Short answer
Inlet pressure is the gas pressure arriving at the appliance's gas valve or governor. Burner (or manifold) pressure is the pressure after the appliance's own regulation, at the injectors. Inlet pressure must be within the manufacturer's range for the appliance to work; burner pressure is what sets the actual heat input and is adjusted during commissioning.
Why they differ
Most commercial appliances have a governor or multifunctional gas valve that drops pressure to a controlled value. If inlet pressure is too low, the valve cannot hold burner pressure and output falls. Modern premix boilers often use a zero-governor valve where gas flow follows fan speed, so combustion analysis rather than burner pressure is used to set them.
Checking both
Measure inlet pressure at the appliance inlet test point with the appliance and others running at full load. Measure burner pressure at the manifold test point and compare with the data plate. On fan-assisted premix appliances, confirm gas rate by meter timing and CO₂/CO readings instead.
Typical faults
Low inlet pressure at full fire causing lockout or poor flame; burner pressure drifting from a faulty governor; and burners set by eye rather than by pressure and analysis.
Figures are typical for UK natural gas. The current edition of the relevant IGEM standard and the manufacturer's instructions always take precedence on site.
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