ShareThisNet Zero Homes – Not Just Design, but Results
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Two projects that aspire to be net zero homes have just been completed in central Oregon. I say aspire, not because they have fallen short somehow, but because the homes have just been finished. It’s too early to know for sure that they have achieved true net zero status. Net zero homes are those that balance their total energy need with on-site energy generation. Over the course of a year, they will generate the same amount of energy as they consume in that same period. Earth Advantage Institute worked closely with the designers and builders to help them develop a home that should achieve net zero. In order to know for sure, each home will be monitored for a full year. The homes were built by Solaire Homebuilders and W.H. Hull Company.
Computer energy modeling of the plans helps estimate the energy use and production. The home’s thermal envelope (walls, ceilings, floors and windows) have been carefully built to reduce space heating and cooling needs to tiny fraction of a typical house. Each home has a solar water heater that should cut energy for hot water by 60 percent. That leaves lights, appliances and anything that’s plugged into the wall. I like to joke that energy models are always wrong, and technically that’s true. It’s impossible to accurately predict future energy use for an individual home. However, space heating and water heating are more predictable than the lights, appliances and “plug loads.” These categories are entirely under the control of the most unpredictable element in the house: people.
The first uncertainty is how many people there will be. Both the Bend homes have three bedrooms. Therefore the energy model must assume four people. Two in the master bedroom and one in each of the others. Will there really be four people? The average household size is around two people.
The second uncertainty is even more difficult to nail down. How much energy will each person use? Behavior is a huge factor in energy use. Big-screen televisions are often the biggest energy users, and the typical home may have three or four of them. How long do these people stay in the shower? The solar water heater may meet the entire demand for hot water during summer months, but winter showers will use at a lot of electricity.
Here’s a telling example. Both homes have energy monitoring systems that track the electricity used by each circuit in the building. During a recent home tour, one home was humming along drawing about 300 Watts as visitors watched the display. Suddenly, the power draw tripled. What did they find plugged into the offending circuit? Someone was ironing the drapes. A simple clothes iron was enough to triple the energy use of the building. Other heating devices can have a similar impact, such as hair dryers, clothes driers and, perhaps, toasters.
So, predicting the actual performance of a net zero home will be impossible. That’s why the Earth Advantage ZERO program grants its final certification only after the occupied home has been monitored for a full year. That means the home as occupied can truly be called net zero. If you want to know more about ZERO contact Bruce Sullivan, .(JavaScript must be enabled to view this email address) .
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