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Minisplit Heat Pumps and Zero-Net-Energy Homes

What we know, what we wish we knew, and an invitation to readers to ask more questions

Posted on Jan 9 2014 by Marc Rosenbaum

For the last several years, just about every project I’ve worked on other than large university buildings has used minisplit heat pumps for heating and cooling. Why?

1 – There is no combustion and no need for a chimney or vent.

2 – In space conditioning applications, heat pumps can provide heating and cooling.

3 – The equipment installation costs and the operating costs compare favorably with other options.

4 – Heat pumps are a natural partner to solar electric systems to achieve zero-net-energy buildings.

In addition, we’re using heat pumps to make domestic hot water (DHW) and even to heat swimming pools. All of these products are air-source heat pumps (ASHP) which extract heat from the air to supply heat to the load.

We’ve been metering the energy consumption at a number of buildings using heat pumps. We’ve seen through our own experience, and through more precise lab-based measurement efforts such as those performed by and others, that the Japanese minisplit heat pumps live up to their performance claims in terms of both output and efficiency.


Interested in learning more?

Since 2012, I've been working with NESEANorth East Sustainable Energy Association. A regional membership organization promoting sustainable energy solutions. NESEA is committed to advancing three core elements: sustainable solutions, proven results and cutting-edge development in the field. States included in this region stretch from Maine to Maryland. and HeatSpring to teach as part of with other experts from the NESEA community. Over 150 professionals have taken my Zero Net Energy Homes design course, and the next course starts on February 3rd.

This course is an opportunity to study with me: to ask me questions for a full ten-week semester. You will walk away with a comprehensive understanding of all of the key components of a zero net energy home — envelope, systems, and renewables — and how they fit together, with key pitfalls to avoid, and numerical calculators for sizing peak heat loss, glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill. amounts, annual energy use, and solar electric systems that will empower you to confidently design a zero-net-energy home.

Successful students will actually do a full design of a zero-net-energy home, and earn NESEA's Zero Net Energy Homes Professional Certificate. The course is approved for 12 AIA CEUs + 6 MA CSL credits (1 hour for Code, 1 hour for Workplace Safety, 1 hour for Business Practices, 3 hours for Energy).

If you’d like to see some free content from the course, you can of my course here, or .

What I know (or I think I know…)

(Caution: unfunded research follows.)

1 – Compact superinsulated homes in climates with design temperatures of 0°F or more can often be heated with a single zone unit with the wall cassette located in the main space. As long as the doors to other rooms remain open, the temperatures in those rooms will usually be within 2°F of the space where the cassette is located. We’ve put electric radiant panels in bedrooms in these homes, and there’s a large variation in how much people use them; the variation seems to be driven mostly by whether the door is open.

2 – Without specific detailed measurement of in situ Coefficient of Performance of the units (COPEnergy-efficiency measurement of heating, cooling, and refrigeration appliances. COP is the ratio of useful energy output (heating or cooling) to the amount of energy put in, e.g., a heat pump with a COP of 10 puts out 10 times more energy than it uses. A higher COP indicates a more efficient device . COP is equal to the energy efficiency ratio (EER) divided by 3.415. ), the amount of energy used by these units, metered separately, agrees well with the usage predicted by simple energy models.

3 – In temperatures below design temperatures, the units have enough capacity to heat the houses even though we don’t size them with an intentional safety factor. Recently it dropped to -5°F here on Martha's Vineyard, and my Fujitsu ¾-ton ducted unit was heating my basement (because the air handler is in the basement, and I haven’t got the ducts installed yet) and one-story house, a total of over 2,400 gross square feet, to 70°F. The load had to have been more than the published output of the unit at that temperature (14,000 BTUBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. /hour). I was impressed. The best single zone systems have little drop-off in capacity down to 5°F.

4 – The cold climate Mitsubishi Hyperheats are rated down to -13°F and we’ve seen them running at temperatures below -20°F. Colleagues are reporting minisplits not specifically rated at these temperatures running happily at them nonetheless.

