Daikin Quaternity
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Fitter30
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Don't know if theres a unit better or worse but the key to controlling rh is the building envelope ( vapor barrier, outside air infiltration and how the space is used). High rh flows to low rh. Take a shower don't run exhaust fan long enough humidity pours out into the space. Blower door test loose house hi humidity outside comes into the house.
Big box dehumidifier rated at 90%rh at 80*f. Dehumidifier with iham rating 80%@80*f The colder the room temp less rh% efficiency falls off big time. So if theres a problem have to look at the whole system.
Big box dehumidifier rated at 90%rh at 80*f. Dehumidifier with iham rating 80%@80*f The colder the room temp less rh% efficiency falls off big time. So if theres a problem have to look at the whole system.
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The nice thing about the Quaternity is that it can dehumidify even on days when there is no sensible cooling load. Only the Quaternity series will control to an RH setpoint independently settable from the temperature setpoint, and the only mini-split that can dehumidify without sensible cooling. This is due to a proprietary split coil and metering controls on the indoor head that gives it the capability to apply some re-heat to part of the coil at the same time that the other part of the coil is chilled to remove moisture.
That said, since only part of the coil is actively removing moisture, it won't remove moisture as quickly as a simpler mini-split operating in "DRY" or "DEHUMIDIFY" mode. Some mini-splits will still cool to the sensible cooling temp setpoint while in dehumidification mode, most will simply control the coil temp to maximize dehumidification, and will ignore the room temp, potentially overcooling the space. Whether either of those modes work for you depends on the particulars. Most people can deal with a dehumidification mode that still follows the sensible cooling setpoint, and it's almost impossible to over-dry the space with any mini-split.
No mini-split can truly keep up with an air-leaky house in a high dew-point climate (as fitter30 points out), and even with a tight house a Quaternity won't necessarily always keep up with the latent loads generated by the occupants on days when there is no sensible load. Wh0le house dehumidifiers are usually a bit like swatting flies with a sledgehammer in all but the MOST humid (as measured by outdoor dew point) climates, but running a portable room dehumidifier can keep up with reasonably tight houses, even though standalone room dehumidifiers convert that latent heat of vaporization into a sensible cooling load that the AC (mini-split/other) would have to handle.
Bottom line- the Quaternity works for most people, most of the time, but may still occasionally need supplemental dehumidification depending on your desired humidity setpoint, climate, and how leaky the house is. Other mini-splits with dehumidification mode may dry it out more quickly, but not without also cooling off the space.
Dana, thanks for your detailed reply. The spec sheet on the smallest Quaternity is really confusing. On the one hand, its SHR appears to be 99%. On the other, it removes 3.3 pints of water per hour. How is this possible?
Either is possible depending on the operating mode and the humidity conditions. If you crank the RH set point down to the bottom of the settable range it'll remove a lot of moisture. If the RH set point is set to the max it'll have a very high SHR.
Somewhere in the fine print it should say under what conditions it would be removing 3.3. pints/hour.
Fitter30
Well-Known Member
3.3 pints an hour = 79.2 pints a day only way it could meet that spec is to meet or exceed aham spec . The system can't heat and cool like a dehumidifier. So it would need a external load not to over cool.
May I describe my situation to y'all and ask for your recommendation?
The current situation is a 2-story tract house on a slab, with ducts in the attic. It is currently served by a 10 SEER, single speed, R-22, 2.5 ton central heat pump that has reached the end of its service life. I believe the heat pump was sized correctly because it runs continuously on the hottest afternoons. In summer, there is as much as 7 degrees disagreement between the thermostat and the temperature in the warmest room. In mild weather the humidity is a bit too high, enough to make the doors stick, but not enough to grow mold. The house does not have an open floor plan, so each of the 5 major rooms needs a heating/cooling supply. There are 2 rooms downstairs and 3 rooms upstairs. There is some chance that in the future, the house may undergo a deep energy retrofit that cuts the heating and cooling loads in half.
So far I am considering 3 replacement options, each with pros and cons:
1. Abandon the central system and replace with Daikin Quaternity. Pros are better humidity control and efficiency. Cons are initial cost and possible oversizing. The smallest available Quaternity is 3/4 ton, but each room only needs 1/2 ton, and the turn-down ratio is quite poor.
