Cleaning Heat Exchanger Assembly; Unit Checkout; Temperature Rise; To Measure Temperature Rise - Lennox G21Q3-40 Unit Information

G − Cleaning Heat Exchanger Assembly
IMPORTANT
Use papers or protective covering in front of fur
nace while removing heat exchanger assembly.
WARNING
If unit has been operating, internal components will
be HOT. Allow unit to cool for at least 15 minutes be
fore placing hands into the heat section opening. To
completely cool to room temperature, the blower
should run continuously for about 40 minutes.
1− Turn off both electrical and gas power supplies to fur
nace.
2− Remove upper and lower furnace access panels.
3− Remove air decoupler box cover.
4− Remove insulation pieces from lower section of air de
coupler box.
5− Unscrew air valve housing, using your hand. Use ei
ther a strap or basin wrench if necessary.
6− Disconnect wiring to purge blower.
7− Remove nut from PVC air inlet fitting.
8− Remove nuts from air decoupler box mounting bolts
and gas decoupler bracket.
9− Remove air decoupler box from unit.
10− Remove rubber gaskets and pad from air pipe.
11− Detach PVC exhaust pipe from coil manifold outlet.
12− Disconnect gas to unit.
13− Disconnect wiring to gas valve.
14− Break union in gas line just below gas decoupler. Re
move gas valve / gas decoupler / piping assembly.
15− Remove remaining gas piping from fitting at vestibule
panel.
IMPORTANT
Hex head fitting contains gas diaphragm valve.
Care must be taken when handling this portion of
piping assembly.
16− Disconnect blower motor wires from control box.
17−Disconnect spark plug and sensor wires from plugs in
combustion chamber. (Access plate is provided.)
18− Remove vest panel.
19− From underside of blower deck, remove four nuts hold
ing rubber heat train mounts.
20− Lift heat train from unit.
21− Backflush heat train with a soapy water solution or
steam clean.
IMPORTANT
If unit is backflushed with water, make sure all wa
ter is drained from heat train before replacing. Use
papers or protective covering in front of furnace.
22− Reverse above steps to replace heat exchanger as
sembly. Be sure rubber seal pad and gasket are in
place on air pipe and that ground wire on gas valve is
put back on. Air flapper valve housing should be hand
tightened only.
V − UNIT CHECKOUT
A − Temperature Rise
Temperature rise for G21/GSR21 units depends on unit
output, blower speed and static pressure of unit setup.
Blower speed must be set for unit operation within range of
AIR TEMP. RISE ° F" listed on unit rating plate.

To Measure Temperature Rise:

1 − Place plenum thermometers in supply and return air
plenums. Locate thermometers in the first horizontal
run of the warm air plenum where it will not pick up ra
diant heat from the heat exchanger.
2 − Set thermostat to highest setting.
3 − After plenum thermometers have reached their high
est and steadiest readings, subtract the two readings.
The difference should be in the range listed on the unit
rating plate. First check the firing rate. If temperature is
too high, provided the firing rate is acceptable, in
crease blower speed to reduce temperature. If the tem
perature is too low, decrease blower speed. To change
blower speed taps see the Blower Speed Taps section
in this manual.
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