RESULTS FROM HOUSE APPLIANCE SAFETY AND DEPRESSURIZATION TESTS CONDUCTED ON SINGLE FAMILY HOUSES UNDERGOING SOUND INSULATION DL Bohac * Center for Energy and Environment, Minneapolis, MN USA ABSTRACT Extensive house appliance safety and depressurization tests have been conducted on houses before and after they have received acoustic treatments as part of an airport sound insulation program. Results show that 25% of the ovens do not initially meet the program carbon monoxide standard of 150 ppm. Only 3% of the natural draft water heaters and 8% of the furnaces exceed the program standard of 100 ppm. For furnaces the failure rate doubles when the test is performed under down-draft conditions. Combustion spillage tests have shown that about 20% of natural draft water heaters and 10% of furnaces fail a worst-case spillage test. About half of the failures appear to be the result of excessive depressurization and the other half to deficient combustion vent systems. Twelve percent of the houses have a measured depressurization greater than the guideline for the combustion appliances in the house. INDEX TERMS Carbon monoxide, Combustion products, Natural gas, Performance, Pressure difference INTRODUCTION Over 6,500 houses near the Minneapolis-St. Paul International Airport have been treated by a Sound Insulation Program funded by the Metropolitan Airports Commission. A customized treatment package is designed to reduce the interior sound level by five dba; six to eight dba is typically achieved. The sound insulation measures are provided at no cost to the homeowner and include, as needed: new or reconditioned prime windows, acoustic storm windows, acoustic primary and storm doors, roof vent baffles, wall insulation, attic insulation, chimney vent caps, air sealing, air conditioning, and replacement furnace. Due to concerns of existing deficiencies that need to be corrected and the possibility that program measures may adversely effect indoor air quality and appliance venting, extensive house air leakage and appliance combustion safety tests are conducted on every house both before and after acoustic measures are installed. Since January 1997, over 3,750 houses entering the program have been tested. In addition, the test was offered to over 3,100 houses that had been previously treated by the program nearly 2,300 of those houses were tested. Results of the tests are used to design properly sized, continuous, exhaust ventilation and necessary combustion venting upgrades. In some cases power vented water heaters or chimney-top draft inducers are installed to help protect against depressurization induced combustion gas spillage. * Contact author email: dbohac@mncee.org 594
METHODS Each evaluation requires two to four hours and includes: measurements or estimates of exhaust fan flow rates mechanical system inspection multi-point house air leakage test multiple measurements of the combustion zone worst-case depressurization appliance flue carbon monoxide measurements combustion spillage vent tests of natural and induced draft appliances Almost all measurements are taken using a data logger that is interfaced to a laptop computer. The data loggers have eight analog voltage and at least three auto-zeroing pressure channels. Computer programs have been written to perform the house air leakage and combustion vent tests. Data is recording at one second intervals and stored in a text file. A separate computer program is used to display summary results and for manual data entry into a central data base consisting of over 500 data fields. The following is a brief description of the appliance combustion safety tests. Flue Carbon Monoxide Carbon monoxide (CO) is measured using a digital type gauge with the probe inserted in the throat or flue of a combustion appliance. The values are recorded as measured and are not adjusted for excess combustion air (i.e. air-free ). CO gauges are auto-zeroed on a daily basis using outdoor air and the span is adjusted monthly using a 100 ppm calibration gas. The Program standard for ovens is a CO concentration less than or equal to 150 ppm after five minutes of operation. For vented appliances (i.e. water heaters, furnaces, and boilers) the standard is a steady-state concentration less than or equal to 100 ppm under normal and downdraft venting conditions. A large blower installed in the house is used to depressurize the house to induce down-draft conditions in the appliance vent. Initial tests (Bohac and Brown, 1997) indicated a two-fold increased incidence of elevated CO under down-draft conditions. Combustion Vent Spillage A combustion spillage test is performed to determine if a significant fraction of the combustion gases are entering the house at the start of an appliance firing cycle. This test is conducted on all natural and induced draft vented appliances. The test is performed in accordance with the procedure recommended in the National Fuel Gas Code, Appendix H (Lemoff, 