[midnightsun]

The directions that come with the cabin filters say to install them with the arrows pointing down, as does the Haynes manual. But when I removed the old filters today, I found that the dealership had installed them with the arrows pointing up. I remembered a contentious thread here about the arrow direction question, and I did a little experiment that convinces me the instructions are BASSACKWARDS! It appears that the arrows should point UP, not down.

Here's my logic and how I came to the above conclusion:

After removing the old filters, I ran the blower fan. The air came strongly from below. To me, this indicated that the filters should be installed such that more air would flow upwards through the filters (arrows pointing up, in contradiction to the instructions).

I thought, "The filters LOOK identical from either face. Maybe it doesn't matter which direction the filters are installed." BUT I also realized that the fibers in the filters could be spaced slightly farther apart on one side (initial pass-through side) than on the other side. Common sense would say that the more porous side should face the the blower air. The less porous side would act as a second-stage filter, trapping the finer particles before venting upward and into our noses. The difference in porosities could be so slight as to not be visible to our naked eyes.

So I tested both sides using my shop vac. I placed the filter over the vac's blow-out port so that the arrows went WITH the air flow going out. Then I turned the vac on and felt the air coming through the filter with my palm. A lot of air passed through.

Next, I flipped the filter over so that the arrows pointed back toward the blow-out port, or AGAINST the outgoing air flow. The air still came through but it felt like it was slightly reduced compared with the first position.

I tried each direction three times, with consistent results. There is more air coming through the filter (and toward our noses) when the arrows MATCH the air flow. In other words, the arrows should point UP when installed, based on the assumption that greater air flow is proper.

My old filters had mosquitoes, gnats, and tiny bits of dried plant matter on top of them, in addition to gray dust. The bottom sides were noticeably cleaner. I assume that the insects end up in there simply because when the fan is off (most of the time), they fly into the vents. I keep my vent "shutters" open all the time, and I usually drive without running the blower fan. Once inside the compartment, some of them can't fly out before their short life span is over. So they drop dead and fall on top of the filters.

Also, my husband showed me the filters for our household hot-air heater. The arrows match the direction of air flow. The filters have metal grids on one side, so in this case it is obvious which direction to install even without the arrows. Again, this would seem to say that the cabin filter arrows should point UP, not down.

Why do the instructions say what they do????? The only reason I can think of is that it is a matter of differences in interpretation (ambiguity). Without the instructions, I would assume that the arrows point WITH the air flow (upward). My husband agrees that is also his interpretation of arrows. This is also how the dealership installed the filters.

However, one COULD interpret the arrows and the accompanying markings that say "Air Flow" to mean that they point TO where the air flow comes FROM (downward). That means the arrows point AGAINST the air flow, and in fact there is slightly less air coming through when installed this way. The person who wrote the instructions probably was a tech writer and not necessarily someone who actually compared physical results.

If you think instructions can't be wrong, don't be so sure. When I was about to change the fluid in the transfer case, I obtained a printed diagram of the transfer case, from the factory owner's manual. Comparing the legend to the diagram, I noticed that the labels for the drain and fill plugs were reversed! Fortunately, in that case gravity (and common sense) makes it very obvious that the diagram labels are switched.

The cabin microfilter question is a little tougher. For now, I am leaving the filters out and I will call the dealership the next business day and ask to talk with a technician. Right now, their installation matches what I think makes sense, but I'd like to hear what they say. Maybe they can even tell me if the filter compartment's front cover is an in-stock part...I broke one of the little f'ing tabs on the bottom.

Oh, yeah, if you actually read this post carefully, you win some kinda prize!

 

[something else]

On any air filter set-up (industrial and commercial and home HVAC, some vacuums, engine, etc.) the suction side of the filter is where dirt and debris are caught. When you removed your old filter, the dirt and debris were on top, because the blower was pulling outside air down through it... So your dealer put in the air filter the wrong way if the arrow was pointing up.

Very rarely is an air handling system designed to have a filter after a blower because of the major reduction in air velocity and blower efficiency... a squirrel cage blower is more adept at pulling against a resistance than pushing against.

Almost universally filters are marked with an arrow designating the direction air is supposed to pass through the filter.

The 'cleaner' side of a used, dirty air filter is the downstream side - or the side that faces the blower intake. The dirt and debris you describe don't build up on one side of a filter just from gravity -- it's because dirt and debris were pulled in and caught there by the air flow. If it was the opposite case, all that dirt and debris would come blowing out your vents every time you used your ventilation.

Hopefully this helps you in finding the correct orientation for your filter.

EDIT .....Unless the filter is marked to indicate vertical orientation and not airflow.......... like this side up... then the manual would be wrong if it indicates installation with arrow down.

 

[Mountain Gold]

Hummm, the arrow indicates the flow of the air. Since the air enters the air box
from the cowl area near the base of the windshield, and since the air box is below the
dashboard....and since the blower motor is even lower, below the filter box, I would
guess that the arrow should be pointed DOWN indicating the flow of air direction.

 

[Aussie Frontier]

Guys sorry to spoil the party but this has been covered at great leangth many times and yes your filters were installed upside down from factory and yes airflow is from top to bottom

Oh yeah the Navara doesnt come fitted with them but it has the housing etc so you can install them

 

[something else]

Guys sorry to spoil the party but this has been covered at great leangth many times and yes your filters were installed upside down from factory and yes airflow is from top to bottom

Yes, it's been covered, and from reading the OP's post it seems that OP has read those threads and is still unconvinced about the direction of the air flow.... and the OP spent a lot of time experimenting and writing out the thread, all that work warrants discussion on the OP's thread.

But if one could not deduce from my lengthy reply about the airflow or the filter arrow; the air flow is down and the filter arrow should point down as Aussie Frontier states, and Mountain Gold correctly guessed.

 

[midnightsun]

Follow-up and "official" word

I agree with the logic that fresh air comes in from above (cowl) but think it odd that the blower fan would blow back towards incoming air. When I called the dealership to ask a tech, (I told him that somebody there had installed the filters with arrows pointing up last time) he confirmed that the arrows should point down just like all the instructions say. But he didn't think it made a big difference, which matches what I found using the vacuum-cleaner test.

I'll replace them with arrows pointing down.

 

[bobblehead]

I'd like to comment on your shop vac experiment if I may. You mention that the filter could possibly be more porous on one side compared to the other but I don't see how that could affect air flow one way or the other. Air molecules would have to pass through both sides of the filter regardless of the orientation so, in my mind anyways, the shop vac test should show the same results either way.

 

[xichlo]

I haven't looked at a micro-filter but its simple really. The filter poristy of the medium is the same ethier way. The flow is different one side, as with house AC filters, they are designed to go in one way. One side is texured differently to catch large particles first and minimize flow compaction/restriction. Put it in the other way and it will plug faster as the tiny pores will fill first. Also some filters have the pleats arranged at a certaian angel to also control flow.

 

[GREY_MATTER]

maybe he reversed his blower and wants to filter the air he put into the atmosphere...

/end thread

 

[Xterra1102]

Air flows from top to bottom....I just changed mine out and all the debris and crap trapped by the filter was located on TOP of the filters indicating a downward flow of air.