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Multi-zone Air Change and Airflow in Two Houses Under Operation of Different Whole-Building Ventilation Systems.


Airtightness of new homes is critical to achieving low-energy consumption, healthy and comfortable spaces, and durability. Airtight homes require rational and predictable 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 (Rudd and Henderson 2007). Higher performing ventilation systems may be able to eliminate unnecessary overventilation, thereby providing equal or improved indoor air quality and comfort at lower cost and risk. A key gap and area of ongoing research is to allow ventilation rate credit for better performing ventilation systems, such as those with a known source and entry path of outside air, and systems with predictable filtration of outside air and recirculation filtration. This would yield energy savings and reduced moisture control risk in humid climates, without compromising indoor air quality relative to the least performing system allowed by ASHRAE Standard 62.2. ASHRAE Standard 62.2 currently assumes that:

a) all dwellings covered by the Standard act as a single well-mixed zone, thus, actual distribution of ventilation air to each occupiable space is not addressed;

b) all ventilation systems perform equally well in terms of the quality of outdoor air delivered the occupiable space;

c) there is no benefit from recirculation mixing and recirculation filtration of indoor air where that is an inherent part of a ventilation system; and

d) the only important performance metric is relative annual average exposure, which ignores the acceptability of shorter-term odor and sensory irritation dissatisfaction.

Building on previous research dealing with ventilation air distribution, this study added new elements of ventilation effectiveness research, accounting for source of outside air, particle contaminants, formaldehyde, and volatile organic compound (VOC) contaminants. The intended result is to provide a basis for understanding whole-building ventilation system effectiveness, which is critical to promoting the best low energy and high value ventilation solutions. These new data provide support for the development of a system of credit factors for better performing ventilation systems.


Description of the Test Houses

The project involved testing of two unoccupied, 1,475 [ft.sup.2] (137 [m.sup.2]), single-story, single-family, detached homes in Tyler, TX that were constructed as lab homes at the University of Texas--Tyler. Refer to Rudd and Bergey 2014 for a full report of the testing conducted in October 2012. The twin lab homes offered a unique opportunity for the direct comparison of nearly identical homes except for House 1 (H1) having a vented attic, and House 2 (H2) having an unvented attic assembly (also known as sealed cathedralized attic with spray foam insulation under the roof sheathing). House 1 had 3.5 inch (8.89 cm) wood frame walls with netted and blown fiberglass insulation, and loose blown fiberglass insulation on the floor of the attic. House 2 had 5.5 inch (13.97 cm) advanced-framed walls with open-cell spray foam insulation in the walls and under the attic roof deck. The homes were completely finished, with kitchen and bathroom cabinets, but were unfurnished and unoccupied. This allowed an evaluation focus on the building elements themselves, avoiding conflation with occupant activities and items that particular individuals bring into their homes. Zone designations for the testing were as follows:

* Main zone included the kitchen, dining area, living area, foyer, and family bathroom;

* Master zone included the master bedroom, master bathroom, and walk-in closet;

* Front zone was the bedroom on the front side of the house;

* Middle zone was the bedroom between the master bathroom and the family bathroom;

* Garage zone was the 2-car garage; and

* Attic zone was the vented attic for House 1 (including the vented attic over the garage) and the unvented attic for House 2 (the vented attic over the garage for House 2 was separate from the unvented attic and the garage but was not monitored as a separate zone).

Despite the greater volume and surface area, House 2, with the unvented attic house and spray foam under the roof sheathing, was more airtight, having 789 [ft.sup.3]/min (372 L/s) at 50 Pa (.0073 in w.c.) pressure differential (CFM50) leakage compared to 1048 CFM50 leakage for House 1 with a vented attic. House 2 had lower cooling and heating loads than House 1, and therefore lower space conditioning system airflow (707 cfm (334 L/s) versus 1137 cfm (537 L/s), measured). Duct leakage to outdoors was 56 [ft.sup.3]/min (26 L/s) at 25 Pa (.0036 in w.c.) pressure differential for House 1 and effectively zero for House 2 with all ducts being inside the building thermal enclosure.