1997). Before the spillage test begins, the combustion zone (CZ) to outside pressure difference is averaged over a 30-second period to establish baseline pressure conditions. All exhaust appliances (exhaust fans, dryers, central vacuums) are then activated, interior doors are opened or closed, and the heating system air handler is turned on or off as necessary to achieve worst-case (WC) pressure conditions in the combustion zone. Another 30-second CZ/outside pressure is then recorded and the baseline is subtracted to determine the WC combustion zone pressure. The appliance burner is activated after the WC pressure test is complete. The technician releases chemical smoke and three temperature sensors are installed at the lower edge of the appliance draft hood to evaluate the occurrence of combustion gas spillage. Complete venting is established when the chemical smoke travels up the vent system. Technicians also consider the difference between the draft hood sensor and ambient air temperatures to determine 595
whether there is significant spillage. Spillage is likely if the average temperature difference for the three sensors is greater than 22C or if any one sensor has a difference greater than 31C. The appliance fails the spillage test if complete venting is not achieved by the end of the spill test period. The spillage time criterion is three minutes for water heaters and boilers and one minute for furnaces. After five minutes of operation in WC conditions the test is continued under natural conditions. While the appliance burner remains on, the air handler and all exhaust appliances causing the WC conditions are turned off. After the appliance venting has stabilized, the combustion spillage and flue CO are recorded. The down-draft flue CO measurement is conducted after the vent tests are complete. RESULTS AND DISCUSSION Over ninety percent of the tested houses were constructed from 1920 to 1955. However, there is significant variety in many construction characteristics. About 60% of the houses are 1 ½ story and the rest are evenly split between single and two story types. About half the houses have stucco siding, 15% have metal or vinyl siding, 15% have wood siding, and 10% have brick siding. Almost all of the houses have basements and a small percentage have crawl spaces. The average floor area, including the basement, is 230 m 2 with a 10 th percentile area of 164 m 2 and a 90 th percentile area of 298 m 2. A high fraction of the houses have natural gas (NG) appliances. Ninety nine percent of the houses have NG furnaces or boilers, 97% have NG water heaters, and 61% have NG ovens. As shown in Table 1, 25% of the 2,891 ovens had a CO level greater than the standard of 150 ppm after five minutes of operation. CO levels were greatest at the start of the burn cycle with 74% of the ovens having a concentration greater than 150 ppm after 2 minutes of operation. However, only 13% of the ovens had a CO level greater than 150 ppm at steadystate burner operation (i.e. the minimum CO concentration reached while the burner stills operates continuously). If an oven does not meet the Program standard, homeowners are required to have the oven repaired and an acceptable level confirmed before they can continue in the Program. When ovens were tested after acoustic treatments were complete (a time delay of approximately one year from the first test) the failure rate was only 7%. In addition, the fraction of ovens with a 5 minute CO level greater than 250 ppm was only 3%. This indicates that there was significant persistence in the oven CO reduction due to the required repairs. Table 1. Frequency of Natural Gas Appliance Carbon Monoxide Range Oven Water Heater Nat. Drft. Furnace (ppm) 2 min. 5 min. St. State Norm. DD Norm. DD <= 25 4% 15% 27% 90% 88% 85% 78% 25-50 4% 21% 27% 5% 5% 5% 5% 50-100 10% 26% 23% 1% 2% 2% 2% 100-150 9% 13% 10% 0% 1% 2% 2% 150-250 17% 12% 7% 1% 1% 1% 2% 250-500 28% 9% 4% 1% 1% 1% 2% > 500 28% 4% 2% 1% 2% 4% 7% Fail Std. 25% 3% 4% 8% 14% n = 2,891 1,290 548 Note Norm. indicates measurements conducted when the appliances are venting properly and DD indicates measurements conducted while a down-draft is induced. 596
The CO levels and failure rates for natural draft water heaters and furnaces were significantly lower than that of the ovens (note the sample size for water heater and furnace CO tests was lower than that for ovens because only the sample only includes results from houses entering the program since 1997 that had pre and post-treatment measurements). Only 3% of the water heaters had elevated CO under normal venting conditions and there was only a slight increase in the failure rate under down-draft conditions. The failure rate for furnaces was greater and the failure rate for normal venting conditions nearly doubled when the test was performed under down-draft conditions. Experience has shown that standard clean and tune maintenance of a furnace is able to reduce