Defrosting - Part 3

I have spent the better part of my free time over the last several weeks sanding and hammering on the Zed. If you do that long enough, eventually you need a break, to focus on something else. Otherwise, you'll lose interest in the job, or maybe working on cars in general, and find yourself taking a few weeks off, like I did last winter when I did the TDI retrospectives. I just didn't feel like working on my cars and the weather was crappy, so motivation was at a premium. Anyway, today's post collects together the bits and pieces of hours I've spent taking a break from the sanding to move one little step forward on Hapy's defroster.

Model First

model from p-side rear

One thing I've learned along the way is to model what you want to build before you built it. With the speakerbox I built for Oliver (the MGB), I built a box out of cardboard first, That allowed me to really understand how it could fit before I started cutting MDF. I did the same thing for the heat register in Hapy when I did the parking heater. So, it should come as no surprise that I built a model of the defroster register with cardboard before starting in on the zinc-coated steel.

This had some aspects that were similar to the parking heater register: I needed to connect a round inlet/outlet with a rectangular inlet/outlet. That is about where the similarity ended, though. The space I had available, and the means of accessing that space made the design of this register quite different.

Piecing It Together

model from d-side rear

I started with a top-plate that matched the 9" wide heater, with 2 partial sides that dropped about 3 inches on each side. This is a simple rectangle with 2 90* bends. For the bottom I tried a trapezoid shape, with a large rectangle in the center, aligning the 9" bottom of the heater with the bottom of the front cross-support bottom. The sides are triangles that bend upwards, overlapping with the 2 sides dropping from the top. These two pieces would be held together with sheet metal screws, and the seams taped with the same contractor-grade metal HVAC tape. This shape is obvious, but it does not route the air towards the round hole. It just compresses the air from a 9" by 6" rectangle to a 9" by 3" rectangle. I figured there would probably be significant air turbulence, reducing the actual air-flow down the pipe to the floor or windscreen.

To reduce the turbulence, and improve the air flow, I added 2 trapezoid-shaped angled sides. Three straight sections were parallel to the heater face and the cross-beam as well as along the top. The one the angled side runs along the bottom from the lower edge of the heater to the lower edge of the cross-beam, just to the side of the round hole. Now, this still wasn't perfect, since the register now ends in a square, around the round hole. I figured this was the best amateur register I could come up with.

Install Planning

nutria and ducks in harmony

Finishing the model took many trips under the bus, fitting and then sliding back out to trim or add material (blue tape). Once I had the piece constructed so I could fit it into place, I needed to figure out how to install it such that I could assemble it in-situ, but could disassemble it and remove it when necessary. Note I didn't say "if"; it's wise to assume that you will need to undo something or you are just making life difficult for your future you.

The top plate was fairly straight forward. It could go in independently, simply by tipping the front (front is front) downward and sliding the rear edge up and over the top of the heater, rotating the front into place and then settling the plate down. The bottom was similarly easy, just offer it up. The challenge was with the angled side-runs.

I had considered pop-riveting the side-runs to the top in-place. This would have been difficult to undo for removal, so I planned on sheet metal screws instead. The side-runs got extra tabs along the top and bottom through which I drilled a pair of holes in each. These holes aligned to holes in the top and bottom so I could screw the side plates first into the top and then into the bottom.

Build and Fit

model to tin

Once I had it all working with a cardboard model, I transferred the lines to the zinc-coated HVAC sheet metal and cut the lines with tin-snips. As could be expected, the lines weren't perfect and some additional fitting was necessary. In fact, I abandoned large parts of the model entirely to get it to fit. First, getting the angled sides to fit in-situ with screws was simply not going to happen. Second, and more important, the cardboard model had flex to it that belied the fit. So, I abandoned the screws and held the sides to the top with that really good metal HVAC tape on both sides. I also used this tape to hold the front of the top section to the cross-beam.

Then, the real fun began. The bottom plate did not fit; at least not on the passenger side. I am not sure how, but the passenger side ended up about 25mm too long. It was just off. Again, I credit the flexibility of the cardboard during the modelling. Boo had asked why I had retained the fuller size of the bottom, stretching all the way across the whole span, when the sides really abbreviated how much I would really need. This is why working in groups is so powerful. Taking her advice, I modeled a new trapezoid-shaped bottom out of cardboard that would fit between the sides, front and back. I transferred to the flashing a cardboard cut up of a new bottom. Again, I used the metal HVAC tape to attach the sides to the bottom as well as the front and back. While this isn't as "perfect", the goal will still be met: air flow from heater/defroster will route into the original air pipe. With careful use of tape, I was able to have only the silver part of the tape visible from the sides.

When I can't take the body-work any longer, I will look into running the wiring and switch for the defroster fan. I'll post when I get to it. I did test the register by running a fused 12V source to the heater unit (and grounding the heater). The register had a couple small leaks that were easily fixed with more metal HVAC tape. That stuff is magical.
Thanks, as always, for following along--