Five Year Mystery - Unable to find source of burning smell from HVAC system

Question

There is a burning smell that wafts out of the vents of my 15-year old, 3-level townhouse (top level is a full unfinished attic with no HVAC).

The bottom level of the townhouse is the main level (with the front entrance door) and sits on concrete slab. The burning smell occurs in all weather, all seasons of the year, with the HVAC unit on or off, HVAC fan on or off. The cause of the smell is not electrical, 3 master electricians ruled that out. It is not the HVAC fan, that was replaced and there was no improvement.

This burning smell has been happening, year round, for over 5 years. It was pointed out to me that one of the HVAC pipes on my roof top is 1/3 the height of all my neighbor's (see the pictures). I have gas heat.

I think this pipe on the roof is related to the gas furnace. If the pipe on my roof is too short, and it is connected to the gas furnace, could that cause a backup of the gas pilot light/fumes to flow back into the HVAC unit to then exit through the ceiling vents into the two floors of the townhouse?

The burning smell is so bad at times that I have to leave the house or put box fans in the windows to clear the air.

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Additional details originally posted as an answer:

First, thank you to everyone who responded to my post!! Your time and response is very much appreciated. Below is response to the responses we received.

We rent and our landlord advised that for the first 7 years he lived here they never had a problem with this smell. Actually, the problem began 2 years after we moved in and has continued for 5 years.

We just heard back from the landlord and he does not believe the short height on the stack pipe on the roof could be the problem. At this point, we are willing to try in hopes it may help. But landlord approval is required.

Regarding neighbors - have talked with both, they never smell it. We have had the smell occur when the neighbors were away on travel. Don't think they are the cause.

We are not the only ones who have smelled the smell in our townhouse, so it is not imagined. The very first time I smelled it, I thought it was an electrical burn.

The smell is intermittent. It starts and builds over a period of 3-4 days with the smell intensifying till it is maddening, then it abruptly stops and we have 2-3 days of relief, then the process starts over. We have taped coverings over ceiling vents in the bedrooms to keep the smell out - just closing the vents was not enough. The smell still got through, although diminished.

I can distinguish between the smell of metal, plastics, animals, mold, chemicals, and decomposition. The smell is not like the pilot light in our fireplace or the smell when using the stove or oven. It is most closely associated with the smell of an overheating electrical unit or electrical burn.

We have literally turned off circuit breakers in sections of the house for days, thinking the smell may be electrical in nature, but that didn't stop the smell.

We are truly baffled by what is producing the smell and why it is coming FROM the vents. After 5 years of doing every process of elimination we could think of, and that others suggested, I am now convinced the smell is originating within the HVAC unit.

The gas water heater sits beside the HVAC unit in the laundry room on the middle floor. There are no smells coming from that unit, it has been checked frequently.

All of this (and more) is why the has been referred to as a MYSTERY.

We have made two discoveries as we have tried to deal with clearning the air - if we run the exhaust fans, they cause the smell coming out of the ceiling vents to increase. If we place the box fans in the windows and blow air into the house, that will cause the smell coming out of the ceiling vents to decrease. Someone told us using the box fans in this way creates a positive pressure in the house and that is why it helps.

I don't know if this additional information helps in further solving the mystery, but I hope so. We are still open to any further thoughts, recommendations, thoughts, feedback you may have.

Answer 1

Do you maybe have a steam humidifier on the furnace? If it was wired incorrectly or was damaged it could be trying to heat water when the furnace is not running, or when no water was present in the boiler.

How confident are you on the diagnosis of the electrical issues? I've heard of underground feeders upstream of the breaker panel becoming damaged and heating the ground.

Defrost cycle on a freezer. These will come on every few days, and they often use an electric heating element that may be damaged or running hot because of a bad thermostat.

If this is a new-ish building, do you have a heat recovery ventilator? These are separate from the furnace, but are generally connected to the furnace ducting, and they will run all year round. I think I remember seeing a recall for bad HRV fan motors burning up in at least one brand.

