This is probably a dumb question, but given the gravity, I thought I had better ask it anyways. We just bought a nice but unmaintained home. My first experience as owner. It has an oil furnace that has been converted to natural gas which is also new to me. I was down looking at earlier when I noticed the vent pipe from the furnace to the chimney isn't a straight shot, there is a 'T' intersection in it. The bottom of the 'T' comes from the furnace - the left horizontal goes to the chimney but the right horizontal extends about a foot into the walking passage and then is simply capped. The weird thing is the cap is has an adjustable door on it and a notice on it not to touch it. Not only was the door open but the cap was not fully attached. This seems very unsafe, yet maybe the open cap performs some function critical to the operation of the furnace I don't understand. Any insight greatly appreciated. I will be calling the gas company tomorrow.