Proceedings: Indoor Air 2002

ROOM CHAMBER ASSESSMENT OF POLLUTANT EMISSION PROPERTIES OF LOW-EMISSION UNFLUED GAS HEATERS

SK Brown*, M Cheng and KJ Mahoney

CSIRO Building, Construction and Engineering, Melbourne, Victoria, Australia

ABSTRACT Pollutant emissions from unflued gas heaters were assessed in CSIRO’s Room Dynamic Environmental Chamber. This paper describes the assessment procedure and findings for major commercial “low-emission” heaters. The chamber was operated at controlled conditions of temperature, humidity, ventilation and air mixing, representative of those encountered in indoor environments. A fixed rate of heat removal from the chamber air ensured that the heaters operated at constant heating rates, typically ~6 MJ/h, simulating operation after warm-up in South-East Australian insulated dwellings. Pollutants assessed were nitrogen dioxide, carbon monoxide, formaldehyde, VOCs and respirable particulates. One heater was lower emitting for nitrogen dioxide, but emitted greater amounts of carbon monoxide and formaldehyde (the latter becoming significant to indoor air quality). When operated with low line pressure or slight misalignment of the gas burner, this heater became a hazardous source of these pollutants. Emissions from one heater changed little after continuous operation for three months.

INDEX TERMS Gas heater, Unflued, Unvented, Pollutant emission, Indoor air, Nitrogen dioxide, Formaldehyde, Carbon monoxide.

INTRODUCTION Unflued gas heaters have been used for many years in most Australian States as primary residential heaters, sometimes with restricted heater sizes or installation of fixed wall vents in the heated room. It is estimated that approximately 600,000 unflued gas heaters (natural gas or LPG) have been installed across Australia, about two-thirds of these in SE Australia.

Health concerns over pollutant emissions from unflued gas heaters in dwellings and schools were raised in the 1980s, particularly in relation to nitrogen dioxide (NO2) and carbon monoxide (CO) emissions (Brown, 1997). Industry response to these concerns led to the development of low-NOx heaters and specification of pollutant emission levels (Gas Installation Standards Committee, 1998), as follows: (a) “The amount of nitrogen dioxide produced by an indoor flueless heater per hour, when divided by the nominal gas consumption, shall not exceed 5 ng/J.” (b) “The CO/CO2 ratio for indoor flueless space heaters shall not exceed 0.004 when operating on available gas at the manufacturer’s specified test point pressure.”

The industry determined these emission levels according to “Method of Test 5.115/6–96 Emission Tests”, in which new heaters were operated at full capacity for a set period (approximately 15 minutes), after which exhaust gases were sampled. However, this is not how heaters typically operate in buildings, where they operate in an on–off mode with thermostat

* Contact author email: [email protected]

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control. Also, there are other toxic air pollutants that are potentially emitted from unflued gas heaters, such as formaldehyde, volatile organic compounds (VOCs) and respirable suspended particulates.

CSIRO investigated the emissions of the above pollutants from several new and used unflued natural gas heaters using a Room Dynamic Environmental Chamber designed to simulate the physical conditions found in buildings. Pollutant emission rates from the heaters were determined at conditions considered “typical” of residences in SE Australia. This report will present the chamber and pollutant sampling procedures used, and results for the performance of heaters.

METHODS Heaters Heaters were placed in the centre of the room chamber and were plumbed to the natural gas line (Victorian gas: ~90% methane, ~8% ethane) through a port in the chamber wall. Gas supply was regulated to provide a minimum gas inlet pressure (at heater bayonet connection) of 1.13 kPa with the heater operating at maximum gas consumption, as required by the Gas Installation Code AG601 (Gas Installation Standards Committee, 1998).

The heaters were natural gas unflued heaters of 17–18 MJ/h capacity, with thermostat operation and piezoelectric ignition. Note that the maximum input rate (heat capacity per room volume) for these experiments was 0.51 MJ/h/m3, which exceeds the AG Code limit for thermostat heaters of 0.4 MJ/h/m3. However, heater operation was limited in the experiments to 5–7 MJ/h heat input, equivalent to ~0.2 MJ/h/m3, which is much below the above limit. Also, this rate of heat input is that expected for a well-insulated dwelling in SE Australia after the heat-up stage (Delsante, 1999).

Gas consumption was measured using a gas meter (American Meter Co. DTM-200A) which was logged several times during each experiment. A line of best fit was fitted to the gas usage data to determine the gas consumption rate; alternatively, the latter was determined from the carbon dioxide emission rate of the heater.

