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Question DetailsAsked on 2/21/2018

Can a leak of the oil additive at the heat exchange coil of AC unit cause an chemical odor when the furnace heats?

Have a new furnace that blows out air with a sweet smelling odor. Ducts are new and heating system has been run long enough to burn off manufacture residual oil. Cooling heat exchanger sits just below the furnace combustion chamber so the hottest air blows across the heat exchanger and then into the air dusts to the room vents.

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2 Answers

Voted Best Answer

Yes - and if this smell only occurs when the furnace is heating or running A/C but NOT, or at least not as strong when furnace is running only on FAN setting, pretty much has to be furnace lubricant, plastics in the furnace, maybe duct sealer or duct joint adhesive if ducts have been installed or sealed within last couple of months or so - or leaking refrigerant. Some heat exchangers have a corrosion-resistant coating on them (especially higih-efficiency, so low-temp ones), so I guess it could be that or the housing on the furnace outgassing, though I would expect that to pretty much go away to below detection level within weeks of installation.

Duct adhesive or tape or such you should be able to smell by sniffing right at some or at duct supply vents as that solvent should saturate the air in the ducts when they are just sitting there idle, as well as smelling when the furnace is running.

R-410a, the most likely gas in your A/C system if less than about 5 years old, has very little smell by itself - I would describe it as slightly acrid or acidic, slightly pungent smell like ether or gray sage brush to me, but the normal synthetic polyester (POE) oil used in most R-410a brands is commonly described as having a sweet or very sweet smell - to me it smells much like overcooked (so very gelled but NOT burnt) tapioca. I have also heard it described by women as like a sweet-smelling gel hair conditioner, whatever that smells like.

If it is leaking refrigerant, I would also expect you to smell it when the A/C is running, though likely not near as strong because it would be cold, so not outgassing - but if you run the A/C for a cycle or too (if not too cold outside to do that) then run the FAN only, as the evaporator coil warms up I would also expect that smell if it is due to refrigerant leak.

And of course, if leaking enough to create a significant and readily detected smell, then I would also expect a pressure test, properly corrected for temperature, to show a loss of gas in the A/C system.

One other thing - you say the evaporator coil (cooling heat exchanger) is "right below the furnace combustion chamber" so it is getting the hottest air from the furnace - probably about 140-170 degrees (or maybe 100-140 if high-efficiency unit). Most A/C units I have read the installation manual on say the evaporator coil should be a minimum certain distance downflow from the furnace - typically 3-4 feet, so it may be overheating the coil and promoting leakage, though would not normally (assuming furnace is operating right and not getting direct combustion gases into it) exceed the maximum operating temperature forthe refrigerant - but could certainly make any small leak or spilled oil on the coil smell.

Guess the only definitve way to tell is check the coolant pressure in the A/C, or open up the coil (sometimes there is an access panel, sometimes it has to be pulled out which usually means draining the refrigerant into storage tank and disconnecting the tubing to pull the coil) to smell on the coil itself if that is the source - or if there is oil spread on it or dripping from it.

Note this is not something to let go (at least not past when you first want to use the A/C), because if you are losing refrigerant and the oil contained with it, that means the unit might be running low on lubricant when it runs - a good way to kill a compressor and to crud up the entire system with carbonized/scorched oil.

Heating and A/C is of course the Search the List category for a well-rated and reviewed tech to check this out - though if this is "new" furnace and since they messed with the evaporator coil during thenew furnace/duct installation, sounds like should be covered under their installation warranty so repair might be free. Might be something as simple as a leaking connection on the coil, or as drastic as a defective or damaged evaporator coil, which can run $1000-1500 range to replace (generally not fixed these days).