Has anyone else also detected the smell? Noses are all different, and you might be sensitive or mis-sensing a smell.

I would make friends with a home inspector or someone else with a thermal imaging camera and inspect the area carefully. If it is caused by something getting hot this is one of the best (only?) ways to find it. Let us know if you solve the mystery.

Answer 2

All things being equal, a shorter vent won't increase the risk of backdrafting unless your vent is designed to rely on the stack effect (e.g., those big black pipes that come out of pit toilets). Ed Beal is right to point out that being below the roof line is relevant but laypeople and contractors often overstate concerns about backdrafting. They do this for good reason—caution not only prevents liability, it also saves lives—but the geometry I see in your image doesn't particularly concern me. (Disclaimer: Though I have some training in the area, I am not an HVAC specialist!)

That said, it would be cheaper to just extend it to match your neighbors' vents than to hire an HVAC engineer to do an analysis. My concern is more that the inconsistency in height could be a sign that this vent was installed either at a time when different codes were in place, or by an installer who didn't follow the code. It might also be totally unrelated. What you could check fairly easily is what the existing code requirements are for gas appliance venting through the roof, in terms of any required minimum elevation or height above roof line; and if the existing vent is not up to code, that could be a sign of some other problem not visible from the exterior. A licensed HVAC installer should be able to tell you just by looking at the picture whether it's a possible code violation; a home inspector would be even better, as a minority of installers unfortunately do not pay as much attention to building codes as they should.

All that said, I think Carl is right that you're likely not smelling either the raw gas or the combustion byproducts from the furnace. It doesn't fit the pattern you describe of a constant odor and natural gas has no odor except when another gas is mixed in specifically to allow gas leak detection by odor (which is common, but doesn't tend to impart a "burning" smell).

Nuisance odors can have a wide variety of sources; off-gassing of coatings and adhesives is the most common culprit in new construction but usually doesn't persist for 5 years. If you've determined that the smell is present:

• inside your townhouse
• AND not in any neighboring townhouse
• AND not outside your townhouse

then the source is probably inside the building. It could be coming from wall space, attic space, basement space (if you had one) or a mechanical system such as your HVAC or plumbing. A plumbing vent problem scares me more than a gas vent problem (but I also live in a place where CO detectors are mandatory); I would never describe sewer gas as having a "burning" odor but you can hire a plumber to come smoke up your drain pipe for a quick diagnosis.

Some other common sources of urban/suburban nuisance odors include:

• decaying animal (dead pests) or vegetable matter (odors tend to be intense and distinctive, don't tend to resemble combustion odors);
• rubbers or plastics, especially when heated (odor varies widely but some are described as burning smell);
• tobacco or marijuana smoke (very distinctive);
• meth labs (not typically burning smells, but have you seen a cranky bald guy with a goatee and mustache in the neighborhood?);
• vehicle exhaust (distinctive and typically exterior);
• ozone/electrical issues (distinctive, your master electricians already ruled this out)
• building product off-gassing, potentially including insulating materials;

etc. My guess—though not very high confidence, these are hard to diagnose even on-site, never mind online—would be off-gassing or electrical, and you've already ruled out electrical unless it's something really peculiar and specific, for three master electricians to have never encountered it.

You're going to have to follow your nose and work by the process of elimination. There are contractors who specialize in odor control—and if you can't find one locally, try looking for a specialist in asbestos abatement or fire damage repair, those can involve many of the same skills. A professional can afford to invest in any of a range of hand-held devices that can analyze and identify the sources of particular types of odor and, even without analyzers, is more likely to have the experience (and trained nose) to know where to check for the source.

If this has been bothering you for five years I'd say it's worth hiring such a specialist, but that's your call.

Answer 3

I suspect your water heater is gas and in a utility closet on the ground floor. Check in there to see if the smell is stronger or if a water pipe looks like it may be leaking a little. My thought is damp insulation inside the water heater shell could be producing this odor.