northern CA - SF Bay Area. Appreciate your responses. Thanks.

  14. GBA Editor
    Martin Holladay | | #14

    B.V.,
    As you evidently figured out, either option -- a conventional air-to-air heat pump connected to conventional forced-air ductwork, or a ducted minisplit -- is all-electric.

    In almost all cases, the ducted minisplit system will use less energy. The main reason is that the blower on the ducted minisplit will use less energy; it will be less powerful, and it will have an ECM (electronically commutated motor), which is extremely efficient.

    The small blower on the ducted minisplit forces the installer to focus like a laser on duct design and reducing static pressure. Ducts end up being generously sized and short, and that's good.

    Most conventional air-source heat pumps with forced-air ductwork have air handlers equipped with powerful motors. That makes duct system designers lazy, and the blower motors have to make up for static pressure problems by ramping up.

  15. Expert Member
    Dana Dorsett | | #15

    Steve: A typical IRC 2012 code-min 1600' house in zone 4 with heat recovery ventilation would have a heat load at +17F (the HSPF low temp test critiera) of 15,000 BTU/hr or less. A high-R 1600' house could have a heat load at +17F of around 10,000 BTU/hr.

    The smallest GreenSpeed (the 2-tonner) at minimum speed delivers about 10,000 BTU/hr, and at high speed about 25,000 BTU/hr. That means the thing will only modulate when it's near the outside design temp- you'd be better off with a 1-ton or 1.5 ton single-speed.

    The 1.5 ton Fujitsu -18RLFCD ducted mini-split has a minimum-speed output of 3,100 BTU/hr @ +47F, and can still deliver more than 18,000 BTU/hr @ +17F. It has a somewhat lower HSPF (11 or so) than the 2-ton Greenspeed (13-ish), but if the 99% design load is under 15,000 BTU/hr (probably is) it will beat the Greenspeed on as-used efficiency, simply because it is modulating nearly all the time rather than cycling.

    Since you'll have a conditioned crawl space available for running ducts it's reasonable to consider a mini-ducted mini-split suspended from the joists and putting both supply & return registers on the floor. There may be framing changes that can make that happen more easily, so try to nail down the load numbers sooner than later- don't leave it until the end.

    B.V: Almost all package gas HVAC systems are ridiculously oversized for typical heating loads in your very temperate climate. With the comparative greenness of the CA's grid, a heat pump is already lower-carbon than gas fired heating in your location (with or without solar on site.) This is not universally the case- in West Virginia's coal-heavy grid it would be greener to go with condensing gas.

    Richard: The thin profile hydronic wall coils are pretty enough, but it's only part of the solution- the rest of the system still has to be designed. Mini-splits are attractive for low-load homes because they are pre-engineered systems, with fewer ways for the installers/designers to screw up (though they more idiot-proof they make them, the more creative the idiots become.) Even with the smallest UFT series (the boiler HTP is touting as good match for the those coils), the min-fire output is probably more than half the 99% design load of Steve's house.

  16. Expert Member
    Dana Dorsett | | #16

    There are many who would dispute the notion that, every house has or needs a gas water heater, especially those not on the gas grid (and don't have or want a propane tank.)

    Depending on location & local grid heat pumps can be quite a bit greener, or sometimes much less-green than condensing gas. But sure, if there's a gas water heater available, it's a fine way to heat a low load (or even an intermediate load) house.

    There are others who would argue that every house needs an air conditioner too, and heat pumps fill that dual-purpose need too.

    Don't get me wrong- from a raw comfort point of view I'm a fan of hydronic heating, but it's not clear that thin profile fan coils are a universal solution, even though they can be a pretty good solution in conjunction with a gas fired hot water heater.

  17. Richard McGrath | | #17

    I was thinking as I have been for years that boilers are needless in the homes that are coming and the ones that are discussed here Dana . 2 ecm circs and a FPHX is what we need . Every house has a water heater . Would work pretty good with other tech too

  18. John Semmelhack | | #18

    Martin & others: the notion that ducted mini-splits are only appropriate for small spaces or short duct runs is not correct. While the designer does have to pay attention to duct layout/design, these systems do not have any hard limits on square footage they can serve or on lengths of duct. I'm attaching an example layout from a 5,000ft2 house (2 levels), served by three ducted mini-splits (9k, 9k, 12k nominal).

    Each of these includes a MERV 13 filter as well!

  19. Charlie Sullivan | | #19

    Richard,
    Those HTP ultra-thin fan coil units look great. Low electric consumption, and more attractive than a mini-split head by my judgement. Presumably they'll be reasonably priced. It's nice to see more sources for that kind of equipment becoming available.

  20. GBA Editor
    Martin Holladay | | #20

    John,
    Thanks for sharing the plan. That's impressive. It just goes to show what's possible when a good system designer (either a good mechanical engineer or a knowledgeable contractor) performs the necessary calculations.

    The bottom line is, you need to pay attention to static pressure. GBA readers may be inspired by your example: If the ducts are large enough, and if the fittings are chosen to minimize static pressure, then you can do a lot with ducted minisplits.

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