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Question DetailsAsked on 5/28/2016

exhaust odor in home

Exhaust odor intermittently throughout 1 story ranch home. Unable to pinpoint exact source. MB: ceiling fan, sometimes odor appears to be coming from outside open window..close window and still smell, closed door closet. MB bathroom: underneath sink in closed cabinet, floor of linen closet, exhaust fan. Sometimes I think I get a whiff through return air vents and registers. Kitchen sink, washing machine, by electric range, floor behind refrigerator. Basement: bedroom closet (adjoins storage room), storage room(sump pump location), bathroom(adjoins mechanical room), mechanical room and outside wall on opposite side of basement. Back drafted water heater and smelled it. Gas turned off for 3 days and odor still present, but not as frequent or strong. Turned gas back on and no back draft from water heater in last 2 days. Gas co. hasn't detected CO from meters. By the time they get here to test the odor has already dissipated. Service Guard has looked at HVAC/water heater. Not dry drain.

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4 Answers

Voted Best Answer

Since odor diminished once the gas was off for a couple of days, particularly if you meant the water heater backdrafting was not intentionally done, I would be looking at an exhaust ducting problem - rusted through, separated, not high enough on roof so exhaust is being pulled back into the house, partly blocked - something like that. Normally if the flue ducting is disconnected above the appliances and the roof cap taken off you can see pretty clearly into the duct using a strong light from the bottom, then a drop light from the top to look for rusted out or separated ducting.

Other possibilities - a poorly burning or partly clogged up pilot on a stove or dryer, if yours has a standing (always on) pilot.

Also, bird or insect nest in the duct cap can certainly cause this, as can one partly knocked off but maybe hanging on by one screw so it is directing wind down into the flue rather than promoting drafting as it should.

One thing on the gas company not detecting CO - if you spend about $25 for a digital reading (I would stay with Kiddee or First Alert brands) CO monitor (which can then later permanently mount in the house), you can put it where the smell is coming from and see if you get any reading - though it is possible to have exhaust gas coming out and still not get a high enough reading to register if the appliance is burning cleanly, because many of the monitors don't start reading till a certain minimum concentration of gas. Since you say the odor dissipates by the time the gas company gets there, pretty much indicates it is coming from the gas appliances, and since it dissipates likely only during the firing phase, not from pilot light - so likely a backdrafting event or leaking exhaust duct.

Another possibility - if you have changed something related to house ventilation (new fan, ducting changes, reroof or attic insulation job that might have messed up the exhaust flue, leaving downwind windows open due to warmer weather, weatherstripped or changed door to the area the appliances are in so they are no longer getting enough fresh air access, etc) then perhaps now the house or at least the room where the appliances are now has negative pressure when the appliances are firing, so when the appliances (water heater, furnace/boiler) kicks on, it could be backdrafting for a few seconds till it builds up enough hot air for gravity venting through the duct - so you get a puff of warm air out the draft hood at the top of the appliance, which smells like gas exhaust and commonly has a fair dose of gas odorant smell too. It is common with gravity (as opposed to fan-driven high efficiency unit exhaust) to have a couple of seconde puff out the draft hood, but should not spread the exhausst smell more than maybe 5-10 feet from the appliance and dissipates very quickly. Or could be from a draft hood that is out of alignment. You could turn the appliances down to pilot for a half hour to hour or so to cool down, then back to ON so you will be there when it kicks on to heat back up, smell around for exhaust smell coming from the draft hood - which looks like this off-kilter one-


One other possibility, especially with newer installations - blockage of a fresh air inlet vent or duct - by a homeowner trying to keep cold winter air out, or bird or instect nest. Or blockage of air inlet screen for water heater - some of the newer ones have a REALLY stupidly placed air inlet screen in the bottom of the water heater (where it cannot be seen or reasonably cleaned) which if it gets lint or duct blocked restricts airflow to the firebox, so can cause weak combustion and blowout of partly burned gas exhaust from the unit base. Saw one so thoroughly dust blocked (on only a 3 year old unit) that the flame was burning red instead of blue or yellow/orange - was almost totally starved for oxygen.

