I pull up to a house for a clean and check. First thing I spot is a tarp over the chimney.
I go downstairs hoping NOT to find a natural draft furnace. $#%?!
The furnace and water heater are both vented into the chimney. Roofer put the tarp on to determine if the roof or the chimney had been leaking thru her ceiling.
I also find flue from furnace completely rusted thru. W/H back drafting. Notice the scorch marks.
HX has crack. Burners covered in rust flakes. I shut it down and told HO to call the roofing co. and give them an ear full and have them come back and remove the tarp.
She had been using the furnace. Her CO detector never went off. I didn't bother to take readings since I was condemning this whole setup.
Oh and yes Mrs HO. You're chimney has been leaking. Back into the flue since there is no chimney cap.