I was explaining to someone why I bought a new car...previous used cars caused respiratory aggravation. The person said that she read about mold problems in used cars due to condensation. It all seemed plausible until I recalled that my past used cars did not have A/C. However, it did get me thinking about how to avoid such a problem on my new 2013 Fit, which does have A/C. The solution was to turn off the A/C 5 minutes before reaching my destination. Since my car is for running errands, drives usually aren't much longer than 5 minutes, so that was not an appealing suggestion. All the less so since I didn't like breaking out into a sweat in sweltering temperatures and sharing my stinky self with the public.

My engineering mind turned to how I could be lazy and not give up my treasured A/C. It reasoned that the air should be dry by the time I reach my destination, and there would not be much additional moisture to condense onto the cooling coils/fins once the initial moisture was squeezed out of the interior air and drained away.

Would this be reasonable?

 

The A/C evaporator is unlikely to grow mold in my opinion. Mold needs not just moisture to thrive, but also some organic material. The evaporator is metal; there's nothing organic there. Also, the air conditioner system is designed so that the water drains away from the evaporator and so it won't stay moist for too long after you park and the car starts to warm up.

Your best bet is to replace the cabin air filter at relatively frequent intervals, IMHO.

Your analysis (of the moisture being already dried out) doesn't hold if you have the ventilation controls set to bring outside air in, which is the usual setting most of the time. Even on recirculate, some portion of outside air is still mixed in. Also, your body is constantly adding moisture to the air as you breathe and sweat, so there is some to condense out regardless. (If you sit in a parked car during cold weather, you can see this from the windows getting fogged up.)

 

Despite frequent (yearly or sooner) replacement of the cabin air filter, my '09 Fit still grew mold. This has been an issue with every car I've owned since R134 replaced Freon as a refrigerant.

 

i just keep the a/c turned on at all times. in the winter keep it in defrost/defog mode. ive never had musty smell.

the worst is you turn a/c off and let it sit. pretty soon the next time you turn on ur a/c, the car will smell like dirty old socks.

just keep it simple. keep the a/c turned on at all times.

 

btw, i think you meant evaporator?

 

@DrewE: I see. Yes, even in the bathroom, mold never grows on metal. Good point.

About air filters, I had a stretch of time in the past where I was looking into HEPA filters. It turns out that, up to a *point*, more clogged filters actually do a better jobs at filtering, though the throughput is diminished. At some point, the throughput is not useful. So if mold going to grow anywhere, it would be on the filter. However, given that the air going through the filter is not moister than the prevailing environment, the likelihood of mold growing on the filter is the same as if you left the filter out in the external environment.

You said that the norm is ventilaton rather than circulation. When I use A/C, I only ventilate for the first 5 minutes because the car is much hotter than the outside. Thereafter, I switch to circulate, because the air being cooled will be mostly drawn from the interior, which is significantly cooler than outside air. It's probably dryer too, even though we're constantly expelling moisture into the air. I say this because the inside air is constantly being dehumidified by the A/C. As an empirical comparison, start by switching to circulate and turn off AC in order to fog up the windshield. Then defog the windshield with AC. The test is whether this is faster on ventilation or circulation. It's a very approximate indicator, with many confounding factors, but I'm not too pre-occupied about *precisely* quantifying the difference. I don't think I'll ever be so bored as to repeat the two scenarios in alternation for a large enough number of times to be able to say which is better with a useful degree of confidence.

@Uncle Gary: Interesting that you're experiencing a mold problem. What is the parking environment like?

@Ken Chan: Yeah, I'm finding that it is turned on much of the time. In the winter because defogging turns it on without without any say from me. In the summer, because it's hot, and opening the window is noisy. I'm getting spoiled by the quiet that A/C permits because I don't need to open the windows to cool down. On the highway, this also means less drag, I suppose; and noise-wise, no need to wear earplugs to stave off hearing loss on long trips. BTW, regarding the evaporator in your last post, who were you directing your question at?

