the HVAC designs of Mark Wall

[kab7]

Hi, I'm currently pondering the cockpit HVAC system, so looking at designs folks have done in the past.

Mark Wall did an interesting presentation in 2006, where he heavily modified the stock AC box, as well as the interior ducts:
https://www.gmcmi.net/wp-content/uploads/2013/02/HVAC-Systems.pdf

He also posted some pictures around 2009 of a very different box configuration, but it looked like a plywood prototype:
http://www.gmcmhphotos.com/photos/hvac-redesign/p30528-hvac-box.html

In the earlier one, he moved the stock blower to the front of the box so it pulls rather than pushes air to reduce the number of bends.
In the later design, he built a flat box with the stock blower on top, pulling air through both the evaporator and heater core at all times
and shooting the air straight through the firewall. I'm curious how that one worked; I'm thinking always going through the heater core might
reduce airflow somewhat, and of course you need a good heater core shutoff.

Does anybody know what Mark finally ended up with?

Getting hot here...

thanks,
Karen
1975 26'

 

[john r. lebetski]

I watched Marks earlier presentation I think at the Berrien Springs convention. All good til he got to the 100% recir air part. That doesn't work for
me as no ventilation and I like fresh air coming in as I drive. Especially when you have to run in cool rainy weather and get fogging inside because
VENT is no longer vent. So you have to run AC and burn more gas wheras the other way fresh air keeps windows clear and you can add heat to it as
needed. I have a 77 so I don't think it applies to mine anyway. There was an article by Wes years ago in their news letter about using the dash blower
to creat positive pressure in the coach to not draw septic odors in as well. That option is no good with 100%recirc. I want to see if there is a more
modern motor/blower like from a 2018 Suburban or Sprinter or something that moves more air less Amp draw. The 90s Caddy's used a flatter design motor
probably more efficient as motor tech may have gotten better as Amp draw hurts CAFE.
--
John Lebetski
Woodstock, IL
77 Eleganza II

 

[russell mehlenbacher]

I have Mark's coach... The 100% re-circ is a problem with moisture on the
windshield. We ran thru a thunderstorm and lost 80% of forward visibility
in the temp / humidity swings. In a bit of desperation we wiped, sprayed
anything at hand. Windex does a wonderful job of clearing the glass on a
temporary basis.
The AC does work really well. (except for the moment, compressor reseal
coming soon) The defroster has not done a good job since we took ownership,
I'm finding that the system was stuck together with foam from a can. Time,
vibration and temperature swings have caused a deterioration of defrost
system integrity.
I did get to replace the plywood box lid with a sheet of plastic, probably
PETE. Just something I had in stock. I expect to add some floor vents
(blast doors) for guaranteed fresh air without the noise of the wind wings.
I'm of the opinion that the fastest way to cold air is the addition of
something like the Vintage Air kit.

Russ

On Fri, Jun 22, 2018 at 7:15 PM, John R. Lebetski
wrote:

> I watched Marks earlier presentation I think at the Berrien Springs
> convention. All good til he got to the 100% recir air part. That doesn't
> work for
> me as no ventilation and I like fresh air coming in as I drive. Especially
> when you have to run in cool rainy weather and get fogging inside because
> VENT is no longer vent. So you have to run AC and burn more gas wheras the
> other way fresh air keeps windows clear and you can add heat to it as
> needed. I have a 77 so I don't think it applies to mine anyway. There was
> an article by Wes years ago in their news letter about using the dash blower
> to creat positive pressure in the coach to not draw septic odors in as
> well. That option is no good with 100%recirc. I want to see if there is a
> more
> modern motor/blower like from a 2018 Suburban or Sprinter or something
> that moves more air less Amp draw. The 90s Caddy's used a flatter design
> motor
> probably more efficient as motor tech may have gotten better as Amp draw
> hurts CAFE.
> --
> John Lebetski
> Woodstock, IL
> 77 Eleganza II
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

--
Russell Mehlenbacher
Rochester Hills, MI
'76 EII
'74 Sequoia

 

[kab7]

Wow, I didn't expect the current owner of Mark's coach to chime in.
What an amazing community!