5 – Treating these units like furnaces and implementing significant temperature setbacks doesn’t appear to save a lot of energy because they will run at full speed when the temperature is set back up, which is a less efficient operating point. And because they aren’t usually oversized for the house load, it can take a long time to get the house back up to temperature. At my house we regularly set down to 66°F overnight, or when we are both going to be gone all day at work, and set back up to 70°F when we're there – unless the outdoor temperature is going to be in the teens or below, in which case we don't set it back.

6 – Properly designed and installed ducted systems are usually really quiet. However, I made the mistake on my new system of installing the air handler with a very short return – about three feet and one elbow to the return grille – and I’m going to have to do some sound absorption. The supply side is really quiet. Most of our ducted systems are hard to hear, most of the time.

7 – The systems with high outputs at low temperatures are also delivering air at higher temperatures than traditional heat pumps, which is more comfortable. I’ve measured supply air at 120°F from my air handler this past month.

8 – If the homeowners let the filters get dirty, output can drop dramatically! These aren’t your father’s 100,000 BTU/h gas furnace. The blowers have significantly lower static pressure capability than what most HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. folks are used to. My air handler allows the external static pressure to be selected at the controller (not by the homeowner) and the maximum is 0.36 inches. So ducts may need to be larger than you think if the distances are significant or there are a lot of fittings.

9 – In cooling, the fact that these systems are variable-speed and have such a wide range of operating points means that oversizing appears to have little consequence (usually in a heating climate, the heat pump size is determined by the heating loadRate at which heat must be added to a space to maintain a desired temperature. See cooling load., so the cooling ends up oversized). Our clients here on humid Martha’s Vineyard have been very pleased with these units' dehumidification performance.

10 – Minisplits are not yet drop-in replacements for fossil-fuel furnaces and boilers in conventional housing, but we’ve been applying them selectively in cases where the existing system is due for replacement. Surprisingly, from a source energy point of view they reduce carbon emissions. On one project, a single oil boiler heated and provided hot water to two adjoining homes totaling 7,500 square feet. It was replaced by three Daikin Altherma air-to-water heat pumps and two electric boilers (for severe weather – the Althermas hadn’t enough capacity, and more importantly, high enough temperature water at low outdoor temperatures). Annual electrical usage into the new system was just under 20,000 kWh. Oil consumption before the retrofit was 3,187 gallons/year, with associated CO2 emissions of 71,389 lbs/year. Using the EPA eGrid figure of 728 lbs CO2/MWh for the NEWE generation region, CO2 emissions dropped to 14,457 lbs/year, an 80% reduction. Fuel cost was reduced by 70%.

11 – Ducted systems need well insulated ducts and they need to be airtight. My system has a design air flow of 353 cfm. Lose 100 cfm of that into the basement, and have ducts wrapped with bubble wrap, and half the output might not reach the living space.

What we’d like to know more about

1 – A consistent and understandable rating system for both capacity and efficiency. It’s frustrating to go to the AHRI Directory and find the heating rating at 17°F (the lower of the two rating temperatures) and then learn that the unit has a higher capacity at 5°F in the manufacturer’s engineering literature.

2 – A good quick method for taking the HSPF and SEER(SEER) The efficiency of central air conditioners is rated by the Seasonal Energy Efficiency Ratio. The higher the SEER rating of a unit, the more energy efficient it is. The SEER rating is Btu of cooling output during a typical hot season divided by the total electric energy in watt-hours to run the unit. For residential air conditioners, the federal minimum is 13 SEER. For an Energy Star unit, 14 SEER. Manufacturers sell 18-20 SEER units, but they are expensive. ratings, assuming they can be made comparable and useful, and modifying them for different climates. I know how the machines we’re using in New England work, but I can’t tell you much about what to expect in substantially different climates.

3 – A rating that includes how much power the system draws on standby, when the thermostat is satisfied. We’ve measured some surprisingly high wattage, and inquired of the manufacturer, to be told that we’re measuring improperly and that because of very low power factors the actual power draw is much lower. I don’t think we really know.

