Hi to everyone. I have a specific issue with ventilation smell, and I am at lost, so wanted to ask for an opinion.

In short: Ventilation puts out ash/dust like smell only when recirculation is ON in winter time when heating.

 

Longer version:

About 2 or 3 winters ago I noticed the same problem. When I drive in winter with heating on (ac also on and heating works fine) If I turn on recirculation to avoid car fumes in traffic I get this strange smell…reminds me of what you get from a bagged vacuum cleaner when full, or also like emptied (but not washed) ashtray. Anyway, next spring I decided to detail my ventilation system. Cleaned all the ducts with alcohol and AC disinfection spray (vent grills off so I could access the tunnels). Also tried to spray as much as I could on evaporator via center vents with grills off. Changed the filter at least once a year but usually twice. Lasted fine until this winter, same issue.

 

Now I do not want to drive remaining months with smell but even worse, the cleaning I did back then was really a pain in the ass DIY job. So if possible, I would not want to repeat it every 2 years.

 

What puzzles me is why this only happens if recirculation is ON. I imagine in that case air no longer passes via the cabin air filter so that falls out of the equation. But even if it did pass through it, I change it regularly. 

 

What else could it be there that would cause it to smell like this in this particular case?

With recirculation on, the air is drawn into the system via an open flap in the footwell.

I can only speculate, but the extra (unwanted) smell would likely be coming from the carpet in the footwell or whatever is in the footwell ( boots etc. ) next to the flap. Assuming of course that the whole cabin does not smell like that.

Thank you for the reply.

 

I vacuum the car on a regular basis, especially the carpet on the driver side. There is no smell like this in the car. Also I did suspect this in the past so I tried to stick my nose as close as possible to that area but I smell nothing. I did purchase a vacuum for cleaning carpets so this spring/summer I will do that as well.  

 

In the next few days weather is going to be warmer where I live so I will definitely investigate this area a bit. I am also thinking to sanitize only this footwall intake part of the vent system. I just don't get it why it behaves like this. My car is really tidy and i take good care of the interior. 

Why are you needing to use the recirculation mode?  I have only ever used when stopped behind busses emitting loads of pollution.  (Edinburgh). And that is during 5 decades of all weathers in all vehicles types. 

Where I live there are still a lot of older cars that really emit a lot of black smoke. Some buses do the same. And I often drive my kids (youngest one is 1 year old). That is the main reason. Then there are various different smells in different parts of the city or around it (mostly factories etc.) I am not using it all the time, but I do use it often.

Can you check the state of the cabin air filter?  There are multiple versions and I've found that if you end up with the non-carbon version of the cabin air filter, it can go mouldy after about 12k miles and give you a fousty smell.  You might also want to put through a can of Air Conditioning saniter after you've changed the cabin air filter.

Edited 20 January, 20251 yr by varaderoguy

I have been changing cabin filter on regular basis. If its that filter that is causing the problem, how come the smell only appears when airflow does not go through it? 

I use those AC sprays every summer, usually those from Sonax. But they do little to nothing.

 

For the sake of me getting to the bottom of this, i just changed cabin filter few minutes ago. Yes, old one was a bit dirty, but nothing alarming. I always but carbon filters or better, depending on current price when I am shopping. 

Do you ever run the car for any length of time with the AC switched off? Could be a build-up of moisture sitting in the ducting that has gone rancid. 

If so, it'll need to be evaporated off by setting the AC to max temperature & max fan speed and take it out for a 20-30min run. Finish off with an AC cabin cleaner bomb.  

No. AC is almost exclusively ON. The only time i turn it off is on summer days after prolonged drive to heat up air ducts and evaporator specifically to avoid these issues.

 

I forgot to mention one more observation. This smell appears only when re-circulation is enabled when car engine is almost at the temperature. Meaning, If I turn it on while car is "heating" (water temp about 60 C and lukewarm air) then it does not smell. As the water gets toward 70-80 and air starts to blow hot, then this issue appears. Changing cabin filter yesterday did not help.

I have a similar question that hopefully may be related.

Just changed the cabin air filter. With A/C set to OFF, blowing regular cold air, the smell of that air is what I would call very 'chalky' - I dont know if this is exactly similar to the OP's 'ash' smell, as its not something I would relate to buning as such, but yes, the smell of a bare ash try does come to mind. It catches the back of the throat.

Any more clues would be much appreciated.

I would guess that you need an air-conditioning clean. Whilst there is bacteria that is caught in the pollen air filter, the HVAC system itself needs a chemical clean.

My go-to - Autoglym has an Air Conditioning Cleaner - https://statuscarcare.com/products/autoglym-air-con-cleaner

You can then use the saniter version for period sanitisation of the HVAC system (I personally try to do it every 6-8 months)