Odor that worsens when heater runs

Odor intensifying when the water heater cycles

Quick Answer:

If the smell gets stronger when the heater fires, it’s often due to hot water releasing gases or bacteria in the tank producing hydrogen sulfide (rotten-egg odor). A simple test helps narrow the cause: listen for the heater to run, then draw water immediately to compare how strong the smell is right after the burner or element comes on versus later.

Why This Happens

  • Warm water releases dissolved gases and volatile compounds more readily, so odors that are mild in cold water become stronger when water heats up.
  • Magnesium anode rods can interact with sulfates and bacteria in the tank and sometimes increase hydrogen sulfide production at higher temperatures. That doesn’t mean the anode is always the root cause.
  • Bacterial growth in the tank, especially sulfate-reducing bacteria, speeds up in warm conditions. When the heater runs it can stir or change conditions in the tank, making odors worse for a short time.
  • Plumbing traps, fixtures, and appliances can hold odors that are released more strongly when hot water flows through them.

Step-by-Step What to Do

1. Prepare to test

Have a glass or cup ready in a few fixtures you use often (kitchen sink, bathroom sink, shower). You want to compare cold vs hot and different locations.

2. Listen and draw

Stand near the water heater or a fixture where you can hear it. When you hear the heater fire (pilot or electric element start), immediately draw a cup of hot water and smell it. Then draw another cup after the heater stops firing or after 1–2 minutes. Note whether the smell is strongest immediately after the heater runs.

3. Compare hot and cold, and fixtures

Draw cold water right after the hot test and compare smells. Test the same sequence at a few fixtures to see if the problem is limited to one spot (for example, Smell only in kitchen sink) or shows up everywhere.

4. Check the first-draw effect

Test the first draw in the morning from a hot tap (before other hot water is used) and then run hot water for a minute and test again. If the odor is strongest only on the very first tap of the day, that points to tank or supply issues like stagnant water (Odor only in first water draw).

5. Try a controlled flush

  • Open a hot tap and run the water for 3–5 minutes. If the smell reduces or disappears, flushing may temporarily clear sediment or stagnant zones in the tank.
  • If flushing helps but the smell returns quickly, the underlying cause is likely in the tank or an ongoing chemical/bacterial process.

6. Record details before calling help

Note when the smell happens, which fixtures are affected, water heater age and fuel type (gas or electric), and whether the smell coincides with heater cycles. This helps a technician diagnose the issue faster.

What Not to Do

  • Do not remove anode rods without guidance. That can void warranties, reduce tank life, or cause other problems.
  • Do not add chemicals to the tank or plumbing unless instructed by a qualified plumber or the water heater manufacturer.
  • Do not attempt gas appliance repairs or dismantle gas connections yourself. If you suspect a gas leak, follow the safety notes below.

When to Call a Professional

  • Call a plumber or water-heater technician if the odor persists after flushing and testing, especially if it is strong in hot water only or returns quickly.
  • If the water heater is older, leaking, or you suspect the anode type is contributing, get a professional inspection to discuss safe options.
  • If you can’t identify whether the smell is coming from the plumbing or the tank, a pro can perform tests for hydrogen sulfide and evaluate tank bacteria or anode issues.

Safety Notes

  • If you detect a strong gas smell in the house (sulfur or rotten-egg odor can also be used as a gas odorant), leave the building immediately and call your gas utility or emergency services from a safe location.
  • Use caution when running hot water tests—hot water can scald. Keep runs short and test with a cup before full use.
  • Avoid DIY work on gas lines, electrical connections, or the tank itself unless you are qualified. These can be dangerous and should be handled by licensed technicians.

Common Homeowner Questions

  • Q: Is the smell dangerous?

    A: A rotten-egg smell from hydrogen sulfide is usually unpleasant and corrosive but is generally not life-threatening at household levels; however, gas leaks are dangerous—if you suspect gas, evacuate and call for emergency help.

  • Q: Could changing the anode fix it?

    A: Sometimes switching anode type or replacing a degraded anode helps, but this should be evaluated and performed by a technician—don’t remove anode rods on your own.

  • Q: Can I disinfect the tank myself?

    A: Shock chlorination can be used in some cases but should only be done with clear instructions from a professional or the manufacturer to avoid damage or safety risks.

For more related articles, see the Sulfur / Rotten Egg Smell in Water hub.

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