Yep, I get the venturi effect, like primary air being pulled into a burner.
Where I'm getting lost is when the system's static pressure is taken into account.
If a furnace blower generates .1" WC static pressure on the supply side and has an a/c coil sitting on top of the furnace, wouldn't the system static pressure kill any venturi effect?
I can see where the velocity on a 3-ton system (for example) might be around 1,000 fpm which would certainly be fast enough to create a venturi effect.
However, with .1" WC pushing on the outside of the heat exchanger, in all directions, how can the venturi effect take place?
Whenever I've encountered holes/cracks in heat exchangers, there's always air blowing into the heat exchanger creating a flame disturbance.
I once had a downflow furnace with a 4-cell heat exchanger where the tops of the cells had almost rusted through. You could see a million small holes all over the top of each cell. There wasn't a CO issue (based on the customer's description, CO wasn't measured) but there was a lot of flame disturbance in every cell. Not to mention a large accumulation of rust in the bottom of the cells.
OK, last question. Why were the burners on the job you discussed generating 400ppm CO, and what was the location of the measurement? I've only seen readings that high when burners were physically misaligned and were directing the flame against the side of the heat exchanger. But, I haven't been checking every job, so I might be missing something,