2. Abandon the central system and replace with 1/2 ton Mitsubishi minisplits plus supplementary dehumidification. Pros are super efficiency and a very flexible turn-down ratio. Cons are initial cost and the annoyance of dealing with dehumidifiers which are notoriously unreliable.
3. Replace with another central system. I am not sure this option is physically possible. SFAIK, R-410A coils are always larger the R-22 coils, and the existing R-22 coil is too big for the tiny closet in which it's installed. (The closet door is only 19" wide.) It once had a washable filter that ran into the door jamb when cleaning was attempted, so the filter was abandoned and a filter grille was installed at the beginning of the return plenum. If replacement with another central system is physically possible, the pro would be initial cost and the cons would be lower efficiency and continued excessive temperature variability among the different rooms.
If there are other options I should be considering, by all means let me know.
In case it's not obvious, my conflicting goals are to minimize carbon footprint and lifetime cost, but maximize comfort and reliability.
The current situation is a 2-story tract house on a slab, with ducts in the attic. It is currently served by a 10 SEER, single speed, R-22, 2.5 ton central heat pump that has reached the end of its service life. I believe the heat pump was sized correctly because it runs continuously on the hottest afternoons. In summer, there is as much as 7 degrees disagreement between the thermostat and the temperature in the warmest room. In mild weather the humidity is a bit too high, enough to make the doors stick, but not enough to grow mold. The house does not have an open floor plan, so each of the 5 major rooms needs a heating/cooling supply. There are 2 rooms downstairs and 3 rooms upstairs. There is some chance that in the future, the house may undergo a deep energy retrofit that cuts the heating and cooling loads in half.
So far I am considering 3 replacement options, each with pros and cons:
1. Abandon the central system and replace with Daikin Quaternity. Pros are better humidity control and efficiency. Cons are initial cost and possible oversizing. The smallest available Quaternity is 3/4 ton, but each room only needs 1/2 ton, and the turn-down ratio is quite poor.
2. Abandon the central system and replace with 1/2 ton Mitsubishi minisplits plus supplementary dehumidification. Pros are super efficiency and a very flexible turn-down ratio. Cons are initial cost and the annoyance of dealing with dehumidifiers which are notoriously unreliable.
3. Replace with another central system. I am not sure this option is physically possible. SFAIK, R-410A coils are always larger the R-22 coils, and the existing R-22 coil is too big for the tiny closet in which it's installed. (The closet door is only 19" wide.) It once had a washable filter that ran into the door jamb when cleaning was attempted, so the filter was abandoned and a filter grille was installed at the beginning of the return plenum. If replacement with another central system is physically possible, the pro would be initial cost and the cons would be lower efficiency and continued excessive temperature variability among the different rooms.
If there are other options I should be considering, by all means let me know.
In case it's not obvious, my conflicting goals are to minimize carbon footprint and lifetime cost, but maximize comfort and reliability.
Rong! Most rooms such as bedrooms don't require anywhere NEAR a half-ton (unless you are hosting a wild party in said bedroom with a half dozen or more active participants.) The "ductless head per room" approach almost always leads INSANE oversize factors. The minimum modulated output of a 3/4 ton Quaternity could easily be 2x or more the actual design cooling/heating load.
Running an FH06 or FS06 in "DRY" mode should work without the supplemental dehumidification if you air seal the house/room reasonably.
There are some 3/4 ton LGs that can throttle back to ~1000BTU/hr at sorta reasonable efficiency (though not nearly as efficient at the Mitsubishis) as well as some that throttle back that low but are ridiculously inefficient at that low low speed.)
That's cheaper up front than the Mitsubishis, but still nowhere near the "right" solution, since oversized mini-splits will still spend a lot of time cycling on/off, and only rarely modulate within the sweet spot range where it's efficiency (and comfort) maxes out.
OK, since you asked...
Start by doing it at least "sorta right", beginning with a room by room Manual-J-ish load calculation. For central air or gas furnaces it's OK to use freebie online tools such as LoadCalc (dot net) or CoolCalc (dot com), as long as you are super-aggressive on the input information, eg- assume it's an air tight house- NO infiltration or ventilation, and the maximum possible R-values for the walls & attic (assume R13 rather than R11 for 2x4 walls, etc, R3.5/inch for any fluff in the attic) and lowest possible window U-factors.
