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Just Two Minisplits Heat and Cool the Whole House

Carter Scott has built 18 homes in Massachusetts without any heat in the bedrooms

Posted on Aug 17 2012 by Martin Holladay

Carter Scott was one of the first builders bold enough to build a cold-climate home heated by only two ductless minisplit units (one in the downstairs living room, and one in the upstairs hallway). Skeptics predicted that the unheated bedrooms would be cold and uncomfortable. Yet Scott was confident that the home’s excellent thermal envelope — with high-R walls, triple-glazed windows, and low levels of air leakage — would keep the homeowners comfortable even when the bedroom doors were closed.

Scott owns a construction company called Transformations in Townsend, Massachusetts. He built his pioneering two-minisplit house in Townsend in 2008; the inclusion of a 5.7-kW roof-mounted photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. array made it into a zero-energy house.

The skeptics’ “cold bedroom” predictions were unfounded. “We have since built several houses in which the upstairs minisplit unit isn’t even being used until the outdoor temperature drops below 20 degrees,” Scott said. “Typically the response from homeowners is, ‘Wow, these houses have even indoor temperatures’ and ‘these houses are quiet.’ And the fact that there are no utility bill makes people excited.”

Carter Scott's first net-zero energy house

A ductless minisplit is a type of air-source heat pumpHeat pump that relies on outside air as the heat source and heat sink; not as effective in cold climates as ground-source heat pumps. that can provide space heating as well as air conditioning. Most of the ductless minisplits sold in the U.S. are manufactured in Japan or South Korea; the best known brands are Daikin, Fujitsu, and Mitsubishi.

The first zero-energy house built by Scott's company, Transformations, was designed by architect Ben Nickerson. Dubbed the Needham model, it was built at 18 Coppersmith Way in Townsend in 2008. Here are the home's specifications:

  • Area: 1,232 square feet
  • Basement insulation: Basement is unconditioned; basement ceiling is insulated with 3" of closed cell spray foam plus R-30 fiberglass batts (total R-50)
  • Wall framing: 12-inch-thick double-stud walls
  • Wall insulation: Flash and fill: 3" closed-cell spray foam plus 9" cellulose (R-50)
  • Sloped ceiling insulation: Flash and fill: 5" closed-cell spray foam plus 13" cellulose (R-64)
  • Windows: Paradigm triple-pane kryptonA colorless, odorless inert gas, often used with argon in fluorescent lighting and sometimes used as gas fill in high-performance glazing.-filled low-eLow-emissivity coating. Very thin metallic coating on glass or plastic window glazing that permits most of the sun’s short-wave (light) radiation to enter, while blocking up to 90% of the long-wave (heat) radiation. Low-e coatings boost a window’s R-value and reduce its U-factor. windows
  • Siding: VinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate).
  • Design heat load: 10,500 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. /h
  • Space heating: Two Mitsubishi Mr. Slim ductless minisplit units (one 12,000 Btuh unit downstairs, and one 9,000 Btuh unit upstairs); installed cost, $5,250
  • Mechanical ventilation: Lifebreath 155 ECM energy-recovery ventilator
  • Domestic hot water: Sun Drum solar thermal system with electric resistance backup
  • PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow. system: 5.7-kW roof-mounted array (Evergreen Solar PV modules) and Fronius IG 5100 inverterDevice for converting direct-current (DC) electricity into the alternating-current (AC) form required for most home uses; necessary if home-generated electricity is to be fed into the electric grid through net-metering arrangements. (cost before incentives: $33,000)

In 2009, this home won the second prize ($15,000) in a utility-sponsored contest called the Zero Energy Challenge. The home has since received plenty of media attention, including attention from GBA; see, for example:

Consultants from Building Science Corporation help refine the specifications

Scott gave a presentation on his recent projects at the Sixteenth Annual Westford Symposium on Building Science in Westford, Massachusetts, on July 30, 2012. Scott is currently working with the DOE’s Building America program, and the program hooked him up with energy consultants from the Building Science Corporation. He’s now building homes at several locations around Massachusetts.

In Devens, Massachusetts, Scott will build 8 superinsulated single-family homes with prices starting at about $330,000 (not including PV). In Easthampton, Scott is building 33 new superinsulated homes at a development called Easthampton Meadow. The homes will range in size from two-bedroom homes to four-bedroom homes, and will be priced at $284,000 to $330,000 (not including PV).