Heating and A/C is the Search the List category for tracking down your type of issue.

One other possibility if you have close neighbor, though unlikely - their exhaust gases (especially if they put in a high-efficiency furnace or water heater which vents directly out the wall near ground level) is close enough to your house and in the generally upwind direction so you are smelling their exhaust gases. Since you said the smell diminished with the gas turned off, however, unless the reduction was just a "hopeful wishing" psychological belief that it had diminished, that would presumably eliminate a neighbor source, as well as wood fungus or overheating electronics/chargers which can sometimes smell like gas appliance exhaust.

One other possibility if not positibve it is linked to the gas appliances - in addition to checking all your electronic devices which use plug-in transformers for possible transformer/charger or battery pack overheating, is to find where your doorbell transformer is (typically looks like following link picture, generally exposed and mounted on an electrical box, commonly under entry stairs or in entry closet) - and make sure it is not overheating or smoking the wood framing it is mounted to - because sometimes when they overheat they can smoke the wood for quite some time without catching fire -

Answered 4 years ago by LCD



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Answered 4 years ago by Member Services


Thank-You for answering LCD. I've read some of your answers to other questions and was quite impressed, which prompted me to ask a question in the first place.

1. Water heater backdrafting was intentially done.

2. Could it be a poorly burning or partly clogged up pilot on the water heater or furnace?

Stove and dryer are electric.

3. New digital Kiddee CO monitor doesn't alarm to anything.

4. New roof was put on in April, 2013. They never went into the attic to check anything when finished. They didn't cover a seam on roof during a brief rain and rainwater did come through the ceiling by front door/living room, which they supposedly fixed. Could there be a problem with venting 3 years later? Could "wood fungus" be in attic? Attic access is through a covered hole in the garage ceiling. Unfinished attic with insulation. What professional should I call to check on this?

5. House is 12 years old.

6. Where is the fresh air inlet vent or duct located? Is it the same as a return air vent?

7. So, checking the flame color of water heater pilot will determine if there's an issue with water heater?

8. Unlikely close neighbor's ground level vents. Cookie cutter neighborhood. Although, lately I smell strong odor of exhaust from neighbors lawnmowers even when the windows are all closed up.

9. Should this be checked on electric stove, washer, electric dryer, refrigerator? "electronic devices which use plug-in transformers for possible transformer/charger or battery pack overheating"

10. A representative from gas company did check the heating/ac. When he was banging on something I did get a slight whiff of exhaust odor up stairs, but he said everything checked out. Should I have another professional come and check it for a second opinion? I have a service agreement that would repair/replace.

11. When gas company turned the gas back on they discovered a disfunctioning gas valve on gas fireplace. Smelled a slight odor of gas, but not the exhaust odor.

12. Representative from gas company briefly checked the gas water heater and said it was fine. Did not check the flu/venting though. What professional should I call to check? Should I call another professional for a second opinion? I have a service agreement that would repair/replace.

13. Conducted a partial peppermint test. Only had a partial bottle and only used a little under a gallon of water. The person on roof covered pipe with towel. I didn't smell any peppermint in master bedroom bathroom until I turned on the exhaust fan....then I got a slight whiff. A lot of rain lately, so haven't been able to get anyone back on the roof to re-do.

14. Someone told me a dead animal was in a wall. The only animals I've seen outside are bunnies, birds and an occassional vole/shrew. Would it smell like exhaust?

15. Exhaust odor has remained less frequent and not as strong since gas turned back on.

I sure do appreciate the answers. I'm just trying to diagnose the source of exhaust odor, so I know who to call to fix it.

Answered 4 years ago by PD


In response back by your numbers in your followup:

1) OK - it did not sound like a flue blockage that could cause backdrafting because that should have set of CO alarm and shoudl have been noticeable at least several times every day, and strongly noticeable in the furnace/water heater area.

2) Poorly burning pilot is possible but not likely to be noticeable in entire house - and pilot flame would most likely be very yellow or orange and probably flickering to cause much smell, and without a flue problem almost all that smell would go up the flue anyway, so unless REAL bad would not cause housewide smell.