But for modulating heat pumps & mini-splits the go-too tool is the HVAC tool developed by the BetterBuiltNW utility consortium. It requires signing up for an account, but it's also free. Unlike the others the defaults U-factors etc are already appropriately aggressive, so as not to oversize the equipment.
Make a drawing of your house floor plans with the room cooling & heating loads marked within each room. Clusters of low-load rooms that are reasonably close together are easily served at high efficiency with a "compact duct cassette", even a "low static" version. Even with the rooms fairly well separated "mid-static" compact cassettes (or even Fujitsu's higher than the industry average low-static cassettes can work.) A 3/4 tonner (such as the Fujitsu -9RLFCD) with a cold-climate type compressor (such as the Fujitsu -9RLFCD) can typically handle the combined loads of 3-4 bedrooms in both heating & cooling mode, yet can modulated down to <<5000 BTU/hr @ 82F (or @ +47F in heating mode) which means it will run VERY long duty cycles. Most vendors will have 9, 12, & 18K BTU/hr versions in the low-static types, 2 tons & higher in mid-static (=beefier blower) versions.
Most compact duct cassettes need to be mounted horizontally, but Fujitsus & Mideas (including those with Carrier nameplates) can also be mounted in an upflow configuration. A low-static 3/4 ton Midea (or Carrier) wouldn't be a bad choice, and probably cheaper than the 3/4 ton Fujitsu, but won't be able to drive as much duct as the Fujitsu version. With the upflow configurations it's possible to build out a <10 square foot "utility cabinet" for ease of service, and run the ducts under ceiling level in soffits (even if you have to go with 3" x 30" or bigger for head clearance reasons), keeping it all fully inside of conditioned space. This is a 1.5 ton Fujitsu that is heating & cooling an entire house in Berkeley CA:
If you have 10,000 BTU/hr of design load for 3 -4 rooms and the minimum modulation flow is split proportionally a 3/4 ton compact cassette won't short cycle, and will spend most of it's run time in the Goldilocks Zone of comfort & efficiency.
For bigger rooms with bigger loads by all means, go with a less expensive and easier to install wall-coil type, setting it up to blow through any open archways/doorways/hallways if it's serving more than just one room.
If possible, set up the modulating mini-splits with a 1.0-1.2x oversize factor for the (cumulative, if ducted) minisplits, and try like hell to hold the line at 1.5x for the worst-case. Above that both comfort and efficiency tends to suffer.
Why should 5, 1/2 ton Mitsubishis be considered insanely oversized? Sure, some rooms have a little less than 1/2 ton peak load, but those Mitsubishis have great turn down ratios.
I believe I noted that con in my previous post.
Sealing of the house would definitely be part of a deep energy retrofit, but as I wrote previously, there is only some chance of that in the future. Is it basically impossible to choose a variable capacity system today that can accommodate today's loads and tomorrow's?
I've not heard good things about the reliability of LG. Have you?
How is sweet spot range defined/determined? What is the sweet spot range for a 1/2 ton Mitsubishi? Is sweet spot range synonymous with your use of the term "Goldilocks Zone"?
And since you answered, let me again express my appreciation and gratitude for your thoughtful reply.
As I wrote above, the system runs continuously on the hottest afternoons, which I take as evidence that the system is correctly sized. I would not claim, however, that the room by room loads and duct sizes are correct, but if they aren't, I have no intention of modifying the duct system. The ducts are in the attic with no floor and only 4 feet of headroom. So it's either use the current duct system or abandon it in favor of mini or multisplits. I am open to the possibility of using the existing duct system for a central dehumidifier.
Very interesting, thanks for bringing it to my attention.
If you are going to group rooms together, why not just go with a multisplit?
Are the Chinese Midea/Carrier as reliable as the Japanese Mitsu/Fujitsu?
The ducts in your photo look really small, almost like high-velocity Unico. I'm pretty sure none of them would adequately serve the highest load upstairs bedroom. Regardless, I'm really reluctant to run new ducts and build soffits, because the ceilings are so low, I'm so tall, the house feels small and cramped, and I'd be worried that I couldn't find a contractor who would go to all this trouble.
On the other hand, your photo does resemble my central installation as shown below. Could the Fujitsu upflow coil and air handler replace my existing upflow system? I forgot to say in my previous post that not all of the existing ducts are in the attic - the 3 that serve the ground floor run through the floor/ceiling trusses. The existing upflow system is located on the upper floor.