Testing Approach

The objective of the testing program was to compare the whole-building, multi-zone, indoor air quality performance between continuous exhaust ventilation both with and without central system recirculation mixing, central-fan-integrated supply ventilation, and energy recovery ventilation. The focus of the testing approach for this paper was on perfluorocarbon tracer (PFT) measurements to determine zone air change rates and inter-zonal airflows with the different ventilation systems operating. Another paper on this project focuses on multi-zone sampling of airborne particulates, formaldehyde, and VOCs to determine indoor air quality impacts as a function ventilation system operation.

Five tests were conducted in each house, including: Baseline, Exhaust, Exhaust with mixing; Central-fan-integrated supply (CFIS) (Rudd 2011); and Energy Recovery Ventilator (ERV). The test configurations were intended to represent normal limiting case conditions for most homes when space conditioning equipment may not operate for long periods (overnight or more) and bedroom doors are closed at night.

Baseline Test. The Baseline test was conducted with no ventilation system or space conditioning system operating as reference for comparison.

Exhaust Test. The exhaust test was conducted using the master bathroom fan because that fan is most often the larger and better of the bathroom and toilet room fans in new houses. The continuous exhaust airflow was adjusted to 45 cfm (21 L/s) to meet the ASHRAE Standard 62.2-2010 continuous fan flow rate for the 1,475 [ft.sup.2] (137 [m.sup.2]), 3-bedroom house.

Exhaust With Mixing Test. The exhaust with mixing test was the same as the Exhaust test except with a central air distribution system fan cycle of 48 minutes Off and 12 minutes On. It was conducted to see the effects of trying to achieve better ventilation air distribution effectiveness and recirculation filtration via whole-house mixing of ventilation air drawn in by the exhaust fan through unknown locations in the building enclosure. The intent of the central system air mixing was to achieve a 0.7 recirculation turnover factor based on prior ventilation effectiveness research (Rudd and Lstiburek 2000, 2001, 2008; Hendron et al. 2007; Townsend et al. 2009b).

Central-Fan-Integrated Supply Test. The central-fan-integrated supply (CFIS) ventilation system test was conducted to evaluate the performance effects of drawing outside air from a planned outdoor air location, then filtering and fully distributing that air to each conditioned space zone. The outside air ventilation supply airflow was set at 135 cfm (64 L/s) by means of a calibrated flow station (Iris damper), and the central system fan was controlled to operate on a 33% duty cycle, 20 minutes Off / 10 minutes On.

Energy Recovery Ventilator Test. The energy recovery ventilator (ERV) test was conducted with a system independently ducted from the central air distribution system. The ERV ductwork in these houses was configured to exhaust from two locations in the main area and supply to all bedrooms. The ERV total supply airflow was measured to be 96 cfm (45 L/s) so the ERV timer control was set for 50% runtime. The ERV included a washable course filter at the inlet of the heat and moisture energy recovery core within the unit. That filter was cleaned before testing began.

A general note for all tests is that all closet doors were left open to allow that air volume to fully interact with the adjoining space, and all bedroom doors were configured to have the same 0.5 inch (1.27 cm) undercut above the decoratively stained concrete floors throughout the houses.

The first 12-hour period of each test was to achieve steady-state for PFT sampling in the second 12-hour period of each test. The test sequence was scheduled such that the 12-hour period of sampling would be overnight. Overnight occupancy with bedroom doors closed and mechanical ventilation is a normal and important limiting condition in homes. The PFT source emission rates are coarsely temperature dependent, which was accounted for in the analysis, but since we were not conditioning the buildings during the mild October testing, we did not want to risk solar heating effects having an impact on the sources. We also wanted to limit wind as a potentially confounding factor in the interpretation of results by taking advantage of generally lower and less-changing wind conditions at night.

PerFluorocarbon (PFT) Tracer Gas Source and Sampler Setup

Each of the two houses was tested with six different tracer gas sources, one for each of the six designated zones. The type and number of tracer gas sources used in each house can be found in Rudd and Bergey (2014). The PFT testing part of the project was set up and executed in consultation with Brookhaven National Laboratory (BNL) staff, and in accordance with the prepared instructions provided by BNL (Dietz 2006) for the Air Infiltration Measurement System (AIMS). Detailed explanation and statistical support for the PFT methods and AIMS analysis is provided in Leadererr et al. 1995 and Dodson et al. 2007.

