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Manifold and Signal Pressure Measurement
To correctly measure manifold and signal pressure, follow
the steps below:
1 - Turn off the electrical power and gas supply to the fur
nace.
2 - Remove the threaded plug from the outlet side of the
gas valve and install a field-provided barbed fitting.
Connect measuring device positive “+” to barbed fit
ting to measure manifold pressure. See figure 44 for
manifold location.
3 - Install hoses and meter as shown in figure 48 for signal
pressure measurement.
4 - Start unit on low fire and after allowing unit to stabilize
for
8 minutes
, record manifold pressure and compare
to value in table 27. Repeat on high fire. If manifold
pressure is within range, rate check is complete move
to step 6. If manifold pressure is not within range con
tinue.
Valve is not adjustable. Do not adjust manifold
pressure.
5 - Record signal pressure and compare to value in table
27. If signal pressure is within range continue. If the
signal is not within range go to “Troubleshooting”.
6 - Shut off unit and remove manometer and signal meter
after accurate readings has been obtained.
7 - Restart unit and check for gas leaks. Seal any leaks
found.
If the unit has difficulty igniting or ignites with loud res
onance the air orifice must be checked and replaced if
necessary.
TABLE 27
Unit
Manifold Nat
Manifold LP*
Signal
High
Fire
Low
Fire
High
Fire
Low
Fire
High
Fire
Low
Fire
040
3.2 - 3.8
1.4 - 2.0
N/a
N/a
0.80 -
0.93
0.32 -
0.42
060
3.6
1.7
0.69 -
0.93
0.28 -
0.45
080
0.73 -
0.93
0.30 -
0.40
*NOTE -
-060 uses LP Kit 19K05 and -080 uses LP Kit 19k06
Air Orifice Replacement
Do not replace the air orifice until the problem has been de
termined. If the unit has difficulty igniting the orifice is over
sized and brings in too much air. If the unit ignites but with
loud resonance the orifice is too small and needs more air.
The air orifice is located inside the gray coupling between
the clamps. Figure 47 show corresponding steps with the
steps below.
1 - Turn off the electrical power and gas supply to the fur
nace.
2 - Remove the black air pressure tube on the air intake
coupling.
3 - Use a 5/16” nut driver to loosen the clamps on the gray
coupling.
4 - Remove the two screws attaching the air intake cou
pling to the furnace cabinet.
5 -
Remove the air intake coupling. If the air intake cou
pling is still too difficult to remove, then remove the two
screws from the right side of the top cap and one from
the far right side of the blower deck that attaches to the
cabinet. See figure 46. Carefully pull cabinet side away
from air intake coupling, then remove the air intake
coupling.
Note: Be careful not to bend the cabinet
side
.
6 - Remove the air orifice. Check the “Part” number
stamped on the air orifice. See table 28. If the part num
ber is incorrect, then replace it with the proper air ori
fice. Repeat manifold check. If air orifice is correct di
ameter, then it must be replaced to resolve ignition or
resonance issue. See table 28 for replacement.
7 -
Reinstall the air orifice on the left side of the coupling
and push firmly into place.
8 -
Reinstall air intake coupling making sure it is fully seat
ed in the gray coupling. Re-install the two screws at
taching the air intake coupling to the furnace cabinet.
Tighten clamps to secure the coupling. Re-install the
two screws on the right side of the top cap and the one
screw that attaches the blower deck to the cabinet.
9 -
Reconnect the black air pressure tube.
10 -Repeat manifold and signal pressure check. If unit ig
nites and manifold and signal pressure are correct,
move on to combustion check. If unit still does not ig
nite or ignites with loud resonance go to Troubleshoot
ing figure 49.