If pretty much blue or slightly greenish blue with possibly a slight yellow center at its base, it should be fine if natural gas - propane ditto but commonly has a slight yellow tip, but the color overall should definitely be described as blue or blue-greenish, not orange or yellow, and flame should be steady - not bouncing all over.. Plus strongest smell would be at the appliances and in that room and pretty noticeable if making it into the rest of the house. Yours is almost certainlyi fine if the gas company guy inspected the appliances.

3) Digital alarm saying nothing - assuming it is reading correctly for operating (mine reads .0 when OK but reading no CO) then probably not a major blockage or total disconenction of the flue within the conditioned space of the house. You say "not alarming" - does not alarm till about 30 ppm minimum, and at varying levels up to about 400 ppm depending on exposure time, so not "alarming" just means it has not reached real dangerous levels. It should read zero - and try the recent high reading button to see what the maximum level it has recorded is. (Be sure to reset this before testing, as it may have been tested at the factory and retain that reading if they forgot to reset it). You could try, if moveable (not hardwired) putting it up relatively high (NOT on top of appliance due to possible overheating), even just temporarily hung on a nail with an extension cord to it (if plug-in type), to see if it reads anything in the appliance room.

4) Roof repair - if you have not noticed it since then (3 years) "probably" not related to that - but if they did a sloppy job of putting the duct back together when they put the roof stickup piece back on, you might have had it rattle loose in recent wind and open up a joint in the flue duct in the attic - which if you have "negative pressure" in the house could pull those fumes down into the house. If when you are smelling the odor you can get a stepladder or such and open the hatch (may get a bit of insulation falling down around you when you do) and see if the smell is strong up there that might be instructive and indicative of a possible duct issue up there. I am assuming here the furnace/water heater are not in the attic - if they are, makes that source much more likely. Could also reset and then place the CO monitor up in the attic just inside the hatch for an hour or so (with hatch closed) and see what it reads after that. Be sure to check the high reading value - not what it reads after you have brought it back into fresh air.

Not likely to be wood fungus - that usually is described as smelling like rotting vegetables or rotting garbage, dank, sticky sour-sweet, mushroomy, or occasionally like sewage - I can't see mistaking that for gas appliance exhaust smell. If you have a Heating and AC contractor checking out the gas appliances and flue ducting, he should go into the attic and check there for CO levels, strong smell and check for damaged or disconnected ducting both.

One possibility - remote - about the roof job. I have seen roofers incorrectly put ventilation fan roof hoods onto flue ducting, which would very easily concentrate fumes in the attic because the hole through the roofing into the hood is usually significantly larger than the ducting and not sealed, so the air circulates around in the hood and some could come back down around the duct into the attic and be pulled into the house through wiring or ducting penetrations. The flue pipe (commonly 6-10 inch metal duct but smaller plastic pipe with some high-efficiency condensing appliances) is supposed to stick up (varies by some codes and areas) roughly 3 feet above the roof, and have a fixed louver or bonnet type rain cap on it, not a "hood" like kitchen and bathrooms fans have.

One other remote possibility on the roofing - if your flue pipe is running hot for some reason, could be heating up the roofing materials if they did not maintain the proper clearance, causing smoldering of the roof materials. Not likely, both because the flue gas is usually only warm at that height (except with attic appliances) and also the outside of proper double-walled flue ducting should not get that hot except right at the appliance.

5) 12 year old house presumably should negate the possibility of old age failures such as corrosion of the flue

6) No, fresh air vent is not the same as the return duct. The return air duct returns the "used" ventilation air to the furnace, and that air should pass through the filter back into the air intake for the furnace fan. The fresh air intake is typically a 6-16" (normally about 8-10" or possibly two of them if large-capacity appliances) duct through the wall of the appliance room or garage or wherever they are, direct to the outside, with no flapper or obstruction other than insect screening. Sometimes this just penetrates the wall, with some higher-efficiency units and in very cold climates sometimes it leads ducting to the appliance firebox. Generally located as close as feasible to the appliance.

