HVAC "cabin filter" for the X1/9?

[Dr.Jeff]

Lately I've been fooling around with the HVAC box for a "AC" equipped X. And one thing I'd like to do is add some sort of filter to its inlet, in an effort to keep some of the debris out. Has anyone attempted to make something like this for the X's heater box inlet?

Due to the way the "fresh air" inlet door is on the top of the box, and it swings upward (out away from the box), there isn't any way to add a real filter over it, like some modern cars do. And there are things attached to the backside of that door to prevent putting something across the opening from below. On older VW's the top of the heater box is similar, in that you cannot add a filter directly over/under the inlet due to its shape. However on those cars there's enough space above the box to allow for a inverted 'dome' shaped screen to cover the opening; commonly referred to as the "leaf basket" because it really isn't a filter so much as a screen for larger items:

Unfortunately something like this cannot be done on the X; the inlet is located in the center of the front scuttle, under the body brace for the frunk latch, that latch and cable, the wiper assembly, some wires, master cylinder feed hoses, etc. So there is no room or access for anything to be above the inlet.

On the AC heater box the evaporator is in the upper portion of it, just like the heater core is in a non-AC box. It sits a little below the opening, flat across the area so all air must pass through it - heat, AC, and fresh vent. The same is true for the heater core on non-AC boxes. I'm wondering if a fine mesh screen can be made to cover above the core (either evaporator or heater, depending on which box you have), but far enough below the flapper door hardware so it won't interfere. It looks possible on the AC box (I haven't looked closely at the non-AC box yet). Here's an idea of what I'm talking about:

The screen could fit above the core (#14) like the red lines, but under the door (#13) like the blue lines, in between the two....

The evaporator has a foam insulation surround sealing it in the box. This forces all of the air to pass through the core, and therefore the screen first. The screen would act in a similar way as the VW leaf basket, but by using a finer mesh it can capture smaller objects from going down inside the box. Unfortunately this would not be serviceable, other than removing the HVAC box (not going to happen). But it is still better than having all that crap go down into the box, out the dash vents, and into your lungs.

Tomorrow I'll do a little mock up of it to see how it looks. Any further thoughts or ideas first?

 

[Dan Sarandrea (Phila)]

Better to seal where the scuttle grilles meet the scuttle and where the frunklid overlaps the grilles. The scuttle grilles themselves are fine enough of a lattice to screen out most if not all tree droppings. If you want to filter dust, perhaps some sort of foam filter attached to the underside of the scuttle grille combined with sealing the gaps between the frunklid, fender, and base of windshield, like modern cars seal body gaps for windnoise control and smoother aero.

 

[Dr.Jeff]

Better to seal where the scuttle grilles meet the scuttle and where the frunklid overlaps the grilles. The scuttle grilles themselves are fine enough of a lattice to screen out most if not all tree droppings. If you want to filter dust, perhaps some sort of foam filter attached to the underside of the scuttle grille combined with sealing the gaps between the frunklid, fender, and base of windshield, like modern cars seal body gaps for windnoise control and smoother aero.

I had the same thoughts. On my VW's I add a thin layer of foam filter material to the "leaf basket" I showed in my last post. It helps. That can be done to the scuttle grills also. The scuttle grills on mine are being replaced with custom grills made from a metal panel with a open "hex" pattern. That's difficult to describe, so I'll add pictures eventually. But the point is it has smaller openings than the stock plastic grills. However either version - the stock grills or my custom ones (even with a layer of foam) - will still allow much more crap inside than I like. Just look at what you find inside the heater box any time it is disassembled. With the stock scuttle grills a lot of rather large items, larger than the scuttle grill openings, found their way inside. Therefore just improving those grills is not good enough. As you note, the X has a lot of openings that feed the HVAC box other than those grills. Trying to "seal" the whole scuttle will be difficult, and I'm not sure very effective, but I'll look into it more.

So I want to add another layer of protection. I'm not trying to keep "dust" out, just all the objects like I always find inside these boxes (bugs, leaves, feathers, bits if trash, twigs/needles, large dirt, etc). Hopefully with most of that gone there is less organic material to breed molds, etc. That's where adding some sort of internal "filter" comes in. I considered making a removable 'tray' that can slide out from the side of the box. A filter of some sort attached to that tray can then be serviced. Unfortunately with the design of the box, its contents, and how it is positioned in the car, that isn't possible. I also considered adding filters to the air outlets, either inside the box or at the dash vents. However those options did not pan out either...just not practical to fit anything in there.

Still thinking about it and listening to any ideas.

 

[Bobkat]

what about a fine stainless steel mesh for the underside? Would be washable. I make CF rear window vents and just use plastic black screen door mesh. works great at keeping wasps from being sucked in.