If you had said Atlanta, I'd be impressed; Berkeley, not so much.
As I wrote previously, the house does not have an open floor plan, so I see no chance at all of a single wall coil serving more than one room.
Please explain why you would intentionally oversize anything, especially if you anticipate a deep energy retrofit.
Despite the low modulation range, there isn't enough load to get it to do what you want it to do. At 82F (roughly the mean binned hourly temp in Atlanta GA during the warmest 6-8 weeks) the FS06 only throttles back to ~1900BTU/hr, (see this) and to achieve that minimum modulation level the sensible heat ratio (SHR) necessarily becomes pretty crappy due to a none-too-cold evaporator coil in the head, since the blower's modulation range doesn't go very low.
That's WAY over a typical room load at that temp (it might be close to the design load) , so it's cycling rather than running continuously, especially when it's muggy/cloudy (when you need the dehumidification the most) rather than bright & sunny. To get decent dehumidification out of it in normal cooling mode it has to spend a decent chunk of time every day in it's mid to high output range.
And that minimum is only achievable with each FS06 head married to it's own dedicated FS/FH06 compressor. When it's on an MXZ multi-split compressor it's minimum is limited by the minimum output of the MXZ, which is several times the minimum output of a single head. That means it will cycle even more, and with multiple oversized heads it can even short-cycle the compressor into lower than advertised efficiency.
Not necessarily. Do load calculations on the house both in it's AS-IS condition, and post-improvements condition. As long as you're careful to keep the equipment sizing relative to the "AFTER" picture <1.5x it will usually work just fine both now and into the future. In a deep retrofit situation that might mean keeping the oversize factor to <1x at the "BEFORE" picture, but it takes more than a little bit of air sealing and insulation to move the needle that much.
A dozen years ago the quality control at LG wasn't as good as it is today, but even then poor reliablity was usually related to poor installation practices. There are a LOT of hacks out there installing mini-splits who don't properly purge & pressure test with nitrogen (>500 psi for at least an hour- 24 hours would be better), followed by pumping it down to <500 microns and verifying that it will stay that low for an hour or more. A lot of hacks always assume the factory charge will be correct for the amount of lineset installed too, rather than consulting the manual, doing the math, and weighing in/out the requisite amount of refrigerant. Unlike Mitsubishi & Fujitsu, LG still doesn't place as many factory certification training hoops for the installers to jump through, and just about anybody legally licensed to handle refrigerants can buy & install their equipment.
The installer matters- look for factory certification levels such as Mitsubishi's "Diamond" contractors or the equivalent from any equipment you're buying. That's still no guarantee that they will do it right, but there is usually a better warranty and support available for equipment installed by factory certified contractors.
My definition would be that it runs nearly continuously with very long on cycles even during the shoulder seasons, at a high enough modulation level for at least several hours out of the day to provide reasonable latent cooling without having to turn the setpoint down to the 60s F. For the FHFS06 that would mean a cooling load of at least 3-4000 BTU/hr @ 82F (a temp at which the FS06would put out nearly 10,000 BTU/hr at the maximum modulation.)
If it's running continuously when it's north of 90F it's reasonably right-sized. Most systems have balancing vanes at the take-off from the supply plenum to help tweak the room to room temperature balance, which doesn't have to be perfect to find comfort. Long cycles improve the dehumidfication, and dry air can be comfortable over a wider temperature range.
As noted above, the minimum modulation of a multi-split is quite high, which leads to shorter cycles for the individual heads, and sometimes can overcool /overheat rooms with very low loads even without the head running (!). This is because whenever any head it cooling, refrigerant is being pumped through all heads (necessary to keep the R32 & R125 % lubricants in the R410A properly mixed). When cold refrigerant is being pumped through the head without the blower running condensation forms on the coil and interior side of the cabinet, causing it to spit water into the room when it initially turns on. Sometimes low load rooms will become colder (or hotter during the heating season) than setpoint just from the recirculation refrigerant.)
Again, it depends on model and the INSTALLER(!) & support. If you want better warranty support it might be safer to go with Carrier labled Mideas. Some "off brand" labels manufactured by Midea are using older designs, and may have fewer QC requirements imposed to get a lower wholesale price. Midea has been using high quality Toshiba compressors (manufactured in China) for a couple of decades now, but the other system components could be from all over. That's not too different from Mitsubishi & Fujitsu & Daikin use components and whole products built in many countries other than Japan. Midea is vying for the #1 slot with Daikin for being the world's largest manufacturer of variable refrigeration volume air conditioning equipment- they are at top tier manufacturer.