Wish list

1 – An efficient, affordable air-source heat pump that provides space heating, space cooling, and domestic hot water.

2 – An air-to-water unit that makes 160°F water at outdoor temperatures of 0°F, to serve as a drop-in replacement for existing fossil-fuel boilers.

What do you want to know more about?

Do you have any questions related to designing a net-zero-energy building? Now's your opportunity to get some of your questions answered by someone who has done it for himself and others. As they say, good judgment comes from experience, and experience comes from bad judgment! Perhaps my experience can help you along with one of your challenges.

Here’s how this “mini-consultation” will work:

  • Step 1. In the comment section below, write a specific problem that you’re facing or question that you have about zero-net-energy home design. Feel free to ask multiple questions if you’d like. Please, be as specific as possible.
  • Step 2. You must submit these questions by Friday January 10 at 9:00 p.m.
  • Step 3. I will select five of the best questions and write in-depth responses to those questions on Thursday January 16th.

The goal is that these responses will have tangible impact on your work, and that it can then help others that are facing similar issues. Think of this like a mini-consultation with me. I’d like to go into more detail that I can in the comment section of an article in a forum.

To read Marc Rosenbaum's answers to many of the questions posted in the comments section below, read his next blog: Practical Design Advice for Zero-Net-Energy Homes.

Marc Rosenbaum is director of engineering at South Mountain Company on the island of Martha's Vineyard in Massachusetts. He writes a blog called . Marc teaches a 10-week online as part of NESEA's Building Energy Master Series. You can test drive his class for free.

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  1. Marc Rosenbaum

Jan 9, 2014 9:21 AM ET

Good Summary of What You Think You Know Re Minisplits
by Sonny Chatum


I've done a deep energy retrofit [Z5, 1440 sq. ft., 1.5 floors, 4ach50, 12% solar glazing, Fujitsu RLS2-12 1st floor, RLS2-9 2nd floor (cooling only to date)].

After experiencing all seasons, now, including some 5F degree nights (design temp here is 12F), I believe I know pretty much the same things that you say you know. Good summary.

I disagree with:

Your #1: A 2 degree delta T among rooms with open doors is about the very best situation that can be expected, and I believe field measurements in "tight, superinsulated" houses would probably show a 2-8 degree variation, based on several other variables, and that doors open/closed would not necessarily be the dominant variable. I have seen 2-8 degrees (steady state), probably depending on solar and outside temperature, with more variation the colder it gets, among other factors. I am not the least concrerned with an 8 degree v ariation, but I believe that some people that like near perfect (2 deg.) temps from room to room could be mislead by your statment of 2 deg. variation.

Your # 6, 8, 11: Forget the ducts altogether. I spent a lot of hard, extremely dusty, work in removing my entire, existing central heat pump and ducts. Do that a couple of times and then see if you wonder why anyone would want to go with ducts. The ducts are dust storms waiting to happen. I don't believe that houses cannot be designed without them. The beauty of point source heating/ cooling, with modern design enlightenment, is similar to solar energy: both are examples of simplicity from the distant past, but now we know how to use them much better than any time in the past.

Jan 9, 2014 12:20 PM ET

Setback Heat up issues
by But Why?

need not be inefficient with a heat pump you just need a "smart" or "learning" thermostat that learns ho wlong it takes to get you home back up to temperature and then it starts moving the temp up that much sooner to get to your set point without kicking into "overdrive" If you want the temp at 70 when you get up, a learning thermostat will figure how long it takes to get your house from 66 to 70 and then it will start moving your temp up gradually over that time span so instead of the system trying to go from 66 to 70 at 7:00 AM it starts its march to 70 at 5:30 or 6:00.

and don't dare let your compatriot read that you recommend frequent filter changes. He'll call you a heretic or worse.

Jan 9, 2014 12:57 PM ET

ductless mini-splits in hot-humid climate
by Scott Tenney

Quick question - In a hot humid climate, like the Dallas area, will ductless mini-splits provide sufficient dehumidification in the summer months or would I also need a separate dehumidifier?