The Easthampton homes will have all of Scott’s usual features — thick R-40 walls, R-60 ceilings, triple-glazed windows, ductless minisplit heating systems — bringing the homes to HERSIndex or scoring system for energy efficiency established by the Residential Energy Services Network (RESNET) that compares a given home to a Home Energy Rating System (HERS) Reference Home based on the 2006 International Energy Conservation Code. A home matching the reference home has a HERS Index of 100. The lower a home’s HERS Index, the more energy efficient it is. A typical existing home has a HERS Index of 130; a net zero energy home has a HERS Index of 0. Older versions of the HERS index were based on a scale that was largely just the opposite in structure--a HERS rating of 100 represented a net zero energy home, while the reference home had a score of 80. There are issues that complicate converting old to new or new to old scores, but the basic formula is: New HERS index = (100 - Old HERS score) * 5. 40 without PV. If a homeowner wants to buy a PV system — an available option — any of the homes can reach HERS 0 (or less).

When the temperature is below zero, Mitsubishi ductless minsplits still perform

For Carter Scott, heating a house measuring 1,700 to 2,000 square feet with two ductless minisplits is no longer an experimental method. It’s standard operating procedure — one he’s used on 18 houses.

Although he has considered using Fujitsu or Daikin minisplit units, Scott continues to specify units from Mitsubishi. He usually specifies the MUZ-FE12NA outdoor unit and the MSZ-FE12NA indoor unit; this pairing is rated at 12,000 Btu/h. Even at an outdoor temperature of 0°F, these units can put out 10,000 Btu/h of space heat at a COP of 1.8. Mitsubishi minisplits will still deliver heat when the outdoor temperature drops to -13°F.

Scott pays his HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. contractor less than $6,000 to install two minisplit units in one of his homes.

After Scott heard that the owner of a home he had built (a custom home in Princeton, Massachusetts) had turned off the upstairs heating unit because the downstairs unit adequately heated the whole house, he decided to try an experiment. At his next house, Scott installed just one ductless minisplit. “We thought that maybe we could just use one unit downstairs. But I didn’t think of cooling. The cooling didn’t rise to the second floor, and the house was hot upstairs in the summer. We went back to the house and installed a minisplit unit upstairs. So if you want AC on the second floor, you need an AC unit up there.”

Since he learned his lesson, Scott always includes a minisplit unit on each floor of his two-story homes.

Anyone interested in following in Scott's footsteps needs to remember that the success of his two-minisplit approach depends in part on compact rectangular designs. Stretched-out houses on a single floor, designs with ells, or designs that include a bonus room over the garage aren't amenable to the two-minisplit solution.

On the other end of the spectrum are the one-minisplit homes; examples include Marc Rosenbaum's house in Massachusetts, the Montague Urban Homestead (also in Massachusetts), Larry and Jill Burks' Up Hill House in New York state, and John Semmelhack's house in Charlottesville, Virginia (see Comment #15, below). The best candidates for this approach are flexible homeowners who leave their bedroom doors open during the day and who have no need for second-floor air conditioning.

What about ventilation?

Carter Scott's first zero-energy house in Townsend included a Lifebreath energy recovery ventilator — an effective but costly ventilation solution.

As a builder of spec homes on his way to becoming a production builder, Scott has always focused on affordability. That's why the standard ventilation system included with his homes is often a simple exhaust-only system. However, he offers a variety of ventilation options to customers who are willing to pay extra.

“The baseline ventilation system we offer is a Panasonic exhaust fan in each bathroom,” said Scott. “They cost us $250 each installed. The next step up — an available option — would be a Panasonic ERV(ERV). The part of a balanced ventilation system that captures water vapor and heat from one airstream to condition another. In cold climates, water vapor captured from the outgoing airstream by ERVs can humidify incoming air. In hot-humid climates, ERVs can help maintain (but not reduce) the interior relative humidity as outside air is conditioned by the ERV. [energy-recovery ventilator] that exhausts and supplies from the same location. That’s $500 installed. A little better would be the Fantech VHR 704 HRV with one exhaust location and one supply location. Better than that would be the Fantech SHR 1504 HRV, which could exhaust three bathrooms and supply all of the bedrooms. The next bump up would be the Lifebreath ERVEnergy-recovery ventilator. The part of a balanced ventilation system that captures water vapor and heat from one airstream to condition another. In cold climates, water vapor captured from the outgoing airstream by ERVs can humidify incoming air. In hot-humid climates, ERVs can help maintain (but not reduce) the interior relative humidity as outside air is conditioned by the ERV., which is about $2,500.”