On the other hand, your photo does resemble my central installation as shown below. Could the Fujitsu upflow coil and air handler replace my existing upflow system? I forgot to say in my previous post that not all of the existing ducts are in the attic - the 3 that serve the ground floor run through the floor/ceiling trusses. The existing upflow system is located on the upper floor.
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Depending on the duct sizing and actual dimensions it may be possible to install a 2 ton or 2.5 ton mid-static Fujitsu in your existing utility cabinet. (consult the manual.) Note that the 2-tonner puts out 29,000 BTU/hr of cooling @ 95F,(almost 2.5 tons), and the 2.5 tonner delivers 35,000 BTU/hr (nearly 3 tons), but the minimum modulation of the 2.5 tonner @ 82F is about 11K, compared to 5.75K for the 2-tonner. In your situation it would be much better to go with the 2-ton rather than the 2.5 ton.
If you had said Atlanta, I'd be impressed; Berkeley, not so much.
Me too, but it's pretty cool that even the wimpier blower of the low-static version can work.
Sometimes (87 hours per year in an average year) it's hotter than the 1% design temp, and cooler than the 99% design temp (especially during Polar Vortex disturbance cold snaps.) If it stays below or above those temps for extended periods the equipment doesn't keep up with the load. If anticipating a truly deep energy retrofit (say, more than a 35% reduction in loads) it might be necessary to undersize for the pre-retrofit existing load to achieve optimal comfort & efficiency later.
BTW: Many vendors of mini-splits have upflow or multi-position high-static air handlers these days, but not all of them have much modulation range. Some of the Carrier Greenspeed versions now have improved turn down ratios (due to the Midea innards of more recent models.)
I had such high hopes for VRF/inverter equipment, but now I'm feeling pretty disenchanted.
It seems like they are only suitable for desert climates. Are the high-SEER VRF central systems from the traditional American brands just as problematic in humid climates?Why didn't Mitsubishi allow the blower to modulate lower?
Apologies, what does x represent?
Off topic, but the battery in my electric car was made by LG in 2018. It is currently estimated to have a 1 in 2000 chance of spontaneously combusting while parked.
I want to choose the equipment and the installer so well that the warranty is irrelevant.
What are the other top tier manufacturers? Does Gree qualify?
I've been thinking about the reasons that heating/cooling energy is wasted in houses. The obvious reasons everyone is familiar with are poorly insulated and sealed building envelopes, and inefficient HVAC systems. But another reason that nobody talks about is that much of the time, unoccupied rooms are being heated/cooled whenever the HVAC system operates. For example, in my house there are 3 people and 5 rooms, which implies that at least 2/5 of the heating/cooling energy is being wasted. Thus lower capacity minisplits are sorely needed. Do you concur?
I had such high hopes for VRF/inverter equipment, but now I'm feeling pretty disenchanted.
The disenchantment is not warranted. There are huge comfort & efficiency benefits from VRF equipment when right-sized and used properly. (Simply counting on a high turn down ratio to quasi-right-size it for you when it's max capacity is ridiculously oversized isn't using it properly.)
Almost all US branded VRF equipment is manufactured and often designed by the bigger Asian VRF companies. Goodman is owned by Daikin, and all of their VRF equipment are Daikin designs, even when the design & manufacturing group is in Texas. Trane VRF equipment is all Mitsubishi. High SEER multi-stage (non VRF) equipment under US nameplates might be made by others, but it's a rapidly consolidating world market. The popular German Bosch heat pump units are all designed & manufactured by Midea. Even Korea's Samsung minisplits are all made by Midea now, along with dozens of lesser known brands worldwide.
When properly applied high SEER VRF equipment works great even in the higher latent load areas such as the Gulf Coast states of the US, but some contractor & user education needed to get the most out of it. The "ductless head in every room" approach is really great for the contractor's ability to make their boat payments, but micro-zoning with ludicrously oversized equipment isn't doing the homeowner/occupants any favors.
With high SEER equipment in an Atlanta climate it's useful to have a dehumidification mode to better manage latent loads, and it's more important to size the cooling equipment correctly than it would be in dry, low summertime dew point climates such as Seattle or Las Vegas.
