The Furnace-free House in Vermont
This document contains a three-way conversation between Donella
(Dana) Meadows (the client), Marc Rosenbaum, PE (the ecological
design/energy consultant), and Amory Lovins (world famous energy
expert). It began when Amory told Donella that it was possible to
build a passive solar home in the Vermont climate which required
no heating system or back-up heat. Marc was very skeptical, and
went on to do computer modeling to verify this claim. The actual
project is a 22 unit cohousing/organic farm/institute being created
in Hartland, Vermont. Amory and Dana have been very generous to
share this unedited private conversation with a larger audience.
Marc responds to Amory's analysis
Dana comments and Amory responds to the difficulty of bringing the ideal to fruition in practice
Marc reports back after further modeling
Amory responds to further modeling
10 July 1998
Hi Dana -
Here is the write-up I promised you on the energy use modeling I did in response to Amory's assertion that the houses shouldn't need heat.
I assumed a 1344 ft2 house, one story (this is a standard unit I have already made up for modeling), R-40 walls, R-70 ceiling, R-40 floor over a basement, very tight construction. 200 kWh monthly internal gains from people, appliances, lighting. 68F thermostat setpoint. Burlington VT weather data file.
Thermal mass 350 ft2 of 4 inch thick concrete floor, plus 500 ft2 of 4 inch thick concrete wall exposed on both sides to the space (interior walls). Net glazing 80% of gross rough opening. 225 ft2 south glazing, 25 ft2 on north, east, and west, each. Triple layer glazing with two low-e layers with high solar gain low-e, argon fill. R overall of windows just about 5, glass-only R-6.
Software used - old program called Suncode (comes from SERI-RES) and new program called Energy-10. Both hourly simulations. Very good agreement, surprisingly, within three percent!
Annual back-up heat required = 6.6 MMBTU
With setpoint of 60F = 3.7 MMBTU
In Eagle, CO = 1.4 MMBTU
In Denver, CO = 0.80 MMBTU
6.6 MMBTU is very low, just under half cord of wood. But you still need something to provide it! A woodstove with chimney costs real money, it's not free. And it doesn't make hot water.
BTW - I played with adding more mass, more glass, changing other parameters - very little impact. the issue is that our weather has occasional long cold cloudy spells, in which passive solar doesn't carry the load.
Hi Amory,
Here's a message from our engineer Marc Rosenbaum, who is really struggling with the challenge of designing a heating-system-free community for us -- in order to save the expense of chimneys, stoves, or heat-distribution systems. He just doesn't think it can be done in our climate, which is perhaps not quite as cold as Snowmass, but often more cloudy. The structure he's modeling is not exactly what we're going to build --most of our individual units will be smaller (though possibly joined in duplex or triplex) and the common house will be considerably larger.
We'd love to add active solar hot water if we can afford it, which could, if we had a big enough, well-enough insulated tank, provide space heat backup, but a) we have the same problem of cloudy days, so we're thinking of wood furnaces to back THAT up and b) we might not be able to afford active solar. We keep getting into infinite regressions of backups, all of which add to the capital cost.
Do you or do any of the bright people at RMI have some suggestions?
30 July 1998
To: Donella Meadows
From: Amory Lovins
Subject: Re: the furnace-free home
Energy10 is an excellent model (I'd forget the other one) and Marc is doing most of the right things. We just need to tune up some inputs. I could tell more if I had elimination parametrics, but here's my intuition based on what you've told me:
First thing, make all the glazings at least R-8.1 center-of-glass (like Hurd Insol-8), and preferably R-11 or -12. That would typically mean three selective films -- either a double-sided Heat Mirror 88 or two separate single-sided HM88 films, plus (say) Cardinal low-E softcoat on one lite (try inner and outer separately to see what's best), all filled with krypton with optimal (around 10-12 mm as I recall) spacings. If necessary, go to wide sashes like Pozzis. Be sure to use krypton, which insulates twice as well as air, not argon, which is only 1/3 better than air and has a wider optimal gap. Extra cost is zero to negligible, because you use not 99.999% lab-grade krypton but only first-distillate-stage Kr (around 60-70% Kr, rest Ar, indistinguishable thermal performance -- same Kr as goes in those lousy "watt-saver" fluorescent tubes). In your climate, R-7 is roughly the threshold above which north glass provides a net winter gain. With such good glazings, you could also consider using a bit more glass. You now have 22% glass-to-floor-area ratio. Ours is 26%. Normal for nonpassive houses is about 10-12%. A bit more gain than 22% wouldn't hurt if you've paid proper attention to glare management and luminance ratios.
