You will be fine using your furnace.
IF you have time, take down the basement walls and look for a Furnace air duct laying on the floor. Take it and put it down the plumbing hole and wedged the end of it between the tanks as far back as it will go. Angle it down and aft to blow towards the valves. This will help keep them from freezing.
Also buy 4" Gorilla tape. Go around underneath your rig and close up all the holes (air gaps). This will keep drafts out and help keep you warmer. Clean the surface first, then wipe it with alcohol and wipe dry. This will help the tape stay on better.
Picture of furnace duct in basement.
Picture is upside down, sorry...
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