Answered 8 months ago by LCD


Wanted to share some comments based on the very helpful answer from LCD to my question. Many people have shared stories on line about odors from new furnaces so I suspect many people have problems with this kind of thing and suspect many just give up sorting out the cause because it is hard to do without help. I have had some other help from a "high-tech" friend who helped me analyze the air at one vent that had the most apparent odor (happened to be the one closest to the furnace). This indicated duct interior polyester film (new chemical for our home) and the duct sealing mastic (also not used in our house before) were not obviously the source of the odor. Many chemicals were detected in the air but nothing in the data suggested the film or the mastic as a likely primary cause of the sweet odor. This was consistent with a "low tech" heating of the duct interior film and the mastic and simply smelling the heated material. Neither gave anything close to a "sweet" odor. Flushing of the furnace for several weeks even now (a year after installation) has not eliminated the odor so a "common" online explanation for new furnace odors coming from residual manufacturing oil still lingering could also be eliminated--but this took time to show it was not likely and is a reasonble thing to have eliminated. So this led to my initial question to Angie's List about the additive oil in the cooling coil exchanger since by smell (another low tech approach) the sweet odor was not apparent at the top of the furnace where the return air is pumped by the blower motor but much more so at air leaking from the cooling heat exchanger chamber that sits only 12 inches below the gas combustion chamber --nowhere near 3 feet below. The installer has agreed to come to do a leak test (charging a service call fee)with a refrigerant sensitive sensor at the cooling coil, but he suggested that if the furnace is cold he could still assess a leak. Is this a good idea or would it make more sense to run the furnace to blow the heated air over the cooling heat exhanger just before leak testing? Also, the installer says he can pump the refrigerant back into the outside AC condenser unit and this would allow a retest of the furnace where the cooling coils were depleted of oil and refrigerant to see if the odor is eliminated. Is this a good idea to prove a cooling coil leak is the sweet odor source? My concern it that while the 410A refrigerant can be transferred back to the outside condenser, the POE oil additive that is the possible root cause of the sweet odor may not be pulled back as well to the condenser--so this proposed test might be invalid. If there is no leak (or too small a leak) and the air temperature is now about 50 degrees can a pressure test be done? LCD suggested a temperature correction can be done to conform the pressure is what it should be in the cooling system but the installer already said he could not do a pressure test due to the cold weather. Is this a valid point or possibly a run around answer? I realize some of these questions are very specific to my own situation but after reading many sites I believe furnace odor problems are quite common and your answers are liklely to help others with similar problems.

Answered 8 months ago by CarlosM


There are several ways to check the A/C for leakage - checking the manufacturer's information (assuming parts are all OEM - that the coil has not possibly been changed out for a different size) and figuring lineset (tubing) volume, to figure (sometimes given in manufacturer charging tables if you are lucky, correctable for lineset length) what the static pressure in the system when it is off should be, corrected using a gas volume table for ambient temperature. However, this is theoretical and of course the amount of the system exposed to inside and outside temps affects the calculated pressure - generally better to properly charge the system, then note what the discharged pressure in cool state is, corrected for temperature, for reference, then recheck over time for a drop-off in pressure.

Not having that info is commonly why the unit is run up to normal operating pressure, to determine what the operating pressures are (high and low side) which tells you if (assuming it was properly charged initially) it is losing gas or not. Which he is right, cannot be done at temps which will freeze the unit up, so commonly can't be done until outdoor temps rise above about 50-60 or so, depending on A/C brand.

Things that could be done to test it, in no rigid order but generally from easiest/cheapest to more expensive:

1) as mentioned, using a refrigerant sniffer to try to detect any leak - not all techs have one, and requires an active leak to work - but they are only about $25-50 these days for ones to detect leaking refrigerant (not to identify which type it is) so no real excuse to not have one

2) use black light to look for an oil leak coming out of a coil or tubing leak - almost all or maybe all A/C lubricating oils (which circulates with the gas so comes out at leaks) show up well in black light

3) inject dye into the refrigerant, which then leaves a telltale sign at the leak - unless REAL cold outside should be able to run the unit long enough (just a minute or so) to circulate the dye in the refrigerant so it is all through the system. Some are designed to be eye-visible only (red or blue that I have seen), some come with single-frequency glasses for the tech to use to detect a unique strong color spectral response to light, some also flouresce strongly under black light to allow for easy detection of small leaks

4) use bubble solution to check for leaks - works for leaks which will drain the unit in weeks or less, not for very slow leaks which drain of enough to be affet performance only over many months or years. Also,, not as effective in your case because work better when nunit is up to full operating temp and pressure.

5) system can be charged to estimated required volume (if it tested to be low on refrigerant) then rechecked in days or a week or two for a pressure drop - this only tells you IF there is a leak, not where it is

6) ditto to 4) but without possibly wasting pricey refrigerant by letting it leak off - with the unit power locked out so it cannot run, remove the refrigerant (held in an extraction pump's tank) and then pressurize the system with nitrogen to rated test pressure (which is a lot higher than operating pressure), shut off the fill valve, then watch for pressure drop from a leak, and detect the leak with dye or with ultrasonic detector (which only a small percentage of techs have because they cost several hundred $) or bubble solution or stethoscope. Quite tiny leaks can be detected that way in most cases

7) pump down the refrigerant, creating a significant vacuum in the system, then watch the gages to see if the vacuum bleeds off as air leaks into the system - not as preferred a method because it puts air and moisture in the system which then has to be nitrogen flushed - also the pressure differential is much less than pressuring the system up to test it

8) clean and degrease and biocide treat the evaporator coil and see if that solves the odor problem - sometimes a coil will grow fungal growth which can smell sweetish. Though normally that would die off within a week or few of the A/C not being run in teh winter (if you have a winterin your area), and the furnace heating and blowing air through it. HOWEVER - if the coil is leaking (and hence needs replacement - commonly about every 10 to maybe 15 years with the bimetalic coils being made these days), then cleaning it before testing for a leak can mean throwing away the $75-150 (200-400 if has to be removed because it cannot be cleaned in-place) cleaning charge.