Subsidies, incentives, and tax credits make PV systems affordable

Before incentives or tax credits, Scott’s PV systems cost him about $5 a watt to install. However, all of his customers benefit from at least one type — and in many cases, several types — of tax credit or subsidy. Homeowners who aren't interested in owning a PV array can also take advantage of a variety of leasing options.

In addition to the 30% federal tax credit on photovoltaic installations available to all U.S. taxpayers, Massachusetts homeowners with PV systems can take advantage of Solar Renewable Energy Credits, or SRECs. The mechanics of the SREC market are complicated, and the value of these credits fluctuates. According to Scott, until recently the SREC market valued residential PV production at 50 cents per kWh — meaning that PV system owners could recoup their investment extremely quickly. The market value of SRECs for residential PV systems is now down to only 28 cents per kWh; although that number is lower than it was in the past, it still provides a quick payback to owners of PV systems — often ten years or less.

The Energy StarLabeling system sponsored by the Environmental Protection Agency and the US Department of Energy for labeling the most energy-efficient products on the market; applies to a wide range of products, from computers and office equipment to refrigerators and air conditioners. program in Massachusetts also offers generous incentives to builders like Scott who achieve the Tier 3 level of performance; Scott receives an $8,000 cash incentive for each high-performance house he builds.

“Solar systems are very cost-effective now,” says Scott. “When it comes to maintenance, PV is pretty simple. Solar thermal is more problematic than PV.”

Propane is sometimes cheaper than natural gas

At one of his zero-energy homes in Townsend, Scott installed an on-demand natural gas water heater; it’s the only gas appliance in the house. During their first year in the house, the homeowners only used $50 of natural gas, but were billed $152 — $50 for the gas, and $102 for the service charge of $8.50 a month.

Once Scott realized that the homeowners were paying more for the service charge than they were for fuel, he concluded, “This dog don’t hunt.” He now avoids hooking up his homes to natural gas lines; he’s concluded that most homeowners with a single gas appliance are probably better off buying propane. Even though propane costs more per gallon, propane dealers don’t charge a monthly service fee.

Letting go of solar thermal

As Scott continues to aim for high performance at the lowest possible cost, he has tweaked a few of his specifications. These days, Scott is installing more open-cell spray foam in his walls, and less cellulose, for two reasons: the spray foam provides a better air seal, and his insulation contractor offers open-cell foam for the same price as cellulose.

To reduce costs, he has switched from triple-glazed vinyl windows from Paradigm to triple-glazed vinyl windows from Harvey. When I asked Scott about the quality of Harvey windows, his answer made it clear that the windows’ best feature was the low price. “The quality of Harvey triple-pane windows is improving,” Scott told me. “They now come with a hole midway up the jambs, which we pin to the studs. This keeps them opening and closing above 85 degrees. We are also careful not to put more than 1/2 inch of low-expanding foam to seal around the windows. Harvey will sell me an R-5 window at an unbeatable price. In quantity, I’m getting the windows for just a few dollars more than our old double-pane windows.”

In the past, Scott tried a variety of approaches for heating domestic hot water, including solar hot water systems. His favorite water heater is now the Navien 180 on-demand tankless unit fueled by propane or natural gas. “An $8,500 solar hot water system doesn't make any sense compared to the $1,800 Navien. The fuel use cost with the Navien is so low that there really isn't much money to be saved doing anything different.”

Ground-source heat pumps are too expensive

Before he discovered ductless minisplit units, Scott built three homes with ground-source heat pumps (GSHPs). Now that he knows about minisplits, however, he has no intention of installing another GSHP.

When I asked him why, his response was simple. “The first ground-source heat pump system I installed cost $38,000. The second one cost $40,000. The third one had a direct-exchange loop and cost me $22,000,” he said. “But I can install two minisplits for less than $6,000. And the whole system efficiencies are about the same, as far as I can tell. Even if the ground-source units have a slightly higher COP, it's not enough to warrant the extra money.”

Last week’s blog: “Living Without Electricity Bills.”

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Image Credits:

  1. All photos: R. Carter Scott
  2. Fine Homebuilding

Aug 22, 2012 11:42 AM ET

Carter Scott's approach
by John Semmelhack

Carter's approach is definitely climate and building-specific, as I think Martin mentioned in the lead-in to the article. The client is the ultimate arbiter over whether something "works" or not (at least to the builder/developer, if not the energy geek). Clearly, the hallway mini-split seems to be working for Carter's clients. Some ideas as to why...

