16
Part 4 – Gas and Combustion
Hardware
Verify that the appliance is supplied with the type of gas specified
on the rating plate.
Heating values of local natural gas are to be
between 950 and 1010 Btu/ft
3
. Consult factory if heating values
are outside this range or if a gas with a mixture of constituents is
being used.
Consult factory for high altitude installations.
4.1
– Gas Piping and Connections
Safe operation of the VTech unit requires that the gas line size
chosen be sufficient to handle the total installed capacity, within
an acceptable range of pressure drop across the piping, and at
the available pressure. Gas pipe size may be larger than
appliance connection, if required for a specific pressure drop, but
it must not be smaller than the water heater connection. Table 3
can be used for gas line sizing data, please verify pipe size
requirements with gas supplier.
Table 3: Recommended Gas Pipe Size for Single Appliance
Input
KBtu/hr
Equivalent Length from NG Meter or LP Regulator
0-100 FT
101-200 FT
201-300 FT
NAT.
L.P.
NAT.
L.P.
NAT.
L.P.
80
¾
”
½”
¾”
½”
1”
¾”
100
¾”
½”
1”
¾“
1”
¾”
125
1”
¾”
1 ¼”
1“
1 ¼”
1”
150
1”
¾”
1 ¼”
1“
1 ¼”
1”
199
1”
¾”
1 ¼”
1”
1 ¼”
1”
250
1 ¼”
1”
1 ¼”
1”
1 ½”
1 ¼”
299
1 ¼“
1“
1 ½”
1 ¼“
1 ½“
1 ¼“
399
1 ¼“
1“
1 ½“
1 ¼“
2“
1 ½“
499
1 ½“
1 ¼“
2“
1 ½“
2“
1 ½“
Installation of a union at the appliance gas line connection is
required for ease of service and removal of the gas train. Install
a manual main gas shutoff valve, outside of the appliance and
before the gas valve, as required by local codes.
Optional gas controls may require routing of bleeds and vents to
the atmosphere, outside the building when required by local
codes. Larger models of this appliance may be supplied with a
gas pressure relief valve. This valve is designed to relieve lockup
pressure in excess of the high gas pressure switch setting. It
must be piped to discharge excess gas pressure through the
valve to a safe location in accordance with local codes.
All gas connections must be made with pipe joint compound
resistant to the action of liquefied petroleum and natural gas. All
piping must comply with local codes and ordinances.
Use new,
properly threaded black iron pipe free from burrs. Avoid flexible
gas connections. Internal diameter of flexible gas lines may not
provide appliance with proper volume of gas. A trap (drip leg)
must be provided in the inlet gas connection to the appliance.
4.2
– Gas Supply Pressure
Gas supply pressure must be maintained within the specified
range in Table 4. Before operating the appliance, the complete
gas train and all connections must be purged of air and tested
using soap solution. The appliance and its individual gas shut-off
valve must be disconnected from the supply piping when
pressure testing the gas supply piping at pressures above ½ PSI.
Table 4: Gas Supply Pressure Range
Pressure Range
Propane
Natural Gas
Minimum (inches WC)
11
3*
Maximum (inches WC)
11
14
*7” WC recommended regulator setting
4.2.1
– Gas Regulators and Lockup Pressure
A stable gas supply pressure is important to achieve stable
operation on gas fired appliances using a 1:1 ratio control valve
for gas pressure regulation.
Lockup pressure (i.e. the pressure upstream of the gas valve
after closing)
must not be in excess of 14” WC. It is paramount
that maximum lockup pressure be confirmed before any attempt
is made to start up the appliance. A suitable lockup regulator with
internal or external relief will not exceed running pressure by
more than 20%. An external relief valve may be required.
Operating the VTech at lockup pressures exceeding the
recommended levels can lead to delayed ignitions and damage
to the appliance.
NOTE: In facilities where the incoming gas pressure is
significantly higher than the pressure required by the water
heaters, it may be necessary for several regulators to work
together to stage the gas pressure down in a stable fashion.
Check with the gas supplier for more information.
The final stage gas regulator is to be located a minimum of 10
linear feet (do not factor in equivalent lengths for elbows) from
the appliance. Even regulators classified as fast reaction type
require appropriately dimensioned volumes of gas between the
regulator and appliance, to absorb the pressure swings caused
by fast flow rate variations, and avoid high lockup pressure.
4.3
– Air/Gas Ratio Control Valve
Operation of the gas valve in combination with the combustion
air fan allows the burner input rate to vary from 20% to 100%.
These utilize a 1:1 ratio dual seat negative pressure gas valve.
The gas valve adjusts fuel supply according to negative pressure
generated by the combustion fan. The valve is dual seat and
serves as a safety shut-off. The inlet gas supply pressure must
be maintained within the specified minimum and maximum
pressures (a reduction of up to 30% is permitted in the inlet gas
pressure between light-off and full fire conditions).
VT80 to 250
– 1:1 Negative Pressure Control Valve
Gas Valve
Venturi
Low-fire adjustment screw (use Torx 40
for adjustment, clockwise increases CO
2
)
High-fire adjustment screw (use slotted
screwdriver for adjustment, counter-
clockwise increases CO
2
)
