The Bay Area Air District's zero-NOx water heater rule changed on November 4, 2026, when its Board of Directors adopted amendments to Regulation 9, Rule 6. As of November 2026, a gas tank water heater over 30 gallons and rated 75,000 BTU per hour or less that was manufactured after January 1, 2028 can't be sold or installed in the Air District unless it emits zero nitrogen oxides (NOx) or an exemption applies, and a companion rule sets January 1, 2029, for gas furnaces. Neither rule makes you remove equipment that still works.
Our guide to heat pumps for mid-century homes covers the equipment side. This one covers the rule itself: what it reaches, the exemption certificates added in November 2026, how the dates fall on older gas equipment, and what to gather before a water heater fails. It draws on the amended rule text the Air District posted on October 30, 2026, its November 4, 2026 press release and flyer, and its appliance FAQ, all read on November 6, 2026.
What Rules 9-6 and 9-4 cover
Rule 9-6 limits NOx emissions from natural gas-fired water heaters and boilers. It was adopted in 1992 and applies to anyone who sells, installs or offers one for sale for use in the Air District, which covers Alameda, Contra Costa, Marin, Napa, San Francisco, San Mateo and Santa Clara counties, plus southwestern Solano and southern Sonoma counties. Rule 9-4 does the same for natural gas-fired furnaces, which it defines as space-heating products with a heat input under 175,000 BTU per hour.
Both are point-of-sale rules. The Air District's flyer on the amendments says the zero-NOx standard applies only to water heaters manufactured after the compliance date, and its building appliances page says the rule applies only when a water heater is replaced, with no requirement to replace a functioning unit. Rule 9-6 also exempts water heaters that burn a fuel other than natural gas (Section 9-6-110.3).
The compliance dates as of November 2026
January 1, 2028, gas tank water heaters. Storage-tank water heaters larger than 30 gallons and rated 75,000 BTU per hour or less, manufactured after this date, must emit 0 nanograms of NOx per joule of heat output (Section 9-6-301.5). The November amendments moved this date from January 1, 2027.
January 1, 2029, gas furnaces. Any natural gas furnace manufactured after this date must emit 0.0 nanograms per joule (Section 9-4-301.3). The rule says this includes non-central installations such as wall furnaces. Furnaces for mobile homes are excluded.
January 1, 2033, larger units. Gas boilers, tank water heaters and instantaneous (tankless) water heaters rated above 75,000 and up to 2,000,000 BTU per hour, manufactured after this date, must emit 0.0 nanograms per joule (Section 9-6-303.5). The amendments moved this date from 2031.
Gas tank water heaters of 30 gallons or less aren't in the zero-NOx section; the rule holds those made after 2011 to a separate 10-nanogram limit. Mobile home water heaters are exempt. The furnace date hasn't been amended, but Rule 9-4 requires a readiness report to the Board at least two years before 2029; the water heater date moved after a similar report in December 2024.
Exemption certificates, added in November 2026
The amended rule creates an Exemption Certificate, issued by the Air District before installation, that lets a person buy and install one gas water heater that doesn't meet the zero-NOx standard. Under Section 9-6-405, the application asks for the verified installation address, the applicant's contact details and proof of eligibility. The flyer says the Air District will set up a website that issues certificates immediately, that owners can self-certify for the financial and space exemptions, and that the electrical exemptions need a contractor. The exemptions:
Financial (9-6-111). Owners enrolled in one of the public assistance programs the rule lists, or who meet its area-median or housing-cost tests.
Ventilation space (9-6-112.1). Replacing a gas water heater in an enclosed space under 700 cubic feet that can't be fitted with a louvered door, transfer grille or duct because it doesn't adjoin a space over 700 cubic feet, attic included.
Ceiling height (9-6-112.2). A garage with a ceiling under 7.2 feet, or any other indoor space under 6 feet.
Electrical, certified by a licensed contractor (9-6-112.3). Knob-and-tube wiring plus more than 50 feet of conduit to the panel or subpanel; a panel under 100 amps; a new 240-volt connection needing more than 50 feet of conduit; or a panel with no open breaker space or expansion slots.
Time-limited (9-6-113). Water heaters over 30 gallons and up to 75,000 BTU per hour, manufactured before January 1, 2033, may still be installed with a certificate in listed cases, including a hydronic water heater (one that heats potable water and also supplies space heating), buildings with five or more units, and deed-restricted housing.
Temporary replacement (9-6-114). A licensed contractor may put in a non-compliant gas unit with a certificate while a permanent one is arranged, then must remove it within 12 months or make it permanent under another exemption.
