Yes, the return originally had an outdoor duct piped into it. I've blocked that off and my furnace heats the house much more effectively now and cut my heating bill tons along with not pulling the humidity to nothing. In the summer, not having a hole in the house directly to my ductwork kept the hundreds of spiders out of my house and the air conditioner ran much less.
Either way, ducting into a plenum is a bad idea. My furnace hasn't run for nearly two months now and likely won't start air conditioning for another month so it wouldn't matter anyway as there will be no fresh air in a system that doesn't need to run in the swing seasons.
If someone is using a ventilation system in Minnesota in the season where the dew point is below 50f, exhaust ventilation via a super efficient bathroom fan designed for continuous ventilation is ideal, of course combined with an appropriately placed passive air inlet with a correct trap. In the summer that's a bad idea as it just introduces tons of humidity, last summer we had higher dew points than what my basement regularly sees, no way I'm pulling that in except through the return plenum. So to me, the scheme would be based on the season if I saw a need to pump unconditioned air into my house but I'm not convinced my air is stale and it seems that there is enough infiltration or I'd still smell the bacon I cooked up this morning after I got home tonight after being gone, but I don't.
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