Minisplit comfort
Is using a single mini split to heat and cool my “dream house” going to result in acceptably even temperature distribution? The house will be well enough insulated with r60 ceiling, r40 walls, r30 basement and r24 below slab and well sealed (under 1 ACH goal) with great fiberglass triple pane windows that barely crack r5. I’ve attached the tentative floor plan to illustrate my concern. Though a mini split will be sufficient heat/cooling power the temperature difference in the distant bedroom vs the main living areas is asking for trouble, I believe. The dashed lines in this plan represent proposed ducts, with the three closest to the right rear corner associated with the ERV that pulls from the two full baths and exhausts to the hallway. The rest of the ducts are outputs from a small air handler in a closet (in the foyer). The intention is this closet has return air grills on 2 sides near the top and it’s output ducts end in floor registers around the perimeter. I’d also place return air grills above doors along the hallway to provide a flow path when the doors are closed. The smallest modular air handler, I can find, is capable of moving 1400CFM, HUGE OVERKILL though it does feature a 1/2hp ECM motor. I’d like to control the air handler based on comparing temperature in the master bath to temperature near the mini split with a 1 degree difference (either way) commanding 400 CFM and flow proportional to temperature difference so equal temperatures result in no flow.
With such a control system the mini split is always “master” and the air handler only moves air to equalize temperatures. Does anyone know of such a control system?
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Replies
Jerry
Delta-T controllers are commonly used on solar thermal systems. Here's one:
http://www.deltatcontrols.com/
If you call up the company and talk to a tech person, they may be able to tell you whether you can use thermal sensors detecting air temperature, and this type of Delta T controller, to control a fan. It should be possible.
Jerry I don't have much of an opinion on your mechanical proposals except that if comfort is your goal rather than experimental curiosity I'd go with either a tried and true conventional system or a much less compartmentalized floor plan.
But there are other things about your layout that worry me. The toilet compartments in the two bathrooms will be unusable and uncleanable with the door arrangement shown. Further, a green home which by definition should be serviceable for generations ought to include at least one bathroom facility which does not present arbitrary and unnecessary barriers to a person with disabilities. The powder room is oddly located and looks too small to meet code. And is that a shower between the tub and the vanity in the master bath? If so you should replan so that the plumbing can go in an interior wall.
James,
The problem with "tried and true conventional" Is simply there are NO ducted heat pump systems that are any where near as efficient as modern mini splits. What is available does not use the heat pump when it's cold outside but switches to resistance heat. As to the comments about the toilet compartments: I'm living in a house which has exactly the dimensions shown and rest assured it's completely usable and cleanable. None the less I've widened them both slightly. and moved the toilets off center to clear floor joist below they now have 2 foot 8" doors. The "odd" location of the half bath is quite deliberate and very central to the 'daytime living area. Again it is dimensioned the same as the 1/2 bath in my current house, I hope it's not a code violation. Yes that is a shower in the upper left corner. The plumbing to the shower will be in the inner wall of the double wall exterior which means that there is R30+ of insulation outside of it. What is an "arbitrary and unnecessary barrier"?
Jerry I'm very aware of the the higher efficiency of ductless minisplits and it would be wonderful to be able to use them in highly compartmentalized homes but as your original post acknowledges air distribution is a problem. I don't know if your workaround will be successful but I'd have a plan B in mind in case it isn't.
With regard to the size and location of the toilet compartments it's your home and your decision, subject to code compliance and the agreement of any significant others in the household. An arbitrary and unnecessary barrier is a bathroom facility that would be hard to use for a person in a wheelchair or using a walker, for example - this can happen to any of us. There's no code requirement for accessibility standards in a private home but it's a smart idea for a new home that's built to last for generations.
The concern about plumbing in an external wall is that it's inaccessible for repair except by tearing out the shower wall, an expensive operation that can damage the enclosure system. On an interior wall the plumbing can be accessed via a cheap and easy opening of the drywall in the space behind the shower.
PS a wider door to the toilet compartments is both good and bad news - wider access is good, but a bigger door will take up more of the interior space. Make sure it will actually enable you to get in there and close the door without having to stand on the toilet seat.
