What is that smell?

A smell of rotten eggs or sulphur around a gas appliance is treated as a gas leak until somebody qualified proves otherwise. Get everybody outside. Do not switch anything on or off, do not use a light switch, and do not use a phone inside the building. From outside, call 911 and your gas utility’s emergency line. They come out for this and they do not charge you for being wrong.

We are a heating and cooling contractor. We are not the right first call for a suspected gas leak, and we would rather say so plainly than have you wait for us.

Turn the system off at the thermostat and at the breaker or disconnect, and leave it off. What you are describing is the kind of smell that comes from something getting hot that should not be, and continuing to run it is how a fault becomes a fire.

If you can see smoke or flame, leave and call 911 first. If it is only a smell, switch it off and ring us; this is the kind of call we will move for.

Nothing here needs you to stop using the system tonight, but it is telling you something and it will not improve on its own. Book a visit and mention the smell when you do — it changes what we bring.

What you are describing is what a heating system usually does the first time it runs after a summer of sitting still. If it clears within an hour or so of running, it is dust burning off the heat exchanger and it is not a fault.

If it is still there the next day, or it gets stronger rather than weaker, it stops being normal. Come back to this page and change the timing answer.

Natural gas has no smell of its own. The rotten-egg smell is an odourant added deliberately so that people notice leaks, which means noticing it is the system working as designed.

• It can come from a supply line, a fitting, a valve, or an appliance that is not lighting properly.
• It does not have to be your furnace — a water heater, a range or a line outside can produce it.
• The gas utility will come out and check. That is what the emergency number is for.
• Once they have made it safe, we can look at the appliance side of it.

A scorching or burning smell that is not the dusty first-run-of-the-season smell usually means a component is running hotter than it should. The most common causes are a blower motor under strain, a seized bearing, a belt slipping on a commercial unit, or wiring insulation getting hot.

• Restricted airflow is behind a lot of it — a blocked filter, closed registers or a collapsed duct makes the blower work far harder.
• A motor that is drawing more current than it should will smell before it fails.
• Do not run it while you wait. There is nothing to be gained and there is something to lose.

A sharp electrical smell, or a fishy one, is characteristic of overheating electrical insulation and components. The fishy note in particular is associated with certain plastics used in wiring and electrical parts when they get hot.

• Common sources are a pitted contactor, a failing capacitor, a loose terminal, or a motor winding breaking down.
• Loose connections make heat, heat makes the connection worse, and it accelerates.
• It is not something to monitor for a week. Switch it off and have it looked at.

If you can smell smoke and there is no other explanation for it in the building, treat it as a fire until you know otherwise. Get out, call 911, and let them tell you it was nothing.

• Do not go looking in the attic or the crawlspace for the source.
• Switch the system off at the breaker only if you can do that safely on your way out.
• Once the fire service has cleared it, we will find out what happened and why.

A furnace sits still all summer collecting dust on surfaces that get very hot when it runs. The first time the heat comes on, that dust burns off, and it smells exactly like it sounds.

• It should clear within an hour or so of running and it should not come back the next day.
• Changing the filter first, and running the system with a couple of windows open, gets it over with.
• If it happens every time the system starts, or it is there all the time, something else is going on.

A musty smell that arrives with the cooling season is almost always water somewhere in the system that is not draining away. In a Georgia summer an evaporator coil produces a great deal of condensate, and all of it is supposed to leave through a pipe.

• A drain pan that is not emptying, a slow or blocked drain line, or a trap that has dried out.
• A coil that stays wet between cycles, often because the system is oversized and runs in short bursts.
• Duct leakage pulling humid air from a crawlspace or attic into the return.
• What that moisture might be growing is a question for somebody qualified to test it. We can tell you where the water is and stop it; we are not going to make health claims about it.

A sewage smell around an indoor unit is usually a plumbing problem that the air handling is simply distributing around the building. The commonest cause is a trap that has dried out and stopped sealing.

• The condensate drain has a trap. If the system has not run for a while it can dry out and let drain gases back up the pipe.
• A rarely used shower, floor drain or sink does the same thing, and the air handler then spreads it.
• We can check and refill the condensate trap. If the source is elsewhere in the plumbing, that is a plumber’s job and we will say so.

A sweet or chemical smell near an indoor unit is sometimes a refrigerant leak, though people describe refrigerant very differently and plenty of other things smell sweet. It is worth checking rather than guessing.

• A leak usually shows up in performance before anybody smells it — longer run times, weaker cooling, ice on the line.
• Refrigerant handling is regulated work. Topping a system up without finding the leak is both bad practice and, for some refrigerants, not legal.
• Cleaning products, a spill near a return, and new building materials all produce sweet smells that the system then circulates.