mysterious floor draft

craigwc

I have a new, forced air, two stage Lennox furnace. My two story home is fully insulated, but very old, with 10 foot ceilings. Two cold air returns on first floor. No cold air returns on second floor. Stairway between first and second floor presumably acts as a cold air return. Furnace in basement.

My problem: when the furnace is on, and even when it’s off to a lesser degree, there is a very cold draft at floor level throughout the entire first floor. Room temperature 1 ft. off the floor is consistently about 7 deg. colder than at shoulder level. There are some small drafts around windows and doors (expected in a 100 year old house, even with weather stripping, outlet covers, etc.) but nothing to account for the degree of cold air movement across the floor. I’ve read that it’s probably caused by cold air “falling” down the walls and moving across the floor toward the cold air returns – although this doesn’t explain the air movement when the furnace is off. The house has no exterior sheathing or wrap, so the walls are quite cold.

I’ve tried many things: leaving the furnace fan on all the time to circulate the air continuously just makes the draft worse; leaving ceiling fans on to draw warm air down just causes more cold air circulation; heating the basement to reduce the chimney effect did not help, nor did insulating the basement ceiling. Years ago the problem wasn’t nearly as bad. At that time, there were NO cold air returns on the first floor. All return air to the basement furnace came by way of a hole at the top of the furnace. I’ve heard this is a bad idea for various reason, but since I didn’t have the draft problem back then, I’m considering it again. Are there other things I could try first?