Thriving On Low Carbon

The right target, Part 2

The Thousand Home Challenge is an initiative of Affordable Comfort. Its goal is the transformation of America's housing stock via a demonstration project of one thousand homes that reduce site energy use by 70 - 90%. What differentiates the THC from other programs is that it is based on twelve months of actual, not modeled, energy usage, and that it allows, indeed encourages, multiple paths to success. It is the only initiative that recognizes that the principal barrier to deep energy reductions is not a lack of technology or insurmountable cost, but rather the choices that people make in how they use energy.

There are two options available to THC candidates. If your house has a recent full year's worth of energy use data, Option A is a 75% reduction from that baseline. This is a good option for an existing energy hog! Option B is an energy budget calculated by the THC Calculator, using as inputs your zip code, square footage of finished floor area, number of occupants, and whether the house is free-standing or attached. The Calculator spits out a site energy budget (see the previous post on Primary Energy for a clearer discussion of site vs. source of primary energy) for heating, cooling, DHW, and electrical loads.

There are some aspects of the THC that aren't optimum, in my opinion. I'd like to see primary energy be the metric, but this would confuse people and conflicts with other programs (no initiative besides Passive House that I'm aware of even calculates primary energy.) This means that one BTU of electricity used to heat water is equal to one BTU of natural gas to heat water, even though in primary energy terms they are different by about 2-1/2 to 1. It means a kWh of exported solar electricity is counted as offsetting an imported one, which I support, although people differ about this. THC does allow twice as many BTU for heating if a combustion fuel is used than is allowed if electricity is used - this is a compromise between a COP of one for electric resistance heat, and a COP of three that might result with a really good minisplit heat pump.

The THC budgets are low. Ours for House 5, with two occupants, is 5,375 kWh/year if the house uses electricity for heat. If a thermal fuel is used for heat, the budget limit is 7,644 kWh/year, with the difference coming from doubling the allowable heating budget from 2,269 to 4,539 kWh/year. Here's how THC calculates the House 5 allowance for Option B:

The percentages allocated to each end use change depending on what type of energy is used for heating, but the cooling, DHW, and all else (electricity not used for the other three) remain at the same absolute levels. The blended space heat is 50% electricity and 50% thermal fuel.

How tough is this budget? As is when we moved in, the house would use 300 - 350 gallons of oil annually for heating, and perhaps another 100 gallons for DHW. Call it 425 gallons. That's 17,277 kWh/year by itself, or about three times the THC allowance for heating with a thermal fuel plus DHW. The allowance leaves 1,893 kWh/year for cooling and all other loads. Average annual electrical usage per househould in MA is about 7,300 kWh/year (but this is undifferentiated as to end use and undoubtably includes all of the above mentioned four categories of energy end use.) Yet I think that it's clear this is a rigorous target.

How might people achieve the THC standard? The three principal components available to a household are energy retrofits to reduce load and equipment efficiency upgrades; on-site renewable energy (but wood is not counted as a renewable, so this really means solar in most cases); and occupant behavior. If a community gathers together to reduce energy usage, community solutions beyond the scale of a house are eligible. Where it gets interesting to me is that occupant behavior can be huge. The range of variation of occupant choice of heating setpoint; cooling setpoint and indeed usage of cooling at all; DHW usage in gallons per day per person (gpd/person); and electricity usage for lighting and plug loads, is surprisingly large. In one group of eight homes built as a neighborhood for which I have been compiling monthly submetered energy usage data, we see DHW gpd/person varying three to one, and lighting and plug load usage similarly. It's easier for households with more people to achieve lower usage per person (many energy loads don't go up in proportion to number of people in the household) yet overall, the usage of energy in this country is in most cases based on occupant choices. And choice is the one input that can change in an instant, with no cost. It's climate change action by epiphany :-)

Once I got my energy threshold from the THC Calculator, I looked at how much we were using and how far we were from the allowable amount. Surprisingly, we're close. Here's an estimate of annual usage:

I think we could qualify for the THC where we are now, without added renewable energy. Here's what's different from the house we moved into a year ago:

- The big move is the replacement of the oil boiler with the Fujitsu heat pump and a very efficient electric water heater (in progress). We take advantage of a number of things here. The first is the excellent COP of the Fujitsu heat pump and the THC 2:1 thermal fuel/electricity ratio. The second is the fact that the oil forced hot water system used two or more kWh/day in electricity to run the burner, the pump and zone valves, and the controls. The third is that in seasons when no heat is called for, the boiler made DHW at a very low efficiency (see Out with the old, in with the new).