The PFT sources supplied by BNL were contained in a metal tube and were always emitting gas at a predictable rate through a stopper at the top. The emission rate of the PFT sources is affected by temperature, so temperature and relative humidity (RH) was monitored in each zone and used by BNL in the analysis. Zone temperature and RH measurement was by small battery powered data loggers recording on a 5 minute interval. A fan was placed in each attic to facilitate distribution of PFT source.

Between each test the PFT sources were sealed in doubled, heavy-duty re-sealable bags (bag within another bag) and left in their respective zone while the house was flushed with outdoor air to a minimum of 10 complete air changes using a blower door and open windows and doors. A fan in the unvented attic aided flushing of that space to the garage attic and to outside.

To start each test, the PFT sources were opened in their respective zone for 12 hours with the appropriate ventilation system operating to approach steady-state conditions. The PFT samplers (CATS--Capillary Adsorption Tube Sampler) were not deployed (capped and not near any sources) during that initial 12 hour period. Then, the samplers were placed in each zone and uncapped for the next 12 hours to complete the test.

A total of sixty primary samples were taken (2 houses, 5 tests, 6 zones per test), and a total of sixty back-up samples were taken to be analyzed if data from any primary samples were suspect, or for general quality assurance (QA) and quality control (QC) purposes. All six backup samplers were analyzed for one test (H2-Test 2) based on an observation question (we wanted to verify the result that the Attic to Main airflow was low in House 2 compared to House 1) and for a general QA/QC check. The results showed only minor differences between the two sets of data and the AIMS air flow analysis, confirming confidence in the measurement system.


PFT testing provided detailed information on individual zone outside air change rates and inter-zonal airflows. The PFT testing materials were provided by BNL, the field testing was done by the author, the AIMS analysis was done by BNL, and the analysis and presentation of the AIMS results was done by the author.

Zone Air Change Rates

Figure 1 shows the individual zone outdoor air change rates for different ventilation systems in House 1. The air change rate is an average over the final 12 hours of each 24 hour test. Infiltration and mechanically induced air change are combined in the PFT measurements. Fortunately, temperature differentials and wind speed were reasonably stable and similar during the testing periods so as to allow good comparison of zonal air change rates between the ventilation systems. The Baseline test (no mechanical ventilation) showed low air change rates throughout all zones, with the lowest being the Master and Middle bedroom zones. Continuous exhaust ventilation from the master bathroom increased outdoor air exchange by about 0.1 air change per hour (ach) over the Baseline in the Main and Master zones, but the increase was less in the Middle and Front zones where the air exchange rate remained below 0.1 ach. Exhaust with mixing (12 min/h recirculation mixing via the central air distribution system) significantly improved the air change rate over Exhaust-only in the Middle and Front bedroom zones. Central-fan-integrated supply (CFIS) showed a significant improvement in air change rate over Exhaust-only in all but the Main zone. CFIS showed an improvement over the Exhaust with mixing system only in the Master zone. Because the ERV system was designed to supply fresh air only to the bedrooms and exhaust air only from the Main zone, the balanced ERV system showed large air change rates in the bedrooms while the Main zone air change rate was about the same as for the other ventilation systems.

Figure 2 shows the measured air change rates in the Garage and vented Attic zones for House 1. The Garage air change rate for all tests, regardless of ventilation system, was similar to the Baseline rates in the living space zones. The vented Attic air change rate was about 0.65 ach for all tests except it was double that for the Exhaust with mixing test. That can be explained by the measured wind speed, which was 4-8 mph (6-13 km/h) for the Exhaust with mixing test compared to 0-2 mph (0-3 km/h) for all other tests.

Comparing the data shown in Figure 1 for House 1 and House 2, it is evident that the living zone air change rates exhibit the same trends for both houses, confirming the reliability of the test methods. Referring to Figure 2, the same is true for the Garage zones.

As expected, the Attic zones respond differently between the houses. In the unvented attic of House 2, the air change rates were very low, about 0.02-0.04 ach, for the Baseline, CFIS, and ERV tests. The air change rate increased five-fold, to 0.16-0.18 ach, for both the Exhaust and the Exhaust with mixing ventilation systems. That points to the Exhaust ventilation system drawing ventilation air from the attic, which will be discussed further in the following section on inter-zonal airflow.

