Specifications

20 IM 685-2
Combination Fan And Limit Control
The fan limit control is a hydraulic action type with a remote
sensing element and connecting capillary tube. The sensing
element is locked into a bracket located on one of the heat
exchanger tubes about halfway toward the back of the fur-
nace, on the side away from the blower. One corner of the
bracket is bent aside to remove the element.
Normal setting of the FAN control: Fan On=125°F, Fan
Off=100°F.
The LIMIT control must never be set higher than the temper-
ature listed below. If the burner is shutting off on high limit at
these settings, it indicates that there is a problem with the fur-
nace not getting enough air or it is being overfired.
Table 4. LIMIT control set points
Maintenance
Preventative maintenance is the best way to avoid unneces-
sary expense and inconvenience. The system should be
inspected at regular intervals by a qualified service techni-
cian. The service intervals below are typical for average situ-
ations but will have to be adjusted to suit the particular
circumstances.
Fuel pressure settings, control settings, and linkage adjust-
ments should only be made by persons thoroughly experi-
enced with the burner and control system and should not be
tampered with by persons without such experience.
Always replace covers on burner controls and boxes as the
electrical contacts are sensitive to dust and dirt. Maintenance
of flame safeguard, controls, gas valves, and other such com-
ponents should be performed in accordance with instructions
contained in the manufacturer's bulletins.
Monthly
1. Check air filters and main supply fan drives, replacing if
required.
2. Check flame signal with a keyboard display module or a
DC voltmeter.
Twice Yearly
1. Burner Air - Check burner fan wheel for dirt build-up
and lint. Check combustion air intake louver and flue box
for dirt buildup and accumulation of wind borne debris.
2. Cleaning - Inspect flue tubes and combustion chamber,
cleaning as required. Keep burner vestibule clean. Dirt
and debris can result in burner air blockages.
Yearly
1. Gas Train - Check all valves, piping, and connections for
leakage. Remove burner gun assembly. Inspect, and if
required, clean the flame rod, ignition electrode, main
burner disc, and blast tube. Check tightness of linkage fas-
teners and bolts that could work loose from vibration and
movement.
2. Combustion - Check quality of combustion. Test CO
2
and
CO and look for irregularities in fire shape. If combustion
characteristics have changed since the last test, determine
the cause. Changes in input, changes in the BTU content
of gas being supplied, reduced combustion air due to dirty
blower wheel, or flue passages in need of cleaning can all
cause changes in CO
2
reading. When a readjustment
seems necessary, do not make the adjustment without first
trying to determine if the required change is not an indica-
tion that something else is in need of correction.
3. Flame Safeguard - Perform a flame failure check and
“pilot” turndown test. See control manufacturer's bulletin
for further information.
4. Motor - Motor life will be increased by proper oiling.
There are provisions in both end shields for relubrication.
Re-oil each bearing with 150 drops (approximately 1 tea-
spoon) SAE-20 oil.
5. If the burner is to be out of service for the summer, turn
off the burner control switch and close the manual gas
cocks.
Flame Safeguard
See manufacturer's bulletin for more detailed information or
for information on flame safeguard other than the RM7897A.
The Honeywell RM17897A is a microprocessor based inte-
grated burner control that will do self-diagnostics, trouble-
shooting, and status indication, as well as the burner
sequencing and flame supervision.
Keyboard Display Module
The Honeywell S7800A1001 module is an optional device
available for use with the RM7897A. It can be a permanent
accessory added to the RM7897A or it can be carried by the
service technician as a tool that is very easy to mount when
servicing the RM7897A. It mounts directly onto the
RM7897A and has a 2 row by 20 column display. The mod-
ule will indicate flame signal dc volts, sequence status,
sequence time, hold status, lockout/alarm status, total hours
of operation, total cycles of operation, and can provide 127
different diagnostic messages for troubleshooting the system.
The module will give a fault history. It can be mounted to the
RM7897A and will retrieve information on the six most
recent faults.
Consult the Honeywell bulletin 65-0090-1 “7800 Series,
Keyboard Display Module” and 65-0118-1 “7800 Series,
System Annunciation, Diagnostics and Troubleshooting.”
BURNER MODEL LIMIT CONTROL
SET POINT
BURNER MODEL LIMIT CONTROL
SET POINT
020 215 079 181
025 160 080 229
032 196 100 170
040 154 110 222
050 229 140 168
064 185 150 194
065 232 200 151