- We tightened the shell up, dropping the blower door number from 900 to 650 CFM50 (see What's a renter to do?), thereby lowering the heating load.

- We ran the main level at 66F - 70F, depending on whether we actively occupied the space, and accepted 2 - 4F lower temperatures in the upstairs, depending on how cold it was outdoors. The upstairs was only heated passively by the heat pump on the main level. We chose to close off the first floor bedroom and bath and run them without heat, except what they got by conduction and leakage through interior walls, so these spaces ran close to basement temperature (which dropped below 50F briefly in January, once we turned to the heat pump for heat, and the energy dumped into the basement by the oil heating system was greatly reduced.)

- We swapped out the refrigerator and the range/oven for much more efficient models (in a future post I'll discuss energy used for cooking, but suffice it to say that electric cooktops are roughly double the efficiency of gas cooktops...)

- We use a lot less electricity than most Americans - no TV for instance. Yet we use the lights, listen to tunes, use power tools and laptops, etc. - we haven't felt particularly abstemious or deprived.

So, it looks as though a year from now we could qualify for the THC. This feels like an admirable goal, yet, in a way, with this house, it feels too easy. We're going to add in other strategies, with a variety of benefits. They include:

- We're going to insulate the basement and rim joist (subject of future posts). This is primarily to warm the basement up, and reduce ground moisture transport in, with the goal of making it less dank in the humid MV climate. Almost everyone here runs a dehumidifier continuously in the warmer months. The rim joist work will reduce air infiltration and puts basement insulation where it does the most good, above grade where it's coldest.

- We're going to do something to upgrade the windows, without replacing them (also a future post or two here...)

- There's a bit of work to do in the main house attic, where the cellulose has settled some in the sloped roof bays.

- We intend to install a 4.76 kW Sunpower solar electric array sometime in the next month. The combination of the falling prices of PVs with the MA SREC program, the MA Clean Energy Center rebates still available though diminished, and the federal 30% tax credit for renewables makes the installation of PVs a better investment than any other available to people like us, who don't have supercomputers to manipulate the stock market.

- We also will install a small solar DHW system. I did some hand-wringing about this, especially vs. a heat pump water heater, but I think I'm going with what I know.

- We're going to put in a woodstove. I think it's a good complement to the heat pump, because keeping a stove going in mild weather is more challenging, and heat pumps are more efficient as the outdoor temps rise, so these could be ideal together. There's a lot of oak here on MV, enough to glean a bit of cord wood each year. Plus, it's nice to have heat, and in an extended emergency the potential for DHW and cooking, when the power is out. Finally, this past winter was the first time in thirty years I didn't have a wood heater, and when you're cold there's nothing like sidling your butt up to one!

Lots of fodder here for more posts!

Anyway, back to the target question. The PV array here will have a bit of shading, and I don't know exactly how many kWh it will make annually. Unshaded it would be 5,700 kWh or more in a typical year. This means we could likely be net zero on an annual basis, even perhaps without the solar DHW system. I think we'll net at least 5,000 kWh/year from the system. Looking at our projected usage more closely by month, I think it is possible that if we primarily use the wood stove to heat for the coldest and cloudiest months - say December through February - we might be able to hit net zero on a month by month basis. It's dicey in December, when the shading on the solar systems will be greatest, but that makes it a worthy target.

To summarize our target:

We will shoot for monthly net zero electrical energy - that is, to use no more electricity than the PV system generates on a monthly basis - and supplement with no more than a cord of firewood. We'll aim to qualify for the THC along the way. Will we get to our goal? Don't change that dial....

