HVAC Recirculate: still bringing in external air?

[jblc]

( Mods, feel free to move to the correct forum, if this isn't the right one...thanks! )

I've been doing some testing on air quality in the cabin, since I live in a smoky climate. I have some concerns about the air recirculation mode not blocking outside air.
Summary: Has anyone found the source for all of this outside air coming in, in recirculate mode?

With all doors/windows closed, and climate control set to Recirculate, I believe there's still a significant amount of outside air coming in. It's less than the non-recirculate mode for sure.

But, I'm unable to get clean air in the cabin EVEN WITH a separate high-throughput HEPA filter I put inside the cabin (running off of an inverter). The faster I drive the more air is coming in.

Two questions:
1) Is this documented anywhere, and is this the expected behavior?
2) All doors/windows are closed. It's also possible that air is seeping through door and window seals, but given how much throughput there is in the HEPA filter sitting next to me on the seat, it would have to be a very LARGE flow.
And, even stationary (ie air not forced through door/window vents by high-speed driving), though, I can't get good air quality.

Has anyone found the source for all of this outside air coming in, in recirculate mode?

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[jblc]

Well, I do wear a mask for short trips if the smoke is bad, but it's not practical and comfortable for longer durations In addition to obscuring vision...

 

[micl9]

Well, I do wear a mask for short trips if the smoke is bad, but it's not practical and comfortable for longer durations In addition to obscuring vision...

OMG you just turned this into a possible mask mandate!?

I understand your train of thought, but I don't think vehicles are that well sealed. There are vents (intended and non-intended) all over the place. But do you get the smell of smoke or actual smoke in the cab?

 

[jblc]

I have a particle meter permanently mounted in the cabin (since we're in smoke country), measuring particulate levels of pm 1.0, 2.5, and 10, as well as volatile organic compounds. It's regularly checked against a calibrated meter, so I know it's accurate.

I use that as the metric for air health.

 

[micl9]

I have a particle meter permanently mounted in the cabin (since we're in smoke country), measuring particulate levels of pm 1.0, 2.5, and 10, as well as volatile organic compounds. It's regularly checked against a calibrated meter, so I know it's accurate.

I use that as the metric for air health.

Wow!
I serioulsy think you would have to tear out the interior and plug all the holes. But too tight and you would have issues when closing the doors! especially when in the truck!

 

[AdamHarris]

Vehicles have a positive airflow system. Even in recirculation you are going to get “some” outside air from the hvac system.

 

[D Fresh]

Vents behind the rear seat?

 

[RBTremor]

Without some fresh air, on a long trip, wouldnt you deplete your oxygen and get drowsy? I get your concern of smoke, but your expectations of a hermetically sealed cabin may be a little high?

 

[DrizzyDrake]

All vehicles (to my knowledge) allow a portion of outside air into the airbox. I've heard many reasons. Lack of oxygen, positive air, mould, or moisture. It's normal operation. I know what you are talking about with the smoke as here in alberta we get smoked out by BC once every two years making it hard to breathe. If it's bad enough, I wear a n95 mask, but if you have really bad issues, you could look into having the box modified so it doesn't let air in, but it may cause wind noise within the box. Your best bet is maybe to find a way to block off the inlet somehow.

 

[jblc]

Without some fresh air, on a long trip, wouldnt you deplete your oxygen and get drowsy? I get your concern of smoke, but your expectations of a hermetically sealed cabin may be a little high?

As I mentioned, it's less an expectation to be airtight -- that doesn't make sense for reasons you said, which I agree with -- and more that I'm surprised at the large airflow in.
That much air exchange is surely not needed to breath safely, even with 4 people inside

 

[AdamHarris]

Vents behind the rear seat?

Those are supposed to be a one-way vent. The open when a positive pressure situation happens in the cabin (door closes and air attempts to "blow up" the cabin like a balloon).

 

[Motorpsychology]

Vents behind the rear seat?

Those are there to exhaust the cab air displaced when the airbags deploy. Do not block them off!

 

[D Fresh]

Those are supposed to be a one-way vent. The open when a positive pressure situation happens in the cabin (door closes and air attempts to "blow up" the cabin like a balloon).

When a vehicle is moving there is ALWAYS some amount of air entering the cabin. Thus there is ALWAYS a positive pressure situation. Thus the vents are ALWAYS open. Thus it ALWAYS sounds like a vacum is running in the bed as you drive down the road.

Those are there to exhaust the cab air displaced when the airbags deploy. Do not block them off!

Don't remember ever saying that he should. But of course that's probably not adviseable.

 

[jblc]

When a vehicle is moving there is ALWAYS some amount of air entering the cabin. Thus there is ALWAYS a positive pressure situation. Thus the vents are ALWAYS open.

Thanks, and totally agreed. I'm just confused by the large AMOUNT of air entering, not that there's some air exchange. A high-flow filter (That's blasting full speed) should definitely be more than enough to account for normal leakage inside a moving vehicle with a small cabin.

Since the filter is ineffective beyond 30 mph, that means there is SO much air leakage, that it's greater than the blasting max-speed HEPA filter.

That seems unreasonable to me, and also unnecessary.

 

[halligan1201]

I have a particle meter permanently mounted in the cabin (since we're in smoke country), measuring particulate levels of pm 1.0, 2.5, and 10, as well as volatile organic compounds. It's regularly checked against a calibrated meter, so I know it's accurate.

I use that as the metric for air health.

Remember that the interior of your truck itself releases VOCs from the various plastics in it, especially for the first few months after new (and even after that new car smell - which is plastic off-gasing) is gone, as well as on hot days.

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