### Image Credits:

1. Steve Larson

1.
Jun 12, 2013 4:50 PM ET

Do the math
by Dana Dorsett

"his house was built 20 years ago! A HERS Index of 65 means it's 35% more efficient than the same home built to the 2004 International Energy Conservation Code (IECC), so it was 35% better than a code that came 15 years after the house was built!"

Lessee, it's 2013 now, so 20 years ago it was 1993, and 15 years after 1993 comes ta 2008.

So by that reckonin' 2004 didn't come around until 2008 or something, right? Tbe IECC just called it 2004 to fool us. :-)

But wait a minute...

"He didn’t have onsite power production in the beginning, but he did start with a healthy dose of energy efficiency when he built the house in 1991-92."

So either we skipped a year, and that was built 21-22 years ago, not 20 years ago, so 2004 actually occurred in 2006 or 2007?

This is too confusing- we shouldn't let physicists do the easy math, unless their name is Tom Murphy!

http://physics.ucsd.edu/do-the-math/

Nice job on the house though- the comfort factor of a well though out lower-load/higher-R house in FL can't be underestimated either. Does it really need a 3-ton air conditioner though, at those R values, with shaded south windows and no west windows? Or was that the smallest high-efficiency cooling available at the time?

2.
Jun 12, 2013 11:40 PM ET

Brute Force Method
by Curt Kinder

Deploying 10kW of PV to achieve net zero for a 2144 SF house in Florida on first blush sounds like favoring generation over conservation. Depending on shading, a 10 kW system should make 40 kWh / day, net of various component losses, way more than this house should need, though shop usage is an unknown.

2144 SF home at reasonable infiltration (3.4 ACH 50 ain't bad here, though the PH crowd would rise up upon their hind legs and wave their antennae furiously at the thought) and described modest glazing should come in at 2 tons, especially with tight ducts.

I assume (info not provided) ducts are outside conditioned envelope...arrayed throughout hot attic as is so typical in Florida.

Sprayfoam insulation, quality checked for leakage with fog or IR would simultaneously deal with attic ductwork in an unconditioned space and reduce infiltration, likely down to 2 ish ACH50, given the other details mentioned. Duct leakage from ducts in conditioned space is immaterial, within reason.

With that done, I'd be looking at a 2 ton 2 zone 2 stage heat pump with an eye toward bedrooms on one zone and public / daytime use rooms on the other zone. Done right, it would stay in low stage 95+% of the time. (~1.3 tons). Know that high SEER rated heat pumps attain rated / advertised SEER while in low stage, so conduct design accordingly.

Heat pump water heater is much more cost effective than solar thermal for households of 5 or fewer people.

SEER 16 two stage system would likely be more cost effective than SEER 18+, as long as the savings went toward other efficiency upgrades (lighting, laundry, refrigeration, to name three)

3.
Jun 17, 2013 3:59 PM ET

Netzero house
by Dennis Heidner

Curt, while the story doesn't say, the house probably had solar hotwater long ago, perhaps before heat pump hot water heaters were widely available. In Florida a good Solar H2O system should perform quite well and probably meet the demand better than a hotwater heat pump heater.

Are the air and duct leakage numbers current -- or back from when the house was built?