1) In the winter, the downstairs unit is probably doing the bulk of the work...perhaps carrying 100% of the entire house heating load over 80-90% of the heating hours (educated WAG). The hallway unit is nearly superfluous in the winter. At least one of Carter's clients doesn't even use the hallway unit.

2) It looks like the 2nd floors on Carter's houses have relatively little glazing, the glazing generally has "OK" shading, and the bedrooms are probably modest...the bedrooms probably have modest cooling loads.

3) Massachusetts has a short cooling season. Some short cycling over a short cooling season, although not ideal, is probably not a big energy penalty.

Aug 22, 2012 4:08 PM ET

I'm all for pushing the envelope, but...
by David Butler

Much of my design practice involves specifying systems and design strategies that run counter to the experience & training of most mechanical contractors. I always try to encourage these guys to push the envelope in their own homes to give them more confidence in the (mostly) correct ACCA design procedures. So here I find myself in the odd position of being the one arguing against outside-the-box thinking.

One thing we must be careful about is not to propagate contrarian designs that might work in a specific case. Designing HVAC for high performance homes requires experience and judgment, not cookbook strategies. As a designer, my responsibility is to meet my client's expectations. A homeowner who expects a cool bedroom at night in summer may be very disappointed with the approach being discussing here. Personally, I know it would never fly in my home.

That being said, I would be genuinely interested in seeing data (e.g., temperature logs) that demonstrate actual performance of a ductless head in this configuration.

Aug 22, 2012 6:52 PM ET

HRV return air cools bedrooms
by Fortunat Mueller

Carter's experience mostly mirrors our own. We've designed and installed mechanical systems (originally lots of mod con gas boilers and now mini splits too) for dozens of homes without heat in the bedroom (or often any heat upstairs) and have virtually never heard a complaint from a client. And we're in a more challenging heating climate than Mass.
The couple of houses where we have gone back to add supplemental heat have all been homes with an HRV or ERV. I wonder if the absence of whole house ventilation contributes (unexpectedly) to the balanced heating of these homes. On a design day in Maine, even the most efficient HRV is bringing in fresh air to the bedrooms at 5 degrees or more below room temperature and though the flow rate is modest, if the bedroom doors are closed I think this can contribute significantly to a cooling of the bedrooms and a desire for supplemental heat.
Recently, I've been encouraging clients to consider a small electric boost heater in the HRV return duct (1000 W) that runs only at very low ambient temperature to eliminate this issue. But maybe exhaust only or the Panasonic ERV is a good option (though I can already hear our ventilation partners protesting).
Finally, just because piling on SHW seems to be the topic of the year, I feel the need to respond to that as well. In my opinion, the viability of SHW (compared to on demand or HPWH) depends on load and incentive environment and defies a one-size-fits all answer.

Aug 22, 2012 7:20 PM ET

Response to Fortunat Mueller
by Martin Holladay

You're the second person today who has posted a comment referring to the fact that during the winter, HRV fresh air grilles tend to cool the room in which they dump fresh air, especially if the room is unheated. (The other person was Jesse Thompson, on this blog.)

Of course, you're both right. I wrote a blog on the issue over a year ago; here is is: A New Way to Duct HRVs. But it makes no sense to deliver outdoor air to the coldest rooms in your house.

The solution is fairly simple: deliver the fresh air to the warmest room in your house (in the case of Carter Scott's designs, that means the living room and the upstairs hallway) and exhaust stale air from the rooms without any heat (the bedrooms). That tends to even out the room-to-room temperature variations.

Aug 22, 2012 7:34 PM ET

Edited Aug 22, 2012 7:36 PM ET.

HRV return air cools bedrooms
by christian corson

In super insulated ,airtight ( and by airtight > 1 ach/50) A properly sized and BALANCED ventilation system will aide greatly in distribution and elimination of atmospheric thermoclines. Distribution will be fine with the unit at the top of the stairs. In fact at higher levels of airtightness like closer to PH the Coanda effect itself will aide with distribution allowing the air to cling to the ceiling until it drops off at the other side of the room or space. It should also be noted that the evaporators that are ceiling mounted have wide ranges 110degrees or so and are continually oscillating. The floor mounted evaporators are less efficient I think because of this. They can not use surface tension for distribution.
Lastly if there is a delta T of 5degrees F from the air from the ventilation system versus room temp then you are NOT using the most efficient units. At 7 degrees F exterior ambient temp we are getting supply air at 66F with a room temp of 68. That is with units with 92% (real 92% not some Stirling tech enthalpy wheel marketing BS) There are no north american manufacturers (that I know of) that can achieve this.
Lastly 2- small windows suck for daylighting
Lastly 3- Why two separate units in Carters homes? Why not use a MXZ-2b-20na hyper heat unit with 12000btu down and 9000btu up. Works great. One condenser and 2 evaporators. 20000btu capacity COP of 3.9ish.
Oh yea edit- remember these thing push like 150- almost 400 cfm.