elevated CO under down-draft conditions. It has been hypothesized that the down-draft test helps identify appliances that are starting to go out of tune since the test puts an added stress on the combustion process. However, no tests have been performed to confirm this hypothesis. Results from the air leakage tests conducted before any acoustic treatments were applied indicate that the average house air leakage was 7.6 air changes per house at a depressurization of 50 Pa (ach50) with a 10 th percentile leakage of 5.0 ach50 and 90 th percentile leakage of 10.5 ach50. Almost all of the houses have a clothes dryer that exhausts to the outside, 41% had bathroom exhaust fans, and 41% had kitchen exhaust fans. The average flow rate of the bath fans was 19 L/s and the average for kitchen fans was 72 L/s. For all the houses the flow rate from the dryer and all exhaust fans averaged 100 L/s. Higher exhaust flows and tighter houses are a cause for some concern since 97% of the houses had a natural draft, natural gas water heater. The Canadian Spillage Test (CAN/CGSB, 1995) specifies a depressurization limit of 5 Pa for natural draft appliances. Section 900 of the Minnesota Energy Code (Minnesota Energy Code Chapter 7672, 2000) also specifies a depressurization limit of 5 Pa for most natural draft appliances and a limit of 2 Pa for natural draft water heaters. The Ventilation Standards Committee for the Program recommended the depressurization guidelines shown in Table 2. It is important to note that the combustion zone WC measurement and depressurization guideline are not used to pass or fail an appliance. The depressurization values are only used as a design guideline for predicting the likelihood of a depressurization problem after a house has been tightened and exhaust ventilation added. Table 2. Program Depressurization Guideline Appliance Type Depressurization Guideline (Pa) Individual (orphan) water heater (WH) 2 Natural draft WH vented with furnace or boiler 3 Nat. draft WH vented with ID furnace or boiler 5 Individual natural draft furnace or boiler 5 Individual Induced Draft furnace or boiler 15 Power and direct vent 25 WC combustion zone pressure measurements conducted on 1,427 houses before they had acoustic treatments indicated that 28% had a depressurization greater than 2 Pa, 14% were greater than 3 Pa, 4% were greater than 5 Pa, and less than 1% were greater than 10 Pa. About 65% of the houses had ND water heaters and furnaces or boilers vented in common (3 Pa guideline), 20% had ND water heaters vented in common with ID furnaces or boilers (5 Pa), 9% had orphan ND water heaters (2 Pa), and 4% had individually vented ND furnaces or boilers (15 Pa). When the measured WC pressure was compared to the depressurization 597
guideline for the combustion appliances in the house it was found that 12% had a measured depressurization greater than the guideline. This indicates that if the appliance vent systems were properly sized and the guidelines were appropriate, slightly more than 10% of the houses had appliances that were susceptible to spillage under WC conditions. In addition, if the guidelines were reduced by 1 Pa, 23% of the houses would have a depressurization greater than the guideline. If the guidelines were increased by 1 Pa, only 7% would have a depressurization greater than the guideline. Table 3 displays the results of spillage tests on 1,303 water heaters and 554 natural draft furnaces that were located in houses that had not yet received acoustic treatments. About 20% of the water heaters failed the WC condition spillage test and about 10% of the natural draft furnaces failed the test. For both appliances, the failure rate is reduced by half when the test was performed under natural conditions (i.e. exhaust appliances not active). This suggests that about half of the spillage failures were due to excessive depressurization and half due to deficiencies in the combustion vent system. The measured WC pressures appear to confirm that assumption. For both appliances the median measured WC pressure was less than -3.3 Pa for the appliances that passed the natural condition test and failed under WC conditions while the median WC pressure was about three times less for the appliances that either passed or failed both tests. In addition, for the appliances that passed under natural conditions and failed under WC conditions the fraction of houses with a measured WC pressure greater than the limit was five times greater than that for the appliances that passed or failed both tests. The common vent (CV) ratio, or ratio of the total gas input rate for the appliances connected to the common vent divided by the common vent capacity specified by the National Fuel Gas Code (Lemoff, 1997), was computed for the appliances. A value less than 1.0 indicates that the common vent is undersized. As displayed in Table 3, the average CV ratio was typically 0.2 greater for the appliances that passed the spillage