This guide is general information, not medical or engineering advice. Nobody can tell from a web page whether a particular house has a problem. If an alarm is sounding or you smell gas, leave first and read afterward.
What a heat exchanger does
In a gas furnace, the burners fire into a set of metal chambers, the heat exchanger. Hot combustion gases pass through one side while the blower pushes house air over the outside. The air picks up the heat, and the gases go out the vent. The metal wall between them is the whole barrier.
Why a crack matters
Metal expands and shrinks every time the furnace cycles. Over many winters, and especially with restricted airflow or an overheating furnace, a seam can crack or a section can rust through. A crack can let combustion gases, which include carbon monoxide, mix with the air that goes to the rooms. Many furnaces also have a limit switch that trips when they overheat, which is why repeated limit trips are worth a closer look.
Carbon monoxide in plain terms
The New York State Department of Health describes carbon monoxide as colorless and odorless, says it can kill before a person notices it, and says CO exposure is the leading cause of poisoning death in the United States.
What New York requires
Amanda's Law, Chapter 367 of the Laws of 2009, extended carbon monoxide alarm requirements to one and two family dwellings, condominiums, cooperatives and multiple dwellings regardless of construction or sale date. The state Fire Code (section 915.3.1) requires carbon monoxide detection in residential buildings in the rooms and sleeping areas that contain a fuel-burning appliance, and the state Property Maintenance Code applies the same section to existing buildings.
- Alarms must run on building power with battery backup, and a 10-year battery is acceptable in existing homes.
- Alarms must be listed to UL 2034 or CAN/CSA 6.19.
Whether your alarms are present and working is for you to check.
If an alarm sounds
- Get everyone out into fresh air, including pets.
- Call 911 or the gas utility from outside.
- Stay out until the responders say the house is clear.
- Do not restart the furnace or the boiler afterward until a contractor has checked it.
What a contractor checks
For a suspected crack the contractor typically looks inside the heat exchanger with a small camera, checks burner flames and vent condition, and measures combustion gases. Other faults can look similar, such as a blocked flue or a venting problem, so a finding should be explained and demonstrated to you, not simply announced.
Flame safeguards are on your side
New York's residential code requires gas heaters in homes to shut off the fuel when the flame goes out. The limit switch, the pressure switch and the flame sensor are part of the same idea. A furnace that shuts down is usually protecting you, and the repair is to fix the fault, not to bypass the control.
What happens if the heat exchanger is cracked
A cracked heat exchanger is rarely repaired. The furnace is usually shut off and the contractor may refuse to run it. Separately, the City of Syracuse can declare premises unfit and placard them when, among other defects, the structure lacks adequate heat or ventilation. If the furnace cannot be fixed you are looking at the replacement conversation, and the repair-or-replace guide helps you ask good questions. It is a decision, not a sales pitch.
Ordinary upkeep
Onondaga County says the HEAP program no longer runs a clean and tune benefit, although a state health page still mentions one, so confirm with the county before relying on it.