[fintstone]

I had another rental (with an oil furnace) that I was renovating, and the workers doing the demo felt sick. It turned out it was the chimney/flue. The house (built in the 20s) had originally had a coal burner in the basement and a wood stove (cooking) on the main floor. When they converted the furnace to oil in the 70s, they simply replaced the coal burner with an oil furnace in the basement and used the same flue. They patched over the hole where the main floor wood stove entered the same block chimney used by the oil unit (no liner) and installed a small electric stove (boxed in the flue). It was leaking very badly.

I had the flue/chimney torn out and a heat pump with electric strips installed there as well. The oil tank was an eyesore, and the new system was much smaller (inside) so it worked out well. A heat pump also added A/C which was not totally necessary in that climate, but really nice for a couple months each year.