Room dynamic environmental chamber This was a 4.0 × 3.0 × 2.7 m high room constructed of inert materials, such as stainless steel, glass and Teflon®, which has been shown to exhibit very low sink effects for VOCs (Mason et al., 1999). Air supplied to the room chamber was cleaned to reduce background pollutants to 3 3 3 levels of NO2 <5 µg/m , formaldehyde <10 µg/m , respirable particles <3µg/m , CO <1 ppm, total VOC (TVOC) 40–80 µg/m3. Supply air was temperature- and humidity-controlled to 23°C and 50% RH and supplied to a nominal ventilation rate of 2.0 air changers per hour (ACH). The actual ventilation rate in the room chamber was determined by dosing the chamber with the tracer gas sulfur hexafluoride (SF6) and monitoring its decay by gas infra- red spectrophotometer (ASTM, 1995). The chamber was operated at a positive pressure, initially 0.2 kPa, but this counteracted the gas supply pressure to the heaters, and subsequently a pressure compensation device was added to limit chamber positive pressure to 0.01 kPa for all measurements presented here, unless specified otherwise.

Air within the chamber was well mixed by recirculating air from floor level to ceiling level at a rate of 18 chamber volumes per hour. This caused air currents of approximately 0.3 m/sec adjacent to the heaters, which are typical of those found in buildings (Christianson et al., 1989). The air recirculation system was equipped with a chiller coil through which water at

638 Proceedings: Indoor Air 2002

16°C was circulated while the heater operated. Water condensation on the coil was prevented at this temperature, so that loss of water-soluble pollutants was prevented. Under these conditions, the chiller coil was capable of removing approximately 6 MJ/h of heat and, since the room was well insulated, this was the major means for removing heat in a controlled manner from the chamber and for regulating the operation of the heater.

Under well-mixed conditions, a pollutant source with a constant emission rate will lead to pollutant concentrations in the room chamber over time of: –Nt Ct = C∞ (1 – e ) (1) where Ct is pollutant concentration at time t; C∞ is pollutant concentration at t = ∞; and N is the chamber ventilation rate. At a ventilation rate of 2 ACH, it can be shown that at t = 2 hours, Ct = 0.98 C∞; and at t = 6 hours, Ct = 1.00 C∞. Thus, the chamber concentrations of pollutants are expected to reach 98% of steady-state levels after 2 hours of operation of a source with a constant emission rate. This assumes that no “sink” losses of the pollutants occur to chamber internal surfaces. If such losses occurred, then a steady state would not occur by 2 hours. Pollutant concentrations in initial experiments were measured at both 2 and 6 hours as a check for potential sink effects by reactive pollutants such as NO2 and formaldehyde. These were not found to differ, and so sampling only at 4 hours is considered to be acceptable in future emission experiments.

Pollutant sampling and analysis NO2 concentration was measured using the standard method ASTM D1607–91 “Standard Test Method for Nitrogen Dioxide Content of the Atmosphere (Greiss–Saltzman Reaction)”, using three fritted glass bubblers in series capturing chamber air, generally as 25 L samples. Duplicate measurements were made at each sampling time. CO and CO2 were monitored using a Q-Trak™ Model 8550/8551 IAQ Monitor (TSI Inc., USA). This instrument logged CO, CO2, temperature and RH at one-minute intervals using a probe positioned inside the room chamber. Data was averaged for one-hour periods at each sampling time.

Formaldehyde concentration was measured using the Australian Standard method AS 2364.6– 1995 ‘Determination of Formaldehyde-Impinger Sampling in Chromotropic Acid Method’, using two fritted glass bubblers in series to capture room chamber air, generally as 60 L samples collected over a one-hour period. Duplicate samples were collected at each sampling time. VOCs were sampled from chamber air onto Envirochem multisorbent tubes (containing Tenax TA/Ambersorb/Activated Charcoal), generally as 3–4 L samples collected over periods of 20–30 minutes. Duplicate samples were collected at each sampling time. These were analysed by thermal desorption/gas chromatography (GC)/flame ionisation detector (FID)/mass spectrometry (MS) under conditions described previously (Brown, 1999). The major 15–20 VOCs in each sample were quantified. The TVOC concentration was also estimated from the total GC peak area from 5 to 35 minutes (approximately C5-alkanes onwards and including ethanol but not methanol), expressed as toluene-equivalent concentration. Since very volatile organic compounds were observed in analyses up to 5 minutes of the GC program, these were also estimated by summing peak areas and expressing this measure as ΣVVOC.

