Gas appliances in a sealed home: CO alarms first, then a yearly inspection

Keep sealing, but put carbon-monoxide (CO) alarms up first. DOE's 2022 Energy Saver guide says sealing uncontrolled air leaks can save 10% to 20% on heating and cooling bills, and CPSC's carbon monoxide Q&A puts the alarms on every level of the home and outside sleeping areas. With a gas furnace, boiler or water heater in the house, add a licensed HVAC contractor's venting check after the sealing, then an inspection every year. Alarms are a small buy that renters and owners can both make; the owner books the inspection, and a renter asks the landlord. One condition stops the work: if an alarm sounds, get outside and call 911.

Sealing can let CO build up when a gas appliance vents badly

A gas furnace or water heater sends its exhaust outdoors through a flue. ENERGY STAR's 2016 do-it-yourself sealing guide warns that after improvements that make a house tighter, there can be more opportunity for CO to build up if gas-burning appliances are not venting properly. EPA's page on carbon monoxide and indoor air lists the sources: back-drafting from furnaces, gas water heaters, wood stoves and fireplaces, and flues that are improperly sized, blocked, disconnected or leaking.

Heating appliances rank second. CPSC staff's 2022 annual estimates, posted May 2026, count an estimated 274 non-fire CO deaths associated with consumer products in 2022, 76 of them (28%) with some type of heating appliance, second only to engine-driven tools such as generators. Among the heating-system cases, the same report describes vents that became detached or were blocked by soot, and a backdraft condition. CPSC's January 2026 winter safety release calls CO odorless and colorless. Your nose will not find it.

Put a CO alarm on every level before the first tube of caulk

CPSC's Q&A sets the placement: battery-operated CO alarms, or alarms with battery backup, on every level of the home and outside sleeping areas. Interconnected alarms are best, because when one sounds, they all sound. The same page does not recommend alarms in kitchens or above fuel-burning appliances. A battery alarm needs no wiring, so a renter can add one; a hard-wired alarm is a job for a licensed electrician.

Example, with the CPSC rule and an assumed floor plan. Take a two-story house with a basement, a gas furnace and water heater in the basement, and every bedroom off one upstairs hallway; the floor plan is the example's own assumption. Under CPSC's placement rule, three levels mean three alarms: one in the basement, not above the furnace or water heater, one on the main floor, and one in the upstairs hallway, which also sits outside the only sleeping area. Put two bedrooms at opposite ends of the upstairs, each off its own hallway, and that level has two sleeping areas. The count rises to four.

If an alarm sounds, CPSC says not to look for the source: move outside to fresh air, call 911 or the fire department, and do not reenter until the responders give permission. The same page lists headache, fatigue, shortness of breath, nausea and dizziness as the low-to-moderate symptoms, similar to the flu but without the fever.

Leave the flue, the ducts and the gas line to an HVAC contractor

The small-buy sealing stays yours: weatherstripping doors and windows, foam gaskets behind outlet and switch plates, caulk where pipes and wires pass through walls, as in seal air leaks before you add insulation, or the removable options in window film and seals for renters. Three spots near a gas appliance belong to a licensed HVAC contractor.

• The gap around a flue. ENERGY STAR's 2016 sealing guide says the pipe gets hot and that building codes usually require 1 inch of clearance from metal flues, and 2 inches from masonry chimneys, to any combustible material, including insulation. DOE's 2022 guide names fire-resistant materials for that gap, such as sheet metal or sheetrock and furnace cement caulk. Keep foam and insulation back, and have the contractor close it.
• The ducts. The same DOE guide says all duct sealing should be tested by a qualified professional to ensure combustion safety and proper air flow; sealing leaky ducts covers the materials.
• Gas lines and burners. ENERGY STAR's maintenance checklist has the contractor check all gas connections, gas pressure, burner combustion and the heat exchanger.

For a bigger job, hire an energy auditor who tests before and after. DOE's Energy Saver guide describes a “test in” with safety and air leak tests, and a “test out” with safety tests and visual inspections of the contractor's work. Not every auditor offers the test out. Ask.

Book the heating inspection every fall, and once after sealing

CPSC's January 23, 2026 winter safety release says to have a professional inspect all fuel-burning heating systems, including furnaces, boilers, fireplaces, wood stoves, water heaters, chimneys, flues and vents, each year. Its Q&A names what the inspector checks: chimneys and flues for blockages, corrosion, partial and complete disconnections, and loose connections. ENERGY STAR's maintenance checklist times the heating check-up for the fall, before contractors get busy.

After a sealing project, ENERGY STAR's 2016 guide asks for one more check: your heating and cooling technician looks at the gas- or oil-fired furnace, water heater and dryer for proper venting. Owners book both visits. Renters ask the landlord. Put three questions to the contractor: does every fuel-burning appliance still vent properly now that the house is tighter, are the flues and chimney clear and connected, and are the burners and heat exchanger sound?

A gas smell is a different emergency. PHMSA's pipeline emergency page says that if you notice the “rotten egg” smell of natural gas, do not use a phone or flip any light switch, leave the area, and call 911 from a neighbor's house or another place well away from the leak.