Forced-air heating is the most common system in metro Denver houses, and it is very good at distributing whatever is in the air. Returns pull air from the house, sometimes from the basement or near a crawl space, and supplies push it back through every room. If odor has settled in the ducts, the return path or the furnace cabinet, each cycle spreads it.
Sellers often prepare a house in summer or early fall, when the furnace is off. Everything smells fine, photos are taken, and then the first cold snap arrives during the first week of showings. Agents in the region know this pattern well. Running the system for a full cycle before deciding the house is ready is the simplest way to avoid it.
Normal versus worth investigating
- Normal
- A brief dusty, slightly burnt smell the first time the heat runs in a season, fading after a short while
- Investigate
- Stale smoke smell from every register, returning every cycle
- Investigate
- Musty or earthy smell when the furnace runs, especially in a house with a crawl space or damp basement
- Investigate
- Sewer or rotten smell near the furnace or from returns
- Act immediately
- Rotten-egg or gas smell, or a burning electrical smell: leave the house and call the gas utility or 911
What usually helps
- IfThe filter is old or the wrong sizeThenReplace it before anything else, and keep a fresh one in place while the house is listed.
- IfSmoke residue was present in the homeThenAsk about duct cleaning by a provider who follows a recognized industry standard, and clean registers and return grilles.
- IfThe smell is musty and the return is in the basementThenLook for basement or crawl space moisture first; the ducts are carrying the odor, not causing it.
- IfReturns or ducts are open to a crawl spaceThenHave an HVAC technician check for disconnected or leaking ductwork drawing in crawl space air.
- IfOdor comes from the furnace cabinet itselfThenBook a furnace service visit. A technician can check the burners, heat exchanger and blower.
Questions to ask an HVAC or duct cleaning provider
- Which parts of the system will be cleaned: supply ducts, returns, the blower, the coil?
- Do you follow a recognized industry standard for duct cleaning, and which one?
- Will any product be applied in the ducts, and does it add fragrance?
- Are there disconnected or leaking ducts drawing air from the crawl space or attic?
- Can you provide before and after photos for the seller's records?
General information for the Denver area, not a quote or professional advice. In an emergency, call 911.