 

 

Refrigerator update

When we moved into House 5, it had the original Maytag 18.5 ft3 refrigerator that was installed in 2000. It had one feature I had never lived with before, an icemaker. It took me some time to realize that the weird sounds I occasionally heard coming from the fridge was it cranking out the cubes. We don't use much ice, and being middle-aged actually learned in our youth how to fill ice cube trays (similar to being able to count, and tell time by the big hand and the little hand, and other lost arts) so eventually I turned that feature off. Researchers at NIST recently reported that they tested four new refrigerators and the icemakers added 12 to 20 percent to the rated energy consumption (http://www.nist.gov/el/building_environment/ice-041211.cfm). The not-so-cool discovery was that 75% of this energy comes from heating the molds to release the ice. Duh. Oh, and the ratings you see on the big yellow sticker don't include the energy used by the icemaker, apparently because someone decided it would be too hard to measure. Double duh.

Anyway, being a complusive measurer, I plugged the Maytag into a kWh meter and measured its usage during March. This is a time of year where the house is perhaps slightly cooler than average. The annualized energy usage was 655 kWh. I knew I could do better, and maybe find a quieter refrigerator also.

I checked the Energy Star ratings (http://www.energystar.gov/index.cfm?fuseaction=refrig.search_refrigerators) and learned that the most efficient refrigerators were 16.5 ft3 models made by GE*, and that they didn't make them anymore. This despite the fact that these units were well reviewed by owners. Moving up to an 18.1 ft3 unit added 11 kWh/year, so after a few days of trying to find the last 16.5 ft3 model left I capitulated to being an American and found that the 18.1 ft3 one I wanted was also discontinued in favor of a newer version that used more energy. Duh (are you sensing a trend here?) I found the one I wanted, on sale as it happened, being discontinued and all, and bought it. It cost $517 shipped to Martha's Vineyard (Land of No Free Shipping Zip Codes). It does not have an icemaker. It is rated at 335 kWh/year.

I've been measuring its energy consumption and after about six weeks it looks like we're headed for an annual energy usage of 260 kWh unless it goes way up in the summer. I gave the Maytag away (I learned to my chagrin that all three of my work colleagues who immediately responded to my offer were intending to use this as a second, in-the-basement fridge). The GE seems quieter than the Maytag, which we appreciate.

One test I use to evaluate whether something is a reasonable energy investment is to look at kWh saved and compare with the cost of solar electric capacity to generate the equivalent amount of energy. This is reasonable IMO as long as the investments being compared have similar service life. I thought that a fridge mightn't last as long as a solar electric system, so this wasn't an optimal comparison. Anyway, here on MV we see that one watt of PV will make about 1.2 kWh/year, and has a marginal cost before subsidies of perhaps $6-7. If the GE fridge saves 400 kWh/year, that's the output of about 330 W of PV costing $2,000 or more. Fridge looks good.

* The most efficient refrigerators are actually still Sunfrost, but the margin is getting smaller, and the PVs cost less per kWh saved, and plus they are a pain to actually use because they are cube shaped so you need orangutan arms to reach half the stuff.

How do you pick the right target?

It's really hard to get somewhere when you don't know where you want to end up! When I set out to alter House 5, I didn't have a clear endpoint in mind. It's not a simple choice, at least to someone who has once or twice overthought a problem. In my consulting work I've helped people achieve net zero energy buildings; accomplish Deep Energy Retrofits; head towards Passive House certification; qualify for the Thousand Home Challenge; and even nail LEED Platinum a few times. What should we be aiming for?

From an energy standpoint, achieving Passivehouse certification with an existing house would be the most challenging task. It has the most rigorous requirements. The house is modeled in the Passive House Planning Package, a densely layered Excel spreadsheet that models buildings in great (some would say mind-numbing) detail. The maximum heating load allowed is 4,750 BTU/sf/year, and that's based on a strict German method for accounting for usable sf. For our house, a ballpark allowable heating number is perhaps 7-8 million BTU (MMBTU) annually. Note that this is a load, not a consumption figure. For example, if the allowable heating load were 8 MMBTU/year, since fossil fuel heating equipment isn't 100 percent efficient, the consumption would be greater than 8 MMBTU. If we stuck with the oil boiler, and it operated at 80 percent efficiency, it would consume 10 MMBTU of oil annually, or about 70 gallons/year.

This would be a very tough target. Passive houses in this climate have a higher percentage of their glazing on the south, and don't shade it in the winter. They are 4-5 times tighter than House 5 is now. They are more efficient in terms of exterior surface area to usable floor area. I pretty quickly concluded that, as much as I have been a proponent of many aspects of the PH approach, this house wasn't going to get shoe-horned into the PH fold in any way that could be described as elegant.

