Image Credit: Alex Wilson
Image Credit: Alex Wilson So far we haven't had to do any snow clearing from the outdoor unit, but in a heavy snow we likely will have to. The blower seems to keep some of the snow around the unit melted. Close-up of the wall-mounted interior unit. The open stairwell does a great job at distributing heat upstairs. Our north bedroom stays between 68°F and 70°F. A summary of the energy consumption data for our house over the past week with this cold weather. The HRV had to work harder than it generally should, due to moisture that got into it before we hooked up the condensate drain.
Image Credit: eMonitor data from Alex Wilson
More Energy Solutions
It’s been pretty chilly outside, if you haven’t noticed. A number of people have asked me how our air-source heat pump is making out in the cold weather. I wrote about the system last fall, well before we had moved in. Is it keeping us warm? We’ve only been living in the house for a few weeks, but here’s a quick report.
So far, so good. Our 18,000 Btu/hour Mitsubishi mini-split heat pump (MSZ FE18NA indoor unit and MUZ FE18 outdoor unit) is doing remarkably well in keeping us comfortable. We don’t have any oil or gas heating in the house, only the electric heat pump and a small wood stove that we’ve fired up twice so far. The indoor heat pump unit is mounted on a wall next to our kitchen, and it’s been operating pretty steadily in this cold weather. (Even though we’ve heated with wood for decades and have all the wood we could ever use, I’ve been curious how the house will do just on electricity, so have refrained from using the wood stove.)
A thermometer in a bookcase on an outside wall diagonally across the kitchen-dining-living space from the heating unit is reading 66°F as I write this, with the outside temperature about 12°F. A thermometer in our upstairs bedroom read 70° when I got up this morning, and has typically been about 68° — and remarkably uniform.
When the mercury dropped to –6°F a few days ago, the house got colder. I saw one reading on the outside wall downstairs as low as 61°F and our bedroom got down to about 65°F. It was chilly enough that I fired up our small wood stove for the first time, and that fairly quickly raised the downstairs temperature to a comfortable 68°F. With our tight construction there are few drafts.
Monitoring our energy consumption
We have an eMonitor (made by PowerWise Systems of Blue Hill, Maine) installed to track the home’s overall electrical consumption as well as the consumption of a number of individual loads. The monitor has clips that clamp onto different circuits in the electrical panel as well as the electrical main coming into the panel, and it somehow measures electricity flow through those cables. We’re tracking consumption separately for our heat pump heating system, our heat-pump water heater, and our heat-recovery ventilator.
Most of the time the air-source heat pump has been drawing about 2,500 watts, with very brief spikes up to about 3,400 watts (I suppose those spikes occur when a pump or fan kicks on). To put this in perspective, the 2,500 watts in the standard heating mode is about twice what our KitchenAid toaster draws (1,200 watts), though of course the toaster operates for only short periods of time.
Since we hooked up the eMonitor and started collecting data (five days ago), our Mitsubishi heat pump has used 221 kWh of electricity — during a fairly cold stretch. This is about what the entire solar-electric system on our barn cranked out during this period — and roughly three times the output of that portion of our PV system allocated to the house. (It’s a “group-net-metered” system, with two-thirds of the output going to neighboring homes.)
It will be interesting to look at this data over the course of months and years to see how the electricity consumption averages out over time and how that compares to our solar production.
Heat distribution with point-source heating
Because our heat source is on a downstairs wall, I had been very curious how effectively heat would be distributed throughout our 1,600-square-foot house. The main kitchen-dining-living space keeps a fairly even temperature in the high-60s. A downstairs study or guest room at the far corner of the house and separated from the heat pump by a hallway and doors (with the door open) stays a little cooler, though watching a movie there last night was fine with a sweater.
Upstairs, the bedroom on the north side of the house has maintained a remarkably constant 68-70°F on all but the coldest nights. When the outside temperature dipped to -6°F, our bedroom dropped to the mid-60s. Last night, with the outside temperature down to 7.5°F, we actually closed our door to keep the bedroom a bit cooler, and the temperature dropped from 70°F to 67.8 by morning.
I don’t have a thermometer in the south bedroom, which is being used as a home office by my wife, but it feels about the same. There are two double-hung windows instead of a single casement window, so there is certainly more air leakage, but there is also solar gain through those windows.