James,
The shower in the master suite of the house I'm in now was also copied. My current house has shower heads in two exterior walls and does present the eventual maintenance hazards you correctly point out. I'll seriously consider using a pair of heads in the partition between the tub and shower instead. The duct system I've shown is no different than what would be used with any ordinary forced air system. The difference is that I'm only using forced air for heat distribution. with the actual heat/cooling system located somewhere else (well not really as I fully intend to locate the mini split in the closet above the air handler). The usual cure for uneven heat distribution with a forced air system is more air flow to the uncomfortable area, I can do that with a combination of higher fan speed and/or closing dampers in areas that don't need all the air they get, just like the "tuning" any HVAC system should get. Hopefully I can find a commercially available control system, but if not I'll just build the one I've designed. Doesn't the very existence of bedrooms and bathrooms force a level of compartmentalization that makes a single mini split " iffy"? What are you thinking as a "plan b"? From what others have said here I'm solving a non existent problem.
I suggest a careful room-by-room Manual J compliant heating and cooling load calculation. Then, armed with that information, look at ways, perhaps unconventional, to move required airflow from the area directly conditioned by the minisplit to the more distant rooms. Panasonic WhisperGreen bath fans operate on as little as 4 Watts and might suffice to move conditioned air between rooms.
You might have to overcondition the central area a bit to provide a source of air able to meet the needs of perimeter rooms. My load calc software allows me to enter system supply air delta-T in order to calculate room airflows. For example, you could plan to overcool the central room to 72*F, and then calculate how much air from the 72*F space is needed to hold a perimeter room to 77-78 or so.
Apply similar reasoning to winter conditions.
Hi Jerry
I realize that you didn't ask for a floor plan critique.
I agree with James about the clearance around toilets....
and providing for the "not-so-able" and/or "aging in place".
Just "eye-balling" your plan ... you may have a code violation.
Make sure that you have at least 21 inches clear in front of the toilet.
...and 21 inches is the legal minimum and not-so user friendly.
I would also avoid placing a toilet so close to the Kitchen and Dining area for acoustical privacy.(not to mention aroma)
Honestly...how long would it take to walk down the hall in a not-so-big house?
...and...
Be sure to carefully draw the toilets and complete door swings "to scale" before construction.
You may be in for a rude surprise.
I have never heard of a house heated with a mini split and fans/ducts to move air to distant rooms. Anyone else? I think you'd be better off with a couple of units, or else a ducted system.
John,
Thank you for the specific dimension. The house I'm in now is in violation of the 21" requirement (it's 20 1/2?) in the "1/2" bath. It's easy to change a "paper tiger" so I will.
Jerry: "Doesn't the very existence of bedrooms and bathrooms force a level of compartmentalization that makes a single mini split " iffy"?"
Yes. They are undoubtedly most successful when installed in single volume spaces. We've had success serving two or three adjacent rooms with a single-head minisplit using small thru-wall fans but what you are trying to do here is an order of magnitude more demanding. We have also successfully used ducted minisplit cassette systems in compact whole-house plans. The efficiency is not as great as the ductless variety because of the energy penalty involved in pushing the air around but they still make good use of the same high-performance compressor units and so represent an efficiency improvement over conventional ducted air systems. I would describe your house plan as labyrinthine rather than compact, so requiring longer duct runs which whether you use a ducted cassette or the home-made system you describe here will further erode efficiency to the point where the benefits of using a minisplit at all will become rather marginal. Given that the home-made version will void the warranty on the expensive guts of the system (as described in the recent 'blogs' discussion of your proposal) a lot of folks would take a more reliable path.
"What are you thinking as a "plan b"?"
Couple options. 1. Looks like you have a basement space below - leave it unfinished so a conventional replacement system is easy to retrofit. 2. Allow for adding supplementary service to boost the areas most likely to be affected by inadequate air distribution, e.g the master bath. That might take the form of overhead IR or baseboard heat for example. Again that would undercut your efficiency gains from the minisplit but a cold master bath might be a serious impediment to household peace and tranquillity.