I don't work for Mitsubishi, so I can't tell you with any confidence all that went into the design decisions. At some low cfm number the velocity is too low to provide adequate mixing with the room air, which is likely to have been part of it.
Off topic, but the battery in my electric car was made by LG in 2018. It is currently estimated to have a 1 in 2000 chance of spontaneously combusting while parked.
LG is a huge company (one of the 4 largest conglomerates in Korea), with many divisions. There is no way to correlate problems related to a relatively cutting edge newer LG-Chemical battery product with more mature LG refrigeration appliance equipment. They don't fall under the same management chain until the highest corporate board room level.
I want to choose the equipment and the installer so well that the warranty is irrelevant.
Me too! Ask the installers why they sell what they sell, and how well the distributor supports them on installation training, technical hotline help, equipment recalls/updates, etc. Local distributor support matters when something goes wrong 10-15 years down the line, even more so if it craps out early for some reason. If there is better local support for one vendor vs. another it moves them to the front of the line. eg: I'm smack in the middle of Mitsubishi country, with the regional training center located 30 minutes from my door, and dozens of factory certified installers less than an hour away. They have more than a 2/3 market share for ductless equipment in my state. But there is still reasonable support for Fujitsu & LG, Daikin not so much, but improving. Most of the local Carrier installers are still pretty clueless about their own VRF equipment, but I'm hoping that will improve.
Worldwide in market share & quality terms, Daikin/Mitsubishi/Fujitsu/Midea/LG pretty much cover the upper tier for VRF equipment design & manufacturing, even when sold under other brand names. (I'm probably missing one or two others...)
Starting a handful of years ago most Gree products are now manufactured (and mostly designed by ) Midea, using Toshiba compressors. (Bosch heat pumps are all Midea too.)
I agree that even smaller ductless mini-splits would be needed to better serve the better insulated better built lower heating & cooling load homes currently being built in the US & Europe. I've read rumors of a Mitsubishi quarter-tonner under development, but haven't heard any details- it could simply be smoke.
But small mid-static slim-duct cassettes can usually be sized correctly even for fairly low load homes.
Are you scolding me?
I've heard of 2-stage. How many stages are possible?
In shame I must confess that this is my idea, perhaps influenced by misleading media, not any contractor's.
Seattle? I thought it rained a lot there.
Safety recalls? Because they might catch on fire?
Mitsubishi headquarters is not far from Atlanta, but I don't think I'll have that many installers to choose from.
Do you think a small minisplit will ever rival the SHR of the best traditional system? Say, SHR = .68?
Yes, but if a single coil and air handler continue to serve the entire house, will I not still be wasting 2/5 of my heating/cooling energy conditioning unoccupied rooms?
wwhitney
In the Trades
But generally not in the hot summer. When it's 60F outside and raining, the dew point is still at most 60F.
Cheers, Wayne
That's right- rain is not the same as high humidity.
The prevailing winds in Seattle are off the Pacific, where the water surface temps stay in the low-50s in summer. It's big news when dew points hit 65F. Even during the hottest 6 weeks of the year the dew points remain below 55F about 70% of the time, and below 60F about 100% of the time.
By contrast Las Vegas dew points stay below 55F about 75% of the time during the most torrid weeks, but unlike Seattle do occasionally hit north of 65F. Most people don't think of Las Vegas having higher humidity peaks than Seattle, but it definitely does during summer monsoon days/weeks when southerly winds bring humid air up from the Baja California. (I've personally experienced monsoon days in Tucson AZ when it was north of 100F and pouring rain in the afternoon.)
Both Seattle & Las Vegas have VERY dry summertime air compared to Atlanta or even Boston.
Are you scolding me?
Not really- it is directed more at the disinformation marketing and the wealth of ignorance among ductless installers in humid climates. "Bigger is better!" (not), and "...a head per room allows for micro-zone control delivering perfect comfort!" (definitely not.)
The Carrier 19VS is a 5 stager, and I don't think they are alone.
In shame I must confess that this is my idea, perhaps influenced by misleading media, not any contractor's.
Sometimes (rarely) equipment gets recalled for unforseen reliability issues or software/firmware bugs. (Some of this equipment smart enough to do stupid things, just like people.)