The bottom line
All in all, we are very satisfied with the air-source heat pump. It works well, in large part because our house is so energy efficient. This is a superb heating option (and cooling, by the way) for a house with a very well-insulated building envelope. Once we install the low-e storm windows on the double-hung windows on the south and east sides of the house, we should do somewhat better. (With our superinsulated house, the south and east windows are a weak point, both relative to air leakage and R-value.)
And on a cost per delivered BTU basis, with the air-source heat pump we’re spending just 58% of what we would spend on oil heat (assuming an Energy Star oil boiler operating at 83% efficiency with #2 heating oil at $3.91 per gallon vs. electric heat in an air-source heat pump with a coefficient of performance of 2.25 and electricity costing 15¢ per kWh). (You can plug in your own assumptions and compare fuels on BuildingGreen’s online calculator.)
Plus, on an annual basis we should be producing as much electricity with solar as we consume — net-zero-energy. So we’re pretty happy. Warm and happy.
Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. In 2012 he founded the Resilient Design Institute. To keep up with Alex’s latest articles and musings, you can sign up for his Twitter feed.
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30 Comments
Power/energy monitoring
Based on Mitsubishi's specs for this unit, the power input at "maximum" compressor speed and 17F outdoor temperature is 2,620W with heat output of 21,600Btu/hr (COP = 2.4)....pretty close to the power you've been measuring. The units are able to maintain that maximum output of 21,600Btu/hr down to about 5F, but it takes more power input to do so…probably explains why you're seeing the spikes over 3,000W (unfortunately, Mitsubishi doesn't list power input at max. compressor and 5F). COP probably drops to the neighborhood of 2.0 at 5F and maximum compressor speed, but I don't have any specs to back that up.
Below 5F, both the power input and heat output will drop off, as will the COP. If the system was running near 100% capacity at around 10F to keep the house at ~68F, it's no surprise that the indoor temperature dropped off when it was -6F…and you'll certainly need to fire up that wood stove once it gets really cold up there!
Clearly, the variable speed heat pumps don't shine when they're running near 100% capacity. Their COP's at 100% capacity are about the same as regular air-source heat pumps…sometimes a little lower! They ARE at their best when they run at medium or low compressor speeds…so once the temperatures warm up slightly, I expect you'll see much lower power inputs and energy use. The overall seasonal COP should be still be quite good, even if it's not-so-good in January.
Melted snow
Interesting to see the snow melting around your outdoor unit. Could this not be from heat pushed out of the unit during it's defrost cycle rather than from simple airflow (which should be colder than ambient air after passing through)?
About the melted snow
Devan, I was wondering the same thing.
Defrost cycles...
When the thing defrosts the outgoing airstream isn't carrying away a huge amount of heat, but it's fairly moisture-saturated air which condenses on the very-cold snow and super-cool soil. Yes, the ice is due to the defrost cycles, but it may not be predominately snow-melt. There has been a lot of single-digit daily highs in VT recently, and even on warmer days the dew points haven't climbed very high, so a lot of that defrost had to have occurred during outdoor temps below +10F.
Under those conditions much of that ice would probably be re-frozen defrost moisture. Even though the heat of fusion is ~7x that of the heat of vaporization, the ground and snowpack temps have been WAY below freezing, and both have some thermal mass to heat up before giving up heat-of-fusion to the condensing moisture in the output air. It would be curious to observe a defrost cycle closely during sub-zero temps, to gauge how far away from the unit the fog forms.
During warmer weather and higher dew points the number of defrost cycles increases, and if it had been hanging in the high-20 in VT for the past month it would likely be predominantly snow-melt.
At least that's my take- haven't measured it, just a WAG.
OTOH...
....Dummerston VT DID get 10 days of "April in January" from the 11th through the 20th, where it didn't even hit freezing every night, both preceded and followed by cooler-than-normal outdoor temps. It could be that ALL of that is snow melt, melting that occurred primarily during the unseasonably warm stretch:
http://weatherspark.com/#!dashboard;a=USA/VT/Dummerston
BTW: The Mitsubish M-series "FH" mini-splits have been released.
