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Rule 4902 – Residential Water Heaters 

Emissions Inventory:

Tons per day – summer season 

Pollutant 2005 2008 2011 2012 2014 2017 2020 2023 

NOx 1.37 1.27 1.30 1.31 1.33 1.36 1.39 1.42 

VOC 0.10 0.11 0.11 0.11 0.111 0.11 0.12 0.12 

(Source: District 2007 Ozone Plan) 

District Rule 4902 Description:

Units in this source category are water heaters located in private residences with heat 

input of 75,000 British thermal units per hour (Btu/hr) or less. The following units are 

exempt: water heaters using fuels other than natural gas, natural gas-fired heaters 

used exclusively to heat swimming pools or hot tubs, and water heaters used 

exclusively in recreational vehicles. 

How does District Rule 4902 compare with federal rules and regulations?

A. EPA – Control Technique Guidelines (CTG) 

There is no CTG for this source category. 

B. EPA – Alternative Control Technology (ACT) 

There is no ACT for this source category. 

C. Standards of Performance for New Stationary Sources (NSPS) 

There is no NSPS for this source category. 

D. National Emission Standards for Hazardous Air Pollutants (NESHAPs) and 

Maximum Achievable Control Technologies (MACTs) 

There is no NESHAP or MACT for this source category. 

How does District Rule 4902 compare to rules in other air districts?

District staff compared NOx limits and optional control requirements in District Rule 

4902 to comparable requirements in rules from the following California nonattainment 

areas: 
San Joaquin Valley Unified Air Pollution Control District April 16, 2009

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 South Coast AQMD Rule 1121, as amended September 2004 

 Bay Area AQMD Regulation 9 Rule 6, as amended November 2007 

 Sacramento Metropolitan AQMD Rule 414, as adopted August 1996 

 Ventura County APCD Rule 74.11. as amended December 1986 

A. South Coast AQMD Rule 1121 

South Coast AQMD Rule 1121 (Control of NOx from Residential Type Natural GasFired Water Heaters) applies to manufacturers, distributors, retailers, and installers of 

natural gas-fired water heaters, with heat input rates less than 75,000 Btu per hour. 

Water heaters in recreational vehicles are exempt from control requirements. For water 

heaters not in mobile homes, has two NOx limits for residential water heaters that are 

considered equivalent – one based on heat input to the unit and the other based on 

heat output from the unit. The emission limits are no more than 10 nanograms of NOx 

per joule of heat output (23 pounds per billion Btu (lb/billion Btu) of heat output) or 15 

parts per million by volume at 3% oxygen, dry (17.5 lb/billion Btu of heat input). Water 

heaters in mobile homes have a different NOx limit - no more than 40 nanograms of 

NOx per joule of heat output (93 lb/billion Btu of heat output) or 55 parts per million by 

volume (ppmv) at 3% oxygen, dry (71 lb/billion Btu of heat input). 

These limits were phased in over three years beginning in 2006; meaning that the limits 

have not been fully implemented until this year. From this perspective, the lower limits in 

South Coast AQMD Rule 1121 represent RACT for future rule-making and not for 

current analysis of District Rule 4902. District Rule 4902 is undergoing rule 

development as part of the District’s 8-hour ozone attainment strategy. District staff 

anticipates incorporating the South Coast AQMD limits for this source category. 

Adoption of the rule amendments is expected in the first quarter of 2009. 

B. Bay Area AQMD Regulation 9 Rule 6 

Bay Area Regulation 9 Rule 6 (Nitrogen Oxides Emissions from Boilers and Water 

Heaters) is an umbrella rule that covers boilers and water heaters up to two million Btu 

per hour. The rule applies to all units regardless of whether the units are residential, 

commercial, industrial, or institutional use. Units with heat input of not more than 

75,000 Btu per hour are generally considered residential units, so the portion of the rule 

that applies to units of this size will be used for the analysis. This rule was amended 

recently. The current requirements for residential-type units is certified units with 

emissions of not more than 40 nanograms NOx (calculated as NO2) per joule of heat 

input. Beginning in 2009 and phasing in over the next three years, the NOx limit will be 

10 nanograms NOx per joule of heat input. The new units would need to be certified to 

this lower limit. The 10 nanograms per joule limit mirrors the South Coast rule for this 

source category, however to date, the rule has not been fully implemented. From this 

perspective, the lower limits in the Bay Area AQMD rule represent RACT for future rulemaking and not for current analysis of District Rule 4902. District Rule 4902 is 

undergoing rule development as part of the District’s 8-hour ozone attainment strategy. 
San Joaquin Valley Unified Air Pollution Control District April 16, 2009

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District staff anticipates incorporating the lower limits for this source category. Adoption 

of the rule amendments is expected in the first quarter of 2009. 

C. Sacramento Metropolitan AQMD Rule 414 

Sacramento Metropolitan AQMD Rule 414 (Natural Gas-Fired Water Heaters) applies to 

any person who manufactures, distributes, offers for sale, sells, or installs any natural 

gas-fired water heater with a rated heat input capacity less than 75,000 Btu/hr. 

Exempted units include water heaters in recreational vehicles, water heaters in 

swimming pools and hot tubs and water heaters that use fuel other than natural gas. 

The rule requires that certified units be used. For water heaters used in mobile homes, 

the rule limits NOx emissions to 50 nanograms of NOx per joule of heat output (116 

lb/billion Btu of heat output). All other subject units are limited to NOx emission of no 

more than 40 nanograms NOx per joule of heat output (93 lb/billion Btu of heat output). 

Both NOx limits are calculated as NO2. The NOx limits in current District Rule 4902 are 

at least as stringent as the Sacramento Metropolitan AQMD Rule 414. 

D. Ventura County APCD Rule 74.11 

Ventura County APCD Rule 74.11 (Natural Gas-fired Residential Water Heaters – 

Control of NOx) applies to any person selling, offering for sale, or installing natural gasfired residential water heaters in Ventura County. The units subject to the rule have a 

rated heat input of less than 75,000 Btu/hour. Water heaters used in recreational 

vehicles and water heaters installed in mobile homes are exempt. The rule requires a 

certified unit that emits no more than 40 nanograms of NOx (calculated as NO2) per 

joule of heat output (93 pounds NOx per billion Btu of heat output). The NOx limits in 

current District Rule 4902 match the requirements of the Ventura County APCD rule. 

Conclusion

There are no federal rules for regulations covering air emissions for this source 

category. After careful evaluation of prohibitory rules in other California nonattainment 

areas, District staff concludes that current District Rule 4902 satisfies RACT for 

residential water heaters.