Aug 22, 2012 7:41 PM ET

Edited Aug 22, 2012 7:42 PM ET.

by christian corson

Ahh snap. Exhaust air from bedrooms? 1 king size bed, two adult humans, two children's, two dogs,15 fish, 4 plants, .7ach/50....................................wont I suffocate?

ASHRAE no likey.

Aug 22, 2012 7:52 PM ET

Response to Christian Corson
by Martin Holladay

OK, your bed sounds crowded. Four people and two dogs? Well, eventually the two kids will probably sleep somewhere else. As for the dogs, it's just a matter of training.

You wrote, "ASHRAE no likey." You're wrong, Christian. ASHRAE 62.2 specifies the ventilation rate for a home, but has no distribution requirements. Plenty of people have installed ASHRAE 62.2-compliant exhaust-only ventilation systems with no fresh air to the bedrooms. (I'm not saying that is necessarily a good idea.)

I'm suggesting something better than that: A ducted HRV system that pulls exhaust air from your bedroom. If you want 30 cfm in your bedroom, there's no reason you can't have it. Adjust the exhaust air flow from your bedroom to 30 cfm. Obviously, air in = air out. You'll get 30 cfm of fresh air. It will be pulled in from the hallway under the door. And you know that the hallway air is fresh, because that's where you are dumping your supply ventilation air.

Aug 22, 2012 8:27 PM ET

Edited Aug 22, 2012 8:35 PM ET.

by christian corson

Yes, its a party in our bed every night! One budding artist and another budding paleontologist. You should hear my wife read Goose and Grim in German. Getting boiled by a which sounds WAY scarier in German. Good night moon.........not so much. The dogs were trained until we had kids, now all bets are off. Plus they are so old they deserve to do whatever the hell they want.
As far as ASHRAE goes you are correct I should have referred to the EN ISO recommendations of 18 CFM per person at rest , but was trying to keep it simple and I couldn't remember the standard off the top of my head....EN ISO 13790? Any way, that is actually where ASHRAE falls short. IMHO. Thanks for keeping me in line. Ill take 36 CFM in all night long in my room, Ide rather have 35 out in the bath.( I only have 22) But I get what your saying. My hallway stays fresh as I blowing 150 CFM through my minisplit.

Oh and exhaust only ventilation is a bad idea. Just throw a wood stove in for good measure.

What did my Grandfather say? ......'doing something wrong three times doesn't make it right.' Scary thought coming from a surgeon with a cigar hanging out of his mouth.

Aug 22, 2012 9:08 PM ET

by Ed Dunn

Have been building homes with just a central space heater here in Flagstaff, AZ for years. Yes, with great insulation (Oak Ridge National Laboratories standards developed in the 70s and 80s) Our bedrooms have no heating system. Bedrooms are no more than 4 degrees cooler than the main living area. Cooling is not necessary in Flagstaff, but a lot of homes with inadequate ceiling insulation are uncomfortable several days in the summer. Our homes are very nice inside. Does not matter if we use Straw, cob, foam or superinsulated stud walls. It works. A tight, well insulated envelope is the foundation for a properly designed passive solar home.

The Department of Energy adopted the ORNL standards a few years ago.

Aug 22, 2012 9:49 PM ET

no bedroom heat here either...
by David Butler

I live in southeast AZ, but nearly a mile above sea level. Every winter I close off my master bedroom supply (actually, I tape cardboard over the inlet inside the cubical boot). I can't tolerate a warm bedroom. I turn down the stat to 64 for the rest of the house. The cats don't seem to mind (not that they have a voice in the matter). Since I'm a night owl, my wife is usually up long before me. A motion sensor outside the bedroom automatically returns the stat 69 for her comfort. However, the bedroom stays cool.

The only time I ever had to open the register in winter was in Feb 2011 when we saw two consecutive nights in the single digits. That's also the only time my heat pump ever needed supplemental heat.