Coming to a Mitsubishi installer near you (any second now!):
http://www.mitsubishipro.com/media/946493/fh_product_guide.pdf
These are an evolution of the ubiquitous & popular -FExxNA series, but seem keenly targeted at Fujitsu's -XLTH cool weather mini-splits, with comparable model-by-model output numbers, but with a quantum step higher HSPF efficiency over both the XLTH and -FExxNA
http://smartgreenbuild.com/pdf/Fujitsu-RLS2H.pdf
http://www.mehvac.com/UploadedFiles/Resource/MSZ-FE12NA~MUZ-FE12NA_Submittal.pdf
The 1-ton -FH has a rated HSPF of 12.5, to the XLTH 1-ton @ 9.3, and the -FE12NA @ 10.6. That's a pretty hefty step up- about 18% more heat per kwh out of the -FH than the -FE at the standard test conditions, and probably similar improvement over the whole arc of operating conditions (TBD.)
the other leading brand
For perspective, I've also got a 1.5 ton system in the Daikin,
FTQ18PAVJU indoor mated with a RZQ18PVJU9 outdoor unit. This
is the ducted system; the ducts going to the second floor are
on the wimpy side but as it's all work space I'm happy to let
it run the 3 - 4 degrees cooler. 1150 sf plus ~600 basement
that also runs up to 10F cooler than upstairs. Setpoint 68
for an average above-grade interior temp of 64 or 65, sort of
deliberately because that makes it easier to track degree-day
records and it's perfectly comfy for me.
This system has also been perfectly happy on the 5F or so
overnights, albeit running close to 100% of the time. It's been
drawing 7 - 8 amps or on the order of 1800 watts pretty steadily
when running, and I'm guessing that the normal three-plus COP
starts to fall off below 15F or thereabouts but it doesn't
seem to change the compressor speed on heating mode at all.
It's less important to do so than when cooling, of course.
Instantaneous current comes from eyeballing an old analog
amp-clamp on the subpanel feed, and overall HVAC energy use
read from the extra meter feeding it. During the real cold
snap the system chewed down around 25 kwh/day on average to make
somewhere north of 200,000 btu / day, plus there's a generalized
aggregate of plug-loads, "wheaties watts" [me] and minor HRV
losses adding another 35,000 btu/day or so.
It has a backup resistance heater but that's been disabled
pretty much the whole time. I used it as a little boost when
I returned to a 53-deg setback in the house after an event
and it got wikkid-cold right afterward, because the temp in
the place was coming up v-e-r-y slowly otherwise fighting
both the envelope loss and interior heat capacity at the
same time.
Between COP falloff and the possible "polyiso penalty" around
5F and below recently discussed in another thread, there's likely
sort of a negative inflection point down around those temps --
where much lower and the system would probably have a hard time
keeping up. Fortunately we rarely get long periods of that
sort of weather around here.
_H*
About the "melted snow"
The "melted snow," with a path cleared around the front of the outdoor unit may not be melted at all.
Can't say for sure at Alex;s place, but I have a nice clear path in front of my minisplit outdoor unit, also, and I know for sure it's not due to any melting.
Several inches of very light weight snow fell around my unit while it was only about 5-10 degrees outside the whole time. Because it's so cold, the outdoor fan runs pretty hard and keeps the light weight snow blown away from the unit. I think it is just that simple; no melting, unless you guys are talking about some totally different phenomenon.
question for Dana
Quoting from comment #4, "Even though the heat of fusion is ~7x that of the heat of vaporization...". Dana, I'm wondering if you inadvertently reversed the relation of these two values in your comment. The heat of fusion for water is 334 kJ/kg (80 cal/g), and the heat of vaporization is 2260 kJ/kg (540 cal/g), rounded to the nearest whole number. So I'm thinking that the heat of vaporization is ~7x that of fusion, rather than the reverse.
I know that you understand this stuff backwards and forwards. If I'm not being too forward, I'd like to suggest that you might have gotten it backward in comment #4. Or maybe it's me that is getting it backward. Can you clarify?
Alex, thanks for the
Alex, thanks for the info…questions,
1. Why did you go with Mitsubishi?
2. What solar system? Do you see much sun there?
3. What HRV system?
4. Why not multi-zone?
5. When do you expect payback for upfront investment in the envelope and systems?
6. What water heater system and how is it working?
7. What wall and roof system?
8. How about a pic of the barn set up?
heat pump report--request for more context
Hi Alex--
thanks for this report; I'm glad you are fairing well in these vortextual times. To better understand how your heat pump is performing, it would be helpful to know a little more about your home's envelope--including R-values of walls, roof, and floor, and U-values of windows.
