Every home energy improvement conversation eventually comes around to air sealing. Seal the attic, tighten the building envelope, reduce infiltration. It's sound building science, and the energy savings are real. What almost never gets mentioned in those conversations — by contractors, by online guides, or by the rebate programs pushing the work — is the safety check that is supposed to happen first.
If your home has gas appliances — a furnace, a water heater, a boiler, a gas range with a range hood — tightening the building envelope without a combustion safety assessment first can create a carbon monoxide hazard that didn't exist before the work was done. I'm not saying this to discourage air sealing. I'm saying it because I've been trained in combustion safety protocols, I've seen what happens when they're skipped, and it matters.
What Combustion Safety Testing Actually Checks
A combustion safety assessment, typically called a CAZ (Combustion Appliance Zone) test, is a structured diagnostic procedure that evaluates whether your gas appliances are operating safely and venting properly. The BPI protocol I use as a certified building analyst covers several distinct checks:
- Spillage test. The auditor runs the appliance, then measures whether combustion gases are actually going up the flue or spilling into the room. This is done using a smoke pencil or CO monitor at the draft hood while the appliance operates under worst-case pressure conditions.
- CAZ depressurization test. This simulates the worst-case pressure the combustion appliance zone might experience when all exhaust fans, the dryer, and the HVAC system are running simultaneously. The question is: under those conditions, does the appliance still draft correctly?
- Flue gas CO measurement. A combustion analyzer measures carbon monoxide levels in the exhaust gases directly. Normal combustion produces very little CO. Elevated levels in the flue indicate incomplete combustion — a sign the appliance needs service regardless of anything else.
- Draft pressure measurement. Measuring the actual draft pressure in the flue confirms that exhaust gases are moving in the right direction with adequate force to exit the building.
- Visual inspection. Flue connections, vent pipe condition, clearances from combustibles, and visible deterioration are checked before the diagnostic work even starts.
Carbon monoxide has no odor, no color, and no taste. Backdrafting combustion gases into a living space is not detectable by the occupants without a CO alarm or a professional test. A properly installed CO detector near combustion appliances is essential in any home with gas equipment — it is not optional.
Why Air Sealing Can Create a Backdraft Problem
Naturally-vented gas appliances — which includes most water heaters and older furnaces — rely on the stack effect: warm exhaust gases are lighter than ambient air, so they rise through the flue naturally. This passive venting system requires that the appliance zone have enough ambient air pressure to support the draft.
When you tighten a house, you reduce the amount of outdoor air infiltrating the building envelope. In a very leaky house, the combustion appliance may have been drafting adequately because there was always enough ambient air available. After air sealing, the house is tighter. If a bath fan, range hood, or dryer is operating at the same time as the water heater, the house may depressurize enough to reverse the draft — pulling combustion gases back down the flue into the house.
This doesn't happen in every home. Modern direct-vent appliances (which draw combustion air from outside through a dedicated pipe) are not vulnerable to this in the same way. Power-vented appliances are also much more resistant. But in homes with older naturally-vented equipment, particularly when air sealing is being done aggressively, the combustion safety picture changes and has to be reassessed.
Field note: I've tested homes where the water heater was backdrafting intermittently under worst-case conditions even before any air sealing work. The homeowner had no idea. A CO alarm placed near the equipment had never tripped because the episodes were brief and the CO levels diluted quickly. The blower door test made it worse — and caught it.
What Happens When It's Skipped
Most of the time when contractors skip combustion safety testing, nothing bad happens immediately. The appliances continue operating the way they were before the work. The homeowners get their insulation, their air sealing, their rebate check, and their improved utility bill. And nobody gets hurt.
But "most of the time" is not good enough when the consequence of the exception is carbon monoxide poisoning. The testing takes 30 to 60 minutes, requires no specialized permit, and is routinely included in a complete home energy audit performed by a BPI-certified professional. The fact that it's commonly skipped by insulation contractors and weatherization crews is a training and incentive problem in the industry, not a reflection of how necessary it is.
What to Do If You Have Gas Appliances
If you're planning air sealing or insulation work and your home has any gas appliances, ask explicitly about combustion safety testing before the work starts. If the contractor isn't familiar with the term or doesn't offer it as part of the scope, look for a BPI-certified building analyst in your area who can perform the assessment independently.
The assessment is also worth doing if you've already had significant air sealing work done without this check, particularly if you have naturally-vented (not direct-vent or power-vent) gas equipment. A one-time CAZ test after the fact won't undo any risk that may have been introduced, but it will tell you what your current situation actually is.
Finally, make sure you have working CO detectors near any combustion appliances and on every level of your home. They are inexpensive, reliable, and the only warning system that works when there's no smoke and no smell to alert you. CO alarms should be tested monthly and replaced every five to seven years per manufacturer guidance.