Fulton Hopkins (HPN) Horizontal Coil Thermal Fluid (hot oil) Heater - Manual (Page 70)

© The Fulton Companies
MAINTENANCE
HPN-IOM-2014-0416
SECTION 4
4-10
burner interlock is the electrical circuit that enables the fl ame
programmer. Your thermal heater needs to have the items in
the burner interlock ‘made’ before ignition can occur. Items in
the burner interlock may include an air switch, air fi lter switch,
auxiliary blower motor starter contacts, high temperature
limit(s), high gas pressure switch, low gas pressure switch, and
/ or low oil pressure switch.
 AIR SWITCH
All C-model heaters have an Air Switch. This is a normally open
switch that closes with proper burner fan outlet pressure. This
switch is generally a diaphragm type switch.
The Air Switch is a safety device that proves that there is an
adequate pressure and volume of make up air for proper
combustion and mixing. There is no manual reset on the air
switch itself to indicate a trip. The most likely time of an air
switch trip is at low fi re purge or low fi re. If this switch trips, it is
generally one of the issues indicated in Table 10-G.
 AIR FILTER SWITCH
C-model heaters with an air fi lter box or ducted supply air have
an Air Filter Switch. This is a normally closed switch that opens
on too high of a suction pressure at the burner fan inlet. This
switch is generally a diaphragm type switch.
The Air Filter Switch is a safety device that proves that there is
not too negative of a pressure at the combustion blower inlet.
This switch is only used on units that have a built in air box for
use as a duct connection or air fi ltering device.
There is no manual reset on the air fi lter switch itself to indicate
a trip. The most likely time of an air fi lter switch trip is at high
fi re purge or high fi re. If this switch trips, it is generally one of
the issues indicated in Table 10-H.
 AUX. BLOWER MOTOR STARTER
All C-model heaters use an auxiliary set of contacts on their
blower motor starter to prove that the burner motor is latched
on. This is a normally open set of contacts mounted on or built
in to the blower motor starter. The Auxiliary Blower Motor
Contacts are a safety device that proves that the blower motor
starter is latched in. These contacts work in redundancy to the
air switch to prove that there is proper makeup air. There is no
manual reset on the auxiliary contacts themselves to indicate a
trip. If the contacts do not make, it is generally one of the issues
indicated in Table 10-I.
 HIGH TEMPERATURE LIMIT
All thermal fl uid heaters have at least one High Temperature
Limit. The high temperature limit(s) is/are normally closed
switch(es) that break on a temperature rise over set point.
The switch may be either a solid state controller or a bulb and
capillary type switch. The High Temperature Limit is a safety
device that protects the thermal fl uid and heat transfer coil
from excessively high temperatures.
Solid-state high temperature limits will have a manual reset.
Bulb and capillary type limits will not have a manual reset.
If this/these switch(es) trips, it is generally one of the issues
indicated in Table 10-J.
 HIGH GAS PRESSURE SWITCH
All gas-fi red modulating or NFPA rated thermal fl uid heaters
have a High Gas Pressure Switch. This is a normally closed
diaphragm switch that opens on a pressure increase over set
point. The High Gas Pressure Switch is a safety device that
protects the burner from receiving too high of a gas pressure.
The switch senses this pressure downstream of the last
gas valve, upstream of the modulation valve on units that
modulate.
This switch is most likely to trip at low fi re. If this switch trips, it is
generally one of the issues indicated in Table 10-K.
 LOW GAS PRESSURE SWITCH
All gas-fi red modulating or NFPA rated thermal fl uid heaters
have a Low Gas Pressure Switch. This is a normally closed
diaphragm switch that opens on a pressure decrease below set
point.
The Low Gas Pressure Switch is a safety device that protects
the burner from receiving too low of a gas pressure. The switch
senses this pressure just downstream of the gas regulator. This
switch will most likely trip at high fi re. If this switch trips, it is
generally one of the issues indicated in Table 10-L.
 LOW OIL PRESSURE SWITCH
All oil fi red modulating thermal fl uid heaters have a Low Oil
Pressure Switch. This is a normally closed diaphragm switch
that opens on a pressure decrease below set point. The Low
Oil Pressure Switch is a safety device that protects the burner
from receiving too low of an oil pressure. The switch senses this
pressure just downstream of the gas pump. If this switch trips, it
is generally one of the issues indicated in Table 10-M.
 PILOT FLAME FAILURE
A Pilot Flame Failure is a fl ame failure that occurs when the
unit is trying to establish an adequate fl ame signal. Solid-state