 

[Dr.Jeff]

what about a fine stainless steel mesh for the underside? Would be washable. I make CF rear window vents and just use plastic black screen door mesh. works great at keeping wasps from being sucked in.

Fine stainless mesh screen is what I have to make a mockup today. It is quite rigid so it will hold whatever shape I form it into. I just edited my prior post to clarify my objective with a 'filter': "I'm not trying to keep "dust" out, just all the objects like I always find inside these boxes (bugs, leaves, feathers, bits if trash, twigs/needles, large dirt, etc). Hopefully with most of that gone there is less organic material to breed molds, etc." I realize it isn't possible to make the air supply "clean" in a old car HVAC system. But any improvement will be welcomed.

@Bobkat, where exactly do you mean by "the underside"?

See the illustration of the AC style HVAC box posted earlier. All air enters at the top section #16 - either through the fresh air door #13 or the recirculate door #11, flows down through the evaporator core #14, through the mid section of the box #18 - where the blower sucks it and pushes through the lower section of the box #25, and through the heater core #19 before exiting the box from any of the pathways (defrost #6, dash #23 & 10, or lower vents #26). This is true for all modes; fresh vent, heat, or AC.

So a filter located anywhere below the top of the evaporator (#14, see red lines) will allow debris to get into it and the other sections. I think it is better to catch the debris before it enters rather than trap it inside after? But I suppose the main goal is to prevent it from going into the car's cabin. Perhaps you're thinking with the filter located somewhere at the bottom it can be made removable for cleaning? However that will not remove all of the stuff that's collected inside the system. Please expound on your thoughts.

 

[Dr.Jeff]

I took a look at the possibility of making a "slide out" tray toward the lower portion of the box, to have a "serviceable" filter of some sort. Unfortunately I could not see any place that it can work. A screen can be installed inside (non serviceable), next to the heater core. But that is the same as putting one at the top as I described previously. I think it will be more useful at the top/inlet than at the bottom/exit. But still the same general principle.

While I was playing with it I explored a couple of other "filter" options, with the same result - not practical. Unless a better idea comes up here I will continue with the original plan to form a screen above the evaporator.

I also realized something else relating to my conversion from the vacuum controls to manual cables. I'll add it to that thread. All of this fooling around ment I did not get the prototype screen formed today. Hopefully tomorrow.

 

[Dr.Jeff]

Here is a little mockup prototype idea for adding a screen "filter" to the HVAC box.
It is rigid stainless steel mesh and sits above the inlet side of the evaporator, below the "fresh" (outside) air supply at the top of the box. Like I illustrated previously:

The screen is shaped with "stand-off" sides to hold it above the surface of the evap core, avoiding abrasion to it (black arrows). The 'front' edge is at a angle to match the shape of the box where the evap core fits (green arrow). And the 'rear' edge is a little longer to reach down where the core rests against the rear of the box (blue arrow):

Otherwise it is the size/shape of the evap core:

Here it is sitting on top of the evap core:

And in the upper section of the HVAC box. The sides are sealed with foam insulation/filler blocks (yellow lines):

Once the box is assembled with all of its internal insulation and seals as designed from the factory, all incoming air must pass through the evaporator's rectangle core area. And therefore through this screen first.

Debris will eventually accumulate on top of the screen, just inside the upper 'flapper door'. It might(?) be possible to get a narrow shop vac nozzle in there from the front scuttle to suck most of it out??? Otherwise the box would need to be removed to fully clean the screen. However keep in mind, that very same debris would otherwise be down inside the box - stuck in the evap and heater cores, in all of the air pathways and various nooks and crannies, where it could not be removed/cleaned without removing the box and disassembling it anyway. So at least this way the trash is prevented from traveling through the box, out of the vents, and into your face/lungs.

I'll also look into @Dan Sarandrea (Phila)'s suggestion of trying to filter some of the air before it enters the HVAC opening - at the scuttle covers. I've explored this idea in the past and it doesn't seem like all of the incoming air can be effectively "filtered" that way. But it should reduce some of the debris, especially the larger stuff. Anything will help; hopefully combined with the internal screen these mods will reduce the contamination that's typical in any vintage car's HVAC box....and the vehicle's occupants.

 

[Dan Sarandrea (Phila)]

Since you are in a hot/dry climate, considerations such as water retention in semi-accessible areas should not be much of an issue, so another idea would be to ascertain what type and density of foam would give you the desired filtration level and airflow (perhaps a combination of screen and foam) and then cut and shape the foam to "block off" the scuttle cross section to the left and right of the HVAC intake "port."

 

[kmead]

I think Dan’s idea will be easier and more serviceable than adding screens you can’t clean to the evaporator and so on.

