17
VT299 to VT399
– Negative Pressure Control Valve
VT499
– Negative Pressure Control Valve
4.4
– Gas Pressure Switches
Gas pressure switches prevent the burner from being activated
if pressure is outside certain ranges. Each switch is a physical
manual reset device, requiring physical depression of the reset
button if it is not closed prior to burner start or during burner
operation.
4.4.1
– Low Gas Pressure
A low gas pressure switch is standard equipment
and monitors
the minimum incoming gas supply pressure supplied to the gas
train. If gas pressure upstream of the valve(s) falls below the
minimum setting of the pressure switch, the switch will open and
the appliance will shut down. An open gas pressure switch alarm
will be shown on the display.
4.4.2
– High Gas Pressure
High gas pressure switches may be ordered to comply with CSD-
1 code. These will either be integrated in to the gas train. If gas
pressure downstream of the gas valve(s) exceeds the maximum
setting of the pressure switch, the switch will open and the
appliance will shut down.
Gas Pressure Switch (Cover Removed)
4.5
– Air Pressure Switches
4.5.1
– Blocked Flue Switch
All models use a normally closed blocked flue switch to shut
down the appliance under the following conditions:
Air intake 50% blocked
Vent outlet 80% blocked
4.5.2
– Low Air Switch
The low air switch is closed by operation of the combustion air
blower, to confirm that there is air flow present. This switch is
adjustable but typically does not need to be altered from factory
settings. Once this switch is closed the unit initiates the pre-purge
counter. If this switch opens during operation the unit will lock-
out.
4.6
– Combustion Air Blower
The VTech use a modulating air fan to provide combustible
air/gas mix to the burner and push the products of combustion
through the heat exchanger and venting system. The fan
assembly consists of a sealed housing and fan wheel
constructed from spark resistant cast aluminum. The fan is
operated by a fully enclosed 120 VAC, Single-Phase EC/DC
electric motor. The fan housing and motor assembly is fully
sealed and SHOULD NOT be field serviced. The power draw of
the motor is proportional to the modulated gas input rate of the
appliance.
VTech Combustion Air Fan
4.7
– Burner
This appliance uses a single cylindrical burner installed vertically
into the combustion chamber at the top of the heat exchanger.
The burner consists of a round mounting flange welded to a
mixing tube. The flange provides the transition from the
combustion air fan into the burner. The mixing tube is covered
with a knitted alloy material that forms the burner port surface.
The knitted burner port material is a metal fiber material which is
a unique alloy of iron, chrome, aluminum and several rare earth
metals. This alloy is designed to operate stress free as a burner
port surface. The burner port surface can sustain operation from
a blue flame down to infrared conditions as the burner input
varies.
VTech Burner
High-fire air gas ratio
adjustment (Use slotted
screwdriver for
adjustment, counter-
clockwise increases CO
2
)
Low-fire air gas ratio
adjustment (use Torx 40
for adjustment clockwise
increases CO
2
)
Low-fire air/gas ratio
adjustment, use slotted
screwdriver for adjustment,
clockwise increases CO
2
Gas Inlet
Lift top cover to access
high fire air/gas ratio
adjustment (use 3mm
Allen key for adjustment,
counter-clockwise
increases CO
2)