We have a gas furnace that vent through chimney. Last winter on cold days we can see "white cloud" (water vapor) coming out of the chimney when the furnace is on.
But this morning, when the furnace is on, I can't see any thing coming out of the chimney. The outside temperature this morning was around freezing point and I did see "white cloud" coming out of some neighbour's chimney (which were built in the same year with similar furnace).
1) Should I be worried? Or the forming of "white cloud" varies a lot from furnace to furnace. 2) If there is a problem with the chimney that causes the furnace not vented probably, is it dangerous? The furnace is 14-year-old and it's in the basement. 3) Just occurred to me that we recently installed a radon mitigation system. The system sucks air beneath the basement floor. This reduces the air pressure in the basement. Could this be the cause, i.e., the exhaust from the furnace actually come back to the basement because of the low pressure.