Next step, use Wolfgang Feist's trick for eliminating edge losses so the entire glazing assembly has the same R-value as the center of glass. You do this by adding about a half-inch layer of foam, dressed with a plastic or textile cover so it looks nice, that overlaps the entire window frame and goes about 1-1.5"onto the glass all around. You lose a little aperture that way, but it looks good and has terrific thermal advantages. (Also eliminates any leftover frame infiltration.) You do it at least inside and preferably also outside. You can design it to be removable in summer on operable units. Next, make sure you're using a good air-to-air heat exchanger. We use Carrier's Venmar (Quebec) condensing polypropylene-core crossflow units, but counterflow would be even better, and we want to retrofit the Venmars from constant to variable speed. In any event, spec several fold oversized (i.e. right-sized) surface and slow, efficient fans at variable speed, ideally with CO2-sensor control, so you have a very low face velocity, and you can readily get efficiencies well up in the 90s of percent. If necessary, use the further Feist trick of tempering the outside air up to at least 50deg.F by bringing in through an earthtube (suitably sloped, drained, screened, etc. to avoid fungus, rodents, etc. -- standard drill for earthtubes).
All this works fine in cloudy, further-north Darmstadt, where the latest Feist houses about the same size need no heating because of the tempered a-a hx (CO-2 controlled VAV) and the edge-covered glazings.
I can't be sure they don't use more mass -- German construction is typically masonry -- but Marc might want to try double drywall or ceramic-tile floor in some of the interior spaces if the modeling suggests a bit more mass would be helpful. (Check also be sure the mass is illuminated by direct beam radiation, or other suitable provision is made for zone coupling.) He should also bear in mind that the more mass he can trap inside the insulation, the more even the radiant temperature will be. Our house, for example, has a 3- to 6-month lag between the outside air temperature and the inside radiant temperature. Since mean radiant temperature is half your perception of warmth (indoor drybulb air temperature is the rest), this helps a lot to keep you feeling cooler in the summer and warmer in the winter.
In short, with these tune-ups, and with proper consideration of radiant as well as air temperature in the space, passive really will carry the load even in Burlington, But finally, if you do need any residual heating, or want a control margin, for those long cold/cloudy spells, just run a radiant coil in the slab and heat it if needed with a hydronic loop from the water heater, which you'll be needing and paying for anyway. You might like to consider a simple French Aquastar (the U.S. distributor is in Vermont and is excellent) for both those functions, run on bottled gas or biogas to taste. (We cast such radiant coils into our floor slabs, but never needed to hook them up.)
Of course, there's a lot to be said for a miniature (watermelon-sized or less) woodstove anyhow, run through an outside wall, because you have to burn the junk mail, bad poetry, incriminating correspondence, and energy studies somewhere; but if you don't want one and prefer to shred and recycle your cellulose, that's fine too. A 50-watt dog, adjustable to 100 W if you throw her ball, also works fine.
If Marc still has trouble getting to the goal, let me know and I'll ask Greg Franta to look at the Energy-10 runs and talk directly to Marc about further fine-tuning; but you're most of the way there. Lots of small further improvements should get you all the way. Cheers -- ABL
31 July 98
From: Marc Rosenbaum
To: Donella Meadows
Thanks, Dana. I'll check this out - I'm darn skeptical - Amory didn't read carefully enough to appreciate how much mass I did include. I can assume all the inputs he described, but can you really get them built? The R values I assumed are huge. For instance, the glazing gets that high R value by coming in large sheets - affects the look of the house a lot - if you want small operable windows, harder to get the R value, cuz the sash/frame isn't as high R as the glass. Do people want to add/subtract foam edges to their windows?
I'll run it with his input assumptions. I don't like earth tubes because it is clear that condensation will occur in them - which means mold to me.
And his solution for minimal back-up heat, which I agree with, is to use the same device as heats DHW. You'll be much better off with a single high efficiency gas boiler than 6 gas water heaters - fewer flues, cheaper, less gas. But you don't want to use gas! You still need DHW, which load is larger than heat in super-efficient homes, and can't be all -solar year-round.
I feel that to go further I need a go-ahead from the design committee, cuz my work is on hold. I also feel that they should authorize this work so that they have a clear sense of the limitations on the design imposed by these at-the-edge strategies. I'll copy this to Don and Ann.
Marc
5) Dana to Balaton listserv, with Amory's comments in reply
>From: Amory B. Lovins
>Subject: Re: energy efficiency
>I need to add to the frustration expressed by John de Graaf
about the tension
>I feel between Amory's wonderful visionary faith in an energy
efficient future,
>and my own perception of the world around me. Simultaneously
I'd like to give
>a partial response to Betsy's question about what really creates
change --
>with a humble personal example of what gets in the way of change.
>
>I AM convinced. I have worshiped at Amory's temple for decades.