A quick search on the Mitsubishicomfort website contractor finder comes up with no fewer than seven "Diamond" certified contractors that would serve ZIP 30303.
Yes- they pretty much all beat that when operating in "DRY" or "DEHUMIDIFY" mode. IIRC Fujitsu's slim ducted (and some other vendors') units still cool to the sensible setpoint while in DRY mode. Operating to a sensible temp setpoint limits it's ability to behave solely as a dehumidifier- the setpoint MUST be a hair below the room temp to operate in DRY mode, and the blower in the cassette cannot be increase while in DRY mode which also limits it's peak capacity, but the SHR is awesome compared to any fixed- speed traditional system while operating in that mode.
Unless you insulate the partition walls between rooms and weatherstripped insulated doors to the other rooms you'd still be heating & cooling the whole house to at least some degree. The "wasting" of fully conditioning the whole house isn't nearly as large as you might think especially in a house with at least some insulation in the walls/attic/foundation. If your goal is a deep energy retrofit it's even less.
The bigger waste to watch is duct losses and air leakage when the ducts are routed outside the thermal & pressure envelope of the house. The magnitude of those duct gains/losses depend a lot on the duct design & implementation, and the overall leakage of the house. While going ductless removes the duct related issues, it won't save much energy if the heads & compressors are way oversized for their loads. Part of many deep energy retrofits involves insulating at the roof deck rather than attic floor when the ducts are in the attic (although that can be an expensive proposition.)
Some other resources to maybe check out for the whole-house improvement approach would be Nate Adams' online videos and detailed case studies on his Energy Smart Ohio site & "Nate the House Whisperer" blog site. (He also has a range of detailed videos on that big video hosting site with stuff are useful to home comfort analysts, efficiency pros, and DIYers alike. ) Nate's approach is the right one- fixing the issues with the house it the necessary prerequisite for home comfort, and makes it a lot easier to spec the HVAC that actually works as-advertised. (He uses a lot more closed cell spray polyurethane than I would go for on environmental grounds, but that's a small nit to pick in the bigger picture.) When there are longer term goals Nate (and I) typically avoid diving into the HVAC as the first order of business- it's easy to make $5000+ mistakes on equipment that won't serve as well in the "after" picture when major efficiency upgrades to the house are anticipated.
There is also a very competent contractor near you in Decatur that does heating/cooling loads and HVAC system design & analysis, and also hosts a pretty good blog site with tons of useful articles, some of which get cross-posted to the GBA site, which also has a lot of stuff on how to improve the overall efficiency of your house (not just the HVAC, which is Energy Vanguard's main biz). More than most HVAC installers, they would be able to analyze your situation with your home improvement goals in mind and be able to spec equipment that would works well both now& later. They're not free, but they might point you in the right direction as where the first-money is better spent (hint- it's usually not he HVAC, though it can be.) Often (usually) testing & re-commissioning the existing duct system and focusing the first tranche of significant money on air sealing & insulation upgrades saving the HVAC for last is the smarter ordering of projects, (assuming the existing equipment still works).
Are those numbers more stringent than given in the installation manuals?
To make this work in my upstairs situation, I would need a duct to pass through a load-bearing wall framed 16" OC.
How low does SHR go? What is the COP at that SHR?
2 theoretical questions:
1. Is there a theoretical limit above which the laws of thermodynamics will not allow SEER or HSPF to rise?
2. We know that when you cool air without dehumidifying it, its relative humidity rises. How high does the SHR need to be to cause the relative humidity to rise?
Depends on the manual. IIRC Mitsubishi wants to see a nitrogen pressurization of 600 psi+ for 24 hours (according to a certified installer I was talking to a handful of years ago.) Sub-500 microns seems to be the default industry standard for a vacuum test, but the duration varies. Time is money- if you watch online videos of pros installing this stuff 20 minutes of holding high pressure nitrogen (barely long enough to test all the flare fittings with bubble juice) and 10 minutes of holding vacuum seems to be "good 'nuff for the kind o' girlz I go with" for a lot of these folks.
And...?
There are many ways to skin that cat. Until you know the cfm requirements and length of that duct (including equivalent lengths of all the fittings) and the max static pressure of the ducted cassette/air handler it's hard to prescribe the good/better/best solutions. With tapered hard duct feeding in & out of a 14.5" wide stud bay penetration a short section of higher-velocity duct won't add a lot of static pressure.
Who knows?
