Tighter housing envelopes help homeowners maintain thermal comfort but also require continuous mechanical ventilation to maintain healthy indoor air quality. While that tighter envelope can reduce homeowners’ heating and cooling energy use and costs, the required mechanical ventilation and the need to condition air brought in from outside cuts into those savings.
Three maxims are key to designing a superior mechanical residential ventilation strategy: use minimal energy, avoid peak electricity costs, and minimize infiltration of outdoor pollutants. To ensure an economical ventilation system that provides healthy indoor air quality and occupant comfort under the residential ventilation rates recommended by the widely used ASHRAE 62.2 standard, all of these factors must be addressed. Energy recovery ventilators can help, but they can be expensive and installation can be complicated.
For years, Lawrence Berkeley National Laboratory (Berkeley Lab) researchers Iain Walker, Darryl Dickerhoff, and Max Sherman have focused their attention on the problem, to simplify the solutions and reduce costs. The result is the Residential Integrated Ventilation Controller (RIVEC)—a control algorithm that is incorporated into heating, ventilation, and air conditioning (HVAC) controls to optimize fan energy use, costs, and indoor air quality. This work has been funded by the U.S. Department of Energy (DOE) and the California Energy Commission’s Public Interest Energy Research Program (PIER).
The control algorithm considers floor area, volume, number of bedrooms, infiltration, target ventilation rate, peak demand hours, and airflow capacities to provide the same annual exposure as a continuously operating whole-house mechanical ventilation system (as required by ASHRAE 62.2). By dynamically accounting for these various considerations, it reduces the ventilation system’s energy demand and conditioning losses and mitigates pollutant exposure.
The amount of energy needed for ventilation can change substantially, depending on the time of day when outdoor air is brought into the system. By minimizing the need to heat or cool the outside air to match the desired indoor temperature, less energy is expended. Ventilating with mid-afternoon summer air in Phoenix or nighttime winter air in Detroit both result in a huge energy penalty. To mitigate that penalty, air can be brought in at different times of the day; but it must be done in a way that still maintains healthy indoor air quality. RIVEC avoids ventilating at times of high energy use, and also can help ensure that ventilation occurs at times when outdoor pollutants such as ozone are lower or absent. It avoids over-ventilation by sensing when other systems that move air in and out of the house are operating and turning off the whole-house fan when it is not needed.
“As RIVEC controls the whole-house fan,” Walker explains, “it considers the operation of the other fans in the house and their contribution to the overall ventilation. By doing so, it reduces the need for the whole-house fan to run as much. Also, despite all the efforts to tighten building envelopes, houses still do leak some air, so we’ve incorporated that into RIVEC as well. You can take a credit for leakage under the ASHRAE 62.2 standard, and we took advantage of that. The system senses the outdoor temperature and wind speed and estimates infiltration rates, and if the infiltration rate is providing sufficient ventilation, RIVEC turns off the mechanical fan.”
RIVEC works on the principle of “equivalent ventilation”— that is, the ventilation achieves the same results as it would if the fan were running all the time. It varies the ventilation rate by evaluating the ventilation options and operating the whole-house fan when the cost and outdoor pollutants are lowest. To decide when the whole-house fan should be turned on, RIVEC uses calculations of dose and exposure that examine how changing the ventilation rates affect pollutant levels indoors. It then compares those results to the pollutant concentrations that would be found if a continuous ventilation rate were used. If the ventilation is off for an extended time, RIVEC calculates how much additional time it needs to run once it starts again, to make up for the time it was idle.
RIVEC has been field tested in homes to show that it can help homeowners achieve the energy savings predicted by simulations. The tests have monitored home heating and cooling energy use, measured air exchange rates, and looked at other aspects of RIVEC operation. Currently, the Berkeley Lab team is working with a University of Illinois team led by Paul Francisco in a field test collaboration with DOE’s Building America program to evaluate the feasibility of using an outdoor temperature measurement to help control fans. Additional efforts in this project are investigating the potential benefits of using RIVEC in a two-speed fan instead of a one-speed fan; potentially, more energy could be saved by the fan having a medium mode in which to operate.
However, simulations are able to provide more information about RIVEC’s efficacy than field tests.
“It’s impractical to field-test RIVEC in the multitude of climates, temperatures, humidities, housing styles, and HVAC configurations that represent the entire existing housing stock,” explains Walker. “Simulations enable us to evaluate a far greater number of situations. I expect we’ll continue to conduct simulations for the next two or three years. Now that we’re done simulating the general case, we’re evaluating the potential benefits of using RIVEC in specific cases.”
This year, simulations are focusing on outside temperature control; next year, on optimizing for humidity control in areas of the country where that’s a pressing HVAC concern. Other future evaluations may focus on: the energy-reduction effects of adding occupancy controls, passive stacks that provide constant ventilation, and an occupancy algorithm to allow pollutant exposure ratios to rise slightly when the house is unoccupied.
“With each simulation, we learn more about RIVEC’s potential to address the spectrum of HVAC issues,” says Walker. “As a result, we continue to add new features to it all the time.”
To date, simulations have shown a 40 percent savings of ventilation-related energy—approximately 5 to 10 percent energy savings from the total monthly energy bill. The peak load energy reduction for a typical home would be about 2 kW. Nationally, the potential energy savings could amount to 1.1 quads.
To help plan RIVEC’s commercialization, the team worked with the Cleantech-to-Market program at the University of California’s Haas School of Business. Walker was impressed with the program, which teams business students with researchers to evaluate the commercial potential of products and to develop marketing analyses and materials.
“The background work they did was very good, and they put together an excellent marketing package for us,” says Walker. “It was quite valuable to get the marketing perspective.”
RIVEC is slated to become commercially available in the United States later in 2014, and Berkeley Lab is coordinating efforts with several potential manufacturers.