And every
>time I have to make a practical decision about my own energy-consumption
>capital, I find myself stopped not by my obsolete mindset, but
by the lack of
>options available in the market to the ordinary, non-genius,
busy,
>finance-constrained householder.
True. As put-upon early adopters, we all need to try to bust these
barriers by being informed and insistent. (No criticism of Dana
intended or implied -- it really *is* a hassle, and she can save
a lot more energy by doing her daily work than by optimizing her
household!)
>For example -- when I renovated two rooms in my house, ripping
out all the
>windows, I could not find a supplier of superwindows -- so I
didn't put them
>in, though I was willing to.
Local contractors often don't know, but there are nationally distributed
brands such as Hurd Insol-8, and they've captured over 10% of the
national market for insulated glass units (over 50% if you count
one-coating low-E glass with argon fill). RMI's outreach tool kit
includes a fact sheet on superwindows, a book called *Homemade Money:
Saving Energy and Dollars in Your Home* that advises on what to
do in what order, etc..; and we maintain staff to answer exactly
such questions (970/927-3851, outreach@rmi.org, www.rmi.org).
>Another example -- the Sears repairman just pronounced my 20-year-old
electric
>stove terminal. First I don't think stoves should be tossed
out after only 20
>years of service, but if I can't find someone to fix it, or
even the parts to
>do so, what can I do?
Not much, alas, other than gripe to Sears and go elsewhere, or trying (depending on what's broken) to get a local non-Sears handyperson -- who has no interest in selling you a new Sears stove -- make or scrounge parts and fix it. Isn't that what rural New England is all about?
> Second, what should I replace it with? What, in a
>sustainable energy future, will we COOK with? I guess I should
clearly abandon
>electricity as an obviously inefficient option.
Not necessarily. It can be very efficient with the right pots, superinsulated ovens, induction cooktops, microwave ovens, etc.. We've published a lot on this over the years.
>Should I go with natural gas?
Good idea if you have it; bottled gas if not, again using efficient pots, kettles, etc.. Again, a good Q for our publications and Outreach staff.
>(We're way too small an operation for biogas, and I don't know
where to find a
>biogas digester anyway.)
An ordinary septic tank may serve meanwhile. Richard Pinkham may know if digesters, which are common in some European and Asian markets, are on the market here as packaged units.
> When I ask suppliers of any kind of stove about
>energy efficiency, I get a complete non-response. They don't
know what I'm
>talking about -- nor do I, really. The only place I can see
efficiency coming
>in would be oven insulation.
There's LOTS more, as Jnilrgen Nnilrgard has long shown -- several fold to many fold savings are available. However, cooking is probably the most backward area of commercializing household energy efficiency; most of the great stuff he and others have developed isn't on the market or is hard to get. (This is also called a business opportunity.) I intend to pursue this with a big whiteware firm that wants to hire RMI on strategic matters.
>My biggest example is the 22-unit eco-village we're planning.
I would love to
>make it state-of-the-art, Amory-vision, no moving parts, no
pipes or wires in
>or out. But when I ask anyone to take PRACTICAL responsibility
for designing
>it that way, even people who are totally convinced of the wisdom
of doing so, I
>find myself being talked into compromises, either because of
non-affordability
>or plain non-availability of the most sustainable choices. It
looks like we
>will NOT put in state-of-the-art passive solar heating, because
it costs way,
>way more than the combination of slightly leakier houses and
wood heat
>(especially because the wood grows on our own land).
Dana, please let me know offline the result of my last communication to you for your engineer. I don't believe the answer, partly based on our own experience here. Someone is making things unnecessarily complicated, methinks.
>It looks like we will be
>talked into gas cooking and probably even gas water heating,
though I have said
>as a mandate: No Fossil Fuels.
This isn't such a bad result, and as reversible-fuel-cell/photovoltaic systems become cheap commodities, you can switch from CH4 to H2.
>Solar hot water systems we can find suppliers
>for, but they need a backup in winter
Ours uses the Vermont-based French Aquastar demand heater to back up the last 1% -- a good deal with or without solar. You too could get to 99% solar (100% in some years) with the sort of quasi-seasonal storage we use -- the normal storage optimization is wrong. By the way, a really good passive downpumper tuned to cloudy climates, caught in an undeserved small-business collapse a few years ago, will go into the public domain next year and could be nicely made by any sheet-metal shop in your area if you can wait until then.
>and we can't afford both the solar and
>the backup, especially since the backup itself will actually
do the job. We
>will probably have PVC pipes, though I have declared: No PVC.
The alternatives
>are too expensive.
Let's talk offline.
>It's really frustrating!
Yup -- even more so to those of us who know what should be at the corner store but isn't yet. Chicken-and-egg -- it's not there because you didn't all demand it yet, and you couldn't demand it because it wasn't there yet.
