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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.
Heating 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 at SLAB DUCTWORK.
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:
ANIMAL SMELLS may be traced to animals in HVAC system duct work or air supply systems
CARBON MONOXIDE - CO hazard levels, poisoning symptoms, & testing - you won't smell CO (carbon monoxide) in a building, but if combustion gases from a heating or hot water system or possibly a wood stove or coal stove are not being safely vented to outdoors you may smell other telltale products of combustion.
DUCT & AIR HANDLER ODORS provides help in tracking down and curing odors that either originate in HVAC air conditioning, heat pump, or heating duct systems & air handlers, or are being picked up and transported by them
HEAT EXCHANGER LEAK TEST - in some cases (not all) a dangerously leaky furnace heat exchanger may release combustion gases at a rate that people smell in the building. See Carbon Monoxide warnings above in this list.
ODOR DIAGNOSIS CHECKLIST, PROCEDURE - this checklist aids in finding the source of building smells or odors: a procedural checklist & an odor event log can diagnose and track down the source of building odors
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?
(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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 ASTM E2600 - 08 Standard Practice for Assessment of Vapor Intrusion into Structures on Property Involved in Real Estate Transactions is available from the ASTM at astm.org/Standards/E2600.htm .
"This practice is intended for use on a voluntary basis by parties who wish to conduct a VIA on a parcel of real estate, or more specifically conduct a screening evaluation to determine whether or not there is potential for a VIC, and if so, identify alternatives for further investigation."
The standard goes on to emphasize the uncertainty in testing any site for gases and vapor intrusion.
 Xiaoyu Liu,*† Mark Mason, Kenneth Krebs, and Leslie Sparks, "Full-Scale Chamber Investigation and Simulation of Air Freshener Emissions in the Presence of Ozone:, Environ. Sci. Technol., 2004, 38 (10), pp 2802–2812
Publication Date (Web): April 9, 2004,
Abstract: Volatile organic compound (VOC) emissions from one electrical plug-in type of pine-scented air freshener and their reactions with O3 were investigated in the U.S. Environmental Protection Agency indoor air research large chamber facility. Ozone was generated from a device marketed as an ozone generator air cleaner. Ozone and oxides of nitrogen concentrations and chamber conditions such as temperature, relative humidity, pressure, and air exchange rate were controlled and/or monitored. VOC emissions and some of the reaction products were identified and quantified. Source emission models were developed to predict the time/concentration profiles of the major VOCs (limonene, α-pinene, β-pinene, 3-carene, camphene, benzyl propionate, benzyl alcohol, bornyl acetate, isobornyl acetate, and benzaldehyde) emitted by the air freshener. Gas-phase reactions of VOCs from the air freshener with O3 were simulated by a photochemical kinetics simulation system using VOC reaction mechanisms and rate constants adopted from the literature. The concentration−time predictions were in good agreement with the data for O3 and VOCs emitted from the air freshener and with some of the primary reaction products. Systematic differences between the predictions and the experimental results were found for some species. Poor understanding of secondary reactions and heterogeneous chemistry in the chamber is the likely cause of these differences. The method has the potential to provide data to predict the impact of O3/VOC interactions on indoor air quality.
 RI Vanhegan, R.G. Mitchell, "Pseudomonas Infection Associated with Contamination of Wick-Type Air Freshener", British Medical Journal, 20 Sept. 1975, pp. 685 [copy on file as Air_Fresh_Study_BMJ75.pdf]
Though unproved, the bottles may have been directly implicated in
cross-infection and they should not be used in intensive care units and
similar places. The practice of topping-up existing bottles from a
stock solution should be discouraged since the resulting weakened
mixture may eventually support the growth of organisms. Since an
increase in the formaldehyde concentration proved irritant we
recommend the use of safe non-volatile disinfectants. The possibility
that organisms may develop resistance to formaldehyde solutions was
not further investigated.
 Mihalis Lazaridis (Editor), Ian Colbeck (Editor), Human Exposure to Pollutants via Dermal Absorption and Inhalation (Environmental Pollution), Springer; 1st Edition. edition (April 1, 2010), ISBN-10: 9048186625
Quoting: The human body is exposed to pollution on a daily basis via dermal exposure and inhalation. This book reviews the information necessary to address the steps in exposure assessment relevant to air pollution. The aim is to identify available information including data sources and models, and show that an integrated multi-route exposure model can be built, validated and used as part of an air quality management process. Many epidemiological studies have focused on inhalation exposure. Whilst this is appropriate for many substances, failure to consider the importance of exposure and uptake of material deposited on the skin may lead to an over/underestimation of the risk. Hence dermal exposure is also considered. Drinking water contamination by disinfection by-products is also discussed. Written by leading experts in the field, this book provides a comprehensive review of ambient particulate matter and will be of interest to graduate students, researchers and policymakers involved in air quality management, environmental health and related disciplines, as well as environmental consultants and ventilation engineers.
 Troubleshooting Split System A/C or Heat Pump Noises, Fujitsu General America, Inc., 353 Route 46 West, Fairfield, NJ 07004, Tel: (888) 888-3424, Tel-Service hotline: (866) 952-8324, Email: firstname.lastname@example.org, Email service: email@example.com , retrieved 8/30/12, original source: http://www.fujitsugeneral.com/troubleshooting.htm [copy on file as Troubleshooting Fujitsu Ductless Mini-Splits.pdf]
 Charles M. McGinley, P.E., Michael A. McGinley, MHS, Donna L. McGinley, " “Odor Basics”,
Understanding and Using Odor Testing", paper presentation, The 22nd Annual Hawaii Water Environment Association Conference.,
Honolulu, Hawaii: 6-7 June 2000, St. Croix Sensory Inc. / McGinley Associates, P.A.
13701 - 30th Street Circle North
Stillwater, MN 55082 U.S.A.
firstname.lastname@example.org, retrieved 9/22/12, original source http://www.fivesenses.com/Documents/Library/33%20
%20Odor%20Basics.pdf, [copy on file as Odor_Basics.pdf]
 Jon H. Ruth, "Odor Thresholds and Irritation Levels of Several Chemical Substances: A Review", American Industrial Hygiene Association Journal
Volume 47, Issue 3, 1986, retrievedf 9/22/12, Abstract: A collation of odor threshold data for approximately 450 chemical substances is presented. The range of odor thresholds reported in the literature is shown along with any reported threshold of irritation to humans. These data can assist the industrial hygienist in determining when an “odor” may be in excess of the Threshold Limit Value®, when an organic vapor respirator is not acceptable due to the lack of an odor warning at the end of a cartridge life, and where odors may not indicate a hazard due to extremely low odor thresholds which may be well below the respective TLVs.
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
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