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Sewer / rotten-egg smell coming out of the air conditioner

Editorial guide · Updated 18 September 2026

In short Short answer: yes, your neighbour is probably right – if the condensate hose goes straight into the sewer without a trap, sewer gas climbs up the hose, through the drain pan and out of the grille. Not exactly dangerous in small doses, but not healthy either, and it usually comes bundled with a dirty drain pan that has a smell of its own.

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Situation discussed

I bought a flat in Berceni, on Turnu Măgurele street, a 2019 block, with a 12,000 BTU LG left by the previous owners. At start-up it clearly smells of sewage, rotten eggs, not mould. I thought it was the bathroom, but the smell comes out of the unit's grille. My neighbour says the condensate drain is plugged into the bathroom pipe. Could that be it? Is it dangerous?

Editorial explanation · Revizie AC

What is really happening

Two sources combine here. First: the hydrogen-sulphide ('rotten egg') smell comes from the sewer; if the condensate route has no water trap (like under a sink), nothing stops gases climbing when the unit is off and the hose is dry. Second: the drain pan of household units has been measured as the most bacterially loaded part of the whole machine – water stands, warms up, and anaerobic bacteria in that sludge also produce sulphur compounds. At start-up the fan pulls air across the pan and sends the whole package into the room.

Explanatory diagramOdours can have several sources
Odours can have several sources
  1. Deposits or moisture inside the unit.
  2. The condensate drainage route.
  3. Sources within the room, assessed separately.
What the studies say

What we do on site

  1. We check where the condensate hose ends (balcony, trap, sewer pipe) and tell you exactly what you have.
  2. Collection bag fitted, then we clean the pan and drain path – the sludge in the pan is usually half the smell.
  3. Cleaning solution on evaporator and fan, pressure wash in place; we flush the hose with water to confirm it is clear.
  4. Sanitising solution on pan, evaporator, roller and hose.
  5. Outdoor unit on the facade – washed in place; at 5 years it is already coated with construction dust from the area.
  6. If the hose enters the sewer without a trap, we tell you plainly: that is an installation change for a plumber – we do not alter the route. See current rates on the Prices page.

What you can do yourself

  • Pour a glass of clean water into the pan (through the filter grille, over the fins) once a month with the unit off – it refills the water 'plug' in the trap, if there is one.
  • Filters washed every two weeks.
  • Run the unit on 'Fan' for 20 minutes before shutting it off.

What NOT to do

  • Do not pour drain unblocker (caustic soda) into the unit's pan – it is plastic and aluminium, not a cast-iron pipe.
  • Do not stuff the hose with a rag 'so the smell does not come' – you block the drain and water comes out on the wall.
  • Do not redo the drain route yourself, that is a plumber's job.

Prevention

A clean pan and a properly trapped route do not smell of sewer. Annual sanitising and, between visits, drying on 'Fan' after each use.

When to call us

If you want to know for sure where the smell comes from before calling a plumber, call 0764 012 943 (WhatsApp works, photos welcome), Monday–Saturday 08:00–20:00.

Louver of the unit with black mould spots on the edge, before sanitising
Louver of the unit with black mould spots on the edge, before sanitising
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The information on this page is educational and based on cited sources; it does not replace medical advice. We do not do repairs, installation or refrigerant circuit work.