Gas Supply Systems; Supply Gas Regulator - A.O. Smith BTR 120-400 Service Handbook

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). Water
heaters 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.
All models require a minimum gas supply pressure of 4.5" W.C. for natural gas and 11.0" W.C. for propane
gas. The minimum supply pressure is measured while gas is flowing (dynamic pressure). The supply pressure
should never fall below 4.5" W.C. for natural gas and 11.0" W.C. for propane gas. 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. 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. 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 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 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 equivalent feet (1 meter)
and no farther than 8 equivalent feet (2.4 meters) from 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.
6 Servicing should only be performed by a Qualified Service Technician