Also, to learn about any modeling you did, before sizing the heatpump.
Thanks in advance for considering these questions!
Alan Abrams
Silver Spring, MD
Our building envelope and answers to Terry Lee's questions
Alan,
We have a pretty well-insulated envelope: about R-45 walls and R-55 roof. Windows on the west and north are very good: quad-glazed Alpen windows with three low-e coatings and krypton gas fill (NFRC unit U-factor of about 0.12), while the windows on the east and south are (currently) pretty mediocre: low-e argon wood double-hung windows; once we install low-e storm windows on the east and south, performance of those windows will be closer to that of good triple-glazed windows.
The house is about 1,600 square feet in southeastern Vermont (HDD about 7,000 I think). Marc Rosenbaum did energy modeling for me, coming up with a heat load at the winter design temperature of about 20,000 Btu/hour.
Terry,
We went with a Mitsubishi because friends of mine at the engineering firm Kohler & Lewis in Keene, NH have had good luck with this system for residential applications. I didn't think we needed a multizone system and still feel that's the case. I've written about our solar system previously; you can find that by looking through older blogs--we have 12 kW in a "group-net-metered" array on our barn, and half of that is allocated to a different account, so right now we have 6 kW allocated to this house. We have a Zehnder HRV that I'll be writing about soon. I'll also write about our GE Geospring heat-pump water heater. I didn't calculate a payback for the house (or garage, for that matter), but it's worth a lot to us to know that we have a house that will require relatively little energy to operate (HERS score 22) and that we will be fine using our wood stove if the grid goes down.
I'm confused!
I been reading the debate with Joe L, etc, on ventilating tight homes, etc, mixtures, rates, owner control vs system sensors and RH, blogs, etc....Love the fact Alex you have an actual house and are nice enough to show how you validated the models, thanks!. What model did Marc use REM? Sounds like it did well based on what we are seeing other than it did not model air distribution (hot & cold spots, etc)?
Alex, I figured you are trying to get to net zero with single zone and a wood burner, how much more electrical load for another air handler? I seen some other minisplits in tight well insulated homes popping up in Vermont home owners pleased, but the cost of wood is not always cheap or available.
PS: I'm wondering if this minisplit, envelope, net zero can find it's way into a home builders product line thats why I asked if you did a ROI check. Also, it the hot and cold spots may be a hard sell?
So now that you have a sanity check how do you feel about all the theory on air ventilation and IAQ?
Is there a simple guide to size these units for even distribution doors closed? (I see the OEMs have some design guides not sure I trust them?
Do you have a balance HRV in all rooms or single point? Would a balanced system that intakes/exhaust each room help distribute cold/hot air better vs adding more minisplit air handlers? I guess that may depend on the rate?
Seems like you are taking care of load just not distribution, and it may get pretty hot upstairs in summer especially if you're further south in hot/humid outdoor air climates?
Location
Is that the front door entrance in the picture? If so, why did you decide to put the outdoor compressor next to the front entrance like that?
Answer for Derek Roff (comment #9)
Thanks for the catch! (I'll chalk it up to late in the afternoon under-caffeinated fuzz on the brain! :-) )
Sometimes I need an editor...
Front entrance?
Peter L,
No, that's the back porch. By summer it will be screened in, so from inside the porch the outdoor compressor unit will barely be visible. In my previous post about our system (link in the blog above), there were a lot of comments about the position of the compressor unit and whether snow would be a problem.
Not ready to join the mini-split bandwagon
I'm having some trouble with the physics of these ductless mini-split solutions for whole-house heating and cooling. The biggest problem I have is the idea that a few indoor registers of a ductless system, without some other means of air circulation, can evenly heat or cool a 'typical' home.
1. As a replacement for a typical ducted AC+Furnace or heat pump+blower setup in a typical, moderately-insulated house built in the last 20 or so years, I can't get past some problems. Take my house for example. 22yo, probably R-15 walls (2x6construction), R-49 attic, air sealed to slightly above what would begin to require mechanical ventilation, half-finished basement with only ceiling insulation, Kansas City, MO climate, 2-stories with an open stairwell, half-vaulted greatroom, 4 bedrooms upstairs (3 above a garage), SE-facing, some 2500+sf, new dual-pane coated argon-filled windows.