Jan 9, 2014 2:34 PM ET

Design, Comparison, and ROI
by Eric Burhop

Thanks Marc. Perfect timing with this article.

We are currently planning a 2100 SQ. ft. home with walkout basement in zone 5. We plan to have a 4" concrete slab for the main floor suspended with Hambro D500 joists. We will be running radiant throughout basement and main floor. Current energy load calcs around 50,000 btu/hr heating and cooling. We were considering forgoing cooling, but with the Daikin Altherma system we could possibly have it as an added bonus to DHW and radiant floors. Unfortunately, the initial cost has us leaning towards a condensing gas water heater....

However, this statement: "4 – Heat pumps are a natural partner to solar electric systems to achieve zero-net-energy buildings." has me pondering if we could possibly be net-zero with a solar/heat pump setup.

I was hoping you could provide an easy method for calculating a real world draw from the Heat Pump to properly size a solar panel installation so we can do a ROI on a solar/heat pump system and compare it to the simpler condensing gas water heater. Daikin's website for the Altherma () under "Energy Efficiency" claims an input of 3.93 kW for ~55,000 btu/hr output from their largest monobloc setup. We would probably have to do the bi-valent configuration with an auxiliary water heater (gas or electric) due to capacity and possibly output requirements (with these polar vortex's and all).

Either way would love some input or resources to do more research.


Jan 9, 2014 3:09 PM ET

Edited Jan 9, 2014 3:13 PM ET.

by Nick Welch

I'll jump in and share my personal anecdote of using a mini split in a very poorly performing old home.

We have a 900sf single-level house with unconditioned basement in Portland Oregon (zone 4c). The only insulation in the whole house is about 8 inches of fiberglass (R-25 maybe) in the attic. I've done some minor air sealing, but not a lot, and not the most important stuff yet. We have low/average quality dual pane vinyl windows that are about 20 years old.

We have a Mitsubishi HyperHeat Mr. Slim FE18NA 1.5ton mini split in a fairly central location that "shoots" down the hallway towards the bedrooms. It was sized assuming more wall and attic insulation, because I was OK with using temporary space heaters until then to make up the difference. If there was an FE24NA, I might have went with that, but there isn't.

I don't really have numeric measurements of the bedroom temps, but as long as we keep the doors open, the temperature difference is small enough to not care about -- and we are wimps when it comes to being cold. The main temperature differential is being in front of the heat pump vs. being away from it. Since our heat loss is so high, the heat pump is blasting out relatively hot air a lot of the time, so you really notice it when you're nearby. I expect this to become less extreme as we improve the envelope.

We leave our bedroom doors open, except each evening when we put our 2 year old down to sleep. That room cools off a bit with the door closed, so when we re-open it at our bedtime, we use a small (6-8 inch diameter?) fan, sitting on the floor, to blow the cool air out and help speed up the process of the warm air getting back in there. We end up leaving this little fan running overnight and often well into the next day before we bother to turn it off, mostly due to laziness.

When we had a cold snap in December that got down to single digits (our design temp is 25F, and remember, the unit was sized with yet-to-be-done envelope enhancements factored in), Mr. Slim (we like to refer to him by name ) couldn't quite keep up, so we used a 1500w space heater to make up the difference, and that sufficed. Our electricity bill jumped way up for that month, but we have very little past data (and it is polluted by the installation of a heat pump water heater last fall) to compare to, so it is hard to extrapolate anything from it yet.

The basement stays at around 50F unconditioned, even in the coldest weather. The heat pump water heater seems to affect this very little in the winter -- the heat loss through the uninsulated walls, the ground temperature, air leaks, and heat gain from the floor above dominate anything the water heater is doing.

Jan 9, 2014 3:17 PM ET

I've heard from separate unverified internet sources...
by Dana Dorsett

... that the Mitsubishi Hyper Heating units auto-stop around -19F to self-protect, and re-start around -18F, but that they DON'T in fact operate at -20F & lower. Got any inside- info on that?

