If a CO alarm is sounding, or people in the house have headaches, dizziness or nausea, skip all of this: get everyone outside and call 911, and don't go back in until responders say it's safe. If you smell gas in Philadelphia, leave and call PGW's 24-hour line at 215-235-1212 from outside; if PECO supplies your gas, call PECO's gas emergency line. When a CO alarm means leave the house covers the alarm patterns.
Can your appliance backdraft?
Backdrafting is a problem for natural-draft equipment, which relies on hot exhaust rising up a metal flue. PNNL's Building America guides explain how to tell:
- A gas water heater with a draft hood (the flared metal collar where the flue meets the top of the tank) is natural-draft and can backdraft. (PNNL water heater guide)
- An older furnace with an open grille on the front and a wide metal flue pipe is an atmospheric unit, typically 80% efficiency or less, and can backdraft or have flame rollout, where burner flames get pulled out of place.
- A high-efficiency furnace with two plastic PVC pipes has a sealed combustion chamber. PNNL calls backdrafting in these almost impossible. (PNNL furnace guide)
What to look for from where you stand
Don't take anything apart. With a flashlight, look for:
- Soot spots on top of the water heater tank or around the draft hood.
- Melted or deformed plastic washers and rings on the pipes right above the tank.
- A flue pipe that has come apart at a joint, or one that runs downhill from the appliance instead of rising toward the chimney.
- On a furnace, soot or discoloration on the metal around the burner area you can see through the grille.
PNNL treats a disconnected or downhill flue pipe as dangerous and says to call for repair as soon as possible: a plumber for the water heater, an HVAC technician for the furnace. Don't try to adjust a furnace that shows signs of flame rollout yourself.
PNNL's water heater guide also describes a smoke check: after heavy hot-water use, while the burner is firing, hold a lit incense stick next to the draft hood opening. Smoke pulled into the hood means it's drafting at that moment. Smoke pushed away means exhaust is spilling into the room, and you should call a plumber. A pass only covers the conditions you tested in.
Why it happens: the house can pull exhaust back in
The EPA explains that when bath fans, range hoods and clothes dryers push air out faster than it leaks back in, the house is under negative pressure, and that pressure can pull exhaust back down a water heater flue or chimney. Fireplaces and leaky return ducts near the furnace add to it. Problems often show up after air sealing, finishing a basement or adding a stronger kitchen hood.
Why a quiet CO alarm doesn't settle it
You still want working CO alarms on every level and outside sleeping areas, per the CPSC. But the CPSC also says alarms are built to sound before CO reaches life-threatening levels, and most people feel nothing at 1 to 70 ppm, so lower levels can go unflagged. The test button only checks the circuitry, not the sensor. For comparison, PNNL notes a home with properly working gas appliances may read up to 5 ppm, and anything above 10 ppm deserves a look.
What to ask for: a combustion safety test
PNNL's combustion appliance zone (CAZ) testing guide lays out what a proper test covers. When you call, ask whether the technician will:
- Check the appliance area for CO and gas leaks, and inspect the vent pipe and combustion air openings.
- Set up the worst case: exterior doors, windows and fireplace dampers shut, the dryer and every exhaust fan on high.
- Run the appliance about five minutes, then test for spillage at the draft hood and measure CO in the flue against the limit for that appliance type.
If an appliance spills only with the fans on, the problem is most likely not enough air for combustion, and the fix is usually makeup air. If it spills with the fans off, the vent or the appliance itself needs a service technician before it's used. Ask for the results in writing, and ask for a retest after the repair.
HVAC companies, plumbers and home energy assessors may offer this test; ask when you book, since a routine service visit may not include it. If you're planning air sealing or insulation anyway, a home energy assessment is a good time to get the test done,, since PNNL notes that air sealing and insulation change the air a natural-draft appliance has to work with. Our furnace tune-up guide shows what else the yearly service report should list. Replacing an old water heater? Our tank-or-tankless guide covers how the new one will vent.