> Maybe people like those at RMI who spend their entire
>lives learning about these things and who get grants to pay
for them, can cut
>their throughput by 2 or 4 or 10. But I would like to demonstrate
what regular
>folks, who are convinced, but who have to spend most of their
time doing other
>things, who are willing to do some research, but not days and
days of it, and
>who have to finance their purchases out of ordinary American
incomes (average
>$31,000/year, among the world's highest, of course), can do.
Sadly, I'm
>concluding they can't do a whole lot.
They can do 2-4x using *Homemade Money*, more if they become intensely involved and really insist on tracking down the best stuff. (We saved 99% on space and water heating, 90% on household electricity, with 1983 state-of-the-art -- all available if you knew where to ask. It all paid back in 10 months. Today's technologies are a lot better and nearly all are more readily available than they were then.)
>We surely need Amory's soaring vision to lead the way. But,
remembering Alan's
>innovation-diffusion game, I'm detecting a weak link in bringing
the vision
>down to practical implementation so that the very convinceable
masses can
>actually practice it.
Absolutely. It's a big opportunity for entrepreneurs willing to put up with the building industry, whose basic unit of production is the pickup truck and which actively resists innovation.
>I have a feeling this observation is relevant to the task we
take up in our
>meeting in two weeks -- make a CONVINCING scenario of a sustainable
energy
>future. I'm convinced that somewhere in the world there are
supercars and fuel
>cells and solar hydrogen generators and rooftop PV systems and
constructed
>wetland sewage treatment systems and passive solar houses in
climates as cold
>and cloudy as mine that need no backup heat. But, as a modest-income
American
>with no mechanical ability and no time to spend becoming an
expert, I can't
>seem to incorporate them into my own life.
Yes, all this stuff does exist (except Hypercars which will very soon), but almost all comes through unconventional distribution channels that your local builders may never have heard of. If you go to appropriate-technology, funkier kinds of designs it's even easier because any handy and open-minded builder can create them from scratch -- but maybe that's not easier if that's not the kind of builder you have.
>Help!!
Patience, courage, nulle bastardo carborundum -- we'll help you get there, one small victory at a time. The problems you describe so eloquently are real. The solutions are also real and can be obtained through persistent manifestation of your correct design intention. Just keep lending "the small stubborn ounces of your weight" and we'll all get that balance tipped.
Love -- Amory
9/14/98
To: Donella Meadows and Amory Lovins
From: Marc Rosenbaum
Subject: The zero-heating energy house in Vermont
cc: Greg Franta
I've taken most of Amory's suggestions and incorporated them into my Energy-10 modeling to further test Amory's claim that zero heating energy houses can be built using only passive gain in Vermont. The enclosed Excel 4.0 spreadsheet shows the results and the input assumptions. You'll notice the windows have center-of-glass R values of about 11, and the overall R is just under 8. You'll notice that the air leakage is lower than any house I know - 5 CFM - and that the heat recovery ventilator is set at 90% efficiency.
Using the current TMY2 data, you will notice that E-10 doesn't think zero heating energy can be achieved, for a variety of mass levels and south glazing areas. As I have stated, it is clear that you can get close, but close doesn't buy you no heating input. And the house as modeled is almost impossible to build - incredibly airtight, almost the entire south wall is glass, and most importantly, the amount of mass modeled couldn't be practically be built into a small house like this (6750 ft2 of 4 inch thick concrete wall.) And the internal gains are actually higher than I expect we could easily achieve - my own home is lower, without extraordinary measures. With internal gains half as high (1178 kWh/year), the back-up heat need rises from 688 to 1030 kWh.
Some additional points:
* TMY data doesn't represent the worst weather year possible - it is an average. Even if Energy-10 concluded that zero heat was possible, it wouldn't guarantee that this would be true every year. Norman Saunders used to say that to reach 100% solar heat you needed to take the average December and add 30% to the solar system to cover weather variations.
* The best realistic result, still achieved with extraordinary measures in glazing and airtightness, and using a pretty massive building construction, is 944 kWh/year. This saves 1262 kWh/year over the base case superinsulated building, at fairly high cost (mass, airtightness, super HRV, the most costly glass, which needs to be installed in large pieces to keep the high R value.) This is about $140 in electricity, $55 in propane, or less than $30/year with wood. Why bother, when you still need a back-up source for domestic hot water? A woodstove is a poor idea - it doesn't make DHW, and it costs a lot to install, with chimney and hearth.
* Note that in Seattle, which is the closest US city to the Darmstadt climate Amory cites, the house uses 1/5 the back-up heat. Average temp in Burlington in January is 16F, Darmstadt is 32F. This makes a difference, even though Darmstadt, like Seattle, gets less sun.