I've also heard 2nd-hand reports of the Fujitsu RLS2s and RLS2-H continuing to run and put out a lot of heat at -30Fand below.

The NEEA Northwest Ductless Heat Pump project (who sponsored that NREL bench test cited in Marc's first link) has also sponsored extensive field performance metering in-situ of dozens of mini-split installations across a range of climates, from US marine zone 4 to zone 6A. The Final Report and the followup Addendum can be found here:

Note, the annualized average COP of the 10 Mitsubishi 1-tons in a zone 6 climate (Idaho Falls, where the 99% outside design temp is -6F) was just shy of 3.0. See the discussions regarding the Eastern Idaho cluster in the Addendum. The Addendum starts on p121. That's a pretty remarkable efficiency average given that many ground source heat pumps don't even do that well in climate zone 6 climates, if the GSHP system designer is less the stellar.

Jan 9, 2014 5:05 PM ET

Dehumidfication (Response to Scott Tenney )
by Dana Dorsett

The duty cycles of mini-splits are long since they modulate with load, but in a high-R low-gain house in a gulf-coast climate you can't count on always getting as much dehumidification you need when controlling only via the sensible load (temperature set point).

The Daikin Quaternity series would cover you though, since it has independently settable humidity and temperature setpoints, and will dehumidify even when there is no sensible load. (It can even dehumidify in heating mode.)

Most better mini-splits have a "dehumidify" mode to optimize the latent cooling efficiency, but it ignores the temperature setpoint in that mode, and is not controlled to a particular humidity level the way the Quaternity does, and requires user interaction to switch it back to standard sensible cooling mode.

Bottom line- you won't have to use a separate dehumidifier for the zone covered by the mini-split to deal with the latent load, and since it's dumping that latent heat outdoors, it's more efficient than a standalone dehumidifer that simply dumps the heat-of-vaporization of the dehumidifying function into the room as higher-temperature air.

Jan 10, 2014 4:04 AM ET

Wishes Come True
by Bruce Gray

Marc, your first wish is available now, and UL/CSA approved in sizes for Zero Energy Homes and larger.

1 – An efficient, affordable air-source heat pump that provides space heating, space cooling, and domestic hot water.

It is called SunPump - short for Solar Energy Heat Pump. It has efficiency like a Geothermal system, but at the cost of a mini-split, and builders can get it on Zero Upfront Solar Heating Lease, repaid by energy savings.

Jan 10, 2014 10:05 AM ET

Efficiencies of minisplits compared to GSHP
by Peter Schonherr


I have client who is building a community centre in northern Missouri. They plan to use forced air GSHPs, (single stage), for heating and cooling. However they are also looking to cut costs to meet their budgets. I suggested using Mitsubishi variable compressor speed minisplits. The lower initial capital costs are obvious. However they would like to a comparison of operating costs and efficiencies as well. Has anyone done a study, simulation or lab testing, (similar to NREL's), of an apples to apples comparison of a GSHP system vs. a minisplit HVAC system? By apples to apples, I mean same loads, climate, usage, etc. of the complete systems including ground loop pumps, fans, etc.

Jan 10, 2014 10:09 AM ET

My question for Marc Rosenbaum
by Martin Holladay

Here's a question to throw into the basket:

As an increasing number of utilities are erecting roadblocks to new gird-connected residential PV systems — based on (possibly fallacious) contentions that too much PV will make grid management difficult and that more PV systems will lead to declining utility revenues and insufficient resources to maintain the grid — has your definition of a net-zero-energy house changed, or have you begun to rethink the idea that we can balance heavy electricity use during the winter with excess PV production during the summer?

Jan 10, 2014 11:10 AM ET

Unfunded reasearch
by Robert Valenti

Marc, thank you for the unfunded research! It's very helpfull to know how these systems are performing.