WRT your toilet compartments: not all commodes are the same depth - add the 21" to the length of the commode you plan to actually install, which may not be the same as the one you currently have. And be sure you have a 5' minimum clear width for the commode and sink side by side - 30" code requirement for each.
Just for reference, here's an example of a compact two-bedroom plan which can be efficiently served by a single ducted minisplit cassette located in the dropped soffit of a short central hallway. With a well insulated perimeter the duct terminations can be just above the door of the rooms which they serve.
Thank you all for the great comments! To address the issues with the 1/2 bath I've adopted the design in a house I built some 35 years ago, hopefully it still meets code, it did then. It also sort of addresses john's concern about being next to the kitchen as it is now entered through a "pocket door" from the 'great room. (note though it's not shown properly the fireplace, masonry heater actually, completes the wall between the dining room and 'great room. How about this for a " plan b" Simply add a conventional heat pump coil and supplemental heat strips to the air handler and connect them to a heating cooling thermostat. Presto nothing is modified and I have a useless minisplit. and a very conventional 3 ton HVAC system located in a closet.
I admit to not being an expert on minisplits but I have started to use them and am very impressed. But I have been building well insulated tight houses for many years and understand how consistent temperatures become in a good house. I believe one non ducted unit located at the left end of the hall would work (properly sized of course) but two would certainly work - one in the hall across from the MBR and one on the small wall to the right of the FP. The key is getting the house tight, and we've learned that getting to Passive House levels is not difficult if you pay attention & use good products.
Bob,
Thank you for sharing your experience and encouragement that mini split(s) can work, even in my "labyrinth". . And thank you for the suggestion of actual locations that might work. I hadn't considered 2 mini splits but now that you forced me to consider that alternative. The right end of the hall looks like a great spot for one of two units but I think the hallway end of the wall next to the stairs (near the location you described but rotated 90 degrees) would be a better choice, giving better coverage of the "daytime" portion of the house, especially the kitchen, for the second, possibly larger unit.. As I said, the fire place is part of a complete wall between the dining room and 'great room' with the missing wall segment being brick and part of the fireplace so the two wall segments shown will be made with metal studs and non combustible materials. I suspect that the even temperatures you've found in well insulated, well sealed houses occur by mostly radiant coupling of all the interior pieces, an effect that can easily be overwhelmed by the drafts in more typical houses. If my suspicion is valid, a way to further temperature equalization would be to deliberately use high emissivity paints and surfaces.
David Meiland,
You asked, "I have never heard of a house heated with a mini split and fans/ducts to move air to distant rooms. Anyone else?"
One person who has done this is Allen Gilliland, the founder of One Sky Homes, a design/build firm in San Jose, California. I described his design in a recent article ("Passivhaus Buildings Don’t Heat Themselves"):
"One of Gilliland’s latest projects is a 1,600-square-foot house with a single ductless minisplit on the living room wall. Gilliland’s HVAC design includes a continuously operating 110-cfm Panasonic fan that will move air from the living room to a duct that will deliver between 15 cfm and 60 cfm to each of the three bedrooms. The project will be monitored to see if it attains Gilliland’s design goal of a maximum room-to-room temperature difference of 3°F."
I was in a house a few weeks ago that had a mini-split in the living room/dining room/kitchen space. There was an adjacent bedroom on the same floor, and they had installed a through-wall vent fan to pull air into the bedroom. There was no control system that I could see, aside from a toggle switch on the wall. Perhaps I should put them in touch with you for controls.
Our house is similar to the one shown by James. It is very tight and mostly well insulated, and we are able to heat it with a single woodstove in the main space. The bedrooms are a bit colder, but we like it that way and rarely run the electric heat that serves those rooms. I debated installing a HRV, with exhaust in the bedrooms and supply in the living room, to move air from warm to cold, but everything I read said that the airflow would not be enough to make a difference.