Respirable suspended particles were logged at one-minute intervals using a Dustrak Model 8520 Aerosol Monitor (TSI Inc., USA) fitted with a nylon cyclone that provided a 4.0 µm cut-

639 Proceedings: Indoor Air 2002

point for particle sampling within the room chamber. Data was averaged for one-hour periods at each sampling time.

Heater test protocol The following heater test protocol was adopted: 1. Install heater and check for gas leaks; rectify all leaks external to the heater; if there are small leaks internal to the heater, record their magnitude and continue with testing, or consider abandoning the experiment. 2. Operate room chamber overnight at 23°C, 50% RH, 2.0 ACH, 18 chamber volumes recirculated per hour to reach stable conditions. 3. Sample chamber air for background levels of air pollutants. 4. Dose chamber with SF6 tracer gas on 2 to 3 occasions during the experiment and measure decay rate. 5. Start heater (remotely) at t = 0, with continuous logging of CO, CO2, temperature, RH and ACH; start chiller at set temperature of 16.0°C. 6. At t = 2.0 hours, sample NO2 and formaldehyde for 1 hour and VOCs for 20–30 minutes. 7. Read gas meter approximately 8 times over the 8-hour heating period. 8. At t = 6.0 hours, sample NO2 and formaldehyde for 1 hour and VOCs for 20–30 minutes. 9. At t = 7 hours, turn heater off.

Pollutant emission rates can be determined by two measures: (a) Emission rate per time (ERt, µg/h) from:

ERt = (C∞ – Cb) . N . V (2) 3 where C∞ is the chamber steady-state concentration at 2–3 or 6–7 hours (µg/m ); Cb is the background pollutant concentration in chamber prior to heater operation (µg/m3); N is the chamber ventilation rate (h–1); and V is the chamber volume (m3). (b) Emission rate per energy consumption (ERe, ng/J) from:

ERe = 1000 ERt/HR (3) where HR is the heating rate (MJ/h).

Gas consumption and heating rate The gas consumption rate was constant over each seven-hour experiment, as demonstrated by the close linear fit (R2 = 1.00) of gas meter readings with time of heater operation. This gas consumption rate can be used to estimate a heating rate (HR) for experiments from: HR (MJ/h) = gas consumption rate (m3/h) . heating value (MJ/m3) (4)

The heating rate was also estimated from the steady-state CO2 concentration (in ppm) achieved in each experiment (C∞), the background concentration (Cb), and the chamber ventilation rate. It was calculated from gas composition that 1 m3 of Victorian natural gas 3 3 would yield 1.11 m of CO2. Using the known heating value (HV, MJ/m ) of the gas, heating rate was estimated from: –6 HR (MJ/h) = (C∞ – Cb).N.V.HV.10 /1.11 (5)

The industry emission test uses CO2 production to estimate pollutant emission rates (Gas Installation Standards Committee, 1998) and so this approach was used in the study.

640 Proceedings: Indoor Air 2002

RESULTS Results for heaters from two manufacturers (A and B) and all pollutants except VOCs are presented in Table 1. VOC concentrations were generally low, especially for heater B for which chamber concentrations of ΣVVOC were 10–50 µg/m3 and TVOC 10–30 µg/m3 (main VOCs: 2-methylbutane, acetone, 3-methylbutanal and acetic acid). Higher VOC emissions were observed for heaters A1 and A2 for which chamber concentrations of ΣVVOC were 3 3 100–1900 µg/m (probably C3 and lower alkanes) and TVOC 20–300 µg/m (same main VOCs as above).

Table 1. Emissions from low-emission unflued natural gas heaters (manufacturers A and B) Heater Hours of use Heating rate Pollutant Room conc. Emission rate (MJ/h) (µg/m3) (ng/J)* A1 <20 6.8 NO2 290 3.0 CO 4 ppm 36 CO/CO2 ratio — 0.001 Formaldehyde 160 1.6 Resp. susp. particles <1 <0.01 A1 1200 6.2 NO2 270 3.1 CO 4 ppm 40 CO/CO2 ratio — 0.001 Formaldehyde 180 1.8 Resp. susp. particles <1 <0.01 A2 4000 5.7 NO2 190 2.5 CO 2 ppm 30 CO/CO2 ratio — 0.001 Formaldehyde 97 1.3 Resp. susp. particles <1 <0.01 B 4000 6.9 NO2 530 5.3 CO <1 ppm <15 CO/CO2 ratio — <0.001 Formaldehyde <10 <0.1 Resp. susp. particles <1 <0.01 A2 (low <200 5.0 NO2 180 2.4 supply P) CO 17 ppm 280 CO/CO2 ratio — 0.007 Formaldehyde 2100 30 Resp. susp. particles 1 0.01 * Industry goals: NO2 5 ng/J, CO/CO2 ratio 0.004.

