Leakage Symptoms
1 .
Odor
- Odors in the building are usually brought in
through the outdoor air intakes and do not indicate
leakage from the furnace . Check for down draft
conditions and check for location of the flue exhausts of
other equipment that may be pulled into the outdoor air
intake . A major and obvious furnace rupture can be a
source of odor . In general, small leaks in a furnace will
not be a source of odor because the pressure created
by the supply fan is greater than the pressure inside the
furnace . Therefore, when the supply fan is operating,
leakage will be into the furnace, not out of the furnace
and into the air stream . If the control system is such
that the furnace comes ON and warms up the heat
exchanger before the supply air fan comes ON, and
there is odor when the supply fan first comes ON, this
could be caused by leakage . During the time the furnace
is ON and the supply fan is OFF, the leakage would be
out off the furnace. Then, when the supply fan came
ON, it would blow those products of combustion into the
supply duct .
2 .
Low CO
2
Readings
- Low CO
2
readings that cannot
be corrected can be caused by air leaking into the heat
exchanger and diluting the flue gas. If this is suspected,
take two consecutive CO
2
readings, one with the supply
fan running and one with the supply fan OFF . If the CO
2
increases with the supply fan OFF, it could indicate
leakage . Note that CO
2
samples must be taken from
inside a tube, not just from inside the flue box.
Checking For Leaks
1 . Open up the rear casing panel
while the unit is shut off
and visually inspect the heat exchanger .
2 .
Visually inspect the heat exchanger
while the burner
is operating, looking for light coming through holes . The
burner must be operated for only a few minutes with
the supply fan off. The heat exchanger can become hot
enough to cause severe burns . take care not to touch it .
3 .
Perform consecutive CO
2
tests
with supply fan OFF
and ON . See Item 2 under
4 .
Smoke Bomb Test
- Cover the flue box openings,
open the rear casing panel so the heat exchanger is
accessible, toss a smoke bomb into the heat exchanger
through the rear inspection port, replace the port cover,
and with a bright light look for smoke leaking through
the heat exchanger . Remove the remains of the smoke
bomb and uncover the flue box openings before
attempting to operate the furnace .
Causes of Failures
1 .
Improper Application
- The furnace rating plate
specifies a “Minimum Airflow CFM.” The furnace must
not be operated when airflow is below this minimum cfm.
If the furnace is being used on a variable air volume
system, the control system must be such that the furnace
will not operate when the supply fan cfm has fallen below
this minimum specified cfm. The furnace rating plate also
specifies a “Maximum MBh Input” which must not be
exceeded . See
.
2 .
Control Failure
- The limit control does not function
properly and shuts off the burner when the heat
exchanger temperature becomes excessive . In most
situations, a properly controlled unit will never require the
limit control to shut off the unit. The limit control should
be a backup control; a problem attributed to a limit failure
generally indicates a control problem in addition to the
limit failure .
3 .
Excessive Condensation
- Applications which will
produce condensation require an all stainless steel
heat exchanger to be resistant to the effects of this
condensation and to give longer heat exchanger life . The
likelihood of condensation increases with:
a . Colder supply air temperature across the
secondary tubes, as on units taking in a lot of
outdoor air in colder weather .
b .
Lower heat flow through the secondary tubes, as
on modulating burners when operating at reduced
input .
c .
High airflow across the secondary tubes such
as any application with a low temperature rise
furnace .
4 .
Chemical Deterioration
- Refrigerant leaks, some
aerosol can propellants, fumes from dry cleaning
establishments, beauty shops, swimming pools,
and others, often have detrimental effects on heat
exchangers when they get into the combustion air supply
and thereby into the combustion . Even fumes from
nearby roof exhaust fans can cause problems .
5 .
Inadequate or Distorted Airflow
- Internal baffles
that have been repositioned or that have loosened up
and moved can distort the airflow and cause failures.
Construction rubbish, shipping cartons, and insulation
that has come loose can end up inside a unit and block
airflow to part of the furnace, resulting in a failure. Or
these items can alter the air or heat flow to the fan limit
or some other control and contribute to a failure .
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