Sometimes people block this off with a rag or with plastic sheeting and tape or insulation because it lets cold air into the area - but it is there to ensure the appliances get enough fresh air for proper combustion and to ensure a good draft up the flue. Usually just a round piece of sheet metal ducting through the upper portion of the wall, or sometimes into a crawlspace if it is open to the outside through screened vents.

7) If furnace and water heater (assuming they have a "standing" or constant-on pilot) pilot color looks good as per 2) above (google for pictures and videos of proper pilot flame adjustment and color) then likely not the problem. Gas flame when the main burner is burning should be same - frequently a bit more yellow at the edges and tip, may have some minor yellow and red sparks in it from burning of dust and rust on the burner, and may be fairly yellowish during the first couple of seconds until the flame steadies down. Note you need to look at this as it is in service - without removing the inspection/service cover, because that will let more air in and might thereby mask an air supply deficiency problem.

8) This could be significant. Mention this to your HVAC guy - strong mower smell when windows are closed might well indicate that you have "negative pressure" in the house when the appliances are firing, or at least when HVAC blower is running. Way to check on this is leave a window full open and open airway to the appliance (so any doors along the way open) or crack the outside garage door open an inch or so and see if that makes a difference. If so, your house may be too tight or the air intake for the appliances is blocked off (or does not exist, though should have one in 12 year old house).

9) Washer, range, dryer, dishwasher, etc - all large appliances can put out a funny burnt or hot plastic smell if they are overheating, and I guess could be called an exhaust type smell in some cases - but would be strongest in that area consistently, and probably usually only when that device is being used, so you would pretty quickly notice that particular room was the source, so not likely the source. Ditto to overheating light fixtures. You could try blocking off all the rooms by closing all doors (with no one/no pets in them in case you are concentrating fumes) for several hours or while away at work or out for dinner or such, and then smell each room as soon as the door is opened to see if any concentration. If coming from an overheated device should be pretty obvious which room the smell is coming from. Don't forget to check seldom used areas - laundry and linen closets, sewage lift pump and sump pump if you have one or the other, etc. Also check closets that the exhaust flue might pass through or next to - sometimes people put in shelving or coat racks or such and puncture the exhaust flue passing right behind the closet wall with nails or screws.

10) I would go with an HVAC company that has CO and natural gas detectors at a minimum. Unfortunately, the companies service agreement/home warranty companies seem, in my opinion, to commonly be those who cannot get work on their own reputation so take on warranty or box store contracts - so I would not trust their competency in general. For a potentially life-threatening thing like this especially, I would go with a long term HVAC company in your area with the best reputation for service, even though it might mean paying more. One other possibility - probably a bit cheaper for the investigation phase but not able to do any repairs - would be a Home Inspector to act as a sleuth, if you are not up to doing this yourself.

11) If you have gas fireplaces or a pellet or wood stove that are in use or have a standing pilot, those are definite possible sources - but again, would likely be concentrated right near them, though not necessarily - can migrate and concentrate in odd places, following drafts to outside doors or such. Could try shutting the pilot totally off on such devices to see if it eliminates the odor.

12) Heating and AC would be the Search the List category for this - to check and repair both gas appliances and any flue ducting issue. Make sure he fires each of the gas appliances up several times while there, so if they are generating the smell he has a chance to smell it himself.

One thought - I am presuming, especially if 12 years old, that your gas furnace and water heater have sheet metal ducting for the exhaust clear to the outside - that they are not just dumped into an old fireplace flue to vent, which is extremely dangerous and contrary to code if you happen to have that. Running up an old chimney is OK if proper type of metal ducting is used to the top and the chimney is large enough to provide code clearances around the ducting. Not likely a problem in 12 year old house. Can also cause exhausst smell if running hot flue duct up inside a chimney that has residual creosote in the flue - can smell like hot railroad ties or power poles in the summer, or like a hot sooty smell.

