What in the world of ductwork?

So I just finished up an inspection today. Everything on the home was pretty straightforward until I got to the attic. Take a look at these photos and let me know If you have any idea what their purpose might have been.
At first glance I thought there was just a air handler up there but that’s not it. Here’s what I know about them.
There is a roughly 18" x 18" plenum making a giant U shape in the attic. It is closed on both ends, runs the full length of the peak then loops back and runs the full length of the eave. It does not connect to any HVAC system or pass through the ceiling into the house. It does not connect to any type of fan or any attic ventilation. It is not connected to the gable vents, ridge or soffit vents Every 24" there is 6" round duct adaptor pointed towards the back of the roof deck, one for the high duct and one for the low one. (Ducts are only on the south side of the roof) The plenum is made of insulated duct board with fiberglass wrapped around that. There is clearly parts of the “system” missing. There used to be insulation stapled to the top chord of the trusses because I can still see the staples and paper facing. At first I thought it was set up to cool that side of the roof but there is no outlet for the hot air to leave the upper plenum. It appears to be reasonably well constructed of professional materials.
The more I look at it the more questions I have. It is clear cut for me what to write up and it is not doing the house any harm just sitting up there. I just would like to satisfy my curiosity. So if any of you on here seen anything like this before help me out??

Maybe at some point it was (or was going to be) an ice dam prevention system.

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That is what I was thinking when I saw those pictures, Stephen.

Arthur, describe what you see and recommend that your client ask the homeowner what it is.

The homeowner was there at the end of the inspection he had never been in the Attic before has no idea what it is. I’m good on what the write-ups going to be I’m just curious myself if anybody knows what this might be. Hoping maybe I can learn something new

If you find out, please post it here so we can learn, too…never saw something like that before.

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I definitely will. This may just be a unique situation where someone tried to engineer their own solution that didn’t work… Or maybe they were using that hot air from the back of the roof deck to dry their marijuana plants. They don’t smoke very easily when they’re wet… so I’ve heard.

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It may have been part of an attic heat recovery system. One thing is for sure, it was not cheap to install all of that.

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How old is the house? It may have been the Ducts for an old supplemental solar heating system fro the 70’s or 80’s.

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The house was built in 1977. That’s a good thought it has a nice big roof plane facing the southern sky. Though we are in northeastern Ohio we get maybe 6 days of sunlight yearly. There are a lot of solar systems going in lately dispite that.

How many HVAC Units in the house?
How many floors?
How many thermostats?

1 HVAC Unit
2 Floors
1 Thermostat

Exact thought I had.

You can’t do two floors with one HVAC verywell w/o a zoning system (2 t-stats required).

It’s my wild ass guess that they were going to add another unit, but didn’t get that far.

I have no legitimate contribution, but lately I have seen some crazy “grow” systems in attics a few times. Powerful lights, irrigation and temp control. I wonder…

My guess is they wanted to insulate the attic but didn’t want to block the soffit venting.