Martin,
Fascinating that someone is using ducted fans to move air in ductless mini split equipped houses. I think I've found a suitable "air handler" that includes an ECM fan controlled by an analog signal. The unit I'm looking at is normally used as a part of large commercial buildings typically hung from the ceiling as part of a big duct system and controlled by a building management system (BMS). It allows air flows from 175 CFM to 1100 CFM with the flow rate established by it's controls in response to a 0-10V DC analog input. At 175 CFM it consumes 32 watts and at 500 CFM it consumes 75 watts. ( Note these power numbers came from a marketing piece and, no doubt, are for ZERO static pressure so they will, no doubt, increase when duct loads are included.) I plan, and have designed, what may be the simplest "BMS" using two semiconductor temperature sensors, one in the air entering the air handler and the other in the most distant room. The 'BMS' uses analog computer techniques to produce a control signal proportional to the absolute value of the temperature difference, with a 2 degree f difference producing a 10 volt signal hard limited to 0.18 volt minimum ( the hard limit is needed to avoid a "lost signal" fault mode).. A one degree difference will command about 500 CFM, and a 0.35 degree difference will idle the fan. BTW the "air handler" is priced at about $1000 and my "micro BMS" has about $5 material cost.
Regarding high efficiency conventional ducted HVAC systems, there are some available now that use the same variable refrigerant flow technology as mini splits. Carrier and Bryant (same company) make one system that has pretty impressive efficiencies and the heat pump works at low outdoor temperatures. You should check out this video: https://www.youtube.com/watch?v=vie4kHN_iYU its about a Mitsubishi VRF flow system that was combined with a traditional air handler and ducted system.
It requires an infinite cfm of 75F air to heat a back room to 75F. Even if a 3F delta is good enough, it still requires a lot of air. Manual J data is needed to calculate the exact numbers.
Hopefully more inverter driven, air to water heat pumps will be available soon at prices close to mini-splits.
Jon R,
The fact you point out is why simply adding fans to move air around a house served by a mini split is far from ideal and requires moving substantial amounts of air. But by placing the mini split in a closet (or other small room) that is part of the air handler's return path, at least some of the air being supplied to remote areas will have been heated ((or cooled) by the mini split so the required air flow is much less. BTW I received actual data on power used on the fan I mentioned above. the data is at 0.25"wc sp.. Even loaded it delivers 175 CFM @ 32W, 400 CFM @ 52W and 800 CFM @ 136 W. really impressive.
Actually not really impressive...Garden variety air handlers with ECM blowers and decent ductwork manage similar or higher CFM / Watt ratios.
Having a minisplit overcool a closet and operating the closet as a DIY supply plenum invites condensation, mold and misery.
Martin H's post #15 supports / quantifies what I was getting at in post #7. In fact that strategy is somewhat similar to what Passive Houses attempt to achieve - essentially balancing room temperatures using ventilation.
Gee Jerry, the manufacturer says don't do it the pros here say don't do it, and you have not done it so how bout not advising someone else to do it. The many reasons not to do it, and easier ways around were listed in your own thread.
"Having a minisplit overcool a closet and operating the closet as a DIY supply plenum invites condensation, mold and misery."
What do you base this assertion on and how do you justify it?
The system I've described does NOT "over condition anything or any place! The mini split will heat or cool the closet to it's set point while the air coming into it (which is where it senses temperature) is return air from throughout the house and of average temperature.. In order for the mini split to change the temperature of the air coming into it it must change the temperature of the air flowing in the ducts. The air flowing through the ducts will be a mixture of air that flowed around the mini split and air that was "processed" by the mini split so it will not be as hot or cold as the air flowing in a conventional forced air system where all the air is "processed" by the heat pump so duct insulation will be more necessary but there is no greater condensation risk anywhere. Certainly there will be condensation, when the mini split is cooling, but it will be on the mini split's coil and drained through the mini split's drain line as intended.
Please show me the "garden variety air handler that delivers 175 CFM and uses 32 watts!
I searched first for a conventional residential air handler and all I found were units that have a few steps in flow rate, big, way to big and even bigger, nothing that is truly controllable and the lowest air flow rates were about 400 CFM.
Kieth,
Which manufacturer say's "don't do it" ? How am I advising anyone to do anything? I have yet to see ANY solution to heat/cooling distribution for a " labyrinth" house like I'll build that uses an air source heat pump that heats at -15f! I'm not masochist enough to re-invent.
