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4. Primary Limit Control (Figure 12)
Primary limit (S10) used on ML193UH units is located in the
heating vestibule panel. When excess heat is sensed in the
heat exchanger, the limit will open. Once the limit opens,
the furnace control energizes the supply air blower and de-
energizes the gas valve. The limit automatically resets
when unit temperature returns to normal. The switch is fac
tory set and cannot be adjusted. For limit replacement re
move wires from limit terminals, remove mounting screws,
rotate limit switch 90 degrees and slowly remove from the
vestibule panel. Install replacement limit with same care.
FIGURE 12
Primary Limit Location and Heat Exchanger
Install limit face down
5. Heat Exchanger (Figure 12)
ML193UH units use an aluminized steel primary and stain
less steel secondary heat exchanger assembly. Heat is
transferred to the air stream from all surfaces of the heat
exchanger. The shape of the heat exchanger ensures
maximum efficiency.
The combustion air inducer pulls fresh air through the burn
er box. This air is mixed with gas in the burners. The gas /
air mixture is then burned at the entrance of each clam
shell. Combustion gases are then pulled through the pri
mary and secondary heat exchangers and exhausted out
the exhaust vent pipe.
6. Burners (Figure 13)
All units use inshot burners. Burners are factory set and do
not require adjustment. Burners can be removed as an as
sembly for service. Burner maintenance and service is de
tailed in the MAINTENANCE section of this manual. Each
burner uses an orifice which is precisely matched to the
burner input. See table 8 for orifice size. The burner is sup
ported by the orifice and will easily slide off for service. A
flame retention ring in the end of each burner maintains
correct flame length and shape and keeps the flame from
lifting off the burner head.
TABLE 8
Gas Orifice Size
Unit Fuel Orifice Size
All Natural 0.0625
All L.P./Propane 0.0340
FIGURE 13
Burner Detail Top View
FLAME SENSOR
IGNITOR
ORIFICES
7. Gas Valve (GV1)
The ML193UH uses an internally redundant valve to as
sure safety shut‐off. If the gas valve must be replaced, the
same type valve must be used.
24VAC terminals and gas control switch are located on top
of the valve. All terminals on the gas valve are connected to
wires from the ignition control. 24V applied to the terminals
opens the valve.
Inlet and outlet pressure taps are located on the valve. A
manifold adjustment screw is also located on the valve. An
LPG changeover kit is available.
FIGURE 14
Gas Valve
MANIFOLD
PRESSURE
OUTLET
PORT
INLET
PRESSURE
PORT
MANIFOLD
PRESSURE
ADJUSTMENT
SCREW
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