He doesn't specify in the first design where he got the intake air from. If you just leave the original blower hole open, it would act as an outside
air intake, but unfortunately from directly above the radiator unless you put a duct on it.
I guess he wasn't happy with that design, so changed it again in 2009 or so. The biggest drawback of moving the blower to the front of the box
is you lose all that real estate for mounting the radiator coolant bottle, etc.
If you instead go with a blower in the cockpit, you lose outside air and get more fan noise.

I'm working on some ideas based on his first design which would have at least some outside air as well as recirc, so we'll see how it turns out.
I've already tried other modifications like a bigger recirc opening behind the glovebox, so our poor firewall will be pretty hacked up after this.

thanks,
Karen
1975 26'

 

[john shutzbaugh]

Karen, I was watching for the results of the modification you were doing with the blower mounted in the cockpit, but must have missed the outcome.
Was noise the main result? Did it cool better? It's pretty hot in my 78 & I too am looking to change things.

John
--
John Shutzbaugh, Vacaville, CA, ncserv;
77 Eleganza, bought it new, can't blame PO, and
78 Buskirk stretch, "What were we thinking?"

 

[john r. lebetski]

78 should have all the best designs implemented. Is it just not cold enough exiting?
--
John Lebetski
Woodstock, IL
77 Eleganza II

 

[john shutzbaugh]

Even though it is a late model coach, I could use MORE air. The temp is ok, just not enough of it.

John

--
John Shutzbaugh, Vacaville, CA, ncserv;
77 Eleganza, bought it new, can't blame PO, and
78 Buskirk stretch, "What were we thinking?"

 

[powwerjon]

John,
I would be checking the duct work under the dash. The type 2 air package in our 77 eleganza would frost your knees.

JR Wright
78 Buskirk Stretch
75 avion still undergoing reconstruction.

>
> Even though it is a late model coach, I could use MORE air. The temp is ok, just not enough of it.
>
> John
>
> --
> John Shutzbaugh, Vacaville, CA, ncserv;
> 77 Eleganza, bought it new, can't blame PO, and
> 78 Buskirk stretch, "What were we thinking?"
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org

 

[kab7]

> Karen, I was watching for the results of the modification you were doing with the blower mounted in the cockpit, but must have missed the
> outcome. Was noise the main result? Did it cool better? It's pretty hot in my 78 & I too am looking to change things.
>
> John

Hey John,
I built the floor box, hooked up the blower and wired it up enough to test, but decided against going any further.
Since we have the max air version, I'd have had to remove the max air door or it would only have been
blowing on the blower. Since that's a lot more work, I decided to think about it some more.
The combination of noise and the difficulty of incorporating fresh air was what made me decide against it.
It's a nice blower though, so kind of a shame I can't use it.

Karen
1975 26'

 

[jim kanomata1]

Make sure the hot water is not flowing through the heater core and the door
is shut.
It is amasing what those things contribut to low cooling.

On Sun, Jun 24, 2018 at 5:15 PM, John Shutzbaugh
wrote:

> Even though it is a late model coach, I could use MORE air. The temp is
> ok, just not enough of it.
>
> John
>
> --
> John Shutzbaugh, Vacaville, CA, ncserv;
> 77 Eleganza, bought it new, can't blame PO, and
> 78 Buskirk stretch, "What were we thinking?"
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

--
Jim Kanomata
Applied/GMC, Newark,CA
jimk
http://www.appliedgmc.com
1-800-752-7502

 

[billvv]

Quote:
> Since we have the max air version, I'd have had to remove the max air door or it would only have been
> blowing on the blower.

