John Wood JW375T75N-AV, Instruction Manual

Page: 10 / 36

10
GAS SUPPLY SYSTEMS
Low pressure building gas supply systems are defined as those
systems that cannot under any circumstances exceed 14” W.C.
(1/2 PSI Gauge). These systems do not require pressure regulation.
Measurements should be taken to insure that gas pressures are stable
and fall within the requirements stated on the water heater rating plate.
readings should be taken with all gas burning equipment off (static
pressure) and with all gas burning equipment running at maximum rate
(dynamic pressure). The gas supply pressure must be stable within 1.5”
W.C. from static to dynamic pressure to provide good performance.
Pressure drops that exceed 1.5” W.C. may cause rough starting, noisy
combustion or nuisance outages. Increases or spikes in static pressure
during off cycles may cause failure to ignite or in severe cases damage
to appliance gas valves. If your low pressure system does not meet
these requirements, the installer is responsible for the corrections.
High Pressure building supply systems use pressures that exceed
14” W.C. (1/2 PSI Gauge). These systems must use field supplied
regulators to lower the gas pressure to less than 14” W.C. (1/2 PSI
Gauge). Appliances require gas regulators that are properly sized for the
water heater input and deliver the rating plate specified pressures. Gas
supply systems where pressure exceeds 5 PSI often require multiple
regulators to achieve desired pressures. Systems in excess of 5 PSI
building pressure should be designed by gas delivery professionals
for best performance. Water heaters connected to gas supply systems
that exceed 14” W.C. (1/2 PSI Gauge) at any time must be equipped
with a gas supply regulator.
GAS PRESSURE REQUIREMENTS
Natural gas models require a minimum gas supply pressure of 4.5”
W.C. (1.12 kPa). Propane gas models require a minimum gas supply
pressure of 11” W.C. (2.74 kPa). The minimum supply pressure
is measured while gas is flowing (dynamic pressure). The supply
pressure (dynamic) should never fall below the specified minimum
supply pressure. The supply pressure should be measured with all gas
fired appliances connected to the common main firing at full capacity. If
the supply pressure drops more than 1.5” W.C. (0.37 kPa) as gas begins
to flow to the water heater then the supply gas system including the
gas line and/or the gas regulator may be restricted or undersized. See
Supply Gas regulator section and Gas Piping section of this manual.
The gas valve on all models has a maximum gas supply pressure limit
of 14” W.C. (3.48 kPa) The maximum supply pressure is measured
while gas is not flowing (static pressure).
SUPPLY GAS REGULATOR
The maximum allowable gas supply pressure for this water heater
is 14.0 inches W.C. (3.48 kPa). Install a positive lock-up gas pressure
regulator in the gas supply line if inlet gas pressure can exceed 14.0
inches W.C. (3.48 kPa) at any time. regulators must be sized/used
according to manufacturer’s specifications.
If a positive lock-up regulator is required follow these instructions:
1. Positive lock-up gas pressure regulators must be rated at or above
the input Btu/hr rating of the water heater they supply.
2. Positive lock-up gas pressure regulator(s) should be installed no
closer than 3 feet (1 meter) and no farther than 8 feet (2.4 meters)
of equivalent length from the water heater’s inlet gas connection.
3. After installing the positive lock-up gas pressure regulator(s) an initial
nominal supply pressure setting of 7.0” W.C. while the water heater
is operating is recommended and will generally provide good
water heater operation. Some addition adjustment maybe required
later to maintain a steady gas supply pressure.
4. When installing multiple water heaters in the same gas supply
system it is recommended that individual positive lock-up gas
pressure regulators be installed at each unit.
MIXING VALVES
Water temperature over 125°F (52°C)
can cause severe burns instantly
resulting in severe injury or death.
Children, the elderly and the physically
or mentally disabled are at highest risk
for scald injury.
Feel water before bathing or showering.
Temperature limiting devices such as
mixing valves must be installed when
required by codes and to ensure safe
temperatures at fixtures.
Water heated to a temperature which will satisfy clothes washing, dish
washing, and other sanitizing needs can scald and cause permanent
injury upon contact. Short repeated heating cycles caused by small
hot water uses can cause temperatures at the point of use to exceed
the water heater’s temperature setting by up to 20°F (11°C).
Some people are more likely to be permanently injured by hot water
than others. These include the elderly, children, the infirm and the
physically/mentally disabled. Table 2 shows the approximate time-
to-burn relationship for normal adult skin. If anyone using hot water
provided by the water heater being installed fits into one of these groups
or if there is a local code requiring a certain water temperature at the
point of use, then special precautions must be taken.
In addition to using the lowest possible temperature setting that
satisfies demand of the application a Mixing Valve should be installed
at the water heater or at hot water taps to further reduce system water
temperature. See Figure 6.
Mixing valves are available at plumbing supply stores. Consult
a Qualified Installer or Service Agency. Follow mixing valve
manufacturer’s instructions for installation of the valves.
TABLE 2.
Water
Temperature °F
Time for
1st Degree Burn
(Less Severe Burns)
Time for Permanent Burns
2nd & 3rd Degree
(Most Severe Burns)
110 (normal shower temp.)
116 (pain threshold)
116 35 minutes 45 minutes
122 1 minute 5 minutes
131 5 seconds 25 seconds
140 2 seconds 5 seconds
149 1 second 2 seconds
154 instantaneous 1 second
(U.S. Government Memorandum, C.P.S.C., Peter L. Armstrong, Sept. 15,1978)
HOT WATER
OUTLET
TO TANK
INLET
CHECK
VALVE
MIXING
VALVE
COLD
WATER
INLET
TEMPERED WATER
OUTLET
12” TO 15”
(30-38 cm)
CHECK
VALVE
FIGURE 6.
INSTALLATION REQUIREMENTS