Did you know that natural gas has no smell? The scent is added to alert people to the dangers when a leak occurs. But do you know what the other characteristic odors are that could spell trouble in your home?

If you switch on the heating or air-con and get suspicious scents, you really can’t ignore it. Read on as we answer the question “What is that smell?” so you can keep your household safe this season.

Safe Smells

When you first turn on your HVAC or heating system for the season, you may encounter a smell of burnt dust. This is perfectly normal and is just an accumulation on the vents, registers, and heat exchangers as the furnace fire up and burns them away. Luckily, this should go quickly and only be noticeable the first few times you start it. 

This can be prevented by cleaning the system before you begin. Hire a professional to do this for you and perform maintenance. This will allow them to alert you to any problems that could arise in the coming months, so you are not left with unexpected HVAC outages. 

There are a few smells that will require immediate assistance. They can signal problems with the HVAC components. In some cases, breathing them in may even be harmful or fatal.

Burnt Electrics

Burnt dust and electrical burning have a similar scent. However, burnt electrics often has a slightly more metallic and plastic smell to them. It will give the scent of something that is overheating or has burnt out. 

When you get this, turn off your system right away. The danger is that leaving it on could result in an electrical fire if you have an old system with fewer safety functions. Once it is off, you can check for issues safely. 

Start by looking at the air filters. If they are blocked, the system works overtime, and the electric resistance heaters can start to get too hot. To prevent this, make sure you are replacing your air filters regularly. 

If air filters are not the issue, it is something to do with the wiring. Turn the electricity to the system off immediately. Unless you are a trained electrician, you need to call a trained HVAC repair engineer to assess the situation. 


The smell of sulfur is characterized by the scent of rotten eggs. If you don’t have any cause for this to occur naturally in the home, you need to act fast. Open your windows, leave the home and call your local gas company. 

A rotten egg smell is often the sign of a gas leak. Gas is naturally odorless, but many companies add scents to tell people when leaks occur. It is not something you should ever ignore. 

Chemical Odor

There is only one chemical that could cause issues in your HVAC system. That is the refrigerant in the heat exchanger. Its job is to remove heat from the combustion chamber and deposit it into the plenum. 

A faint smell of chemicals such as formaldehyde signals a leak. This can start electrical fires. However, more worrying is that it can send deadly carbon monoxide gas into your home. 


Mustiness is most associated with HVAC systems. They contain a lot of moisture which can pool in their many vents and spots. If left unattended, this can turn into mold. 

Luckily, mustiness is not a smell that is usually harmful to you or the system. It will, however, make your air quality suffer. Run the heater for a few hours and see if the smell vanishes. 

If not, you will need to get professional assistance to remove the smell. An HVAC technician will know the nooks and crannies where mold builds. They will also have to tools to get into these spots and solve any issues, as well as preventing them from returning. 


A burning oil smell can be the result of two problems. Neither of them is dangerous, but it is a very unpleasant odor that needs removing from the home. 

Check around the system. If you can not see any oil, then you may have issues with the oil burner. Soot and smoke indicate more serious issues, along with odd sounds. 

If you do find oil, then something may be wrong with the fittings. Check and tighten any oil lines, filters, and fittings. If leaks still occur, the threads may have gone, and the part may need replacing. 


As well as a rotten egg smell, gas also has its own characteristic tinge. This most commonly occurs when you first put the heating system on at the start of the season. 

A faint gas smell that dissipates should be fine. However, you should still get a technician to look at it. It may be a case of dust getting burnt off. 

A lingering or strong smell is a serious issue. Follow the same instructions your would for a rotten egg smell, opening windows, and leaving the home. The gas could ignite and could make you and your family sick. 



An odor that persists, or smells of gas or smoke, should be dealt with immediately. Anything else will end be more unpleasant than dangerous. However, it may be hard to get someone to deal with this at the start of a season, as many people fire up their systems and find the same problems. 

The best way to deal with this is prevention. Hire a regular maintenance service to clean the system and notify you of possible problems before they occur. 

What Is That Smell?

Now you can answer the question “What is that smell?” you should know if you have a major problem. In any case, the issue will need dealing with unless you can stand to live with the odor in your property.

Your first stop for HVAC maintenance and repair should be P&M HVAC. We provide services, financing, and reviews to the Houston area. Contact us here to discuss your needs and schedule our services starting today.