In Ed's climate in northern AZ at 7000 msl, I wouldn't hesitate to forgo direct supply to bedrooms, provided the house is tight and well insulated.

Aug 22, 2012 10:29 PM ET

Edited Aug 22, 2012 10:32 PM ET.

One Mini-Split, 2 Indoor Units, One Ducted, One On The Wall.
by Dave Robinson

Kudos to Carter Scott on his use of Mini-Splits and the Envelope to make them work. I've been doing a similar approach for several years in renovations - first in California's central valley and now in the Blue Ridge Mountains of Virginia. I totally focus on renovation of existing housing stock, usually foreclosures. In 2009 I reported in Home Energy on my first of what we call the Mini Split Hybrid approach. Hybrid because the single outside unit drives two indoor units, one conventional unit on a wall in the main living space and one running an air handler and a small duct system to feed the bedrooms. I usually keep this ductwork inside the envelope, in a lowered hallway or stairwell ceiling. I use Fujitsu, but some others make an indoor duct able system. My initial results were outstanding with the owners reporting good comfort and very low bills. I've been invited to participate in a DOE program and two houses are now in a year long monitoring program, one in California, one in Virginia. These are both about 1700 square feet and have 2 ton units driving 2 12,000 BTU indoor units, one ducted, and one not. I always use the 2 ton. It's always oversize, but the smallest they make. As has been stated here, oversizing is not such a bad thing with inverter Mini-Splits.

Because this is not new construction, I cant get as good an envelope as Carter is providing. Typically we dense pack the walls and add R-60 above. On the most recent project in Virginia, we drywalled the basement ceiling and dense packed the first floor as well. This one has a short hard pipe duct system currently exposed in the top of the stairwell. Initial results (summer) are surprisingly low watt draw on cooling. I'll report more results as they become available. DOE and NREL will report also.

The Hybrid approach of Ducted and Ductless addresses the concerns about even temperatures in all rooms. The installation cost is about the same as two separate MSHP systems. So if you dont have the renovation budget for a deep retrofit with new rigid foam and triple panes, ... consider running ducts to the bedrooms and let a single unit handle the main living area (which you've opened up in the renovation anyway)

Aug 22, 2012 10:30 PM ET

Wall hung heaters.
by Zolton Cohen

Someone earlier in this thread mentioned the concept of "task heating and cooling." That's an interesting way of looking at things.

Has anyone ever done anything with simple wall hung gas heating console units like those made by Empire? I have one in my garage shop that outputs about 10k BTUs. It is a direct-vent unit that intakes combustion air and exhausts through the same concentric pipe. As simple and as dumb as dirt.

But it has a thermostat and is almost completely quiet - other than some small expanding sheet metal noises when it first fires up. There is an optional fan, but I never bought one.

My wall hung heater was about $550 a number of years ago; I'm sure they're probably double that now. The efficiency rating is 80%; not great compared to modern day furnaces, but at a small fraction of the cost.

So, put a couple of these heaters in a house and a couple of window A/C units into wall openings and you've got the output of a mini-split at very little cost. Is this an idea worth exploring?

The only drawback I see is that I believe "space heaters" can't usually be used as a primary heat source. But maybe that point could be finessed...


Aug 23, 2012 6:11 AM ET

Response to Zolton Cohen
by Martin Holladay

Your approach works fine, and is one I advocated in my article, Heating a Tight, Well-Insulated House. I like systems that are "simple and dumb as dirt." And as many people have repeated, once you have an excellent thermal envelope, it hardly matters how efficient your heater is, since you will be using very little fuel.

Add me to the list of people who live in a house without any heat in the bedrooms. I've never seen it get any colder than -38°F, but that's cold enough. I have one wood stove in the living room -- that's it. If I ever go away on the weekend during the winter, I keep my house warm with a single gas-fired Empire space heater in the living room, just like the one you describe. That keeps my pipes from freezing just fine.

Aug 23, 2012 8:43 AM ET

Installing the heads pre drywall?
by matt berges

As shown in one of the photos at the bottom of this article, I have recently had a contractor who wanted to install my mini split heads on the "rough in". I had previously had 8 installed in other homes (with other contractors) and had never been asked to jump out of sequence like this. It was explained to me by this current installer that this was the best and only good way to do it (for sake of making the proper connections), but I am not comfortable installing my finishes on the rough in. I would like to hear advice from the GBA community on this sequencing issue. My concerns are dust, damage, added difficulty, added risk (theft or other). I like to have the mini split heads go in at the very end, and be commissioned and tested at that point.