Inter-zonal Airflows

As understood by the standard deviation results provided with the BNL AIMS analysis (1), the measured inter-zonal airflows have a higher degree of uncertainty than the zonal air change rates. However, they serve a valuable purpose in at least confirming airflow in expected directions and indicating the reliability of the PFT measurements. As shown in Figure 3, and as could be expected, the airflow from Garage to Main zone was the highest for the Exhaust ventilation systems. Airflow from Garage to Main zone was the lowest for the CFIS ventilation system and between Exhaust and CFIS for the ERV system. As a theoretically balanced system, the ERV system might be expected to behave just like the Baseline, but the fact that the ERV system was designed to supply to the bedrooms and exhaust from the Main zone set up a mechanically induced airflow imbalance within the multi-zone structure that shows up in this measurement (Main zone being negative and bedrooms being positive). Airflow from the Garage to the bedroom zones was small for all tests (the garage was connected to the house by only one wall adjacent to the Main zone), but even so, airflow to the Master zone was slightly higher for the Exhaust systems, as makes sense since the exhaust fan was located in the Master zone.

Airflow from the vented attic to the living space zones for House 1 and House 2 is shown in

Figure 4. The Exhaust with mixing system consistently shows the highest airflow from the attic, followed by Exhaust, CFIS, ERV, and Baseline. By comparing the results between House 1 and House 2, it becomes clear that:

a) The Exhaust system was moving 20% of its ventilation air (10 cfm (5 L/s)) from the vented attic in House 1 to the Main zone. About 7 cfm (3 L/s) or another 14% of the Exhaust ventilation air in House 1 was moving from the Attic to the bedroom zones. A total of 34% (17 cfm (8 L/s) out of 50 cfm (24 L/s)) of the ventilation air for the Exhaust system in House 1 was coming from the vented attic. In comparison, for the unvented attic of House 2, the Exhaust system moved only 2% of its ventilation air from the Attic to the Main zone. This indicates that the exhaust makeup air path to outside was more resistive through the unvented attic spray-foamed roof than through the vented attic ceiling with recessed light penetrations.

b) In both houses, some central air distribution system return side leakage was causing the CFIS and Exhaust with mixing ventilation systems to move about 10 cfm (5 L/s) of attic air to the Main zone. However, there is a big difference in ventilation effectiveness between the 10 cfm (5 L/s) in a) and the 10 cfm (5 L/s) in b). In a), it is 10 cfm (5 L/s) out of 50 cfm (24 L/s) of what was expected to be good ventilation air, whereas in b) it is 10 cfm (5 L/s) out of about 1000 cfm (472 L/s) of recirculated and conditioned/filtered air. For the CFIS system, the full amount of expected outside air was still being delivered from a known outdoor intake location, whereas for the Exhaust system of House 1, 34% of the expected outside air was from the vented attic.

Inter-zonal airflows between the four living space zones (common area and three bedrooms) were calculated in both directions from the PFT measurements. The Baseline system showed little of this inter-zonal airflow in all cases, which was expected and confirmed that at least the trends indicated by the inter-zonal airflow results were reliable. All systems showed little inter-zonal airflow between bedrooms. As expected with the largest central air distribution system return air inlet in the Main zone, the most inter-zonal airflow was between the Main zone and bedroom zones for the CFIS and Exhaust with mixing cases. The Exhaust cases showed significant airflow (about 15 cfm (7 L/s)) from Main to Master and no airflow in the reverse direction, as expected with the exhaust fan located in the Master zone. Otherwise, the Exhaust systems showed little inter-zonal airflow and distribution of ventilation air. The ERV system showed little airflow from Main to bedrooms and between bedrooms, but relatively high airflow (10-20 cfm) (5-9 L/s) from bedrooms to the Main zone, as expected since the ERV system supplied fresh air to the bedrooms and exhausted stale air from the Main zone.