* Note that in Eagle, CO, closest to Amory, you can get incredibly close to zero heat. VT is slightly colder but much less sunny.
* I am possibly not up on the latest from Darmstadt, but the houses built there in the early/mid 90s were row houses (less exterior surface/volume ratio) and were backed up by a central gas boiler. They were not expected to be zero heating energy houses. Please send info on the latest versions if they indeed have achieved zero energy.
* In summary - this conversation has gone in a similar vein to my conversations with Amory back in 1996 (?) about the possibility of doing a law school classroom building in VT which needed no mechanical cooling to maintain comfort conditions. At that time, I chased down Amory's suggestions and examples, read his suggested references, spoke to some of the authors, and concluded that there were no examples of buildings in this climate or similar climates which were controlling humidity in the cooling season passively. Thus far, I am concluding that Amory doesn't understand the climate here well enough to make these assertions. Greg has been included on this distribution because of Amory's note to Dana in the last exchange:
"If Marc still has trouble getting to the goal, let me know and I'll ask Greg Franta to look at the Energy-10 runs and talk directly to Marc about further fine-tuning; but you're most of the way there. Lots of small further improvements should get you all the way. Cheers -- ABL"
Greg - if you know something about how to achieve zero heating energy houses in this climate, I'd be very open to your "fine-tuning."
Date: Fri, 18 Sep 1998 10:41:30 -0600
To: Marc J. Rosenbaum
From: Amory Lovins
Subject: Re: The zero heat house again
At 13:12 9/14/98 EDT, you wrote:
>Enclosed is a spreadsheet with Energy-10 modeling results testing
Amory's assertion about zero heating energy houses in VT.
>A MS Word 5.0 memo is also attached which has my comments. Your
responses would be appreciated.
>
>Marc
Dear Marc,
Thanks for your thoughtful note. My reaction is that actually you're getting the results I'd expect for your climate (I used to have a house in the NE Kingdom, spent a lot of time in NH/VT over 15 years, and do know a bit about the climate). You're really close to converging on the results I hoped for. What I'd suggest you add next is the following small but important details: - Check that you've included heat gains from lights and appliances. If my recollection of the ASHRAE Fundamentals is anywhere near right, your internal gains might be just for people, not people plus equipment (even very efficient equipment: our superefficient household lights and appliances average about 110-120 W, but that's about 1/10 of normal). A 110-W internal gain from these sources is 964 kWh/y, which approaches your resistance-heating load in the better cases. Most of the lights-and-appliances heat, too, is normally released in the evening when you want it. Putting the kitchen on the north side helps too.
- The trick that Feist used lately to eliminate heat in his original Darmstadt test house (yes, it's a row-house as you say, and achieves amazing predicted-vs-measured agreement): eliminate glazing edge losses by putting ~1 cm of foam sheathing over the entire frame assembly, inside and outside, and running ~3 cm onto the glass -- slightly reduces aperture but makes center-of-glass rating identical to whole-glazing-assembly rating. If you model R-11 instead of R-8 whole glazing assemblies, I presume -- though only the parametrics will tell for sure -- that it will help markedly.
- Think whether you really want to assume a constant 50 cfm. Demand-controlled (CO2) ventilation, as used by Feist, works much better, using a variable-speed fan on the a-a hx. With a stove-hood a-a hx, perhaps little bathroom units, and careful materials choice and radon precautions for good indoor air quality, I'd have thought you could use the performance rather than prescriptive option for ach/replaceance and get plenty of fresh air with considerably less than 50 cfm. (Of course, you can also make the heat exchange so efficient that 50 cfm hardly matters:...)
- Temper the air-to-air heat-exchanger input with an earthpipe, so all air coming into the hx starts at 8+ deg.C. Then specify the hx for low face velocity, i.e. big, slow fans and extended surface area, for average efficiency ~92-95% with, say, Venmar crossflow, even more with counterflow.
- Make sure you're counting the *radiant* temperature in your comfort conditions, not just the air temperature. In our high-mass superinsulated building -- and we do get up to 39 days of observed continuous midwinter cloud here -- the radiant temperature lags the outdoor air temperature by some months. This makes indoor air temperature a very conservative metric for comfort sensation. I suspect that with this taken into account, and a floorplan that provides excellent zone coupling (good solar gain to the mass in each zone for a fairly isotropic radiant field at night), your medium-mass variant will already be at the goal. For example, our winter indoor air temperature often drops to 15-18deg.C at night, but it stays comfortable because of the steady 25+deg.C radiant field. Another way of saying this is that a good radiant temperature from passive solar design can let you lower the thermostat setpoint in the model (and in the house -- before you eliminate the thermostat) by several Cdeg. w/o loss of comfort.