This home, as would be expected, has big temperature differentials between upstairs and downstairs in summer and winter, and is obviously a challenge given the 2-story configuration. I'd love a high-efficiency ductless system to solve my problems, but I don't think it would. How is my upstairs going to be managed without a cassette in every room? Put it in the hall and all the air will just flow down the hall and down the stairwell. Keep the doors closed and none of the bedrooms or bathrooms get much.
Likewise, put a cassette in the fairly-open downstairs, and the North vs. South facing sides may vary in temp, though even temperature is much more likely down there. And then there's the issue of heating and cooling the finished part of the basement. I'm afraid to make it work I'd be talking about 6+ cassettes; rather expensive.
2. I know most of you talk about using the mini-splits in a high efficiency house, so super-insulated, super-tight. Keep it small, few bedrooms, open floor plan, ranch configuration, and I can see managing it with a couple of cassettes. A more "average" sized home these days (1800-2800sf) in a climate zone that gets both extremes and lots of humidity (midwest), and might not be a ranch, and I'm not sure you could get away with only a couple cassettes on even a super-insulated, super-tight house.
Does a mini-split system rely on mechanical ventilation (HRV/ERV), fans, and open doors to help keep the air in the house circulated well enough for even heating and cooling? Does it need to have the very low outdoor heat transfer of a super-insulated, super-tight house to forego the ducts?
Otherwise, as much as I dislike ducts and forced air systems, it seems that a well-designed zoned system would be best at managing loads throughout the home, where you need it, when you need it.
Can you just hook a high-efficiency mini-split outdoor unit like what's used in the ductless configurations to a zoned duct system for better balance and control, and forego the expensive and limited-location cassettes?
I am thinking in a retrofit scenario like mine, with an "average" house especially, that would be the way to go. And I'd love it if I could have one unit supply the could air from ceiling ducts in the upstairs during the summer, and let the cold air fall downstairs to cool, and vice versa with heat in the winter...
Critique me?
Drain Pan Heater
The power spikes are most likely a drain pan heater (resistance) kicking on. Just got to see four of these go in a project and they are very impressive. I'll be watching them for the next year. Apparent the heater is optional but necessary for running at low temperatures.
Sublimination?
With it being cold and dry already, then the air dried even more when going over the coil - result in the perfect conditions for sublimation of the snow?
Alex, you are on the home stretch now... longer, warmer days ahead. Perhaps two months away before netZero over the period of a month?
Response to Stuart Millar
Stuart,
You are absolutely correct that a mini-split system with a single outside unit paired with a single inside unit isn't the right solution for a typical house--particularly a larger house that is not well insulated. For that application, you need either multiple indoor units, or ducting to distribute heat from a single (larger) indoor unit. I believe this will be the primary growth area for companies like Mitsubishi--larger systems that are better adapted to houses like yours. Over the next year we can expect to see the product lines expanded to air-source heat pumps able to deliver up to perhaps 50,000 Btu/hour; some will be able to directly replace gas or oil furnaces, integrating to existing forced-air distribution systems.
Follow up to Alex Wilson
Thanks, Alex
If you're confident about better, larger, whole-house duct-based heat pumps coming out this year, I may wait for that. I've been getting varied and conflicting advice from HVAC contractors. My furnace is 22 years old, so I'm trying to be ready to replace it at any time.
Regarding the mini-splits, even in a new super-insulated house, how does the air get distributed and balanced if you've got 2 stories, closed doors, and >1800sf? Particularly, how would you handle an upstairs (bc of closed bedroom doors and a hallway that will spill down a stairwell in any 2-story house)?
Also, what are peoples' opinions of zoning as a solution to potentially handle the distribution problem (ducts, of course)? Is zoning a bad word around here? In theory, it makes sense to me--you even can focus the conditioned air to the areas you are using and leave the rest of the house less controlled. Like a mini-split with many cassettes, but it seems zoning ducts is cheaper than buying and installing a bunch of cassettes.
Response to Stuart Miller
Stuart,
Many people with 2-story homes have installed one or two ductless minisplits upstairs, and one or two downstairs. During the winter, the downstairs unit(s) do most of the work, while in the summer, the upstairs unit(s) take over. As far as I know, this approach results in fairly even temperatures -- as long as bedroom doors are open during the day.
Reply to Martin
Martin, the air distribution works even if you have to place that upstairs register in a hall that probably ends up pouring most of the conditioned air down the stairwell? And in the winter, with heat radiating up, how does it warm the bedrooms at night when everyone's sleeping with the doors closed?