13) Not clear on what you did with the peppermint oil test - was this on the gas appliance flue ? Or in the sewer vent pipe, which is where it is normally used to test for leaks ? IF dumped down the sewer vent pipe and yuou smelled a bit after the bathroom fan was turned on, assuming the vent pipe was still blocked at the top, could be you have some odd smell (though would usually smell like sewage or hydrogen sulfide gas) coming through a cracked drain pipe, an undersink vent [google for "under sink DWV vent" for picture of this], or a drain pipe with inadequate trap water depth - like an infrequently used guest bathroom drain or floor drain. Sniff around them to see if the source when you smell the odor.

14) I suppose it could be, but I have never heard a dead animal described as smelling like exhaust - and especially since you smelled the same smell when the water heater was backdrafted, I don't see it being that. That smell is generally described as rancid grease, putrid, rotten meat, old slimy rancid greasy leather or shoes, or puky - not like exhaust gas.

15) This could jsut be a deceptive impression or hopeful wishing - can't see that turning gas off and on would make any difference. Might make a difference if the smell were a gas odorant smell and a minor leak at the meter valve stopped after the gas was turned off and then back on, but I presume you know what your natural gas odorant smells like ? If not, call or go by your gas company and get a scratch and sniff card from them or have them mail you one, because many people can detect the odorant smell long before the gas shows up on a detection meter (which detects the gas presence iself, not the odorant). Or, as I said before, stand by furnace or water heater (furnace draft hood probably closer to floor, or stand on a chair or stool to get close) and smell the short burst of air that comes out the draft hood when it first fires up in response to thermostat demand - have someone turn up the thermostat to make it turn on - or if tankless water heater have someone run on hot water to make it fire up. The first burst tends to be a lot stronger on the odorant smell than once the burnears settle down, because they burn it up pretty well in normal operation.


One other possibility I coincidentally ran into just a couple of days ago - if a high-efficiency unit like a furnace, water heater, or tankless water heater, IF it is not a totally sealed unit, then if they ran the air intake and the exhaust PVC piping together up to the roof (typically through an old 6-10" gravity draft appliance flue pipe) and terminated them right next to each other, it is possible for the exhaust to be partly pulled right back down by the air intake. If that opens into open air at the appliance (rather than into a sealed firebox) then it could dump exhaust air back to the appliance room - would normally be strongest smelling right near the appliances when you are first smelling it, and right when or immediately after a firing cycle. Ditto if they vented multiple devices right next to each other (supposed to be at least 3 feet laterally apart in most code areas so exhaust from one does not go into the other) - but again, unless your ducting has changed recently this should have been unchanged since the roroof job. Of course, if nothing has changed in the appliances or venting recently, then that would not explain this "new" smell.

You did not say how often this happens - but if you have a neighbor you are on good terms with who is home a lot, you might ask them to come smell around when it happens, as a secone opinion as to what it smells like and where the smell is strongest.

Bottom line - from all you have said, unless you are smelling sewer gas venting into the house and thinking it is exhaust gas, my first guess would be a separated or rusted through fllue duct, which a good inspection with strong light from both top (preferably a drop light from the top) and strong flashlight or inspection light from the bottom can usually identify even if a small gap or hole (including rusted out nail holes). On tall houses or ducts with bends soemtimes you have to run a camera down there - some HVAC contractors have them, otherwise they have to call in a sewer and drain cleaning contractor with a sewer camera to run down there (with the appliances off, of course - the cameras are expensive).

If you would, once you find the cause let us know here what it was from, for instructional purposes.

Answered 4 years ago by LCD



1) if you have a unit heater or wall heater - gas or electric - check that out as possible source too, from leakage or overheating.

2) if you have any sort of attic vents (including ridge vents) through the roof consider4 those as possible sources of furnace/water heater exhaust coming back in those vents into the attic. Especially since you said you smelled it through open window - indicating maybe exhaust gas (or sewer vent pipe gas) flowing down the roof as it cools and coming in a window - especially if that window is "downhill" of the stacks on the roof and on a side of the roof that is close to where they come through the roof (which might a both sides of the ridge if located near the ridge and basically sticking up above it).

Answered 4 years ago by LCD

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