Fulton Alliance (FT-HC) Horizontal Coil Thermal Fluid (hot oil) Heater - Manual (Page 74)

© The Fulton Companies 2012
MAINTENANCE
FTHC-IOM-2012-1001
SECTION 4
4-10
 CALL FOR HEAT/BURNER INTERLOCK
The call for heat circuit is the circuit that enables burner
operation. Fulton Thermal Corporation has used a variety of
Temperature Controllers to act as the Call for Heat. Generally
these controls work in combination with a control relay. When
the Temperature Controller calls for heat, a signal is sent to
the coil of a control relay that closes a normally open set of
contacts in series with the burner circuit.
When the call for heat is met, the signal is removed and
the contacts return to their open state. Situations that may
interfere with the Call for Heat circuit are in Table 10-F. The
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 TEMP. 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.