When I turned the heat on for the first time this year, I noticed a foul mildew-ish smell. It smells like mold and mildew is in my vents. They probably need cleaning- the intake entry area is filthy. But I can't afford it right now. Spraying a bit of Lysol helps some, but it doesn't eliminate the problem. What can I spray in the intake duct to kill the stuff once and for all?

Galland

Mildew or mold will only grow in those vents if there is moisture condensing in there.  Try to solve that problem.  It may be contained to a small area - if you can get a look into the ductwork through an access panel or the end of a run you might be able to see where it is and remove that section to clean it.

And it may not even be mildew per se.  My forced air furnace is also stinky when it's first fired up.  The heat exchanger collects dust and rust over the summer and baking that off always smells up the first cycle or two.  

I do clean my vents but on the cheap.   I have an extra 15 feet or so of flex hose and I just string all the sections into the ducts with a brush on the end hooked up to a 5 hp shop vac .

I securely duct tape it all together do I don't lose a brush attachment deep in the ducts.  It's amazing the amount of crap that collects.


Edited to Add:  good advice from richardh247  further down!  I didn't even think about the A frame air conditioning coil being crudded up  -- as I don't have central AC, just gas heat

Thanks for the advice. I hope you are correct- that it isn't a systewide problem. I may replace the flexi piping (I don't know the word for it) at the mouth of the vent intake door. It is some sort of plastic wrapped around a metal spiral and it is absolutely filthy. You'd probably recoil in horror if you saw it. But that area doesn't smell bad- it just looks gross, so I doubt it is the source of the odor. I'll post results.

Galland

They do stink when you first start them up, but I doubt it is mildew per se unless your condensor is collecting moisture.  Even then, mildew does not normally build up on tin.  Is this the first time you've noticed this particular smell?

If it is mildew, use bleach to kill it by making a rod and patch with a broom handle and "J" hook and a large beach towel.  Soak the towel in bleach. wring it out, and then swab as much of the ducting as you can reach, just like swabbing the barrel of your rifle.  The "J" hook screwed into the end of the handle will grab the towel so it doesn't get stuck and lost n the vents (make a hole in the middle of the towel by ramming the end of the hook through it).

But more than likely, you need to clean the coils, not the vents (just need to dust the vents as the poster above suggested).  Buy some coil cleaner, or use a solution of 1/3 ammonia to 2/3 distilled water and spray the coils (interior and exterior if you have a split system with an air handler), let sit 1/2 an hour or so, and then wipe down with plain water.  Repeat as necessary until the coils are clean.  

The musty smell is most usually due to non-use.  You probably smell it twice a year:  the first time you use the AC, and the first time you use the heater.  It's normal, but cleaning the coils and dusting the vents will help keep the smell to a very bare minimum.  

Quoted: Even then, mildew does not normally build up on tin.  Is this the first time you've noticed this particular smell?

No. It gets worse every year.

If it is mildew, use bleach to kill it by making a rod and patch with a broom handle and "J" hook and a large beach towel.  Soak the towel in bleach. wring it out, and then swab as much of the ducting as you can reach, just like swabbing the barrel of your rifle.  The "J" hook screwed into the end of the handle will grab the towel so it doesn't get stuck and lost n the vents (make a hole in the middle of the towel by ramming the end of the hook through it).

Bleach sounds like a good idea. Though the entry duct is so deteriorated that I probably should replace it. It is caked with dark brown dirt.

But more than likely, you need to clean the coils, not the vents (just need to dust the vents as the poster above suggested).

Where are the coils and what do they look like? There is a unit outside and a large metallic box inside of a small closet.

The musty smell is most usually due to non-use.  You probably smell it twice a year:  the first time you use the AC, and the first time you use the heater.  It's normal, but cleaning the coils and dusting the vents will help keep the smell to a very bare minimum.

Thanks. I'm going to buy a bottle of bleach this weekend.

Galland

The coils look just like the freon coils behind your frige.  On the outside, they ring the compressor and fan in a circle or square.  Those are the actual compressor coils and you'll find those very easy by removing the outer unit shell.

On the inside, you have what's called the air handler - hence, those coils are called the handler coils.  The only difference between those and the compressor coils is that the handler coils run vertically, just like the back of the frige, instead of in a circle or square as in the compressor case.  

Think of the coils as the air conditioning and heater's radiator.  Freon is pumped to the compressor, where it is compressed (well duh, LOL).  The freon, once compressed, changes from an oily gas to a dry gas.  But first it has to be cooled, and that's what the compressor coils do:  work like a vehicle radiator to cool it to prepare for compression.

From there, the compressor sends the gas to the air handler, where it is once again cooled, this time in dry and moisture-free gas form.  The air handler is actually the sending unit that blows the cold air - called a split, it is the difference between the return air temperature and the output air temperature.  The nominal split temperature for maximum efficiency is 18 degrees.

If the coils are dirty, you'll lose this degree of split, spend a lot more money heating and cooling the home, and it smells bad.  That's why I think you have dirty coils, rather than mildew (which is virtually undectable to the human nose except in extreme cases).  You can verify this if the utility bill is higher than normal, continually climbs higher and higher the more you use the heat or air conditioner, or if the smell keeps getting worse.  

If the coils are dirty:

In the AC cycle, the dirt and dust works to insulate the coils, so the gas doesn't cool fully - the forced air is warmer than it should be, so the AC works harder and longer, running hotter, so the utility bill goes up and the smell is more pronounced.  

In the heat mode, the coils do almost the same thing (keep the air away from the coils), but the coils heat up and dissipate the forced heat as all that heat is lost through the transfer that allows the dirt to act as a radiator, versus the coils.  Since the coils are a pourus metal (aluminum or copper in most cases), this stench invades the lines and blows through the system.  You're baking filth and grease, especially from the air handler.  

The other option is a pin leak in the shrader valve or compressor itself, which will bake freon on the lines slowly and freon stinks when heated.  But if you haven't had to charge your freon every year, it's highly unlikely you have a pin leak or a bad shrader valve.  If you have had to, your AC tech is violating environmental rules as no charging of the system is allowed if a leak is suspected.

If you really want to confirm the coils,  do this:

Go down to Home Depot or some other home place and get an infrared (thermal) thermometer.  They can be had for about $30.  

Now, turn your AC on (not heat) to the lowest temperature on the thermostat, and let it run for about 10 minutes.

Use the thermal thermometer and measure the temperature of the air coming out of the register (vent) that is farthest away from the air handler.  It should read about 60 to 67 degrees, depending on the tonage of the unit and the cubic air footage conditioned.

Now, go to where your AC filters are - AC still running, of course - and measure the temperature of the air being sucked into the air handler.  

The air coming from the register should be about 18 degrees colder than the air being sucked into the return.  If it is more than 10 degrees different, your compressor, evacuation fan, or run capacitor are going bad.  If it is between 10 and 15 degrees, your system may need to be charged or it has dirty coils.    

Like I said, your best bet to get rid of the smell AND lower the bills AND heat and cool the house better is to keep those coils clean.  The commercial coil cleaners are best, but ammonia and water will do the trick with a little more elbow grease and a few more applications of the solvent.  

Since you don't even know what the coils are, my money is on dirty coils, even if you have a yearly service (which is doubtful, or you'd see the coil cleaning on the service invoice).  Get 'em cleaned, and 99% of your problem will go away in a day.  Then clean them yearly, and have a professional company come out to service the unit once a year for about $50.  Make sure you MAKE them test the split.  

If you need more info or have more questions, post 'em up!  I love doing this shit!  

Pull out the frige and lookat the back, and you'll know what the coils look like.