What’s the Big Deal About a Furnace Heat Exchanger Crack?

That depends on the furnace, and the crack.

upflow furnace with a-coil and water heater

Fig 1

If you have a standard upflow or downflow furnace with an a-coil on the discharge end of the furnace, then cracks don’t mean much.  If your system looks similar to this one, you’ve got a “standard” installation.  This is an 80% AFUE gas-fired furnace with a 35,000 btuh gas-fired water heater.  Both appliances are common vented through the roof, above the ceiling.

A small furnace heat exchanger crack in this scenario won’t make much difference in operation, and won’t allow carbon monoxide to enter the indoor air stream.


Hairline crack in furnace heat exchanger

Fig 2

It’s the size of the crack, and the amount of blower air it lets into the heat exchanger that determines whether there will be an operating problem.  Hairline cracks, like the ones shown here, have no effect on operation.  The “crack” is on a seam, and won’t let air get inside the heat exchanger.  It’s a cosmetic defect, but plenty of technicians will use it as a reason to “red tag” your furnace and force you to buy a new one.

Small hairline crack at crimp of clam-shell on furnace heat exchanger

Fig 3


This crack may cause a flame disturbance when the blower starts, but might also be a benign crack that doesn’t allow blower air to push inside the heat exchanger.




furnace heat exchanger drawing

Fig 4

Keep in mind that the blower in a furnace pushes a high volume of air, at high pressure, across the outside of the heat exchanger.  So, any crack in the heat exchanger would allow blower air enter the exchanger and upset or blow the burner flame around.

There is never a case, with a standard furnace installation with an a-coil on top, where combustion products or carbon monoxide can move from the inside of the heat exchanger to the blower side (indoor air stream.)  It is physically impossible since there is considerably more static pressure on the outside of the heat exchanger than there is on the inside.

Technicians that tell customers that a heat exchanger crack can cause carbon monoxide poisoning are wrong.  They don’t understand how a residential gas furnace operates and are repeating what they heard someone else say!!



furnace heat exchanger pressure drawing

Fig 5

The blower in a residential gas furnace creates between .8 and 1.5 inches of water column of static pressure inside the furnace cabinet.  This pressure is uniformly pushing against the furnace cabinet and the outside of the furnace heat exchanger.  If there is any kind of hole, crack or split in the heat exchanger, blower air will be forced INTO the opening.  Under no circumstances will combustion gasses pass from the inside of the heat exchanger to the indoor blower air stream.  It will not happen.


This phenomenon only applies to residential gas furnace with air-conditioning coils on the discharge.  

It does NOT apply to:

  • Duct furnaces
  • Roof-top package units
  • Gas-fired unit heaters
  • Open-throated forced air furnaces used to heat warehouses
  • Any other configuration of gas-fired heater.  

All of these configurations fail to create sufficient static pressure around the heat exchanger to prevent combustion products from being sucked out of cracks and holes and into the indoor air stream.


Myth-1 – Heat Exchanger Cracks