test under natural conditions compared to the appliances that failed under natural conditions. This indicates that compliance with the recommended CV capacity has some ability to predict proper combustion venting under natural house conditions. Table 3. Worst Case Depressurization and Spillage Test Results Natural Draft Water Heater Natural Draft Furnace Spillage CZ WC WC < CV CZ WC WC < CV WC Nat. Freq. (Pa) Limit Ratio Freq. (Pa) Limit Ratio Pass Pass 81% -1.0 8% 0.97 90% -1.1 10% 0.91 Fail Pass 9% -3.5 51% 0.96 6% -3.3 50% 0.86 Fail Fail 11% -1.1 9% 0.79 4% -1.2 5% 0.65 Note: CZ combustion zone, WC worst case, and CV common vent. The spillage failure results are consistent with those reported by Nagda, Li, Koontz et al. (2001). For that study the WC failure rate for water heaters was slightly more than double that of furnaces. The water heater failure rate of 28.6% was greater than for this set of houses, but the median WC pressure of 3.4 Pa was also significantly greater than the median level of 1.2 Pa found for the sound program houses. CONCLUSIONS AND IMPLICATIONS Measurements on natural gas ovens indicated that 25% of the ovens had carbon monoxide levels greater than 150 ppm after five minutes of operation. When the ovens that failed the 150 ppm standard were repaired and the tests repeated after one year, the CO failure rate dropped to 7%. This implies that the repairs have a persistent effect, but maintenance may 598
need to be repeated annually for ovens to maintain proper operation. Only 3% of the natural draft water heaters and 8% of the furnaces failed the CO standard of 100 ppm. However, the furnace failure rate almost doubles when the test is performed under down-draft conditions. A higher level of safety can be obtained by performing the test under down-draft conditions particularly for appliances in a situation where depressurization inducted down-drafts are more likely to occur. Combustion spillage tests for houses that had not yet received acoustic treatments have shown that about 20% of natural draft water heaters and 10% of furnaces fail a worst-case spillage test. About half of the failures appear to be the result of excessive depressurization and the other half to deficient combustion vent systems. Appliances that fail under natural conditions are likely to spill combustion gases under most conditions and steps should be taken to remedy the deficiencies. Twelve percent of the houses have a measured depressurization greater than that recommended for the combustion appliances in the house. Spillage failures due to excessive depressurization will likely only occur at the same frequency as the worstcase depressurization conditions. However, even an infrequent, depressurization-induced spillage event coupled with high levels of contaminant (i.e. carbon monoxide) can have catastrophic occupant health effects. Given the magnitude of the potential impact, it is reasonable to protect against these occurrences by performing worst-case spillage tests and taking steps to remedy any failures. In some cases combustion appliances can be protected against depressurization by redesigning the venting system to comply with code requirements. In most cases natural draft appliances need to be replaced by induced-draft, power vent, or direct vent type equipment. Externally mounted draft inducers can also be a cost effective option. Appliance combustion safety tests are standard practice for federally funded low income weatherization programs in the United States. At the current time only the Minneapolis-St. Paul and Milwaukee County sound insulation programs conduct these tests after acoustic treatments are complete. The carbon monoxide and combustion spillage results from the Minneapolis-St. Paul program indicate that testing appliances and addressing failures would provide a significant benefit to occupants and reduce liability concerns for program funders. ACKNOWLEDGEMENTS The indoor air quality evaluations are being funded by the Minneapolis St. Paul Metropolitan Airports Commission as part of the Part 150 Residential Sound Insulation Program. The high quality of the test results are only possible through the dedication of the technicians who perform this work. REFERENCES Bohac D, and Brown T. 1997. Results From IAQ Evaluations on Cold Climate Single Family Houses Undergoing Sound Insulation, Proceedings of the Healthy Buildings/IAQ 97: Global Issues and Regional Solutions conference, Vol 3, pp325-330. Washington DC. CAN/CGSB. 1995. CAN/CGSB Standard 51.71-95, The Spillage Test, Ottawa: Standard Council of Canada. Lemoff T. 1997. National Fuel Gas Code Handbook. Avon, MA: National Fire Protection Association. Minnesota Energy Code Chapter 7672. 2000. Detached Single-Family and Two-Family Dwellings, State of Minnesota. Nagda N, Li Z, Koontz M, and Natarajan S. 2001. Depressurization-Induced Backdrafting and Spillage: Assessment of Test Methods. ASHRAE Transactions. Vol. 108 (1). 599