Jan 10, 2014 1:00 PM ET

I'd like to get on your list of "best" questions
by Eric Friedman

My questions have to do with the ducted system we installed in our attic for the 2nd floor. Specifically:

1. Our thermostat for the ducted system only goes down to 63 degrees, which seems to set the actual temperature at 67. Two issues here – one, we would like the flexibility to lower the actual nighttime temps to 65 as that’s comfortable sleeping weather to us and two, when we go away for a week or more, we’d like to be able to lower the temperature even further, knowing that it may take a while to come back to temp when we return. Do you know of any way to reset the thermostat to have a lower minimum temperature? I’ve asked Mitsubishi but have come up empty.

2. Our duct runs in the attic are relatively short but use flexible duct material. Someone suggested to us that these ducts should be replaced with rigid ducts. While I’m sure that this would improve efficiency somewhat, I am wondering whether this is really worth it and whether there would be any other benefits we would see (e.g. quicker rise in temps, etc.).

Jan 10, 2014 4:41 PM ET

Distribution systems
by Aaron Birkland

How do you decide when ducts are worth the time and expense to install them?

Many comments made on this site related to mini-splits are variants of a theme: in 'well insulated' or 'compact' homes, the point-source heat of ductless mini-split is just as comfortable distributed heat. I believe Dana Dorsett once provided some back-of-the-envelope calculations demonstrating that with sufficient insulation and sufficiently small fenestration, even with a doors closed, heat diffusion through interior walls plus a couple of sleeping bodies can be at least theoretically capable of keeping a room at design conditions comfortable with a ductless mini-split happily humming away somewhere else.

That being said, where do you believe that this phenomenon breaks down: What is the point (if any) at which ducts (or distribution systems in general - e.g. hydronic) start to provide value to the table? How do you evaluate your house designs for suitability of installing ducts? Are there some concrete metrics (such as room-by-room heat loss exceeding a certain threshold) that point you in a certain direction? How about soft metrics (what the homeowner is willing to tolerate)?

I have an large old Victorian home that we're renovating (but not performing a deep energy retrofit) so it's not really going to be in the net-zero category, but it will be a vast improvement over what it was. Loss from some of the bedrooms at design conditions, factoring in improvements, will still be upwards of 3000 BTU/hr, in part due to the presence of a number of 18 sq ft windows.

For my own project, I'm apt to come to the conclusion that relying on a ductless in the hall would present too much comfort risk for us - but I've never heard a single comment suggesting any meaningful compromises in comfort when a ductless mini-split is deployed, and that has been nagging at me, leading me to question my objectiveness (and sanity!)

Jan 10, 2014 5:10 PM ET

"Loaded" heat pump question
by Rheannon DeMond

From Rheannon DeMond

Thank you for making this cameo on GBA. Like you, for the past few years our company has seen most of our homes use air source heat pumps for both heating and cooling. We tend to build mostly in climate zones 5-7 and are frequently challenged by the question “what do you do when it is -13°F outside and your heat pump stops working?” Or better yet, “how will the heat pump perform when it is -13°F outside?” While I have seen some unit’s power through the cold, I have also seen other units fail, and as a homebuilder that is the absolute last phone call you want to get.

The question I would like to propose to you has to do with how heat pumps are rated. Something I noticed while researching Daikin units a while back is how much more efficiently their ducted units are in comparison to their Minisplits. A Daikin 2 ton ducted system is rated to output 23,000 Btu/hr at 14°F, but a Daikin 2 ton Minisplit is only rated to output 11,300 Btu/hr at 14°F, according to their Engineering Capacity Tables.

From the other end Mitsubishi rates their standard 18,000 Btu/hr Minisplit to output 17,200 Btu/hr at 17°F, but their 18,000 Btu/hr SEZ units with ducts are only rated for 13,100 Btu/hr. Then you learn about their Hyper Heat Inverter Technology where the 18,000 Btu/hr Minisplit is rated to output 21,600 Btu/hr at 5°F and everything you thought you understood about heat pump technology collapses on its self.

Now for my “loaded” three part heat pump question. How exactly are these companies determining the capacities of these units? What is the most efficient delivery method, ducted or minisplit? And lastly is there a way to actually calculate, even an estimate, of how much heat these units will actually output at -13°F?

Thank you for doing this, I would feel honored if my question was picked!