"Blowing on the blower"... Isn't the max air door chamber above and separated from the evap coil chamber?
--
Bill Van Vlack
'76 Royale; Guemes Island, Washington; Twin bed, full (DS) side bath, Brazilian Redwood counter and settee tops,455, 6KW generator; new owner a/o mid
November 2015.

 

[kab7]

The Max Air slot opening is just below the dash, exactly where you would have to mount the new blower.
So if you don't move the vents, you'll mostly just be circulating cold air to the blower and back.

I've updated my drawings of the various HVAC years:
http://www.gmcmhphotos.com/photos/hvac-original-and-mods/p64915-airflow-all.html

I tried to make it more legible, but the upload process appears to shrink the file and reduce the quality of the image.
There are separate images available for each model year which hopefully are more readable, but I wanted to show them all on one page.
Really makes clear the progression of the design.

After looking at this stuff so closely, I'm not at all surprised the earlier air flow was so bad.
The only opening is through the top into two small ducts (~6sq" each) and to the defroster duct.
The defroster door doesn't close all that well, so even less air goes to the pitiful vent openings.
Not to mention all the right angle turns the air has to make before it even gets there.

Karen
1975 26'

 

[billvv]

I see what you mean... I did some sketchup drawings that show ducts added to the fan inlets to prevent that short-circuiting.

I envisioned them routing up under the dash, and covering the exposed parts with a little cabinet, but for a lot of reasons, I agree that this seems
like the wrong way to go. The fan on the front of the HVAC box seems like it provides the shortest air paths, at least forward of the firewall.

I think any solution needs to allow modulating between 100% recirc and 100% outside air for max cooling and defrost/defog. The Type II unit would need
an additional damper for that. For defog, the AC and heat have to run at the same time, which I think means a control change as well.
--
Bill Van Vlack
'76 Royale; Guemes Island, Washington; Twin bed, full (DS) side bath, Brazilian Redwood counter and settee tops,455, 6KW generator; new owner a/o mid
November 2015.

 

[kab7]

I upgraded my drawings to try and show the air paths better, color coded.
Red is warm air, yellow is air in the blower/pressure chamber, blue is cool air.
http://www.gmcmhphotos.com/photos/hvac-original-and-mods/p64915-airflow-all.html

Ironically, it's a lot easier to go from a 73/74 box to the later design than from the 75/76 version.
The 73 already has the recirc door and you don't have to remove the max-air slot. They used the recirc door
vacuum pot to power the max air door, and didn't cut a hole for recirc.
You can see why: they were trying to separate the max air output from the blower inlet air.
Once they added the type-II external duct under the dash, that's no longer a problem.

I tried to find a unit from a modern vehicle, but it's all so convoluted and tied to the exact shape of each vehicle that
it's not really feasible to fit in our available space.
I also thought a lot about doing Mark Wall's later iteration, but decided there was no real advantage in terms of space efficiency.
The problems are the same, and there's a lot more vertical than horizontal space available outside the firewall.
Plus, we already have the box. So, basically, I'm just re-creating the 77/78 design in a 75 coach, with a little variation
from how Mark did it. This version has both recirc and outside air, and the defroster door should work as designed.

It really requires you to remove the box from the coach, but I found that you can (very gently) remove the evaporator core
with the hoses still attached and tie it out of the way. It's easy to disconnect the heater core, and (with someone
stationed to protect the evaporator) you can slide the box down and out through the grill opening.

Karen
1975 26'

 

[billvv]

From the diagrams, it looks like the major difference btw 74-74 and 77-78 is mostly the new duct plus the openings in the firewall and box to make the
connection. Certainly see how the '76 is 'boxed' into a corner in all of this. I think if I could provide some recirc to the blower inlet, the rest I
could live with.
--
Bill Van Vlack
'76 Royale; Guemes Island, Washington; Twin bed, full (DS) side bath, Brazilian Redwood counter and settee tops,455, 6KW generator; new owner a/o mid
November 2015.