Rotten smell after winterization

Quick Answer:

After winterization you might smell rotten eggs. Most often it’s from trapped air, old standing water, or bacteria in low-flow traps, not antifreeze residue. The quickest fix is to run every faucet, shower, and appliance to flush air pockets and stale water so fresh water replaces it.

Why This Happens

When systems are winterized, lines are drained and air is introduced into pipes and traps. That air can carry hydrogen sulfide or let bacteria multiply where water pooled. A small amount of antifreeze used properly for winterizing household lines is typically odorless when flushed out. More commonly, the smell comes from:

• Stale water in traps where flow was minimal during winter.
• Bacteria that grow in warm, oxygen-poor pockets of water.
• Sulfide gases released from organic buildup; this is related to issues like Sulfur odor in laundry water.

Step-by-Step What to Do

1. Purge trapped air: run all fixtures

Open every faucet and fixture in the house — cold and hot — and let them run for several minutes. Include outdoor hose bibs, showers, sinks, and any appliance connections (dishwasher, ice maker). This flushes trapped air pockets and replaces standing water.

2. Run appliances and hot water systems

Run a full cycle on the washing machine, dishwasher, and any water softener regeneration to move fresh water through. Turn on the water heater and bring it up to temperature so hot lines are flushed too.

3. Check traps and drains

Pour a bucket of water down seldom-used drains (floor drains, basement sinks) to refill traps. If a trap dried during winterization, refilling it will block sewer gas and remove odors.

4. Vent and re-check

After flushing, ventilate the area and smell again. If odor is reduced but returns, repeat the flush and run fixtures longer. Persistent intermittent odors may follow a pattern; see notes below.

What Not to Do

• Do not re-winterize without purging air. Re-applying antifreeze or re-draining lines while air pockets remain can trap odors or spread contaminants.
• Do not pour undiluted chemicals or household cleaners down drains in an attempt to mask the smell.
• Do not ignore outdoor vents or septic alarms that could indicate a separate problem.

When to Call a Professional

Call a plumber if:

• The smell persists after you have flushed every fixture and refilled traps.
• You notice recurring odors over time, or the odor returns after a few weeks despite flushing — this pattern can point to deeper plumbing or sewer issues such as biofilm, a failed trap, or a venting problem. If the odor keeps coming back, have a pro inspect; for example, issues like Smell returns every few weeks usually need a diagnostic check.
• You suspect a sewer gas leak, hear gurgling drains, or have multiple fixtures affected at once.

Safety Notes

• Avoid breathing concentrated sewer gases in enclosed spaces; ventilate the area while troubleshooting.
• If you used antifreeze during winterization, do not mix different types of chemical additives. Most household winterizing antifreeze is propylene glycol and is low-toxicity, but avoid ingesting or letting pets drink from areas with standing antifreeze.
• If you have a septic system, use caution when flushing and consult septic care guidance before using treatments.

Common Homeowner Questions

Is antifreeze causing the rotten egg smell?

Unlikely. Proper winterizing antifreeze is usually odorless; odors are more commonly from trapped air or bacteria in standing water.

How long will the smell last after flushing?

Often it fades within a few hours to a day after a thorough flush. If it remains, repeat the flushing steps and check traps and seldom-used drains.

Can bacteria grow in trapped air pockets?

Yes. Stagnant pockets with low oxygen can encourage bacteria that produce sulfide gases. Thorough flushing usually removes the conditions they need to persist.