Kindest Regards,

Rheannon DeMond
Bensonwood & Unity Building Technologies
Energy & Sustainability Specialist
NESEA ZNEH Course Alumni, Class of 2012

Jan 10, 2014 5:24 PM ET

Edited Jan 10, 2014 5:25 PM ET.

Domestic hot water
by Martin Holladay

Here's another question to throw into the basket:

As you know, it's hard to make blanket recommendations on water heater selection, because of the number of variables (some families use little hot water, others lots; some locations have natural gas, others don't; heat-pump water heaters make sense in some climates and in some homes, but not others).

Here's my question: Are there ever any situations when electric-resistance point-of-use tankless water heaters make sense? If so, please describe the type of family, the type of house, the climate, and the electric rates where such a strategy makes sense.

Jan 10, 2014 7:39 PM ET

outside concrete slab skirted insulation
by richard turner

I live in north Georgia, well water temperature average 55 degrees and yearly air temperature average about 61 degrees. Copying the passive annual heat storage idea, with a slab home placing foam board insulation on the ground, 12 feet out away from the home and cover with a liner to drain water away from the home. Foam should be bury about 2 feet of top soil and grass sod place on top. There will be no west wall windows and a generous 2 foot soffit overhang on the south. I own the blower door machine and insulated my 1st attic in 1976 so this is not my 1st rodeo. The mini split system with the dehumidifier, any idea on the concrete slab with constant ground temperature having a positive effect on a home in warm humid north Georgia. Thank you for doing this, I would feel honored if my question was picked......................................Richard

Jan 10, 2014 9:07 PM ET

Edited Jan 10, 2014 9:14 PM ET.

Distribution, Oversizing, and a Mix and Match of Indoor Styles
by Leigha Dickens

When your heating/cooling loads in a net-zero home are already smaller than the capacity of anything but a single wall-mount unit, how do you balance correct sizing with adequate distribution? What are rules (second to Aaron's question) of how much one should worry about distribution?

My specifics:

A super-insulated net-zero and passive solar home in Virginia, CZ4 and a solid need of heating, cooling, and adequate dehumidification. (R35 walls, R50 roof, R30 floor, triple pane windows U-0.21ish, goal 0.80ACH50) The heating design load is 10,000btus/hr and cooling load is 9,000btu/hr, so both could in theory be satisfied with a single one ton wall mount--but! distribution! The house is 1620 square feet, and the layout can be divided into two halves. The front half is open living/kitchen/dining space, this is the main passive solar space. The back half is three bedrooms and two bathrooms. This design and climate is very typical of the kind of homes I work with all the time. There is also one mud-room off to the side, mostly open to the living/kitchen area.

My instinct until now has been to design so that all the main living areas and bedrooms have an indoor unit, either wall-mounted or connected to a ducted unit. I thought that there were limits on how much ductwork you could expect one ducted indoor mini-split to sustain in not-quite-compact-but-still-not-giant houses, necessitating a combination of indoor units. The problem is that in NZE houses, the heating and cooling load can be lower than the capacity you can get with an outdoor unit that can support multiple indoor units. I always thought that was ok because of the ability of the mini-splits to work at partial capacity, but now I wonder if maybe I'm crazy.

Thus the plan in this house was to use a 18,000btu/hr capacity outdoor unit and two indoor units, one 9,000btu/hr wall-mount for the kitchen/living/dining half, and one 9,000btu/hr short-ducted unit connecting to all of the bedrooms, supposing I could even get a short-ducted unit that could duct to those three rooms. The bathrooms, closets, mudrooms, etc, figured we would leave out. Is it ok to leave those out? Or should I be leaving the bedrooms out too, rather than doubling the capacity needed just to get distribution to all the bedrooms because I'm stuck in an old way of thinking that worries about that when I shouldn't?

