Some measurements are very direct like the height cleared by the Olympic high jumping gold medalist this summer. Efficiency has a number of nuances, though, that make measuring it difficult in terms that allow consumers to make informed decisions. When it comes to an entire house or building, or comparing homes or buildings, what should the measurement be? And how should you compare energy use on-site versus off-site? How do you determine what is efficient about the building versus the operations of the building? And how should renewable energy use or the time of energy use, and time-dependent cost of energy use factor in? Should the emissions of the source of the energy used be factored? To learn more, read the paper written by FSEC’s Deputy Director Philip Fairey and Natural Resources Defense Council’s David Goldstein.
Most electric water heaters use an electric resistant rod in the tank. But there is another option for electric water heaters – one that is familiar to most Floridians, and that is a dedicated heat pump for heating hot water. Just like a regular heat pump for heating the air in your house, the heat pump water heater has a small compressor unit on top that uses vapor compression to heat the water. Prior FSEC research on heat pump water heaters (HPWH) in Florida showed that they saved approximately 66% of the energy needed to heat water with an electric resistance system. HPWHs also create a quantity of cooled, dehumidified air from the compressor section of the unit as a by-product of their operation. FSEC researchers found out that a HPWH coupled to the conditioned living space can reduce space-conditioning energy in a cooling-dominated climate, but with qualifications a lab test was undertaken to investigate the effect of coupling a garage located HPWH to the conditioned space with ductwork. With the HPWH ducted to and from the interior, cooling energy dropped by 4% or 0.8 kWh/day. Effect on space heating energy for this configuration could not be determined. Experiments also investigated using an outdoor air source for the HPWH, to supplement ventilation. During the cooling season, the HPWH tempered the outdoor air with only a minimal impact on cooling energy. Space heating energy increased by 18% or 1.4 kWh/d. The space coupling of the HPWH had a minimal impact on water heating efficiency.
In later field evaluation, eight occupied homes were retrofitted with a HPWH coupled to the conditioned space. Results were more pronounced than the lab evaluation: cooling energy savings averaged 8% (1.1 kWh/day). Space heating energy use increased by 24%, although with considerable variation and little application in Florida’s mild climate. The evaluation suggested the coupling eroded some of the HPWH water heating energy use savings, reducing it by 0.4 kWh/day or 11%. If not located in the house, you often end up with a slightly cooler garage.
Monitoring results over a four-year period document a phased retrofit applied to a central Florida home with very high electricity consumption, eventually ending in a home with near-zero energy use. The retrofit included simple pass-through measures, such as the installation of efficient lighting and low-flow shower heads, as well as deeper measures which included a high-efficiency space heating and space cooling controlled by a smart thermostat, a heat pump water heater, and ENERGY STAR® appliances. The average household electricity use was reduced through a combination of these efficiency measures and photovoltaic power generation by 82%. Results from the case study, and nine other deep retrofits suggest how an effective zero-energy home (ZEH) program can be implemented in otherwise poorly performing existing homes.
Electric clothes dryers represent 5% (790 kWh) of annual energy use in Florida homes. Clothes dryers with heat pump (HPCD) technology, which use substantially less energy than standard resistance dryers, are relatively new to the domestic market. In eight FSEC project field homes, electric resistance clothes dryers were replaced with a new unvented HPCD. The estimated median energy savings are 34% (264 kWh/year or 0.72 kWh/day), and average annual savings are 36% (308 kWh/year or 0.9 kWh/day).
Although HPCDs use less electricity than standard resistance dryers, they still release a significant amount of heat from their operation. The unvented units that were located inside the home led to very high utility room temperatures and increases in space-cooling energy that may compromise identified savings; this is an issue the manufacturer is addressing. Given the heat issues, these unvented appliances are appropriate in Florida only if they will be installed outside of the conditioned space—typically in the garage. We further speculate, based on observed findings, that another technology—vented heat pump clothes dryer—may be the most appropriate dryer system type for Florida conditions.
In Florida, space heating and space cooling is customarily accomplished by central systems with unavoidable air distribution losses as ducts – often leaky – pass through unconditioned space. High-efficiency 1-ton, supplemental, inverter-driven ductless mini-split heat pumps with a 25.5 seasonal energy efficiency ratio (SEER), and 12 heating seasonal performance factor (HSPF), were installed in the main living area of 10 central Florida homes with the goal of reducing space heating and cooling energy by decreasing runtime of these less efficient systems. The SEER rating is 1.8 times as efficient as the new federal minimum level for conventional central systems and even more efficient than the older systems in these homes
Installed as a supplement, the mini-split installations showed median energy savings of 33% (6.7 kWh/day) for space cooling and 59% (6.5 kWh/day) for heating in the existing homes where they were added. An added consumer benefit is a redundant heating and cooling system, creating tolerable interior conditions during main system failures.
Two additional homes received high-efficiency, ductless heat pumps as complete central system replacements – a single ducted unit and a multi-split design. These homes exhibited cooling energy use savings of 37% (7.8 kWh/day) and 29% (3.5 kWh/day), respectively. While significant cooling savings were measured, the multi-split installation suffered comfort issues. The mini-split replacement, however, showed superior interior moisture control and maintained the space 1oF lower on average.
Thermostats are the central switch that controls operation of heating and cooling systems—commonly the largest energy end use in homes. That energy setup/setback has potential for energy savings has been demonstrated repeatedly in well-controlled evaluations. Thus understanding how the occupants and thermostat interact is key to controlling energy use. Programmable thermostats have often been bypassed by occupants. Newer “smart” thermostats get around these problems by self-programming depending on heuristic or machine learning evaluation of user control habits as well as sensed occupancy. These modern devices use a combination of data on occupancy, weather, and thermostat-setting preference to help consumers with automated setback/setup schedules.
Researchers at FSEC installed smart thermostats, primarily the Nest Learning Thermostat, in more than 30 research homes. A full year of sub-metered hourly temperature and heating and cooling system operation data were available prior to the installation of the smart thermostat allowing detailed evaluation of temperature-related changes. Overall measured heating and cooling energy savings averaged 9.5% with some significant variation among homeowners.
In another study of mechanical ventilation in homes, two lab homes, constructed to represent characteristics of typical existing Florida homes, were monitored. They were configured with tight and leaky building envelopes, and with and without mechanical ventilation. Simulation results of high performance new homes with mechanical ventilation, and typical older homes with and without air tightening and mechanical ventilation, were also presented.