McQuay SuperMod IM-685-1, Installation And Maintenance Manual

14 

IM 685-1

Air and Gas Adjustments

The burner has been adjusted and tested at the factory with
accurate instruments. There should not be a need to readjust
the burner after the unit has been installed.

Verify that the gas supply pressure is correct, the electrical
power is correct, and test the burner thoroughly. Do no make
adjustments unless there is a clear indication that there is a
problem, and proper instruments are available so the adjust-
ments can be made correctly.

Gas Supply Pressure

The maximum pressure rating of the combination gas con-
trol(s) used on this burner is 0.50 psi (13.9 in. w.c.), as mea-
sured at (2), Figure 16a. If the gas supply pressure is higher
than this an additional regulator must be installed so the
pressure will not exceed 0.50 psi. 

Many gas burner problems are due to gas supply pressure
problems. High or low gas pressures can cause nuisance
lockouts of the flame safeguard and combustion problems.
Low gas pressure will reduce the heat output of the furnace,
and if extreme, can cause combustion problems and flame
safeguard lockouts. Every gas supply system has a high pres-
sure regulator somewhere upstream. Perhaps it is at the
meter and adjusting the outlet pressure is not an option, the
following discussion on the "High Pressure Regulator"
would still apply.

High Pressure Regulator

If a high pressure regulator is included as part of the burner
gas train or is included elsewhere in the gas supply line, it
should be adjusted so the pressure at the inlet tap to the com-
bination gas valves is 7.0 in. w.c. The inlet tap is (2) on Fig-
ure 16a. Check that the pressure is relatively consistent as
the firing rate changes. If any other equipment is served by
that same gas line or pressure regulator, check that the gas
pressure also remains relatively consistent when that equip-
ment is turned on and off. Verify that the regulator closes off
tightly at zero flow by observing that the pressure does not
creep up when the unit is off. If it does, excessive pressure
will have built up over the off period, possibly exceeding the
pressure rating of the combination gas controls, and causing
other problems at light off.

Gas Adjustments

See the sections on "Gas Valve Pressure Regulator Adjust-
ment", "Gas Supply Pressure", and "High Pressure Regula-
tor". The gas flow rate is determined by the gas pressure and
a characterized element within the modulating gas valve.
The stem of the valve connects to the bracket that positions it
with lock nuts that are adjusted at the factory and determine
the minimum firing rate of the burner. Other than gas pres-
sure adjustments, this is the only adjustable control of the
gas. Adjusting the minimum rate is not intended to be a rou-
tine field adjustment. Properly adjusting the minimum rate
requires clocking a gas meter at very low flow rates, or con-
necting a test flow meter into the gas train.

Air Adjustments

Airflow and the resultant combustion characteristics have
been preset and tested at the factory and no further adjust-
ments should be required. Airflow to the burner is deter-
mined by the characterized plate on the air valve outlet (1)
and an adjustable plate (2) on Figure l0a. The adjustable
plate can increase or decrease airflow across the entire stroke
of the valve. If burner airflow is in question, measure the
static pressure at Ports (4) and (5), Figure 16, and compare
those readings with Columns 6 and 8 in Table 5. A signifi-
cant difference should be checked out.

Air and Gas Control Linkage

An L-shaped control rod is connected to the actuator and
passes through the burner housing and into the control box.
This control rod positions the valves that control the burner
air and gas, and actuates switches in the control box to prove
when it is at the maximum and minimum position. When the
actuator positions the control rod to the minimum rate posi-
tion, the bracket on that rod that connects to the air valve and
gas valve should be firmly bottomed against the end of the
gas valve which acts as its stop. The linkages to the air and
gas valve should be straight and in alignment. Although the
bracket is to bottom out, the plate connected to it which
slides from right to left to control airflow should slide freely
and not be forced against either the right or left side member
of the air box.

At the minimum rate position maintain a gap according to
dimension "D", Figure 10a. With that sliding plate in this
minimum rate position, check dimension "E", Figure 10a. To
gauge opening "E", use a drill blank held perpendicular to
the plate. For Models 050-200 opening "E" can be accessed
through the grommeted opening in the primary air collector.
See (3), Figure 10a. for Models 020-040 the primary air col-
lector must be removed. Generally, it is easier to remove the
collector with the inch diameter tube still attached by discon-
necting the tube at the other end.

At the maximum rate position the sliding blade should be
full open, but it should not be forced against the left side
piece of the air box.

The control rod bracket that connects the air and gas valves
must be correctly positioned on the control rod such that the
bracket will be firmly held under tension against its stop
when the actuator is in the minimum rate position, and so the
sliding blade will open fully without making contact at the
end of the stroke. When modulating towards the minimum
rate position the actuator will continue to travel after the
bracket contacts the stop, flexing the vertical end of the con-
trol rod so the bracket is held under tension. To correctly
locate the bracket on the rod, first adjust the air damper link-
age so there will be a gap per dimension "D", Figure 10a,
when the bracket is bottomed out against the end of the
valve. Second, loosen both set screws on the bracket assem-
bly so the bracket is free to slide on the rod. Third, position
the actuator to the maximum rate position. Position the slid-
ing blade to the wide open position.