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I have a similar system in my basement. The furnace draws ambiant air from the basement and there are no supply vents in the basement, yet it isn't cold down there.
If your furnace is like mine, there is warm air being blown out the top of the heating unit. You can feel it when it's running.
If you have a relatively new house, the basement will be well insulated. I have a walk-out with windows and glass french doors and it still stays warm, even with the blinds blocking the sunlight.
However, I think you may have a problem if you finish the basement. You will probably end up isolating the furnace from the living space, thus eliminating what little heat is blowing out of the top of the furnace. In that case you will have to tap into the existing ducts with small diffusers to bring heat into the finished space. It's not a big deal, but it will reduce conditioned air pressure upstairs. That's why the basement vents must be either small or adjustable or both. Your furnace is sized for the first floor, not both floors.
If you end up walling off the furnace, make sure that all doors leading to the furnace have vent louvers so you don't starve the furnace of air or create a CO2 problem. A lack of supply air to the furnace and water heater will result in both of them drawing air DOWN the combustion stack, resulting in dangerous levels of CO2. Modern furnaces have an air sensor that will shut the unit off if the exhaust vent isn't drafting.
However, I think your reply clearly answer my concern about isolating furnace who is sucking in air form basement. I'm just hit with a new gas bill which quite fascinating(and its just Novemeber!) and I am trying to lower it down.
Thanks for sharing your thoughts.
PS: What do I need to in order to get a reply from HK )