Solar Thermal Hot Air Boxes Must Have An Airtight Seal On The Outlet Duct!

There are solar thermal hot air boxes (STHABs) videos and write-ups all over youtube (videos) and BuildItSolar.com (pictures& text) that show how to make them yourselves for maybe a couple hundred dollars USD (or even a lot less if you have the right stuff laying around) but I have yet to find one that includes an airtight seal for the inlet/outlet ducts. An airtight duct seal makes the difference between getting free heat in the dead of winter and getting free heat AND unwanted free freezing air conditioning in the dead of winter. The laws of physics are pretty reliable, and they work the same way during a sunny day and a dark freezing night – air temperature is changed by what the exterior environment is doing to the inside of the the solar thermal hot air box. On a sunny day, the sun sends photons through the glazed front of the solar thermal hot air box, which hits something that changes the sunlight into heat, and thereby heats the air being moved through the STHAB (solar thermal hot air box). In the middle of a freezing cold winter night, the ambient temperature will chill the interior of the STHAB and thereby chill any air that passes through it. You would wipe out any benefits from the daytime heating if you don’t have an air-tight seal covering the exit duct during long winter nights. You might not care about that if the STHAB is just being used to provide heat during the day to a place that won’t be negatively impacted by severely cold weather – for instance, if this is attached to an unheated shed/garage with no plumbing or items that will be damaged by freezing temperatures. This also doesn’t apply to STHABs that are entirely inside a conditioned space – in that case, the cold doesn’t get to it, and it doesn’t become a cold generation machine.

The STHAB shown in my picture is a commercially made box, from Northern Comfort / Sunsiaray, but I think the guy who made them retired in 2018?

I also made one myself, which my brother sort of remade, but that is not as good looking, and probably not as productive, though I haven’t been able to test that carefully. I used two old window sashes that I got for free as the front glazing, which has the benefit of being able to easily withstand the heat generated inside the box on a sunny day, though putting a sheet of Lexan over those window sashes would trap more heat inside the box, and thereby make it more productive. If you do a really good job of building one of these boxes and then put an acrylic or polycarbonate sheet on the front of it, instead of a glass sheet (window sashes, screen door, etc), you run the risk of that potentially expensive sheet melting. I was able to measure temperatures of over 200 degrees Fahrenheit coming out of my homemade STHAB on a sunny day when no fan was running, and how long would polycarbonate or acrylic stand up to that?

This is really great and durable technology, and I encourage you to give it a try, but remember to make sure you have an airtight seal on the ducts when the sun is not shining on it!

2 thoughts on “Solar Thermal Hot Air Boxes Must Have An Airtight Seal On The Outlet Duct!

  1. Should I leave the comment here or should I follow up with your listserv. I have had the solar hot air box for about 8 years. I live in California so we have lots of sun in the winter. I connect my hot air box to a duct that leads into my living room and I have a sensor that detects the temperature in the panel and then kicks on a fan when it’s above 90 degrees. The bottom line for the panel is that the ambient temperature makes a huge difference. It will increase the temperature in my house to 10 degrees over ambient. For example, if it is 60 outside it will increase my house temperature to 70°. If it is 50 degrees outside it will increase my house temperature 60 degrees. I try not to burn my wood stove on sunny days and the hot air box really makes a huge difference. It works a lot better in the fall and spring transition. It works less well in the depths of winter. I got the design from BuildItSolar.com.

    Liked by 1 person

    1. Interesting! Thanks for sharing your experience. Yes, these STHABs are most useful in the fall and spring.

      Like

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