Musings of an Energy Nerd

The NAHBNational Association of Home Builders, which awards a Model Green Home Certification. Green conference is being held this week (April 29 to May 1, 2012) in Nashville, Tennessee. Several GBAGreenBuildingAdvisor.com employees and bloggers — including Dan Morrison, Michael Chandler, Peter Yost, Ted Clifton, and me — are attending.

Nashville has a number of famous buildings, including a full-scale replica of the Parthenon. Nashville's Parthenon isn't made of quarried marble, however; it's made of concrete. So how's the concrete quality? Do the columns resemble marble?

To continue last week’s discussion of heat-loss calculation methods, let’s consider a simple rectangular building, 20 feet by 30 feet, with 8-foot ceilings. Let’s assume it has an 8-foot-high basement with uninsulated concrete walls; the below-grade portion of the basement is 7 feet high, with 1 foot above grade.

To keep things simple, we’ll assume that the house has a flat roof, and that each side of the house has two windows (each 3 ft. by 4 ft.) and one door (3 ft. by 7 ft.). The house doesn’t have a chimney.

I’m going to devote the next several blogs to a discussion of heat-loss and heat-gain calculations. These calculations are the first step in the design of a home’s heating and cooling system.

In order to address this big topic in little bites, I’ll start by discussing heat-loss calculations. I’ll get around to heat-gain calculations and cooling equipment in a future blog.

The least expensive way to heat domestic hot water is with natural gas. Homes without access to natural gas usually choose an electric water heater, since electricity is generally cheaper than propane.

Although tankless water heaters are, on average, more efficient than traditional tank-style water heaters, they’re also more expensive — so expensive, in fact, that many potential customers wonder whether their high cost can ever be justified by likely energy savings.

Before you can decide whether to buy a tankless water heater, you’ll need to know how much energy you’ll save. Can you trust the information provided by tankless water heater manufacturers — for example, the estimate from Rinnai’s online calculator that you’ll save $178 per year?

Energy consultants and auditors use energy modeling software for a variety of purposes, including rating the performance of an existing house, calculating the effect of energy retrofit measures, estimating the energy use of a new home, and determining the size of new heating and cooling equipment. According to most experts, the time and expense spent on energy modeling is an excellent investment, because it leads to better decisions than those made by contractors who use rules of thumb.

When workers need to insulate the walls and roof of an existing building with exterior rigid foam, it often makes sense to cut off the roof overhangs first. With the eaves and rakes removed, wrapping the building in rigid foam is a snap. The missing roof overhangs can later be rebuilt by scabbing the necessary framing on the outside of the foam.

In the northern half of the U.S. — and even much of the South — installing a residential solar hot water system doesn’t make any sense. It’s time to rethink traditional advice about installing a solar hot water system, because it’s now cheaper to heat water with a 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. (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.) array than solar thermal collectors.

In short, unless you’re building a laundromat or college dorm, solar thermal is dead.

Electric resistance heating systems have a bad reputation. While the required equipment is cheap (and sometimes cheap-looking), homes with electric heat are known for their high fuel bills.

Energy experts often repeat the cliché, “There’s no such thing as a zero-energy home — just zero-energy homeowners.” Energy monitoring data from two well-publicized Massachusetts homes — the so-called Montague Urban Homestead house in Turners Falls and the home of Matt and Laura Beaton in Shrewsbury — prove the cliché to be true.