The ventilation, ceiling and electrical exemptions can be used once per installation address and don't apply in new construction. The flyer says this gives owners time to upgrade before the next replacement, roughly 13 years out. From January 1, 2028, anyone selling non-compliant units must post a notice about certificates, and sellers and contractors must keep sale and installation records for at least five years.
How the dates fall on mid-century gas equipment
Which date applies depends on the fuel, the type of unit and its rated heat input, all shown on the rating plate.
A gas tank water heater in a closet or garage. If it holds more than 30 gallons and is rated 75,000 BTU per hour or less, a replacement manufactured after January 1, 2028 must meet the zero standard unless an exemption applies. A tight closet or low garage ceiling may qualify for a space exemption, so measure first.
A gas boiler feeding radiant floor tubing. Boilers rated above 75,000 BTU per hour fall under the 2033 date. If the plate shows 75,000 or less, ask the Air District which section applies, because the 2028 section is written for storage-tank water heaters. Our radiant heat guide covers the system itself.
One gas unit for both hot water and heat. The rule calls this a hydronic water heater. In the class larger than 30 gallons and 75,000 BTU per hour or less, one manufactured before January 1, 2033 can be installed with a certificate.
A tankless water heater. The rule's term is instantaneous water heater. Those rated above 75,000 BTU per hour are in the 2033 group.
A wall furnace or forced-air furnace. Rule 9-4's 2029 standard covers both.
Propane. Not covered, according to the FAQ.
What the rule means when you buy or sell
The rules apply when an appliance is sold or installed, not when a house is sold. Nothing in Rule 9-6 or Rule 9-4 requires a seller to replace a working gas water heater or furnace before closing. Section 9-6-403 puts the model number, certification status and date of manufacture on the rating plate, so a photo of it shows which category a unit is in.
Two state rules still apply. Health and Safety Code Section 19211 requires water heaters to be braced, anchored or strapped, and the seller must certify that in writing; our home sale requirements guide covers it. And the California Energy Commission's 2025 compliance manual treats replacing a water heater or a space-heating system (furnace, wall heater, heat pump or radiant floor) as an alteration, with the Energy Code compliance form submitted with the building permit application. Ask for permit records on any recent replacement.
What to check now, before the unit fails
Photograph the rating plates on the water heater, boiler and furnace: fuel, input in BTU per hour, tank size in gallons and date of manufacture.
Measure the water heater's room (length, width and height) to get its volume in cubic feet, and note whether it adjoins a larger room or attic where a louvered door, grille or duct could go.
For a garage installation, measure the ceiling height.
Note the main panel's amp rating, whether it has open breaker spaces, and how far the water heater is from the panel. Our electrical panel guide covers what to look for.
Write down any knob-and-tube wiring an electrician has found.
Ask a licensed contractor to price a compliant replacement and, if you may qualify, the exemption route.
Check the Air District's building appliances page for the certificate system; the flyer says it will connect owners to available incentives.
This is general information as of November 2026, not legal advice. The dates have changed before, so confirm the current rule and any exemption with the Bay Area Air District, use a licensed contractor for the work, and check permit requirements with your city's building department.
Frequently asked questions
Does the zero-NOx water heater rule make me replace my gas water heater?
No. The rule governs what can be sold and installed, and the Air District says it doesn't require replacing a functioning unit. As of November 2026, when yours is replaced, a new gas tank over 30 gallons and up to 75,000 BTU per hour that was manufactured after January 1, 2028 must meet the zero-NOx standard unless you hold an exemption certificate.
Can I still install a gas water heater after 2028?
In some cases. The amended rule allows one with an Air District exemption certificate for a qualifying financial, space, electrical or time-limited situation, and tanks of 30 gallons or less sit outside the zero-NOx section. The space and electrical exemptions can be used once per address.
How do I get an exemption certificate?
Apply to the Air District before the unit is installed, with the installation address, your contact details and proof that an exemption applies (Section 9-6-405). Each certificate covers one water heater. As of November 2026, the flyer says the Air District will set up a website for this, so check its building appliances page.
Are gas furnaces included?
Yes, under Rule 9-4. Natural gas furnaces manufactured after January 1, 2029, including wall furnaces, must meet a zero-NOx standard. As of November 2026 that date hasn't been amended.
Related reading
Heat Pumps for Mid-Century Homes: Going All-Electric in the Bay Area
Eichler Radiant Heat: Copper vs. Steel, Repair, Replace or Retire
More guides on this topic: Owning & Renovating a Mid-Century Modern Home
Planning around a gas water heater or furnace before you buy or sell? Call or text the Boyenga Team at Compass at (408) 373-1660 or (650) 383-8606, or contact the Boyenga Team to talk through the equipment and the timing.
