Climate: Defrost only recirc mode

[jmueller065]

My coworker gets the credit for coming up with this idea.

Here is the deal: You've preconditioned the car to a nice & toasty temperature but the windows still fog/ice up if you have the climate system off during your commute.
You can run the defroster with the temp set to LO but when its really cold out (<= 0F) it will start icing the windows and/or getting you really cold (killing the preconditioning).
I've found that you can set it to recirc mode with the vents set to floor + defrost but, again that darn cold weather, this doesn't direct enough air at the windows to defrost them.
If we could have a recirc + defrost only mode:

• - The warm air already in the car can be redirected to the windows
  - The windows will cool the air but since its on recirc it won't be as cold as outside air and thus re-heating it won't take as much power (if you want to set a temp to warm it)
  - Unlike an ICE there isn't much danger of CO accumulation (as the only CO/CO2 generated is by the passengers which is far less than an ICE)
  - This may only require a software change (if the internal duct plumbing in the Focus allows for such a setup)

 

[WattsUp]

They could call it Eco-defrost.

 

[dmen]

Interesting.
I have been wondering why the climate control doesn't allow recirc to be selected when on defrost setting. Does it have something to do with CO buildup in an ICE? Does CO build up differently when using defrost vs using dash/floor vents? Seems recirculated air coming out of any vent would carry the same risk, no?
I guessed that recirc + defrost was not an option because it wouldn't be very effective. My thinking is that if you don't get the moist air out/ dry air in, you won't effectively defog/defrost. The fog/frost comes from the combination of glass surface cooling and air moisture rising. The moisture mostly comes from passengers breathing. The longer you drive, the more you breathe, the more humid cabin air becomes, the more the windows fog. Heated defrost works best because it both warms the windshield and brings in dryer air. Lo defrost works ok because the cold air coming in is super dry, so it pulls away some moisture, but as we've noticed sometimes the moisture freezes before being pulled away. Lo recirc defrost would only blow moist cool air against the cold windshield, so probably wouldn't clear anything. Heated recirc defrost would take care of the cold windshield problem, but not the moist air problem. Unless of course the air can be dehumidified as it recirculates. A/c dehumidifies but there isn't a separate dehumidifier is there? Could a/c run to dehumidify and heat coils run to heat at the same time? Would that end up saving any energy?
The other issue is side window fogging. If your windshield needs defogging, your side windows are going to be fogging too. In my experience this is tolerable when I'm alone, but needs attention if I have any passengers adding to the humidity. Vent flow is pretty limited towards the sides, so dry warm air is really necessary to keep them clear. Sure, there are vans and trucks with basically no side windows, but it's still a lot safer to be able to see out of these windows if you have them.

 

[davideos]

I'm pretty sure it is as you say...recirculating moist air isn't going to defrost your windows very well. AC + heat is usually the mix you want on a cold day in an ICE, but in a BEV, one tends to reconsider. AC dehumidifies all the air, pulling the moisture out and will eventually clear all your windows. I suppose it blows on the windshield almost exclusively in that mode because it is the most important and will clear it faster. Other cars do have vents pointed at the side windows...my Explorer has them in the door. Outside air is better because it has a lower moisture content if you are needing to defrost....it is already cold and has less ability to carry moisture inside the cabin vs the warmer moisture laden air inside.

 

[jmueller065]

Keep in mind that if you heat the air its ability to hold moisture increases: Air at 0F may be at 100% humidity, but heat it up to 40F or so and that value drops. So even recirc + heated defrost is still useful (and it also would heat the window).

http://www.engineeringtoolbox.com/moisture-holding-capacity-air-d_281.html

I never said there wouldn't be a limit: After using this for some time yes its effectiveness will drop and this it would only be useful for people with a relatively short drive. For those people though it would save some juice (it is a short range car to begin with).

The Focus does have some vents pointing to the side windows..