Improper Exhaust – Don’t Breathe This!

Can you tell what’s wrong with the exhaust in the picture above?  I put a nice red arrow to show the problem area so it should be pretty obvious.  The unfortunate truth is that this hazardous setup can appear perfectly normal to the untrained eye, but can be deadly if left uncorrected.

The picture shows a water heater and furnace venting into the same flue.  This is acceptable if the vents are arranged properly, with the lower output unit’s exhaust on top, but the appliances in the photo are installed with the vents upside down.  The problem with having the higher output unit vent on top of the lower output unit is that when both appliances run, the higher BTU furnace can take up all of the chimney flue’s flow capacity and choke out the lower output exhaust.  If the entire flow capacity is spoken for when the lower unit tries to vent, it can backdraft and allow hazardous gasses, such as carbon monoxide, to remain in the home.

When the exhaust vents are properly installed with the lower output unit venting on top, both units can exhaust simultaneously without a problem.  The high output from the furnace will simply force the lower output from the water heater up and out.  If improper venting is discovered in your home it should be corrected by a qualified HVAC technician immediately.