Appendix 5A: Designing for zero cooling equipment in a hot climate

The Pacific Gas and Electric Company's Advanced Customer Technology Test (ACT2) program was designed to test the hypothesis that an integrated package of energy efficiency measures could compete with new energy supply. As part of the program, Davis Energy Group was challenged to improve an initial design for a house that already met California's strict Title 24 energy code, which is supposed to include all efficiency measures that are worth buying from a societal perspective. It shouldn't have been possible to find any more. But these clever designers did.

By improving the floorplan of the 1,672-ft2 tract house, they made the living space more usable, yet eliminated some unnecessary corners that had added 23 feet (11%) of length to the outside walls. Visual interest was achieved by changing the roofline instead. The designers then put the windows in the right places, used windowframes that would transmit less heat, and invented an engineered wall that saved about 74% of the wood, reduced construction costs, and nearly doubled the insulation. These measures together saved 17% of the energy use. (Of the saved construction cost, 53% came from the shortened perimeter wall and the other 43% would be applicable to more typical designs. Normally one would also save about a third of total energy at no extra cost by improving orientation, but that wasn't possible in this case; nor, under subdivision rules, was a light-colored roof.)

A long list of small improvements to the building envelope, windows, lights, major appliances, and hot-water system raised the total energy saving to 60% and increased the cost by nearly $1,900. Some of the savings were small but impressive, like the way carefully selecting the kitchen and bathroom exhaust fans saved 80% of their electricity at no greater cost than standard models, which are only about 1-3% efficient—in effect, electric heaters that use 1-3% of their energy to move air. All the measures were conventional except using waste heat from the refrigerator to preheat hot water. This reduced water-heating energy needs, made the refrigerator more efficient because it was water-cooled rather than air-cooled, and made the refrigerator cool rather than heat the house. (Normally, the food in the refrigerator warms up toward the temperature of the surrounding household air—the less the refrigerator's insulation, the faster—and then the refrigerator's compressor removes that heat, adds its own waste heat, and puts both into its condenser coil, which heats the kitchen. This refrigerator, in contrast, moves all the heat into water. A little of that heat gets back into the house, but most goes down the drain with the used hot water.)

Along the way, the thicker insulation and better windows eliminated any need for the $2,050 furnace and its associated ducts and equipment. Instead, on the coldest nights, a little hot water—about a shower's worth—from the 94%-efficient gas-fired water heater could be run through a radiant coil cast into the floor-slab.

At this point, the designers had reached the limit of conventional cost-effectiveness. Any further savings they bought wouldn't save enough energy to pay for itself compared with the projected long-run costs of electricity, about six cents per kilowatt-hour, and of gas. Yet one-third of the original three-ton air conditioner still remained. Could no more be done?

Yes—by giving proper credit to measures that saved not only energy but also space-cooling capacity. The designers had set up a special basket called "potential cooling elimination package," into which they put all the potential energy-saving measures that weren't cost-effective from just their energy savings but that could also reduce cooling loads. As soon as proper economic credit was given for that reduction, seven such measures became cost-effective, such as better superwindows to block more summer heat, and double drywall and, in the central zone, ceramic floor tile to store coolth so the house could better "ride through" daily heat peaks. Together these seven measures cost $2,600. But because they eliminated (with a 44% safety margin) the last $1,500 worth of air-conditioner and $800 of its future upkeep costs, their net cost was low. Counting their energy and capital-cost savings made them attractive additions to the package.

Factoring out small electrical appliances (one-third of initial electricity usage), which offered many savings opportunities but would be brought along by the buyer rather than installed by the builder, the resulting final design would save about 80% of total energy or 79% for electricity alone: 78% for space heating, 79% for water heating, 80% for refrigeration, 66% for lighting, 100% for space cooling, and 92% for space cooling plus ventilation). If such construction techniques became generally practiced—so-called "mature-market cost"—then those savings would make the house, in a mature market, cost about $1,800 less to build and $1,600 less to maintain.

The measured savings, adjusted for some last-minute design changes requested by the homebuyer, agreed well with these predictions. The house proved very comfortable even in a severe hot spell. Since by law the Title 24 code is supposed to include all cost-effective measures, the Davis house may mean that this influential state standard has to be rewritten from scratch.

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