Wow- this article has generated a lot of postings!

Aug 24, 2012 8:39 AM ET

Back to minisplit capacity and dewpoint
by Marc Rosenbaum

I'm going back to post #38 about minisplit capacity and dewpoint. The information in that post contradicted what I understood to be correct, so I checked in with my engineering contact at Mitsubishi. Here's what he wrote:
"The only relevant factor in determining the available heating capacity is the outdoor wet bulb temperature. This graph below represents guaranteed heating capacities at various wet bulb temperatures."
This graph shows that the Hyperheat models FE09 and FE18 produce 100% of their nominal capacity down to about 3F wet bulb, and the FE12 down to about 12F wet bulb (I'm eyeballing this graph). In the case of the FE18, I take this to be the capacity they publish at 47F dry bulb, 21,600 BTU/hour, which they show as the maximum capacity in their submittal sheet at 5F dry bulb as well. This graph states that it includes the correction for defrost.

He goes on to say:
"Rated capacity is the capacity as the equipment was tested in a lab setting to obtain the published efficiency values, maximum capacity is the actual capacity that the unit is capable of producing as it operates in the field, you should always use maximum capacity when selecting/sizing equipment for an application."
So I don't believe people need to be concerned about dewpoint of the outdoor air when sizing the Mitsubishi Hyperheats. Of course, at temperatures approaching 0F, the wet bulb and the dewpoint get closer together than they do at higher temperatures.

Aug 24, 2012 8:47 AM ET

Response to Marc Rosenbaum
by Martin Holladay

Thanks for taking the time to contact Mitsubishi to clear this issue up. So, the outdoor wet bulb temperature is relevant; dewpoint isn't.

Bottom line: check the heating capacity of the unit you are considering at the outdoor temperature you are concerned about, and you should be all set.

Aug 24, 2012 1:45 PM ET

Needham Temp / RH data, exhaust from bedrooms
by Mike Duclos

I worked on the Needham design with Carter and we did some temperature / RH logging, which we can share.

This data includes the week without power due to the ice storm that hit Central MA in Dec. 2008.

If anyone is interested, send me a note at mduclos (at) and let me know if you want the HOBO files or .csv

As Martin has described in the reference above, exhausting from the bedrooms is an interesting concept.

During the design we were concerned about the bedrooms getting too cool, so Carter decided to try this in the Needham, this from Carter:

"On this home we exhausted the three bedrooms and supplied the fresh cooler air in front of each of the two mini-splits to be heated up.
We used return air grills to allow the air to get to the bedrooms even if the doors were shut."

Aug 24, 2012 3:37 PM ET

Edited Aug 24, 2012 3:54 PM ET.

Thanks to Marc Rosenbaum from me as well!
by Dana Dorsett

The "rated capacity" without a graph or an explanation of what it implied (the output at which the stated COP at that temp was valid) led me astray (and leading me to spread dis-information with this confusion- mea culpa!)

Is the more detailed data published on the web anywhere?

I find the real explanation very comforting relative to my long-standing misinterpretation of the spec, since some of the people I've recommended them to (some of whom have in since installed them) had some degree of capacity-anxiety related to those numbers.

Aug 24, 2012 4:05 PM ET

dew point vs wet bulb
by David Butler

Marc wrote:
> So I don't believe people need to be concerned about dewpoint of the outdoor air when sizing the Mitsubishi Hyperheats.

Dew point and thermodynamic wet bulb temperatures are both non-relative indications of now much moisture is contained in the air. The HVAC industry standardized on wet bulb because it can be directly measured. On the other hand, dew point is calculated, not measured. But is commonly reported in climate archives. Online calculators are available to convert from one to the other, at a given dry bulb (typically assuming standard atmospheric pressure).

Since the outside coil is acting as an evaporator in heating mode, both wet bulb (dew point) and dry bulb affect capacity. I've always been puzzled why Mitsubishi only references wet bulb. This should not be construed to mean that outdoor dry bulb doesn't independently impact capacity, as your contact apparently believes. If you look at any expanded table for US heat pumps, they only reference dry bulb. Dakin's engineering books reference both wet bulb and dry bulb.

Aug 24, 2012 5:19 PM ET

But the result is the same, from a practical point of view
by Dana Dorsett

If Mitsubishi is saying that you'll get at least the full nominal capacity down to where the wet-bulb temp is +3F independent of how close it is to the dry bulb temp it's easy to spec a unit with some confidence based on only the nominal output rating in areas where outside design temps are in positive single-digits F. That covers most of US climate zone 5.