- Don't be afraid of the "extra" airtightness We did <0.03 ambient ach -- almost unmeasurably small -- after dampering the a-a hxs.
- Do go for the "extra" glazings -- well worth it. Greg can advise you whether your climate justifies tuning to elevations.
- Think about where the mass is. Are partition walls optimal? Why not slabs and outsulated exterior walls?
- Since you'll have a water heater -- perhaps a modulating-valve propane Aquastar if you can't get a solar geyser-pump (whose plans will go public-domain in '99) -- you could provide very cheap backup by hydronic radiant slab coils. We installed such coils, but never hooked 'em up.
- If you want to get fancy, you can use a small heat pump to recapture latent heat from the exhaust air, discharging it saturated at 0deg.C, and bring it back into the space as sensible heat. Also a good way to do water heating. You can also normally do graywater heat exchange from showers etc.. That's as far as I can get without actually seeing the parametrics from Energy10. Just an aggregated residual heating load doesn't tell me what I most need to know -- where the residual loads are coming from and (via elimination parametrics) how the measures interact. Greg invented the elimination method and can briefly describe it if you're not familiar with it. It's very revealing.
Greg, as more of an Energy10 jock than I, what do you spot that I missed? This is an important example that will have a huge teaching value, so thanks in advance for whatever you can do, perhaps directly with the Energy10 file. Marc, I now think, even more than before, that "you're most of the way there. Lots of small further improvements should get you all the way."
Thanks to all -- Amory
9/21/98
To: Donella Meadows and Amory Lovins
From: Marc Rosenbaum
Subject: The zero-heating energy house in Vermont, again
First I'll respond to Amory's specific points:
- Internal gains are 0.269 kW continuous, 2355 kWh/yr, which exceeds my own not-super-efficient house. Some runs cut this in half, which is more realistic as lights and appliances get more efficient.
- I changed the window R-value to 11 overall. I didn't reduce the aperture accordingly, so the solar gain is undiminished.
- The infiltration modeled is very low - 5 cfm for leakage (and 0.03 AC/H is harder in little houses) and 5 cfm equivalent from the ventilation system, which is 50 cfm at 90% efficiency. Demand-controlled ventilation based on CO2 in very tight houses here would likely not control humidity - in my own house and in a more recent house (EBN Vol. 7, No. 2), we run about 50 cfm continuously to keep RH down to about 40% in the cold months. Much higher RH risks dust mites. However, you can see from today's runs that I modeled zero infiltration also!
- earth tubes - please see previous memo
- radiant temperature - I'm very skeptical that your walls have a several month time constant - their thermal diffusivity is much too high for that to be true. Rather, they are being recharged on a schedule much shorter than months, which is unlikely to happen in VT. More to the point, I don't believe the scenario of wall/slab surface temperatures of 25C, and air temperatures of 18C. Consider the simple physics (in English units, sorry!): assume 2500 ft2 of surface at 77F (25C) and a heat transfer coefficient of 1 BTU/hr/ft2/F. If the air is at 65F (18C), then the heat transfer rate is 1 x 2500 x 12 = 30,000 BTU/hour. This is in a house with a design load of ~2 kW, or 7,000 BTU/hour. How long can the air temp remain more than marginally lower than the wall/floor temp? This scenario only happens in houses with lots of low R glass and/or leaky construction.
- air tightness and glazings - see previous memo
- mass location - the model uses partition walls for convenience of modeling, but they are better than exterior walls because they aren't coupled to the exterior, and they are accessible to heat transfer from both sides. However, in practice there is more area of mass in the models than the building has for total interior plus exterior walls plus slab, so I can safely say it is optimized!
- The Copper Cricket was very overrated - I have data showing what a poor performer the under-tank thermosyphon heat exchanger is. Khanh Dinh, the inventor, abandoned this design due to inefficiency. The Cricket, or any other solar water heater, is not an alternative to back-up heat for DHW in this climate, unless the system is truly huge.
- air to water heat pumps are poor choices - complex, low COP, noisy - the one on the market uses a glass-lined tank, which won't last as long as this kind of appliance should.
Today's results (SEE BELOW FOR SPREADSHEET)
If you make the glass R-11 overall, and have very high mass, and turn infiltration to zero, the model predicts zero heat energy. This assumes the 2355 kWh/year internal gains. Cutting the mass from 6750 ft2 to 5000 ft2 of mass area brings the result above zero, so here we are right on the edge. Cutting the internal gains in half brings the heat energy back up above zero. R-11 windows, very low but not zero infiltration, and half internal gains has an annual load of 509-671 kWh/yr, depending on mass.
