Southcentral Ventilation Study

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The Cold Climate Housing Research Center (CCHRC) commissioned John Freeman of Sunrise Energy Works to study the Skuttle ventilation technique in new housing in the Anchorage area during the winter and spring months of 2004. This study monitored nine houses in a new subdivision to assess the effectiveness of their Skuttle ventilation system and their compliance with the Alaska Building Energy Efficiency Standard (BEES) ventilation requirements.

The Skuttle system consists of a six-inch duct bringing outside air into the return side of the furnace plenum, thus pulling fresh air into the house when the furnace fan comes on. Either two or three bathroom fans exhaust stale air. Each bathroom exhaust fan has one of three different controls: a manual switch, a dehumidistat, or a timer. The control for each bathroom fan operates independently and none of the controls interconnects with the furnace fan supply. The bath fans operated a very small percentage of the time, thus, the Skuttle system operates primarily as a "supply only" ventilation system when the furnace fan is on.

This study compared the total effective ventilation rate calculated from measurements in each house to the BEES requirement. Each house in the study has a furnace-fan-integrated supply duct (Skuttle) and bathroom exhaust fans. During the study, motor loggers monitored the runtimes of the furnace and the bathroom fans. These fan runtimes and initial airflow measurements provided estimates of the mechanical ventilation rate in each house. Blower door tests of the houses were used to estimate the natural air leakage contribution. Results were used to calculate the daily averages of the total effective ventilation rate. All of the houses were occupied, and data recorded reflected the occupants' normal living patterns.

The BEES allows for a combination of mechanical ventilation and natural air leakage to provide the required ventilation flow rates. AHFC testing policy excludes air coming through the crawl space and garage in calculating natural ventilation flow rates. That exclusion left an average of 38% of natural air leakage usable for ventilation.

The daily average mechanical ventilation airflow provided by the Skuttle ventilation system, as operated by participants in this study, ranged from 6.4 to 40.8 cubic feet per minute (CFM). The runtime for the furnace fan supply was more significant to the amount of mechanical ventilation than the type of bathroom fan control. The total mechanical ventilation flows alone did not provide the 90 to 140 CFM ventilation rates required by the Alaska BEES for these houses. Homeowners may turn off noisy fans, thereby reducing the contribution of bathroom fans to mechanical ventilation.

Estimates of the natural air leakage contribution to ventilation varied widely. Several participants left crawl space vents open for the study period, while vents were closed in other houses. The air leakage model assumes leakage is evenly distributed throughout the house envelope. If actual leakage is largely in the crawl space, and the upper house is relatively tight, then airflow through the upper envelope will be reduced, and will be overestimated by the model. Calculations of the daily average total effective ventilation were likely over-estimated for the houses in this study that had crawl space openings. Calculations range from 73 CFM to an unlikely high of 657 CFM.

To better assess ventilation effectiveness, several other indoor air quality (IAQ) parameters were monitored for the four-month study period. Measurements of carbon monoxide and benzene levels showed a pattern of garage-to-house pollutant transfer, carbon dioxide (CO2) accumulated in bedrooms at night, and decayed slowly. These results support the estimates of relatively low total effective ventilation. They also reinforce the probability that the higher estimates of natural infiltration misrepresent the actual contribution of infiltration to the total ventilation. The pollutant transfer pattern and the CO2 buildup also indicate that mechanical ventilation is more important than natural infiltration in distributing ventilation air effectively.

If the system design had provided balanced flow by linking the bathroom exhaust fans and the furnace supply, it would have been possible for it to meet the BEES ventilation requirement. Balanced flow is possible with an interlocking control that operates the furnace supply and bathroom exhaust at the same time.

Balanced flow gives better source control for clean supply ventilation air. Exhaust-only ventilation (when just bath fans are on) tends to pull air from polluted crawl space and garage zones and likely increases ventilation needs. Supply-only ventilation (when just the furnace fan is on) tends to drive moisture into walls and ceiling assemblies.
 
The recommended improvements for the Skuttle system in this study are to provide interlocked control on the furnace supply and bathroom exhaust fans and to switch to low-noise bathroom exhaust fans. This would increase the effective ventilation flow rates with better distribution to bedrooms and give balanced ventilation with cleaner source air.

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