Your approach although interesting will just add an additional permeable shelf on top of a permeable shelf…

I know the wiper arm swings through there but there should be enough clearance to at the very least make your screen work above the opening and below the wiper arm.

 

[Dr.Jeff]

cut and shape the foam to "block off" the scuttle cross section to the left and right of the HVAC intake "port."

How did you get in there to take that picture?
It would be easier to partition off the middle section of the scuttle than trying to seal the whole scuttle area. But how much of the incoming air will that actually catch? Isn't there also a lot from around the frunk lid, latch, etc? I'll have to look at it again.

I know the wiper arm swings through there but there should be enough clearance to at the very least make your screen work above the opening and below the wiper arm.

That was the problem I found with trying to make a filter above the HVAC box inlet. Due to the flapper door coming upward when it opens, there really isn't a good way to effectively seal off the opening due to the bracketry for the hood latch, wiper mechanism, etc.

I'm thinking a combination of both approaches; preliminary filters on the scuttle openings and partitions for the center portion of the scuttle, combined with the screen I made over the evaporator. Frankly none of it will be serviceable in a reasonable manner.

Even a relatively thin foam layer over a screen as Dan said will do a lot. There's a foam filter sheet that I put over the air intake of my shop AC unit. It is made for that purpose and is cheap/easy to get. It may seem too thin but actually I'm surprised at how much dust and dirt it collects on the shop's unit.

I planned to test it over the SS screen I made (last post). But that may be too much filtration - as in collect debris too quickly and clog up? However if Dan's scuttle partitions are accessible(?), then possibly(?) they could be serviced, along with the scuttle grill filters. By doing that, then the screen inside the box won't build up as quickly....as in the life of a typical ownership.

 

[dllubin]

How did you get in there to take that picture?
It would be easier to partition off the middle section of the scuttle than trying to seal the whole scuttle area. But how much of the incoming air will that actually catch? Isn't there also a lot from around the frunk lid, latch, etc? I'll have to look at it again.



That was the problem I found with trying to make a filter above the HVAC box inlet. There really isn't a good way to effectively seal off the opening due to the bracketry for the hood latch, wiper mechanism, etc.
View attachment 47254

I'm thinking a combination of both approaches; preliminary filters on the scuttle openings and partitions for the center portion of the scuttle, combined with the screen I made over the evaporator. Frankly none of it will be serviceable in a reasonable manor.

Even a relatively thin foam layer over a screen as Dan said will do a lot. There's a foam filter sheet that I put over the air intake of my shop AC unit. It is made for that purpose and is cheap/easy to get. It may seem too thin but actually I'm surprised at how much dust and dirt it collects on the shop's unit.
View attachment 47253

I planned to test it over the SS screen I made (last post). But that may be too much filtration - as in collect debris too quickly and clog up? However if Dan's scuttle partitions are accessible(?), then possibly(?) they could be serviced, along with the scuttle grill filters. By doing that, then the screen inside the box won't build up as quickly....as in the life of a typical ownership.

I bought that exact same air conditioner filter material at Home Depot to make a new air cleaner element for my carbs (New filter elements for my dual DCNF Sprint filter no longer available). Unlike the stuff I bought in the 90s, that filter material is not particularly resistant to gasoline - not that it would likely be an issue for your application. I ended up getting some foam meant to be used for fuel system air filters.

 

[Dr.Jeff]

A little off topic but I don't know of a better thread to add this to....

After playing with the "filter" idea for a couple of days I bagan reassembling the HVAC box. And I noticed something really interesting about the factory design. The evaporator core has a foam/rubber seal that surrounds it inside the box. It is what forms the filler material as shown in my pic in post #6 (yellow lines):

This blocks the airflow from passing around the core - making all air pass through the core. This is what my evap seal looked like when I took things apart:

It's the one on the right (the other one goes on one end of the heater core). Mine is dry, broken in one place, and beginning to look a bit ragged.

This is what the evap seal looked like on @LarryC's box, as shown in his rebuild thread:

He described it as "tar-like gunk". I'm guessing some sort of chemical attacked his to dissolve it.

Given this factory seal is 'custom' formed to perfectly fit the core and the box I decided to restore and reuse mine. Some clean up, a couple repairs, and re-sealed the surface with a rubber coating. Then as I assembled the box and installed the seal I realized a flaw in it's design.