This is still based on average weather data. It assumes virtually unavailable construction: R-11 overall windows (how are they operable and yet remain R-11 overall?), huge amounts of mass, earth tubes, very large ventilation heat exchangers with active controls, all the south facade in glass,etc.. As soon as you put any device on site which makes thermal energy available to heat DHW, you may as well use it for heat, too, and then why take the building far past the economical point of construction to save tiny amounts of energy (as well as making it into an unlivable energy machine instead of a house?) The payback on increasing the glazed south area from 228 to 312 ft2 is on the order of 100 years, yet the glass itself has seals which are likely to fail in 10-20 years. Investing in equipment in which the payback greatly exceeds the service life is not good!
I also don't see the virtue of separate gas water heaters or woodstoves with tightly clustered homes (and I see tightly clustered homes as a significant virtue - less land used, builds community, opportunity to share laundries, central thermal systems, etc..). VT has plenty of cordwood. What makes sense to me is to share a wood gasifier for several houses, using it for heat and DHW. A tank of several hundred gallons allows firing at convenient times, and allows the wood system to cover summer DHW loads without an onerous firing schedule. The large tank also allows relatively low marginal cost solar DHW, although its use is mostly for convenience, certainly not for avoided fuel cost once wood is used. This system seems far preferable from a sustainability standpoint than 22 distributed Aquastars, burning propane, a fossil fuel.
Date: Wed, 04 Nov 1998
To: Marc Rosenbaum
From: Amory B. Lovins
Subject: Re: Posting the discussion
At 12:21 11/4/98 EST, you wrote:
>Hello Dana and Amory -
>
>I have compiled the discussion we three have had about zero
heat houses in VT into one document, and I'd like to get your blessing
to post it to the Hartland cohousing list, the greenbuilding listserv
at CREST, and the EEBA (Energy Efficient Building Assoc) listserv.
I think it would stimulate lots of discussion and would serve a
real educational purpose.
>
>Please let me know if this is OK with each of you.
>
>Thanks
>
>Marc Rosenbaum
Sure, although they weren't written with this in mind, and your intro should say they're unedited private correspondence reproduced by permission. (I presume I didn't make nasty remarks about anyone -- if I did, please let me know first so I can redact accordingly.)
I also haven't yet had time to add one final footnote to our helpful dialogue, because I've been running around too much to analyze your latest results. My quick reaction at first glance is that, as I think I said before, it probably does make a lot more sense to leave a small residual heat load and to handle it with a hydronic loop from the water-heater (which you'll need anyway) into a radiant slabcoil. That's cheap, effective, and probably a lower-total-capital-cost solution than fighting for the last iota of heat savings: it still eliminates the furnace, but adds very little capital or operating cost for the backup. (Perry Bigelow has done >1k tract houses in IL this way for a decade with no furnaces and without even using superwindows.) However, this doesn't mean that some of the improvements that are well down the list, like the Feist trick to eliminate edge losses, or the earthtube to preheat OSA to 8deg.C min. before entering the a-a hx, aren't also a good idea. I'd also be surprised if tuned superwindows with COG R>8, probably ~10-12, weren't a great idea, especially for radiant comfort.
Best -- ABL
Amory B. Lovins
Senior Vice President, Chief Financial Officer, and Director of
Research
Rocky Mountain Institute, Inc.
and Director, The Hypercar Center
and Principal, The Lovins Group, Inc.
Top of File
| Results of Energy-10 Modeling for Hartland Farms | ||||||||||
| Marc Rosenbaum, PE, Energysmiths | ||||||||||