The COP of mini-splits in dehumidification modes isn't usually spelled out in the short-sheet specs. The actual COP depends a lot on the indoor dew point temp, the speed of he blowers, and the actual temperatures (indoor & out). Whatever it is, it's going to be better than a dehumidifier first converting the heat of vaporization into sensible heat, then pumping that heat out in high SEER mode.
Unlike simpler heat pumps & air conditioners a modulating mini-split's controls monitor in real time the indoor coil temperature, the superheat on the evaporator coil & subcooling on the compressor coil and adjusts the blower speeds & compressor speeds for optimal efficiency as long as it's keeping up with the sensible load. In DRY mode the blower speed on the evaporator coil is fixed at a low or intermediate speed, but the compressor speed & condenser blower speed are adjusted to keep the coil at a very effective minimum temperature that stays near the freezing zone without being so cold as to frost up, independent of the entering air temperature & humidity at the cassette/head. That simply can't happen with simpler equipment's simpler controls- some margin has to be built in to avoid the risk icing up the coil completely. Most high SEER 1 or 2 stagers run a thermal expansion valve (TXV) at the evaporator coil to meter the refrigerant at a fixed superheat, whereas modulating mini splits have variable refrigerant volume, metered inside the compressor (condenser, when in AC mode) unit, which allows it to adapt to the operating conditions.
Most homes (even in the Gulf states) won't need to run in DRY mode the majority of the time to keep the indoor humidity at or near 55F (=50%RH @ 75F). With a few cheap AcuRite monitors around the house to monitor the humidity just switch it to DRY mode when the indoor RH is heading north of 55% at mid-70s temps. If it's not keeping up in DRY mode (for either the humidity or the sensible temp) setting the standalone dehumidifier to hold the line at 60% is good enough for most people, though those with severe dust mite allergies may need it lower.
Yes, there is a limit, but the range of operation is refrigeration type & pressure dependent. Just as there isn't a theoretical limit on motor efficiency or coil surface area size, there are always practical as well as cost limits. Every real-world heat pump is a compromise, with many of the design choices being application-specific. Cryogenic deep freeze equipment doesn't work with the same refrigerants or pressures that would be useful for heating & cooling homes with.
Smart vapor-reinjection compressors have made big strides in low temperature space heating efficiency & capacity over the past 12-15 years. Even so, current equipment isn't even half way to theoretical limitations imposed by the physics for using R410A to heat a 70F house pulling heat from 0F outdoor air. In Japan there are home air conditioners that would test well into the 40s or even low 50s on an AHRI SEER test, and Carrier/Midea is currently selling an R410A refrigerant mini-split in the US that tests in the low 40s.
In short years R410A is going away (due the extremely high global warming potential (GWP) of R125, which constitutes half of the mixture that makes up R410A (the other half is R32, which is somewhat flammable.) Many products (including window-shaker ACs are already using plain-old R32, which has only 1/3 the GWP of R410A, but there are also R454B versions coming. Like R410A R454B is a mixture of two different refrigerants- R32 & HFO1234yf, both of which are modestly flammable and needs to be handled differently by air conditioning techs (you can't just use the same old evacuation pumps that worked fine with R12, R22, R134A, R410A) so there is going to be a lot of re-training necessary in the residential markets. Commercial refrigeration techs already have training & equipment needed for handling highly flammable R290 (= pure propane), commonly used in many applications in lieu of R22. But I digress...
There is no magic number- it depends on the humidity sources (both indoors and ventilation air), ventilaton rates, etc. No simple AC at any SHR is going to deliver the best comfort & efficiency under all conditions. But the tighter the home, the more control one has over it. A large fraction of the PEAK latent loads in air leaky GA homes is outdoor air infiltration & ventilation, but only during the worst weeks of the year.
From late September through mid to late May the outdoor dew points average less than 60F, with outdoor air contributing very little to the latent loads. The period from half past June to late August is a different story, but most of the time any reasonable SHR will still handle it, except during the most torrid weeks. Those are the weeks when having a DRY mode (or an ultra low SHR) &/or a whole house dehumidifier becomes useful. During most of the cooling season hours even a high SEER unit will handle the latent loads, as long as the run times are sufficiently long. In that location there will also be days during the shoulder seasons with high outdoor dew points and low or almost no sensible load where DRY cooling mode can keep it from getting clammy.
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