The best minisplit comfort I've had came from the traditional HVAC contractors I had giving me estimates for my minisplit installation. It was unbelievable fun playing with them when they came to size up and estimate my place. They couldn't believe my plans to take OUT my existing ducted heat pump system from the late 90's. I had tightened and insulated real good and my calculated load was about half of what it used to be (3 ton heat pump). They tried to talk me out of minisplits---it won't be comfortable all over, all the heat from unit downstairs will go upstairs. They just havent learned--I already experienced that the upstairs didn't get all the heat anymore, after superinsulation. I also was pretty sure my house was open enough--I'm seeing, typically, about maximum 3 deg. temp difference and that's fine with us. It's one thing to read about temperature uniformity with superinsulation, but great to live it! The contractor I selected even put right in his proposal that heat would be inadequate, poorly distributed and that this proposal is solely based on the customer's specification--oh, and also, he put right in the proposal that his sizing software doesn't even accept values of insulation as high as I have. Short story ending is that now I know that my minisplits are adequate for both MD winters and summers, although moisture removal seems to come up a little short.
Boy, it sure was a minisplit comfort to see reactions and comments of traditional HVAC personnel!
Jerry,
Q. "Which manufacturer says 'don't do it'?"
A. Mitsubishi. The manufacturer's judgment on your plan was published on the Q&A Spotlight. The judgment was pronounced by “David Hazel, Regional Manager–Channel Development with Mitsubishi Electric. 'No way,' he said unequivocally when I described Jerry Liebler’s proposed hidden and ducted installation. 'Any type of restricted, ducted, or pressurized installation such as this will void the warranty.' ”
Martin,
How can I access the "Q & A spotlight"? I would like to contact Mr. Hazel with a few questions. For example what is the minimum room size? Where is it specified? What is the allowable air movement rate through that room where is it specified? Absent such specifications the warranty holds!
Jerry,
The words "Q&A Spotlight" in my last comment were a hot link to the article.
Here is the link again:
https://www.greenbuildingadvisor.com/blogs/dept/qa-spotlight/putting-duct-back-ductless
Martin,
Thank you for the link! In reading the article you describe the "system" as I initially posted it. BUT after Dana's comments it was changed ELIMINATING THE SHELF. This is not mentioned in the article. Also between the first and second post I made on this topic I eliminated the basement as the return path, it's now the main floor with "jump ducts"over doors. Mr.Hazel is quoted with " show me the detail" after hearing what I'm sure was an in accurate description of my plan. FWIW Some of the details include the top of the mini split 'head will be 2 feet below the closet ceiling, mounted on a 3'9" wall at the back of a 2' deep closet. there will be 1'9"x1'9" "grills"on two walls above the mini split head. The "air handler" 'inlet' will be .about 2 feet below the bottom of the head. The air handler will move 175 to 1100 CFM. The air handler will be controlled by sensing temperature difference between air at the top of the closet and the air in a distant room. Initially I've chosen a one degree f difference causing 500CFM but that can be changed as I trade temperature spread for fan power.
Cockamamie
:ridiculous, ludicrous as in.... Who dreamed up this cockamamie idea?
So.... I say.... do the install... and report back young man... we all are interested in the actual now verses the back and forth ad infinitum
From what I understand, one key factor to their reasonable efficiency is the ability to modulate the compressor to the lowest level possible to maintain the temperature of a large space (ideally matching heat gain/loss exactly). With an added air handler, the "space" that the mini-split is trying to control becomes more complex. I think what will end up happening in reality is that the heating load observed by the mini-split will completely be determined by the whatever the air handler is doing. Therefore, the excellent and very efficient mini-split control logic will be rendered irrelevant. It will try its best to track the widely varying heat loss observed in its little closet, and in doing so will deviate from the "low and steady" region that is responsible for the mini-split's perceived efficiency.
This whole discussion is made necessary because the floor plan of the house is a complete mess. It's hard to understand spending so much time on countless iterations of wall sections and air handling details while the basic design cries out for professional architectural help.
Malcolm,
Please be specific.
