HVAC contractors have the opportunity to give their customers lower air conditioning costs and improved air quality, and all it takes is a few quick measurements on your next service call.
Wouldn't it be great to eliminate unnecessary over-ventilation in homes, thereby providing equal or improved indoor air quality and comfort at a lower cost?
The team of Armin Rudd and Daniel Bergey of Building Science Corporation, partnering with the University of Texas and TxAIRE Institute, set out to test at two single-family homes constructed as lab homes by the University of Texas at Tyler. (The images in this article are from their study.)
The twin lab homes in Tyler offered a unique opportunity for the direct comparison of nearly identical homes. The difference: House 1 had a vented attic; House 2 had an unvented attic assembly.
In this test the two homes were compared for both energy consumption and overall air quality. Here's what they learned.
The ASHRAE Standard
ASHRAE Standard 62.2-2010 may be considered to currently contain the "standard of care" for ventilation system design and operation in residential buildings, yet there are considerable technology gaps with that Standard. ASHRAE Standard 62.2 uses an approach that assumes the entire house is a single, well-mixed zone, and that there is no difference between different home whole-building ventilation systems in providing effective ventilation. To try to facilitate that assumption, the ventilation rate has to be high enough to accommodate the worst performing system, which is single-point exhaust.
Utilizing high-performing systems that draw outside air from a known fresh air location and filter and fully distribute that air to the occupants' breathing zone (including bedrooms where occupants spend the most continuous time) should allow optimization of the ventilation rate to avoid the problems of over-ventilation.
Over-ventilation unnecessarily consumes energy and raises the risk of comfort and indoor air quality complaint problems due to elevated indoor humidity in warm-humid climates.
Higher-performing ventilation systems may be able to eliminate unnecessary over-ventilation, thereby providing equal or improved air quality and comfort at lower cost.
Results from the houses
In House 1, CFIS (central fan integrated supply) and exhaust with central system mixing showed the best uniformity of zone air change rate. One-third of the outside air for the exhaust ventilation system in House 1 came from the attic. Exhaust ventilation showed the highest particle counts; CFIS showed the lowest particle counts due to air filtration.
The testing showed that single-point exhaust ventilation was inferior as a whole-house ventilation strategy. It was inferior because the source of outside air was not directly from the outside (much of it came from the attic); the ventilation air was not distributed; and no provision existed for air filtration.
Central system air recirculation/mixing can help improve the exhaust system by way of distribution and filtration. In contrast, the supply and balanced ventilation systems showed that there is a significant benefit to drawing outside air from a known outside location, and filtering and distributing that air.
In House 1 (vented attic) all ventilation systems reduced the formaldehyde concentration over the indoor baseline concentration, which was roughly 20 times higher than what would be expected outdoors. Exhaust-only ventilation reduced the indoor formaldehyde concentration the least, followed by exhaust-with-mixing, CFIS (central fan integrated supply), and ERV (energy recovery ventilation). In general for both houses, the CFIS and ERV systems showed a 60 percent to 70 percent reduction in formaldehyde concentration over exhaust.
How the airborne particulate sampling was done
During the 12-hour quasi steady-state period of each perfluorocarbon tracer (PFT) test period, air sampling for airborne particulates was conducted in the main (common area) and master bedroom zones of each house. During some tests, additional particulate sampling was done outdoors, and in the garages and attics. Particulates were monitored at six particle sizes (0.3, 0.5, 1.0, 2.0, 5.0, 10.0 micrometer) with a Fluke 985 Particle Counter.
The particle counter has a counting efficiency of 50 % @ 0.3 µm and 100% for particles > 0.45 µm. The sample flow rate was 0.1 cfm (2.83 L/min). The meter was programmed to complete 48 cycles of 15 minute samples over the second 12-hour period of each test, gathering a sample volume of 1.5 ft³ (42.45 L) each cycle. Data was recorded electronically and imported into a worksheet for analysis. Only the last twenty-one 15-minute particle counting cycles (cycles 20 through 40), or the last 5.25 hours before researchers re-entered houses, were used for analysis. This was to analyze the data closest to steady-state and to isolate the particle load attributable to the operation of different ventilation systems from any occupant (researcher) interaction. Occupant interaction can be significant, especially in the larger particle sizes.
- Compared to the exhaust systems, the CFIS and ERV systems showed better ventilation air distribution and lower concentrations of particulates, formaldehyde, and other VOCs (volatile organic compounds).
- System improvement percentages were estimated based on four system factor categories: balance, distribution, outside air source, and recirculation filtration.
- Recommended system factors can be applied to reduce ventilation fan airflow rates relative to ASHRAE Standard 62.2-2010 to save energy and reduce moisture control risk in humid climates.
- HVAC energy savings were predicted to be 8 to 10 percent, or $50 to $75 per year.