Which one the HVAC tech (which should be one with factory training in leak detection, not just be a refill and walk away tech) starts with or uses depends on personal preference and what equipment they have, but running through the gamut selecting options from the list can run $150-500 depending on how many tests are needed to find out if there is a leak, and then find where it is if there is one. The direct dye and pressure tests are most reliable, because the charge-and-let-sit option depends on either having a significant leak which shows up within a week or two (which should be your case if you are smelling the odor strongly), or on properly figuring the effect of varying ambient temperature on the pressure in the system.

The option you mentioned of pumping down the refrigerant to see if that makes the smell go away might work, but would require that the evaporator coil and that area of the duct at a minimum be thoroughly cleaned and degreased to remove the smell of the oil already there, so I don't think that would be a very workable solution because you risk paying for the cleaning and then finding that money was wasted if there is a leak requiring the hewly cleaned coil replacement anyway. I also am not confident that without complete wet duct cleaning, assuming you have metal ducts, (another few hundred to $500) that you would get rid of the smell from oil particles already on the interior surface of the ducts.

One other thing I did not mention - though most people can tell the difference between sewer gas smell and a sweet smell, but sometimes sewer gas in a house can smell pretty strongly but not "sewer smelling" if scented cleaners or heavily odorized laundry detergent is used, especially if the scented water is going into the same trap as the A/C drain tube is going into. If the condensate drain leading from a pan under the evaporator coil goes into a sewer line without a trap (contrary to code) or because it has not been running for quite a while the trap has gone dry or if trap is weet but possibly with scented waer, check by smelling at the entry point to the drain tube from the condensate tray for the odor. Depending on accessibility and whether the coil is a pul-out type of accessible via removeable panels in the ducting, may have to be done by the tech - otherwise you may be able to do yourself, maybe with a piece of tubing or scrap pipe or wrapping paper cardboard tube or such as an extension wand to inhale through, with the end of it at the drain tube. Careful of the coil - the fins can be very fragile.

You also did not say if the friend or the HVAC tech smelled the smell or not, and if so whether they agreed it is a sweet smell, or maybe thought it was lubricating oil or hot plastic smell ?

One sort of last-resort method, which means taking the furnace out of operation for a day or so, would be using plastic sheeting to seal off the inlet to the ducting (and hence the evaporator oil) and also (after totally shutting off power to the furnace) wrapping the furnace in visqueen to make an airtight box, then after about 12-24 hours of it sitting idle, smell at the supply vents from the ducts, and also inside the furnace wrapping to see where the smell is strong. if at the vents, contaminated/chemical outgassing ducts/duct selant or evaporator coil growth or leak would be the presumed source, if strong only in the furnace then lubricating oil or hot plastic or possibly exhaust flue (if plastic flue from high-efficiency furnace) plastic getting hot or a piece of plastic debris on heat exchanger would be the presumed cause. This of course presumes that the ambient air around the furnace does not have any odor under normal conditions. BUT - if the source is the furnace itself, I would bet after it has sat for a half hour or hour without running that if that is the source, it would smell pretty strongly even if you just took off the access hatch. So maybe just an hour or so shutdown, then smelling furnace and at supply vents, would give you a pretty good idea, assuming the smell is pretty strong.

You also did not say, I don't think, how old the unit is - there are a LOT of previous questions about unit repair versus replacement, with answers and some guidanceon evalulating when it is worth repairing versus replacing an A/C unit - if it does turn out to be an A/C leak. These can be found in the Home > HVAC link, under Browse Projects, at lower left.

One last thought if you have small kids in the house - take off the register grills and check there is nothing shoved in there - had a similar caseonce where the toddler was shoving something like gummy worms down in there, which then put out a sweet sugary smell when they heated up and partly melted as the unit ran. Same thing could happen (probably at only one or two grills, but who knows what is going on in the mind of kids) if a drink is being poured down there. Looking and running a hand around should tell you pretty quick iof that is the cause.

Good Luck - and please let us know what it turns out to be, for future reference.

Answered 8 months ago by LCD

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