Jan 11, 2014 12:25 AM ET

Wish list No.2

Marc, et al:
Your wish list No.2- an air source heat pump that can serve as a drop in for a boiler system is available and widely distributed in many size capacities from residential to industrial in Japan as well as by Japanese companies marketing in Europe. It is an R744 (plain old CO2) refrigerant system operating very efficiently, but at very high pressures. The name it goes by in Japan is "Eco-Cute," the "cute" being a derivation of the Japanese name for a water heater. Downsides? Initially very expensive, and as we have few technicians trained in installation and servicing of high-pressure CO2 refrigerant systems, not at the present time in the scope of serious consideration for installation in the US. Why the Japanese have not decided to market in the US is a bit of a mystery to me, but it may just be that our energy alternatives are just too competitive to justify a market for a high-priced, and not understood new appliance technology. These units have been on the market in Japan for almost 10 years now and have become rather ubiquitous, with a market penetration of over a million units in Japan.

Jan 11, 2014 1:37 AM ET

in hope i will not disturb the intent of the thread...
by Jin Kazama

DANA: my 9RLS2H ( 4 units ) worked very well down to -32~-34c a few weeks ago ...
though i can tell you that the heat was almost half as hot as when the temp is near 0c
But they never stopped working.

Sifu Martin : i still don't get why the utilies aren't using methods to store the excess summer electricity in cold climates to use during winter.
Even with somewhat a great loss, something as simple as producing hydrogen through electrolysis of water could be easily stored and turned back to energy when required.
There must be alot of other ways to do it somewhat efficienctly .

HPWH ...
Why are we ( NA ) so late on this compared to CO2 water heater used in Japan for example ?

Lastly, about dehumidifying ..i don't undertand how the RLS2/H models work on that .
There seems to be no other setting than " dehumidify mode " ..without any power or target setting.
Last summer while testing the dehumidifying of those units, i had much greater sucess using regular cooling mode than the dehumidifying mode ..something is not right on this.
( i was dumping in a pail so i was able to calculate the removed amout of water for a certain period of time )
How does other brand such as Daikin and Mits work on this ??
I wouldn't rely too much on this mode if i was a critical humid climate
( although this past summer we had crazy high humidity all the time .. )

Jan 14, 2014 1:09 PM ET

comfort in a small home on a slab
by Rachel Wagner

The energy load calculations show that a mini-split will heat the whole house, or at least the lower level, of, say a 1200 ft 2 slab on grade in a very cold climate. But, even with super-insulation, what about comfort on a concrete slab? Do you or does anyone else have a case study of a super-insulated slab on grade home with a single mini-split delivering comfort in the middle of winter? Bathroom floor? Bedroom floor? Thanks.

Jan 14, 2014 2:08 PM ET

the real world isn't so kind
by Hobbit _

So, where do I get a system with a COP of 10 ??!

The Daikin here has a "dry" mode which I still haven't figured
out how to reliably invoke, but basically it runs a very low
fan and lets the compressor chug along at *just* enough
capacity to pull out the latent load but not blast a lot
of cold air into the premises. Works about as well as simply
running the unit in cool mode with low fan anyway -- when
it's close to setpoint it backs off capacity and will run
for an extra hour or two that way, pulling lots more water
out of the air. I was dumping buckets on this for a while
too, and getting 4 - 5 pounds per hour on a 1.5-ton system
if the indoors started off relatively humid.


Jan 15, 2014 7:06 PM ET

best location in 2 story + basement
by Hallie Bowie

Where would you locate the mini split in a compact, super insulated house with 2 stories plus a basement with living space in a mixed climate (northeast Ohio)? On main level, so bedrooms get some cooling in summer, or on lowest level with an open stair to allow heat to rise? If the clients aren't concerned about mechanical cooling, would that influence the decision? What if they also want a woodstove?

Jan 16, 2014 5:50 AM ET

Response to Hallie Bowie
by Martin Holladay

A single ductless minisplit unit cannot effectively heat and cool a three-story house, even if the unit's rating matches the design heating and cooling load, because of distribution issues.

For heating, the unit should be installed in the main living area of the basement or the main living area of the first floor -- wherever the occupants spend the most time. Expect areas that are distant from the unit to be uncomfortable, unless you have another heating system in the building.

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