Here's a weird one. I am having an intermittent CO problem. I have two induced draft furnaces and a natural draft Hot water heater with combined flues.
The FAU's are 100k BTU's each and the Water heater is a State 75 gal with 75k BTU's. They are all located in a basement, (finished), mechanical room.
The problem we are having is an intermittent rise in CO. We cannot reproduce the problem at will. The Water heater has a 4 inch flue with a length of 2 ft before it connects to the common WYE. It has a 10 rise. It is brand new. The FAU's have 3 inch induced outlets increased to 6 inches at a combined tee. Then just 2 ft to the combined WYE. The flue pipe is 7 inches and rises 30 feet to the cap. The combustion air is 2 - 6 inch rounds, half to top and half to bottom of the mechanical room.
All engineering data I have seen shows we are with in spec.
The CO rise only occurs when the flue pipe is cold, (i.e. the FAU's or Water heater haven't been used in awhile).
I have done everything I can think of. I have raised the flue to even with the ridge of the roof and installed a high wind cap. I have done extensive CO reading around the whole house. I don't want to switch to an induced draft water heater because I have no way to run a new flue and combustion air pipe. I am thinking it's a negative pressure problem but I need some more help before I foot the bill to have a house envelope and smoke test done.
Any advise, my customer is running out of patience.