| Building description: 28x48 ranch, 1344 ft2, wall height 7.5 ft (a standard house I use repeatedly.) | ||||||||||
| R Values - wall, 40, ceiling, 70, floor over basement, equal to 84 (E-10 doesn't handle basements well.) | ||||||||||
| Air leakage - "standard" is 0.1 AC/H leakage (17 CFM) plus 50 CFM @ 70% efficiency | ||||||||||
| "extra" is 0.03 AC/H leakage (5 CFM) plus 50 CFM @ 90% efficiency | ||||||||||
| Windows - "standard" is triple glazed in fiberglass frame with 2 high SHGC low-e layers and argon | ||||||||||
| - U glass = 0.16, SHGC = 0.60 | ||||||||||
| "extra" is quad glazed with 2 HM88 films and one high SHGC low-e and krypton | ||||||||||
| U glass = 0.093, SHGC = 0.47, U window = 0.13 | ||||||||||
| Window areas (gross) - 12 ft2 on N, 24 ft2 on W,E; 120, 228, 312 ft2 on S (varies) | ||||||||||
| Doors - 2 standard doors - U = 0.18 | ||||||||||
| Mass - | lightframe - 150 ft2 of 4 inch thick interior concrete partition wall | |||||||||
| medium mass - 1350 ft2 of 4 inch thick interior concrete partition wall | ||||||||||
| heavy mass - 6750 ft2 of 4 inch thick interior concrete partition wall | ||||||||||
| Internal gains - 269 Watts continuous (2355 kWh/year), 3 occupants | ||||||||||
| Heating system - electric resistance - 68F setpoint | ||||||||||
| Location - Burlington, VT (others modeled as check points are Seattle, WA, and Eagle, CO) | ||||||||||
| CASE |
Air Leakage |
Glazing type |
S/N Glass, ft2 |
Mass |
Location |
Heat, kWh/Yr | |
|
1 |
standard |
standard |
228/12 |
1350 ft2 |
Burlington, VT |
2206 |
|
|
2 |
extra |
extra |
120/12 |
light |
Burlington, VT |
1912 |
|
|
3 |
extra |
extra |
120/12 |
1350 ft2 |
Burlington, VT |
1830 |
|
|
4 |
extra |
extra |
120/12 |
6750 ft2 |
Burlington, VT |
1686 |
|
|
5 |
extra |
extra |
228/12 |
light |
Burlington, VT |
1463 |
|
|
6 |
extra |
extra |
228/12 |
1350 ft2 |
Burlington, VT |
1244 |
|
|
7 |
extra |
extra |
228/12 |
6750 ft2 |
Burlington, VT |
1063 |
|
|
8 |
extra |
extra |
312/12 |
light |
Burlington, VT |
1346 |
|
|
9 |
extra |
extra |
312/12 |
1350 ft2 |
Burlington, VT |
944 |
|
|
10 |
extra |
extra |
312/12 |
6750 ft2 |
Burlington, VT |
688 |
|
|
11 |
extra |
extra |
120/60 |
light |
Burlington, VT |
1995 |
|
|
12 |
extra |
extra |
120/60 |
1350 ft2 |
Burlington, VT |
1904 |
|
|
13 |
extra |
extra |
228/60 |
light |
Burlington, VT |
1551 |
|
|
14 |
extra |
extra |
228/60 |
1350 ft2 |
Burlington, VT |
1328 |
|
|
15 |
extra |
extra |
228/12 |
6750 ft2 |
Seattle, WA |
222 |
|
|
16 |
extra |
extra |
312/12 |
6750 ft2 |
Seattle, WA |
127 |
|
|
17 |
extra |
extra |
228/12 |
6750 ft2 |
Eagle, CO |
102 |
|
|
18 |
extra |
extra |
312/12 |
6750 ft2 |
Eagle, CO |
13 |
|
|
19 |
extra |
extra |
312/12 |
6750 ft2 |
Burlington, VT |
37 |
Thermostat set at 60F |
|
20 |
extra |
extra |
312/12 |
6750 ft2 |
Burlington, VT |
1030 |
Int Gains reduced to 1178 kWh/yr |
|
21 |
extra |
R-11 overall |
312/12 |
2500 ft2 |
Burlington, VT |
671 |
Int Gains reduced to 1178 kWh/yr |
|
22 |
extra |
R-11 overall |
312/12 |
5000 ft2 |
Burlington, VT |
562 |
Int Gains reduced to 1178 kWh/yr |
|
23 |
extra |
R-11 overall |
312/12 |
6750 ft2 |
Burlington, VT |
509 |
Int Gains reduced to 1178 kWh/yr |
|
24 |
extra |
R-11 overall |
312/12 |
2500 ft2 |
Burlington, VT |
376 |
|
|
25 |
extra |
R-11 overall |
312/12 |
5000 ft2 |
Burlington, VT |
287 |
|
|
26 |
extra |
R-11 overall |
312/12 |
6750 ft2 |
Burlington, VT |
253 |
|
|
27 |
zero |
R-11 overall |
312/12 |
2500 ft2 |
Burlington, VT |
72 |
|
|
28 |
zero |
R-11 overall |
312/12 |
5000 ft2 |
Burlington, VT |
25 |
|
|
29 |
zero |
R-11 overall |
312/12 |
6750 ft2 |
Burlington, VT |
0 |
|
|
30 |
zero |
R-11 overall |
312/12 |
2500 ft2 |
Burlington, VT |
196 |
Int Gains reduced to 1178 kWh/yr |
|
31 |
zero |
R-11 overall |
228/12 |
6750 ft2 |
Burlington, VT |
94 |
|
|
32 |
extra |
R-11 overall |
228/12 |
2500 ft2 |
Burlington, VT |
1127 |
Int Gains reduced to 1178 kWh/yr |
|
33 |
extra |
R-11 overall |
228/12 |
6750 ft2 |
Burlington, VT |
1000 |
Int Gains reduced to 1178 kWh/yr |