 

I try not to use AC but when I do, I've always been in the habit of turning off the AC about two minutes before I anticipate the engine being turned off. This eliminates the condensor from being cold and condensing moisture on the interior and reducing mold buildup.

 

The condenser is the part of the air conditioner by the radiator where the heat removed from the interior is removed to the outside air. The evaporator is the part that's in the cabin ventilation system that gets cold. I think we all understood what you meant perfectly well, even if you used the incorrect term.

Evaporation tends to cool the surroundings down because it requires heat for the fluid to change from the liquid to the gaseous state. Condensation warms the surroundings up (or makes them less cold) because the heat must be removed from the fluid to go from the gaseous to the liquid state, and that heat enters the environment.

 

@tbFit: Good suggestion. 2 minutes is way more tolerable than 5. There is likely to still be a cooling effect for 2 minutes after turning off AC. Even if not, the air in the cabin is still cool, so having it blow over you still has a cooling effect on your body. However, if you have the air directed at the windshield, you don't control whether the A/C kicks in. So in order to deactivate the A/C in the last 2 minutes, you also need to switch the air direction to any setting that doesn't include the windshield.

@DrewE: Oh. Thanks for that. It's a bit of a revelation, this terminology. I find it odd that the condenser is the *outside* portion of the heat exchanger, because I always pictured the *inside* air leaving condensation on the cooling fins. So intuitively, it seems to make more sense to have the inside portion called "condenser" rather the "evaporator". However, I'm there there are good scientific reasons for the current terminology.

For example, from your 2nd paragraph, it seems that condensing applies more to the refrigerant on the inside of the closed A/C system. It gets compressed from gas to liquid on the outside, which can be viewed as "condensing" (if one squints a bit). Then on the inside of the cabin, it expands to gas, i.e., "evaporates" (sort of). I never realized that there was a material state transition involved. I just thought it was all liquid, and you just squeezed the heck out of it on the outside. But a phase transition makes more sense becaue fluid (if it's like water) is not very compressible.

 

The "condenser" and "evaporator" do indeed refer to what the refrigerant is doing; it's partly liquid and partly gaseous, depending on where in the system/cycle it is. (By the way, both gasses and liquids are fluids—that is, they both flow and conform to the shape of their containers. Liquids are essentially incompressible, so have a fixed volume, while gasses are compressible.)

The refrigerant is a liquid when it enters the evaporator (via some sort of a metering valve, of which there are a few possibilities). In the evaporator, it transitions to a gas, and absorbs heat from the air flowing through the evaporator, cooling the air. The gas then goes to the compressor, which makes it a higher pressure gas and incidentally also raises the temperature of the gas due to the ideal gas law. The hot, high-pressure gas goes to the condenser where the heat is extracted to the outside environment, condensing the refrigerant back to a liquid, which flows back to the evaporator.

There are a few other refrigeration cycles in use in some other applications—ammonia absorption refrigerators still derive the cooling from the evaporation of a liquid (ammonia in this case), but use a cunning arrangement of water and heat and hydrogen gas to move the ammonia around and alter its vapor pressure without needing a mechanical compressor. Peltier coolers rely on a completely different phenomenon wherein an electric current through a junction of dissimilar metals directly causes some heat transfer (and vice-versa). It is certainly also possible to make a heat pump that relies on pressure changes in a gas rather than primarily on phase change to move the heat, but that's not as commonly done because much more refrigerant and much higher flows are required (or, put another way, the amount of heat absorbed or rejected from pressure changes is lower than that for phase change, so more flow is required for the same cooling effect).

 

Good point, but I did the mod that disconnects the automatic AC in that position. I sometimes like defrost air position without the automatic AC happening.

 

When I looked under the dash, I didn't see anything that looked like the pictures that I found online, so I didn't bother. I'm getting use to having the A/C and the heater fight it out.