Checking for Cracked Heat Exchangers
Let us begin by saying there is absolutely no known technique for finding a
crack 100% of the time.
Probably the best is to visually locate a crack. However, particularly in modern
80% and 90% forced air furnaces, a thorough visual inspection is time consuming
and difficult at best.
In the past there have been many suggested techniques. These include:
A smoke bomb test - a smoke generator is placed in the heat exchanger, the
outside of the heat exchanger visually accessed to watch for smoke passing
through the crack.
The wintergreen test - oil of wintergreen is sprayed into the combustion chamber
(with the blower operating), then see if the smell is present at the supply
registers.
The salt test - a salt solution is sprayed into the combustion chamber, a hole
drilled in the supply ducting and a torch held in the air blowing out of the
hole to see if the flame changes color from the salt.
The pressure test - all openings in the heat exchanger are sealed, the blower
energized and a pressure sensor or draft gauge is inserted inside the heat
exchanger to see if a crack is allowing distribution air to blow into the heat
exchanger (through a crack or hole).
The tracer gas test - all openings in the heat exchanger are sealed, a methane
tracer gas is injected into the heat exchanger and a combustible gas leak
detector passed around the outside looking for areas where the tracer gas is
escaping.
The flame distortion test - watch the flame when the blower comes on and watch
for the flame color, shape, etc. to change.
Most of the above mentioned tests make several fatal assumptions:
Leakage is going to be from the flame side to the distribution side of the heat
exchanger. This may be the case if the blower is not on and the vent system is
not drafting, however, when the blower is on, for example, there is very little
chance flue gases, smoke, etc. is going to pass through a crack into the
distribution air. When a blower energizes, there is a tremendous amount of
pressure around the outside of the heat exchanger, therefore, there is a much
greater chance of distribution air being blown into the fire side of the heat
exchanger.
The crack is open when the heat exchanger metal is at (or close to) room
temperature. Some cracks have been known to only open when the unit is fired and
the heat exchanger metal is hot.
The service technician has the time, experience or even reason to suspect the
exchanger is cracked.
To test for cracks using a combustion analyzer, simply watch the O2/CO2 readings
and the CO reading when the blower comes on - usually several minutes after the
burner(s) ignite.
Typically, the O2/CO2 or CO readings will stabilize within 30 to 60 seconds
after ignition. If a crack is present, when the blower energizes, air (at 20.9%
O2) may be blown through the crack in sufficient quantities to raise the O2 (or
decrease the CO2) reading on the combustion analyzer.
Using a combustion analyzer to test for cracks in a heat exchanger also has
limitations, however, there are some distinct advantages:
It tests under actual operating conditions.
It may provide additional information as to how dangerous a crack is. For
example, if a crack is visually observed and a combustion test finds that when
the blower comes on the carbon monoxide reading rises to excessive levels, a
service contractor can be more confident that a dangerous situation exists and
has the documentation that the unit needs to be immediately condemned and taken
out of operation.
It can be easily done during the normal course of a
service call where
combustion testing is performed.
It is important to keep in mind that changes in combustion test readings may
also be caused by other factors:
On oil fired equipment, loose clean out ports in the heat exchanger will most
likely result in O2/CO2 or CO readings changing when the blower comes on.
Depressurization of the mechanical room due to leakage in the return side of the
distribution system may be sufficient to change the readings when the blower is
energized.