Lurk no more… I lived in Portales, NM for a few years and spent time vacationing in the Cloudcroft area. I thought it would be a beautiful place to retire.
If you’ve never built a solar heater, you sure don’t start small. But in truth, building larger only takes more time and money. The experience needed is the same.
From what I read you have all your bases covered for the most part, at least from a material standpoint. Attaching or holding down the glazing can be a small design sticking point, but only because there are many ways in which to accomplish this.
It’s always easier to use existing openings into a home, such as a window or in your case, a vent from an old gas heater. For the size of your heater the 8” ducts are likely the best. Any smaller and you will have trouble getting enough airflow. Any larger, and you need a bigger opening.
A couple of suggestions to consider due to the dust issue. Use an air filter to keep dust out of the system as much as possible. A quality pleated furnace filter should be enough. Just stay away from the $.99 spun fiberglas filters. They not only do a poor job of filtering smaller particles, but the fibers themselves will end up in the system. I would also recommend pushing air if at all possible. I would suggest a slight overpressure to help keep dust out of the system. If you are drawing air, dust will be drawn into the smallest opening. And as your heater is larger, a second, smaller booster fan may be needed to help balance the pressure in the system to achieve the slight positive pressure.
Something else to consider during the design phone of the build is access. Since dust is an issue where you live, you may want easy access to the innards of your collector. Access may never be needed. But if it should be, you’ll be ready.
And lastly, the most interesting, and varied part of any heater design is the absorber. Without an absorber, a collector is simply a black box which isn’t very efficient. You mentioned using 3 layers of screen. There are several different ways to use screen as your absorber. Do you know how you want to configure these 3 screens inside the collector box?
Greg in MN