Soot and oil burner puffback was heralded by horrible oil smells and soot in the buildnig © Daniel FriedmanHeating & HVAC System Odor Diagnosis & Cure
Sources & cures for odors & smells from air conditioning, heating & other HVAC systems & components

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Heating & air conditioning system odor causes & cures:

This article describes & lists sources of smells that are traced to HVAC systems such as air conditioners, heating boilers, furnaces, duct systems, heat pumps, and water heaters.

Smells blamed-on heating systems that use ductwork may actually be coming from something contaminating the ducts or may be simply transported from another building location. But odors often are traced to improper or even unsafe heating equipment operation or to heating equipment fuel (gas or oil) leaks.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

HVAC Equipment Odor Sources, Implications, Hazards & Repairs

Photo of tranite cement-asbestos material used for air ducts in a slab over a sewer pipe (C) Daniel Friedman and ConradHeating System Odors: smells from improperly functioning heating or air conditioning equipment can come from a wide range of sources, as we will list here.

Watch out: odors of combustion gases or heating oil or natural or LP gas can all be indicators of serious safety hazards as well as malfunctioning building heating or water heating equipment.

[Click to enlarge any image]

The open concrete floor slab shown in our photo at left finally explained how sewage odors were being transported in the HVAC duct system. Ducts in the slab had collapsed enough to admit sewage from a leaky sewer pipe that ran in parallel to the ductwork.

Details of this mess are


After a catalog of articles that address specific building odor complaints traced to heating or cooling systems, we include field reports of several smell complaints and how they were tracked to their source.

See these detailed articles describing possible causes of or sources of heating system odor or noise complaints:

Reader Question: Chemical or Paint Odors: strong odor when heating furnace first kicks on, before the blower operates

We are experiencing a strong, very troublesome odor when our furnace first kicks on, before the fan begins blowing. The furnace company has come out several times and they are satisfied that it is NOT the furnace. They thought perhaps it could be in the ducts. We had them vacuumed today, but to no avail.

The smell, if anything, is getting worse.

We first noticed a similar problem last year, but it is much worse this year, and coincidently? we noticed it strongly right after painting our basement with "Painter's Select Porch and Floor Coating." We painted it on Oct 10th of this year.

We were fixing the house up to sell, and wanted to brighten up the basement, and put up a moisture seal on the field stone walls. It was painted when we moved into the house in 2005, and the paint had peeled off, probably due to moisture leaking through the field stone.

Before painting, we had it "pointed" in areas that clearly needing re-mortaring. The paint we used was supposedly a low-odor paint, good for poorly ventilated areas, and indeed, the basement does not seem to have a particularly strong smell.

But, when the furnace (oil) burner kicks on, about 1 minute later, a powerful odor wafts up out of all of the vents, which our furnace people think is a paint smell, and not an oil smell. It seems much stronger than just paint though, and is truly disturbing.

We are trying to sell this house (unsuccessfully so far) and we are also worried that this cannot be good for us. I personally am still not persuaded that this is a paint problem. It is SO strong! But, we cannot seem to solve this problem on our own and are truly desperate for help. We have thought about buying a dehumidifier. Would this be a good idea? - Susan 12/5/11


Susan, you are quite right to focus on the heater start-up cycle when you notice the smell particularly at that time. You didn't mention the heater fuel but I'm guessing it's heating oil not gas. Odors coming from gas heating appliances are particularly dangerous because of the possibility of a chimney, draft, or combustion air problem that can be a telltale for conditions producing potentially fatal carbon monoxide.

If the smell were due to poor venting or a chimney problem or a startup problem with an oil fired heater I'd expect you to recognize the fuel oil odor. Because you think it's a paint-related odor, I too am led to focusing on the prior cleaning and painting history in the home.

But let's ask why that odor would be particularly severe when the heater starts and before the blower turns on. Foundation or stone wall sealants as well as paints (but not likely mortar) often outgas strong odors when new but it would be unusual for such an odor to persist.

However if a nearby painted surface were being abnormally heated - say by a hot supply air plenum before the blower comes on to cool down that area in your heating system - that could be a factor.

I think that to get to the bottom of this you want to home in on exactly where you smell the odor (coming from air supply ducts?) in the home, and where the odor appears to be strongest, keeping in mind the possible role of heat from your heating system in increasing outgassing.

Reader Question: burning smells from heating boiler: suspect burn-off of oil coatings?

(Jan 15, 2015) MLizbeth said:
Hi, We've got a boiler mainly for hot water heating. (House is heated with wood stove.) Four days ago, out of the blue, the electric switch box on the front of the boiler started smelling like paint or like heated-up electric components.

My plumbing/heating man first said it had just overheated (from turning it on more often to run water through baseboards during this cold snap, so water wouldn't freeze in pipes).

But he checked it out at my insistence, and replaced the switch - he said it was, after all, damaged. But there is still a smell when the boiler goes on, i.e. it gets heated up. (He also bled out air that he said was in the pipes. Could be totally unrelated.)

Smell dissipates until next time boiler kicks on. This all happened literally out of nowhere. I hate to bother him yet again with more phone calls but this smell shouldn't be the norm. What do you think?

This question was originally posted at OIL BURNER OPERATIONG PROBLEM FAQs


Burning smells are not normal and should be fixed, including the cause not just the damaged part. For example backpressure in a combustion chamber can overheat nearby components.

Just be sure you distinguish between left-over odors from a prior problem and new smells.


(Jan 21, 2015) MLizbeth said:
Thanks for the good ideas. I had my heating/plumbing man check it out again, and he was surprised to find that the toggle switch was cracked inside. He replaced it. I wasn't surprised; I knew something had to be different.

But it hasn't solved the smell, which is kind of petroleum-ish, a little like paint, not really a burning occurring anyway. (He and his assistant checked everything immensely thoroughly, which was reassuring.) The smell comes only from the toggle switch box on the front.

I am thinking to have him simply replace that. Though there is no problem per se, to have that smell when it has never been there before should not be the case. Thank you again.



Indeed if the switch was overheating or arcing other components in the switch box may be burned; open, examine, replace. Use the email at our CONTACT link found at page top or bottom to send me some photos for further comment if you can.

(Jan 23, 2015) MLizbeth said:
Well, my plumbing/heating man along with his assistant were here, opened it, looked at it plus basically every last aspect of the system, checked out EVERYthing in my basement. Very kind and helpful! Everything was fine.

The one thing he mentioned that resonated with me was that components inside are given a light coating of oil, even the electrical wires inside.

That could have overheated before the (cracked) toggle switch was replaced. I also spoke with my own oil company who helped me out with information and it looks like that is the solution to the mystery.

My husband has already called him, and he'll be replacing the whole switch box and wiring.

If I can get a photo that shows anything at all I will, but as of a couple days ago, visually, it was "picture perfect" and nothing to see other than what it should be. It appears that that oil coating just heated up.

Strange the littlest things that can occur (that I a layperson would never have known of prior)! I appreciate the vast repository of information you have placed here for home owners to access, to become knowledgeable about their home systems. Thank you!

Reader Question: Burning or Electrical Heater Odors: tracking down a burning smell when the air conditioning is on

For the past 3-4 months, some of our employees are complaining about a burning smell ( like the smell when you turn on the heater after a very long time) in the building when the air conditioning is on. I had a couple of HVAC companies come and check out our system but they couldn't find anything wrong. Any ideas? - Fieldpiece 9/12/11



If you believe people are really smelling something burning, it could be unsafe and certainly justifies a more expert onsite inspection. Some simple tests like turning off suspect equipment might help too.


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