Mnrenovator, under ideal conditions the references I've seen show a minimum of about 60% conversion efficiency for a flat plate air solar panel. Conditions are never ideal however, especially in Minnesota. To get the actual efficiency you have to downrate that according to the performance parameter:
(Collector inlet temperature - ambient outside temperature)/insolation
There isn't a lot of insolation in MN and the outside temperature is usually pretty low also. I think insolation tables for your area in BTUs per hour can probably be found on the internet. If you can locate the collector inlet vent (inside the house in a closed loop system like you are talking about) so its drawing air at a time and a place when its the coldest you can maximize actual efficiency. You still have an uphill battle I think.
For instance, to get maximum efficiency would require one to let the temperature drop in the house to a point you probably wouldn't be comfortable in. Most solar heating does not start kicking in until about noon. Assuming you work and there's no one at home to complain (big "if") then you could probably get a reasonable amount of efficiency out of it on workdays when you're at work.
For most people I don't think that would be reasonable. I don't know how well you're insulated but my preference would be to just insulate as well as can possibly be attained. Then even if you don't maintain maximum efficiency the temperature in the house will have such a small drop from the last heating period that any low actual efficiency will be sufficient to heat the house. I think you will still need a relatively huge flat plate array to get the output you need at your location.
A good older book that will tell you everything you need to know and what you might be up against is "Solar Air Heating Systems" by Kornher and Zaugg
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