Carbon Monoxide From the Water Heater
The Short Answer: A CO alarm at a gas water heater is a blocked or disconnected vent, draft-hood spillage, a dead power-vent blower, overfire, or a tank dumping water into the fire. Carbon monoxide has no odor. If anyone is dizzy, leave and call 911 from outside. Do not tape B-vent or fire a silent blower. A licensed tech proves draft.
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Search Intent & The Real Problem
The detector chirped in the hall. Or someone got a headache every time the tank recovered. Forums sold “it’s just condensation” and a roll of HVAC tape. Carbon monoxide is incomplete combustion plus a flue that is not taking the products outside. Mercaptan in the closet is a fuel leak, not CO — closet smells like gas. White vapor at a draft hood or T&P is often steam — white smoke vs steam. This page is combustion products in the room.
A water heater is not a furnace with a cracked secondary exchanger. Atmospheric tanks use a flue tube and a draft hood. Power-vents use an inducer and a pressure switch. Condensing tanks and tankless use PVC or polypropylene on purpose. You name the vent before you name a “leak.”
What other guides get wrong
They tell you carbon monoxide smells like rotten eggs. That is odorant in fuel gas. CO is silent. They recommend running the burner with a $20 detector as a “test.” If people are already sick, that is an occupancy problem, not a science fair. They tape B-vent joints that have pulled apart. They mix tankless outdoor vapor with indoor spillage. And they skip the shared chimney: a furnace that is already spilling will take the water heater with it — heater and furnace both dead.
Step-by-Step Troubleshooting
Step 1: Alarm, bodies, soot — decide if you stay in the building
| What you have | What it is | First move |
|---|---|---|
| CO alarm in full alarm, headache, nausea, pets sick | Products in the living space | Leave. 911 / fire. Do not relight. |
| Detector chirp every 30–60 s, nobody sick | Low battery on many units — still treat the heater as suspect until a tech says otherwise | Fresh air. Do not “test” by firing it. |
| Soot on the jacket, yellow lazy flame, scorch at the door | Spill / rollout / overfire | Gas cock off. Licensed. |
| Rotten-egg odor in the air | Mercaptan / fuel leak, not CO | Leave, utility — closet |
| Rotten eggs only at the hot faucet | Anode / bacteria | Not this emergency |
| White cloud, no odor, T&P wet | Steam | White smoke |
- What to look for: Timing vs burner recovery. Windows sweating in one room. A detector that only screams when the tank fires. A draft hood that is wet, sooted, or rust-streaked.
- Required tools: Your feet if anyone is sick. Eyes from the doorway. No wrenches.
Do not start a cold heater to “see if it spills.” If you already watched the burner run and exhaust washed out the hood instead of up the B-vent, that was the test. Shut it down.
Step 2: Name the vent — B-vent, power-vent, or plastic
| Vent | CO path if it fails | Do not |
|---|---|---|
| Type B double-wall, draft hood on top | Disconnected joint, bird nest, downdraft, negative house pressure | Tape a 2-inch gap |
| Power-vent / power-direct-vent, 120 V inducer | Dead blower, jumpered pressure switch, blocked sidewall | Fire it silent — blower |
| PVC / PP condensing tank or tankless | Disconnected concentric, ice, condensate backing into the cabinet | Assume “plastic cannot leak CO” |
| Common vent with a furnace | Furnace taking the draft, liner collapsed | Relight the water heater “to check” |
- What to look for: Rating plate. PVC at the top is not an atmospheric tank. A vinyl hose on a pressure switch. Wet or pulled joints in the attic — vent pipe leaking, vent collar. Outdoor tankless odor at the cap is a different page: tankless exhaust.
- Required tools: Flashlight. Phone photo of the termination and the hood. No foil tape.
Cool drip on B-vent in a cold attic can be flue-gas condensate. A joint that pulled off the next section is a flue, not a drip.
Step 3: Flame, rollout, and water where fire belongs
- What to look for: Sharp blue on the main with the door positioned as the listing allows — you are looking, not tuning. Yellow, lifting, or fire at the door is flame too high or rollout — shut the cock. Lint packed in an FVIR arrestor under the tank starves air and makes CO. Water or a rust ring in the burner tray is a dead tank; keep firing it and you steam the chamber — dripping on the burner. A bang at light-off dumped a chamber of fuel, then lit it — loud bang.
- Required tools: Eyes. Gas off if the door is sooted or wet. Do not pull an FVIR screen apart if you cannot reseal it.
Manifold pressure is licensed: natural gas typically ~3.5–7 in. w.c., LP ~10–11 in. w.c. at the appliance — confirm the rating plate. Overfire from a wrong orifice or a high meter is a CO and scald problem. You do not “turn the gas down” at the cock.
Step 4: Power-vent and condensate — the silent-fan CO event
- What to look for: No whir at the top of the heater, then someone “fixed” it by sucking the pressure-switch hose. That jumper lets the valve open with no proved draft. Condensate drowning the inducer. Sidewall termination packed with snow, a dryer vent, or a bird screen. A heater that runs a few minutes then dies as the switch heat-soaks — shuts off after 5 minutes, thermal switch tripping.
- Required tools: Ears. 120 V at the disconnect, from a dry place. Do not jumper the switch.
Drip at an outdoor cap after a storm can be rain, not CO — exhaust drip. Indoor spill at a draft hood is not rain.
Step 5: What a licensed tech actually measures
- What to look for: You are done with DIY when the alarm sounded, the hood sooted, the blower is dead, or a vent joint is open. The truck brings a combustion analyzer, a manometer, and a draft gauge — not a roll of tape. They prove CO at the hood and in the flue, clock the meter, and open or replace vent, blower, or tank. A flue tube leaking tank water into the fire is a replacement, not a “tune-up.”
- Required tools: None on your side. CO alarms on every level of the house are not optional after this call.
Do not “air out the closet and relight.” Products that spilled once will spill on the next recovery until the vent is fixed.
When DIY Ends: Calling a Licensed Pro
It ended when carbon monoxide was in play. You may shut the appliance cock on the way out, leave the building, and photograph a disconnected pipe from a safe distance. You may not tape B-vent, jumper a pressure switch, fire a silent inducer, or keep resetting a rollout. A licensed plumber or HVAC tech proves draft and combustion, then repairs the vent or condemns the tank. Same-day beats a detector you unplugged because it “kept going off.”
Frequently Asked Questions
Q: Can a water heater leak carbon monoxide? A: It does not “leak CO” like a gas cock weeps fuel. It makes CO when combustion is incomplete or the vent does not carry products outside — blocked B-vent, spillage, a dead power-vent blower, overfire, or water in the chamber. Treat an alarm as an evacuation.
Q: How do I know if my water heater is leaking carbon monoxide? A: A sounding CO alarm, flu-like symptoms that improve outside, soot at the draft hood or door, a yellow lazy flame, or a vent joint that has pulled apart. You cannot smell CO. Do not keep firing it to confirm.
Q: Is a carbon monoxide leak from a water heater an emergency? A: Yes if the alarm is in alarm, anyone is sick, or you see spill or soot. Leave, call 911 or the utility from outside, then a licensed tech. Do not tape the flue or relight.