Why not use a remote bathroom fan. I have one that ducts two bathrooms controlled by switches in either bathroom. While a bathroom fan exhausts outside, this would exhaust into the living room. you could control it with switches, timer, thermostats.......
I would put one minisplit in the living room across from the fireplace more or less, and another in the master.
I can see 30 feet of wall I would eliminate, but that is just me. The master is surrounded by bathrooms and laundry. I would move the guest toilet so it does not abut the master, and move the laundry machines away from the head of the bed, but that is just me.
Kieth,
I don't know if you saw it but on the second plan I posted. The ERV exhausts both bath rooms and returns fresh air in the hallway. I'm strongly considering eliminating the "air handler" in favor of 4 or 5 of the 130 CFM Panasonic fans that offer two speed operation and selectable low speeds. these would be mounted on extra thick walls of the closet below the mini split and run at the low speed selection whenever the mini split runs. With separate fans the duct work becomes separate from end to end and each duct has it's own flow rate selection (by changing a switch setting)..
Aaron,
If the flow rate of the air handler is greater than the flow rate of the mini split the temperature in the closet cannot rise simply because all the heat being added by the mini split is being moved out of the closet by the air handler. Under these conditions the mini split will control it's heat output based on the temperature of the return air flowing into the closet just as if it were in a very large room.
Jerry, All good floor plans have an inherent logic to them. You don't need to see the elevations to know whether a house is a modernist one by Corbu or an Arts and Crafts one by Webb. Whatever the style they also speak to something larger than the simple subdivision of spaces. Even the rather modest rancher linked to by James Morgan makes it's ambitions clear. Your plan doesn't. None of the spaces evoke any sense they were thought out with any underlying understanding of what architecture does.
Why I think this is particularly important is that if you don't manipulate the fundamental elements of the building (it's shape, fenestration, integration into the site, climate - using the building volumes themselves as part of any energy saving strategy) in a "Green" design, all you are left with is a rather singleminded focus on making a highly insulated envelope. It's kind of hard to reconcile that way of thinking with your description of this as a "dream house". It needs someone with a bit of training and vision to loosen it up a bit.
Malcolm,
Perhaps if I'd shown the orientation and covered porches and the street side elevation you may be a bit less harsh. The street side faces south, has a central covered porch with 4 large pillars with symmetrical windows to the kitchen, dining room and 2 bedrooms. The central south facing roof will be covered by PV. The north side has a highly desirable view and an even larger covered porch which can be acccesed by a door from the master suite while the view can be enjoyed from the master bedroom, great room, family room and even kitchen. The north facing view is seen through fixed glazing to control heat loss as much as possible,
Panasonic vent fans are incredibly efficient air movers! It is hard to beat eleven CFM per watt. My current thought is to use 5 of them and control them with two current sensing relays that sense the power being used by the mini split. The first relay would switch all 5 on at 70 CFM each for 350 CFM total at 3/4 amp which is less than the current at which the mini split shuts off. The second current sensing relay would be set at 3.7 amps and would switch 3 of the fans to their high speed (130 CFM). The 3 fans that go to high speed would 'serve' the north side of the house. At low speed the fans use 28 watts delivering 350 CFM at high speed they use 47 watts to deliver 530 CFM (which just happens to be the "high" fan setting on the 15RLS2H). High speed starts when more than about 12000 BTU/h is being delivered by the heat pump (hopefully never). To be completely accurate the controls will add 2 watts in low speed and 5 in high speed. With the fans mounted in the walls of the closet, the mini split indoor unit high on the back wall of the closet and the mini split's fan speed set at low (336 CFM) this closet will be a great place to dry clothes. I'll also include switches to turn on the fans continuously at both speeds and a couple of power outlets in the closet to plug in portable electric heaters when it gets too cold for the heat pump (again hopefully never). Mounting the mini split in a closet allows ALL of it's output to be moved, while a more conventional mounting would 'capture' but a small fraction of that output to move.
There is a bomber design of old that had 4 props and 4 turbines... (A tad past the "kiss" idea maybe)
Anyway, build that puppy. Experiment with the actual soon.
And do blog some actuals.
The theoretical is gettinga bit long in the toothy me thinks...