This study compared two nearly identical houses under baseline conditions and under operation of four different mechanical whole-building ventilation systems. Testing of exhaust ventilation from the master bathroom showed low uniformity of outdoor air exchange among the living space zones compared to the supply and balanced ventilation systems. Indoor air recirculation by a central air distribution system can help improve the exhaust ventilation system air change uniformity by way of air mixing. The exhaust ventilation testing showed that while only a small amount of air was drawn from the garage (which was connected to the Main zone by one wall) a total of 34% of the ventilation air came from the vented attic. Inter-zonal airflows between bedrooms were small for all systems. The most inter-zonal airflow was between the Main zone and bedroom zones for the CFIS and Exhaust with mixing cases.


This project was supported by the U.S. Department of Energy, Building Technologies Office, Building America Program. The Texas Allergy, Indoor Environment, & Energy Institute (TxAIRE) at The University of Texas-Tyler, under direction of Roy Crawford, Ph.D. and John Vasselli, provided the two lab houses for this research. Daniel Bergey, formerly of Building Science Corporation assisted in the testing.


Dietz, Russell N. 2006. "Instructions: Brookhaven Air Infiltration Measurement System." Brookhaven National Laboratory, Upton, NY. Revised: February 2006.

Dodson, Robin E., Jonathan I. Levy, James P. Shine, John D. Spengler, Deborah H. Bennett. 2007. "Multi-zonal air flow rates in residences in Boston, Massachusetts." Atmospheric Environment 41 (2007) 3722--3727.

Hendron, R., A. Rudd, R. Anderson, D. Barley, A. Townsend 2007. Field Test of Room-to-Room Distribution of Outside Air with Two Residential Ventilation Systems. IAQ 2007: Healthy & Sustainable Buildings Conference Proceedings. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

Leadererr, Brian P., Luc Schaap, Russell N. Dietz. 1995. "Evaluation of the Perfluorocarbon Tracer Technique for Determining Infiltration Rates in Residences." Environ. Scl. Technol., Vol. 19, No. 12, 1225-1232.

Rudd, Armin and Daniel Bergey (2014). "Ventilation System Effectiveness and Tested Indoor Air Quality Impacts." Prepared for the National Renewable Energy Laboratory, Golden, CO, for the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Building America Program. February.

Rudd, Armin (2011). "Ventilation Guide--Fully Updated." Building Science Press, Somerville, MA. September. ISBN-10: 0-9755127-6-5.

Rudd, Armin, and Joseph Lstiburek 2008. Systems Research on Residential Ventilation. Proceedings of the 2008 ACEEE Summer Study on Energy Efficiency in Buildings, Pacific Grove, California, August. American Council for an Energy Efficient Economy, Washington, D.C.

Rudd, Armin, Hugh Henderson, Jr., 2007. Monitored Indoor Moisture and Temperature Conditions in Humid Climate U.S. Residences. ASHRAE Transactions (17, Dallas 2007). American Society of Heating Refrigeration and Air-Conditioning Engineers, Atlanta, GA.

Rudd, Armin, Joseph Lstiburek 2001. Clean Breathing in Production Homes. Home Energy Magazine, May/June, Energy Auditor & Retrofiter, Inc., Berkeley, CA.

Rudd, Armin, Joseph Lstiburek 2000. Measurement of Ventilation and Inter-zonal Distribution in Single-Family Homes. ASHRAE Transactions 106(2):709-18, MN-00-10-3, V.106, Pt.2., American Society of Heating Refrigeration and Air-Conditioning Engineers, Atlanta, GA.

Townsend, A., A. Rudd, and J. Lstiburek 2009b. A Method for Modifying Ventilation Airflow Rates to Achieve Equivalent Occupant Exposure. ASHRAE Transactions 115(2).

Armin Rudd


Armin Rudd is Principal at ABT Systems LLC, Annville, Pennsylvania, and formerly Principal at Building Science Corporation, Somerville, Massachusetts when this research was conducted.

(1) The AIMS error analysis uses a 5% error in the estimate of the volume of the room, a 7% error in the source emission rate, and a 10% error in the CATS PFT concentration when there is only a single CATS in the zone. A full error analysis description is in Leadererr et al. 1995.
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Author:Rudd, Armin
Publication:ASHRAE Conference Papers
Date:Jun 22, 2014
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