If mini-split room cassettes were cheap and easily-concealable, popping them in everywhere would be obvious. But it seems that they are expensive compared to even zoned ductwork and usually large and visible. I'm just not seeing proper air flow and air distribution in any non-open environment without lots of cassettes.
But I've never studied them in the real world, either. Just thinking through the physics of it. Would ERVs/HRVs fix the circulation, in a well-enough insulated house that exterior heat transfer in indirectly-vented rooms is slow? (I thought ERVs/HRVs normally exhaust from Kitchens and Bathrooms, as Lstiburek has talked about, which wouldn't help the bedroom problem much, particularly upstairs bedrooms in summer/winter)
Response to Stuart Miller
Stuart,
It's probable that ductless minisplits aren't appropriate for your house. If a homeowner wants perfectly even temperatures from room to room, even when the bedroom doors are closed, you need a conventional heating and cooling system that distributes heat and cool air to each room.
Other homeowners like minisplits. Obviously, the indoor unit has to be located carefully. It wouldn't make much sense to install an indoor unit at the top of the stairs and aim the airflow down the stairs. It would make more sense to aim the airflow away from the top of the stairs.
Sensibly located, these systems work for homeowners who leave their bedroom doors open during the day. If there is at least one upstairs unit and one downstairs unit, the upstairs unit will keep the second floor cool during the summer, and the downstairs unit will keep the first floor warm during the winter. Bedrooms with closed doors are a whole 'nuther issue.
On closing bedroom doors
I agree with Martin. I typically recommend minisplits for compact, well-insulated, and fairly airtight homes, not large, leaky homes. Interestingly, my wife and I like to sleep in a cooler room and we find ourselves closing our bedroom door to keep it somewhat cooler. With the door open on nights when it drops to say 10°F our bedroom will stay between 68 and 70°F. If we close the door it may drop to 67°F. Last night, with the outside temperature dropping to -3.1°F our bedroom dropped to 65.8°F with the door closed. Again, our sole heater is a minisplit on the first floor.
Thanks!
Thanks for the replies!
Report on ductless minsplists
Great discussion, Thanks very much for reporting on this Alex. It would be very helpful to know the modelled design heat loss (and design temperature) for your building envelope.
I have an energy star home
I have an energy star home with a City Multi 66,000 BTU system with 8 air handers and I also have a 20,000 Daikin Heat pump in the basement. I heat the basement ot 71 degrees and basically keep the house at about 72-74 during the day. I get 105 degrees off the system at 0 degrees. I am in CT and this winter so far i have spent about $550.00 for heat. I have ahouse in Nh with City multi and my sister spends about 700 a season on heat for the little cape.
Set point
What is the set point of your FE18 (are you setting it at 66, 68, 70F?) when it can't keep up at -6F?
What is your calculated heat loss at -6F?
Minisplit set point
Patrick,
With our cold Vermont winter, I've kept my set point of our FE18 at 75°F. I think if I got the remote thermostat (rather than use the one built into the wall-mounted indoor unit) I would keep the set point lower. It's keeping up very well--with the outside temperatures at 0°F last night, a thermometer in a bookcase at the other corner of the house from the minisplit got as low as 66, and our upstairs bedroom dropped to 69°F. I'm working upstairs right now without a sweater, just a shirt. We have just the one indoor unit, installed on the first floor. As the outside temperature warms I'll probably lower the set point.
At one point, the output dropped significantly, and I discovered (to my great embarrassment) that I hadn't been cleaning the filters. Once vacuumed and washed off, the performance was better than ever. So keep the filters vacuumed off!
I'm not so worried about using all that electricity to keep the place on the warm side, as we're running a significant surplus with of solar system. Once we get a plug-in hybrid (next purchase--probably a Volt) I may have to watch our household electrical consumption more closely.
Multi Splits for "average" houses
I see that Mitsubishi now has also extended its Hyper Heat technology (see Dana Dorsett's comment #5 above) to multiple head units . These are the "MZX C" models (such as MXZ-4C 36 NAH). These seem to have very high efficiency at low temperatures (down to -13F like the single head FH models Dana described above), but in multi head configurations. Has anyone tesed these yet in the real world? This seems to be just what those of us with average houses who like to keep our doors shut have been asking for.
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