Here is the area of the box below the evap core that houses the blower assembly (looking up from the bottom). Air is drawn down through the evap (the area below in this pic), into the ends of the squirrel cages, then forced out if the blower into the next chamber. You can follow that airflow with the red arrows:

The seal around the evap core goes between this section of the box and the one with the core. In the above pic if you look to either side of the squirrel cages you can see it - now white in color due to the rubber coating I put on it. Here are better views of either side....
On this side you see the evap core (green arrow) and the seal (blue arrow). The seal is up against the wall of the box, so air only travels where the green arrow is:

As you can see there isn't a lot of open space for that air to pass. Here's a better look (blue arrows):

On the other side of the squirrel cage it's a different story. The seal almost completely blocks the entire air space (yellow box):

A little hard to see due to the shadows, but here's another look at the only open passage for any air to pass (red arrow):

The rest of the passage is completely blocked by the seal, leaving only a very tiny opening.

This creates a significant restriction to air entering that squirrel cage - to almost zero! Since there are only two squirrel cages, that means the entire HVAC box can only get about half of the potential airflow through it, and therefore out of the box to the cabin. Remember that ALL air - vent, heat, AC, defrost - must go through this same route. And therefore has the same consequences.

When Larry rebuilt his box he had to make a new seal for the evaporator due to its condition (as shown above). Here is what he made:

It appears to be less obstructive to the air passages. So it was fortunate that his original seal was in such bad shape. But this makes me wonder about all of the AC equipped X's - do they all suffer from low airflow due to this seal? Keep in mind this sort of air restriction into the squirrel cage will add a significant load to the blower motor, leading to premature failure. When we add relays to boost the current to that motor we may just be adding to its demise.

I will make a custom seal for mine as well and toss the factory one completely. Looking at it I think it is best to place the seal on the opposite side of the evap core. It will still block any air from bypassing the core. But it will not be in the space shown here, and therefore open up the air route from the upper section of the box into the blower.

 

[Dr.Jeff]

I took a second look at possibilities for filtering the scuttle area, to clean the air before it reaches the HVAC intake. There will be some challenges.

To "partition off" the scuttle on either side of the HVAC hole there are some obstacles that need to be accomodated. From the passenger side it isn't too bad, mainly the hoses from the hydraulic reservoirs going across to the other side and some wires. The drivers side has those hoses, plus the sheath for the hood latch release, the wiper mechanism (which moves), and some more wire harnesses:

If they can be worked around with the filter partitions, then there is the question of the frunk lid gap:

The green line represents approximately where the seal is. So everything beyond that is open. Naturally that gap is smaller when the lid is fully closed, but there is a gap.

And not much opportunity to add a seal for that gap; the landing below is quite a ways down, and the hood recedes around the scuttle grills. So that will also need to be sealed somehow:

Then there's the large air gaps around the latch mechanism for the lid:

Kind of a lot to work around. But if it could be done it should help to keep the HVAC cleaner.

I see how the idea Karl had would be simpler, putting a filter directly over the HVAC inlet door. This was my first thought as well, but it has its own challenges. The flapper door opens upward so the filter would need to extend up over and around that. There is also a metal panel that deflects the incoming air, which the filter will need to go over. But just above those is where the wiper mechanism (again, a moving item that must be kept clear), the hood latch, latch release cable, wires, etc, all live. So not much room to get the "house" shaped filter in there. That's why I opted to look inside the box instead. But again, it will certainly help if it can be done.

Servicing either of these approaches would not be easy, but to some extent may be possible. That's an advantage. I now see it will not be possible to do any cleaning (e.g. vacuuming) of the internal screen over the evaporator from above (no real surprise).
For my project I will install the internal screen either way. After I've made my custom scuttle grills I'll revisit these additional possibilities.

 

[kmead]

I think making a small ‘tent’ which is more or less just the height of the door at it maximum opening and is more or less conformal to the raised box the door rises up through.

This would at least keep the chunks out of the air box, certainly not the dust or anything smaller.

Not a happy situation. Nothing, a little something you may be able to service but of limited capability or something which you can’t remove or clean at will.

 

[Dr.Jeff]

Agree.

This afternoon I switched my efforts to the next link in this puzzle....the heater tubes in the center tunnel. Mine leak so need to be replaced. In my case I will do a complete redesign of the heater pathways, so either way those tubes need to come out. I'll post about that elsewhere.

 

[dllubin]

Agree.

This afternoon I switched my efforts to the next link in this puzzle....the heater tubes in the center tunnel. Mine leak so need to be replaced. In my case I will do a complete redesign of the heater pathways, so either way those tubes need to come out. I'll post about that elsewhere.

I replaced the metal tube in the passenger compartment tunnel (was leaking where the bracket was welded to it) and the rubber hoses at each end with exactly 10 feet of Gates 28411 Safety Stripe 5/8" heater hose. No junctions between the engine and the heater. I also determined that there was room for two of those hoses in the tunnel should the lower heater pipe needed to be replaced (It didn't). I used 3/4" EMT straps to hold the hose against the side of the tunnel.