DISCUSSION Under the tightly controlled conditions of the chamber test procedure, the pollutant emission behaviours of the heaters were able to be assessed. For example, heater A1 emissions were identical whether the heater was new or had been operated for 1200 hours. Also, even though nominally identical heaters from the same manufacturer, heaters A1 and A2 exhibited small differences in their emission rates. Heater B, from another manufacturer and using a different burner design, exhibited emissions much different from those of heaters A1 and A2.

The heater from manufacturer A was promoted as being much lower in NO2 emissions than the code requirement of 5 ng/J, and this was found for both heaters A1 and A2. However, even with these emission levels, it was observed that the one-hour WHO goal for NO2 of 210 µg/m3 (WHO 1997) was exceeded in the room chamber at a ventilation rate of 2 ACH and much greater concentrations are probable at lower ventilation rates. Also, these heaters

641 Proceedings: Indoor Air 2002

emitted CO and formaldehyde, the latter reaching chamber concentrations above typical indoor air guidelines (60–120 µg/m3). By comparison, heater B emissions of CO and formaldehyde were below detection, and emissions of VOCs were much lower than observed for heaters A1 and A2. Heater B did, however, emit NO2 just above the code requirement level of 5 ng/J. These results are consistent with a decrease in the combustion efficiency of the burner in heaters A1 and A2 compared to heater B. Clearly the definition of “low- emission” needs to consider more than just NO2 emissions.

In our initial experiments, the chamber backpressure of 0.2 kPa caused a low gas supply pressure to heater A2; emissions from these experiments are also presented in Table 1. It is seen that this had no influence on NO2 emissions but significantly increased carbon monoxide and formaldehyde emissions, chamber concentrations of the latter exceeding occupational exposure standards. In a further experiment we fitted a T-piece to the burner regulator of heater A2, causing a slight misalignment of the burner, and CO concentrations of approximately 30 ppm were reached despite correct gas supply pressure to the heater. These observations show that this heater was susceptible to much greater pollutant emissions if incorrectly installed (e.g. undersized piping) or maintained.

CONCLUSIONS A chamber test procedure for measuring pollutant emissions from unflued gas heaters has been developed and applied to commercial products. The procedure ensures the heater operates at a heating rate that is similar to that expected in practice, which entails cyclical operation at below full capacity. A broad range of air toxics can be evaluated since the heater is isolated in a room chamber supplied with clean air. Classification of emissions from unflued gas heaters needs to include pollutants in addition to nitrogen dioxide and carbon monoxide, especially formaldehyde, and to consider the potential for heater misuse at installation and operation.

ACKNOWLEDGEMENT The Department of Human Services, Melbourne, supported this project.

REFERENCES ASTM. 1995. Method E741–95, Standard Test Method for Determining Air Change in a Single Zone by Means of Tracer Gas Dilution, Philadelphia: American Society for Testing and Materials. Brown SK. 1997. Indoor Air Quality. Australia: State of the Environment Technical Paper Series (Atmosphere), Canberra: Department of Environment, Sport and Territories. Brown SK. 1999. Assessment of pollutant emissions from dry-process photocopiers. Indoor Air. Vol. 9, pp. 259–267. Christianson LL et al. 1989. Simulating residential room conditions. In Building Systems: Room Air and Air Contaminant Distribution, Christianson LL, ed. Atlanta: ASHRAE, pp. 218–220. Delsante A. 1999. CSIRO Building, Construction and Engineering, personal communication Gas Installation Standards Committee. 1998. AG103–1998, Approval Requirements for Gas Space Heating Appliances, Canberra: The Australian Gas Association. Mason MA et al. 1999. Comparison of the sink characteristics of three full-scale environmental chambers, Proceedings of Indoor Air ’99, Edinburgh, Scotland, 8–13 August, Vol. 5, pp. 149–154. World Health Organization 1997. Nitrogen Oxides, Environmental Health Criteria No. 188, WHO Copenhagen.

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