Having worked on a number of very successful Deep Energy Retrofits (DER - see both of my keynote addresses and my Client Case Studies on the energysmiths web site) I of course did some modeling around a major envelope retrofit. Components of the existing house are:

Basement walls are uninsulated; there's one inch of foam beneath the basement slab

First floor over the basement is insulated with R-19 fiberglass batts, installed at the bottom of the 2x10 floor joists, which means that there is about four inches of air above the batts, which means that the rim joist area at the perimeter of the house is not insulated.

The above grade walls are 2x6 with cellulose insulation

The sloped roofs are 2x10 with cellulose, and the attic has about fourteen inches of cellulose (the ceiling area is about 50/50 sloped/fat)

The windows are fiberglass Thermotechs, which means that the sash and frame of the window are fiberglass pultrusions filled with molded polystyrene foam, and therefore are more insulating than typical wood windows. The double glazing has a layer of a hard coat low-e called Energy Advantage, which is optimized for solar gain rather than insulating value (Thermotech recognizes that cold climates benefit from solar heating!), and is filled with argon. The overall window R value is perhaps 3.3, which is not really much better than typical wood windows, except that the glazing Solar Heat Gain Coefficient (SHGC) is 0.68, which is much higher than normal windows these days, which may have a glazing SHGC of 0.4.

When we moved into the house, the blower door test was about 900 CFM50 - a very good number for a new house, compared to average new homes. Some targeted air sealing dropped this to 650 CFM50, and I know there's more work to do, especially at the rim joist area in the basement.

The house definitely has winter solar shading, from trees (fewer than when we moved in) and in the depth of winter, the house to the south.

I put all of this into the model, and it reports that the house would use about 300 gallons annually for heating. I think this is pretty close. A previous tenant used 435 gallons per year for heat and DHW. Knowing what I know now about how inefficient the oil boiler was at making DHW in the part of the year when there was no need for heat, it seems possible that they used 300 gallons of oil for heat and the balance for DHW.

I then cranked up the R values in the model - R-40 walls, R-50 roof, R-20 basement walls, windows to R-5 triple glazed, and cut the blower door number to 325 CFM50, half of what it is now. This drops the heating demand by a factor of three, to about 100 gallons of fuel oil annually. It's worth noting that at this point I would still be well above the PH heating load criterion and still close to double the air tightness requirement.

This level of investment would be high. It's adding the equivalent of four inches of polyisocyanurate foam to all surfaces - walls, roofs, basement walls - and doing a major window upgrade (Thermotech windows can be triple glazed, and this could be retrofitted to the windows without replacing the window, a marked advantage over most windows.) Two things would make me consider doing this to a house:

1 - The house is in need of re-cladding - the siding and roofing are in poor condition

2 - The house is fairly simple geometrically and has a good surface-to-floor area ratio.

House 5 fails both these tests. At eleven years old, its surfaces both inside and out are in very good condition. The exterior trim is unpainted cypress, which will never in our lifetimes need replacement. The wall cladding is cedar shingle, another low maintenance, durable material. On the inside, the trim is unpainted cypress, and the walls and ceilings are skim coat plaster, all in good shape. Plus, an interior DER reduces usable space (tough with small rooms, especially bathrooms and kitchens), doesn't fix as many thermal bridges, and is disruptive to the occupants.

On the simplicity front, this house probably set a record for corners, roof planes, and lineal feet of perimeter for a home designed to be affordable. We have sixteen corners, seven roof planes, and 164 feet of perimeter for a house with 1,589 gross sf. Contrast that with a 28 x 36 cape - four corners, two roofs, 128 feet of perimeter. That's a less interesting house, to be sure, but it certainly would be easy to retrofit. I see myself adding insulation to the roof when it comes time to re-roof, but for now, the house exterior is in too good condition to tear into it for a DER, and it's too complicated a job.

The next possible target I evaluated was the Thousand Home Challenge. I'm going to save this, and the rest of the story, for another post. It's 8 pm on Saturday night, I spent the day plumbing, and I hear Sam Adams calling me.