Jack was a customer of mine years ago and a respected friend still. We are both fitters. You will always find him with a piece of soapstone in his pocket. He knows whereof he speaks. Other than his choice of tractor color, I agree with about everything he has said here. VF htrs are equipped with oxygen depletion systems (ODS) pilots. The air we breath is 20.9% O2. If you run a VF htr without adequate make-up air the unit can deplete the O2 in the space. Once the O2 level gets into the 18-18.5% range the pilot flame kind stalls & feathers off the t-couple and the unit goes into a hard lock-out. Some complain of moisture problems with VF. I will say that they are running them incorrectly or they are oversized. They are intended as supplemental heaters. Jack gave you the numbers on moisture inclusion in combustion by-products. I lived in MA when Jack was my customer and all of my wooden chairs would fall apart in the winter months because cold air cannot hold moisture. A little vent free heater improved the climate in the house by adding a little moisture. Whatever is airborne, paint/finishing products, cleaning products, perfume, po pourri (sp) will go through that heater and whatever it smelled like going in I guarantee it will not come out smelling better. Paint a room with a VF htr in it and it will knock your socks off. Keep an IR unit clean and it will not give you problems. Clean them with compressed air and pay attention to the pilot assembly. Dust and crap can...will over time, build up on the inlet to the pilot. A can of compressed air will do the trick. I prefer infra-red units over blue flame as the flame is hotter down in the meniscus of the plaque. I would not have a vent free fireplace or log set, period. I won't pick up an odor from a BF or IR unit but the log-set and fireplace people sell "big flame" and as a result they are in the 30-40 kbtu range. There are millions them out there, but I can walk into a house or a hotel lobby (Rutland, VT Holiday Inn for instance) and within 4' inside I can tell there is a VF fireplace running. Jack is also correct that Empire is the best wall hung plaque unit on the market today. I used to sell the Rinnai vent free IR and BF and they were outstanding. The cheap Chinese units hit the market in the early '00's and the large southern LP companies (the biggest VF market in the USA) beat a mad path to cheap. They got what they were looking for and today beg Rinnai to bring back the IR's. The very best IR on the market today is the Rinnai 510 (5-10kbtu) and the 824 (8-24kbtu). They are floor mounts. I occasionally run the old Rinnai Bantam or Glo-Ray IR's I keep close, but my go to is the 824. Modulating gas valve and blower, cool to the touch and quiet. They are not cheap, but you get what you pay for. They are an excellent value.
To the OP, you say you cannot vent a unit in the location of your basement. I would suggest that you look at the Rinnai direct vent wall furnaces. They have vent extension kits available that will allow you to vent up the wall and out. Again, they are not inexpensive, but you get what you pay for. Since '91 when I introduced that product to the New England market I would not be without them in my home, basement, garage and shop. Check them out.
Merry Christmas, Jack and everyone else!
Jack