I suppose you'd need the graphs to spec one for sub-zero design temps though. But in a high-R/low-U houses you can undershoot by quite a bit for hours and never feel the chill, due to the longer thermal time constant.

Sep 6, 2012 4:23 PM ET

Problems with mini-splits
by Chris Baddorf

You mentioned that Carter Scott has satisfied customers to this point. when I spoke with my HVAC contractor about mini splits he mentioned that they have had a terrible time with sensors going out on the units at the rate of about 1+ per year. Efficiency and cost savings are nice but on going maintenance issues would quickly eliminate any gains. Anyone else experienced these issues?

Sep 7, 2012 10:54 AM ET

Edited Sep 7, 2012 10:56 AM ET.

Response to Chris Baddorf
by Martin Holladay

Anecdotal reports in New England (chiefly for Mitsubishi and Fujitsu units) are that installed units are performing well, with very few (if any) glitches or problems.

Sep 26, 2012 7:36 AM ET

Edited Sep 26, 2012 7:39 AM ET.

Late to the party
by Doug Snyder


This has been a long and fruitful conversation! I don't know that I can add much, except that Coldham Hartman architects and myself,, did a deep energy retrofit last year in Amherst, MA and also only installed one Fujitsu on the first floor. Temperature differentials in the bedrooms were negligible, according to Hobo recorders. So, DERs are viable also. Our clients were OK in the summer as well, but I can imagine some folks wouldn't go for it.

I'm in Vietnam now, and people deal with the heat a whole lot better than in the US. A lesson in human behavior for myself.

Principal, DS Greenbuild LLC

Sep 26, 2012 8:02 AM ET

Response to Doug Snyder
by Martin Holladay

Thanks very much for sharing your experience.

Another satisfied minisplit user without any bedroom heat -- the list of satisfied customers is growing.

Dec 31, 2012 2:00 PM ET

Mini Split installers
by Bob Lobsiger

I had a heck of a time getting realistic bids for doing Mini-Splits.
I finally used my neighbor but he only did it since I was a friend. Otherwise he won’t do mini-splits. When I contacted the Mitsubishi Diamond dealer, he gave me, IMHO, a ludicrously high quote and it wasn’t even what I asked for him to bid. He “knew better” than what I wanted and gave me a quote for a less efficient MXZ2A20NA outdoor unit and two MSZA12NA heads for 8,546.66 which included a discount since it was “off season” for an open stud, can’t get much easier install.
I asked for a quote for two individual 12,000 BTU Hyperheats. Not a HVAC guy but I work in the data center world where I deal with HVAC issues day in and day out. I knew what I wanted, had researched the heck out of the topic. Got the units I want and they work great.....

Jan 1, 2013 6:48 AM ET

Edited Jan 1, 2013 6:49 AM ET.

Response to Bob Lobsiger
by Martin Holladay

In some areas of the country, the installation of ductless minisplit units is still rare, and in these areas, there isn't enough competition yet for contractors to quote reasonable prices. Hopefully, as the installation of ductless minisplits becomes more common, this situation will change.

Apr 26, 2013 7:30 PM ET

Edited Apr 26, 2013 7:31 PM ET.

Robin Hood?
by Stephen Masek

If you use somebody to take something from others who do not want to give it, what does that make you? Fine Homebuilding makes Scott out to be some sort of hero, but he is most certainly not to those of us who do not want our tax money taken and given to him in the form of subsidies, all in the name of the "global warming" hoax. Note that Robin Hood took ill-gotten money from the government and give it back to the people, just the opposite of what those receiving subsidies are doing. Shame on them!

Dec 31, 2013 9:01 PM ET

Wow, I don't know where that last guy came from,
by Gordon Taylor

but I know that it's a strain obeying the "Be Nice" directive.

It's New Year's Eve 2013, we signed our contract yesterday, and we'll be breaking ground by the end of January. (You can do that in Zone 4C.) I've spent the last hour rereading all the posts on this thread, and it reminds me what a terrific asset GBA is. A big Thank You to all, and have the most prosperous and happiest of New Years!

Gordon Taylor
Willamette Valley

Jan 1, 2014 7:49 AM ET

Response to Gordon Taylor
by Martin Holladay

Thanks, Gordon -- and Happy New Year. Good luck with your construction project.

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