Water Heater Carbon Monoxide — Warning Signs and Prevention

Quick Answer

Carbon monoxide (CO) from a water heater is rare but deadly. It occurs when a gas heater’s venting is blocked, cracked, or improperly installed.

Symptoms:

Headache

Dizziness

Nausea

Confusion

These can feel like the flu.

If you suspect CO poisoning:

Evacuate immediately

Call 911

Call Enbridge: 1-800-767-1689

Prevention:

Install a CO detector ($30–$50) near the heater

Annual vent inspection: $100–$150

Do not assume your heater is safe just because it is working. Faulty venting is invisible.

Why Water Heaters Produce Carbon Monoxide

Gas water heaters burn fuel, producing CO as a byproduct.

Normally:

Venting systems (chimney or sealed duct) remove CO outside

When venting fails:

CO accumulates inside the home

Common Causes

Blocked or damaged vent pipe

Improper vent installation

Backdrafting (air pressure pulls CO back inside)

Older, non-sealed systems

Risk Factors

Basement installations

Unfinished or poorly ventilated basements

Tight homes with limited airflow

Older heaters

High elevation (6,000+ ft)

Warning Signs of Carbon Monoxide Poisoning

Immediate Symptoms (Acute Exposure)

Persistent headache

Dizziness

Nausea or vomiting

Chest pain

Confusion

Loss of consciousness

Slow Symptoms (Chronic Exposure)

Recurring headaches

Fatigue

Memory problems

Difficulty concentrating

Feeling “off” in a specific room

Red Flags

Symptoms worsen in winter

Symptoms improve outside

Multiple people affected

Pets acting lethargic

How to Tell CO Poisoning vs Flu

SymptomCO PoisoningFluFeverNoYesSore throatNoYesCoughNoYesMuscle achesPossibleYesOnsetFastGradualWorse in specific roomYesNoImproves outsideYesNo

Rule: When in doubt, assume CO and evacuate.

Prevention: Actions You Can Take

1. Install a Carbon Monoxide Detector ($30–$50)

Place near the heater (basement)

Optional: one in each bedroom

Test monthly

Use a battery or hardwire with a backup

Minimum 85 dB alarm

Brands: First Alert, Kidde, Nighthawk

2. Annual Vent Inspection ($100–$150)

A plumber will:

Check for cracks or blockages

Verify proper vent slope

Inspect exterior vent

Test for backdrafting

Recommended annually, especially before winter.

3. Maintain Basement Ventilation

Avoid sealing basement completely

Allow airflow from upper levels

Use a fan if enclosed

Open windows periodically

4. Check Exterior Vent

Inspect roof or wall exit:

Blocked by snow or ice?

Bird nests or debris?

Too close to windows/doors?

Clear obstructions immediately.

5. Upgrade Older Heaters

Older: B-vent systems (less safe)

Newer: FVIR sealed combustion

If heater is 8+ years old and venting is questionable, consider replacement.

Safe Venting Checklist

Proper venting should:

Rise upward from the heater

Connect to the chimney or sealed duct

Exit is above the roofline

Maintain 1/4″ rise per 12″ horizontal run

Use proper materials (Type B vent or sealed pipe)

Have no gaps or cracks

Terminate away from windows/doors

If any condition fails, call a plumber immediately.

When to Call 911

Call immediately if:

Multiple people feel sick

Loss of consciousness

Severe chest pain

Confusion or breathing difficulty

Combined symptoms (headache + nausea + dizziness)

Emergency Procedure

Leave the home immediately

Go outside

Call 911 from outside

Report “possible carbon monoxide exposure at my home”

Do not re-enter until cleared by emergency services

Call Enbridge (1-800-767-1689) if:

You suspect gas + CO issue

Venting is visibly damaged

You need gas shutoff confirmation

What Happens If CO Detector Alarms

Immediate Actions

Leave home immediately

Open windows while exiting

Call 911 or Enbridge

Do not re-enter

Optional: turn off the heater if safe

Professional Response

Fire department or Enbridge will:

Measure CO levels

Inspect venting and heater

Determine if a shutdown is required

Recommend repairs

Find Local Help

Carbon monoxide is invisible and deadly. Annual inspection ($100–$150) is considered an essential prevention technique.

Enbridge gas emergency: 1-800-767-1689

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Frequently Asked Questions

How common is CO from water heaters?

Rare, but extremely dangerous. Prevention is essential.

Is a CO detector enough?

No. It alerts after exposure begins. Inspection prevents exposure.

Is a finished basement safer?

Not necessarily. Venting must still be verified.

Can I inspect venting myself?

You can check visually, but professionals use CO meters and smoke tests.

My heater is 15 years old. Do I need an inspection?

Yes. Venting deteriorates over time. Annual checks are critical.

Sources and Update Policy

This article was last updated March 2026. CO safety guidance aligns with CDC recommendations.

Sources:

CDC carbon monoxide poisoning guidance: https://www.cdc.gov/

Enbridge gas safety: 1-800-767-1689

CPSC carbon monoxide detector standards

International Fuel Gas Code (venting requirements)