Yesterday I purchased a used (2009 model year) USSC 6041HF for my basement that looks really nice yet and the seller said it worked good but was selling it cause they moving. I was reading the tech manual for it and it says DO NOT vent into a chimney. I currently have a, what I call tuned in, USSC 8500 used to heat our 2700sqft house and vent it into a chimney with no problems. I’m familiar with the control panel as the 6400 and the 8500 have the same one so I know the draft fan constants. I have a chimney not being used(my house has two chimneys) and I can get fresh air to intake of 6041 so why should I not vent the 6041 into a chimney like I do my 8500?