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SERVICE & TROUBLESHOOTING
Blocked Vent Safety Switch
A manual reset blocked vent safety switch is supplied on all
Model Digit 3=G units and is designed to prevent operation of
the main burner in the event there is spillage of flue products
into the space. This spillage may occur due to a restricted vent,
inadequate vent draw, uninsulated vent pipe in cold ambient,
long vent runs, excessive vent diameter, restrictive vent terminal,
negative pressure within space, etc. After the cause of the
spillage has been corrected, depressing the button of the blocked
vent safety switch found on top of the unit may reset the switch.
See Figure 22.2 for additional troubleshooting information.
Figure 22.2 - Blocked Vent Safety Switch Troubleshooting Flow Chart (Model Digit 3=G only)
Is there a negative pressure in space?
YES
Can negative pressure
be corrected?
YES
Correct
Install aftermarket
negative
power venter
pressure
To determine spillage, place lit match stick
:
(6" - 8" match stick if possible) 2" - 3" into
diverter relief opening and determine direction
of the flame (or direction of smoke if flame goes
out). If flame or smoke comes back at you, there
is spillage. The flame or smoke should be pulled in.
One indication of negative pressure is that outside
:
doors tend to be pulled toward inside of space.
Aftermarket power venter systems are readily
:
available for these applications. While Modine
has not tested and therefore does not endorse
any specific system, example manufacturers
include Tjernlund and Field Controls. It is the
responsibility of the installer/specifier to properly
size and select the system to be used.
22
Is vent blocked or restricted?
YES
Remove restriction
NO
Is vent in compliance with
National Fuel Gas Code (NFPA 54)
Category I requirements?
NO
YES
system
Figure 22.1 - Blocked Vent Safety Switch Location
Is there spillage after 5 minutes?
YES
NO
Correct vent
Is unit overfired?
YES
Can one or more of the following be done?
Reduce input
• Lengthen vertical vent run
• Insulate vent
• Use larger diameter vent
• Use less restrictive weather cap
Does unit still trip?
YES
Install aftermarket
power venter
system
5-564.4
NO
Replace blocked vent switch.
Does switch still trip?
YES
NO
YES
NO
Install aftermarket
power venter
system
NO
OK
BLOCKED VENT
SAFETY SWITCH
NO
NO
OK
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