When you commission or service gas-fired equipment, you’ll see two different pressure readings on your manometer: static (burners OFF) and dynamic (burners ON). Which one should you use? Here’s a clear, field-ready guide for technicians and installers.
The short answer
- Use dynamic pressure for anything that must be set or verified in operation.
- Use static pressure only to confirm you aren’t exceeding a listed maximum with the gas OFF.
What each term means
- Static pressure: Supply pressure with no flow (all burners OFF). No piping/regulator losses are showing yet.
- Dynamic pressure: Supply/manifold pressure under load (burners ON). This reflects real operating conditions.
How to read manufacturer specs correctly
1) If the spec says “Manifold pressure”
- Measure/Set: Dynamic pressure at the manifold test port with the burner(s) ON at high fire.
- Adjust: Appliance regulator to the rating-plate value (e.g., NG ≈ 3.5″ w.c., LP ≈ 10–11″ w.c.—always follow the data plate).
- Also check: While holding high fire, confirm inlet dynamic pressure meets the appliance’s minimum supply requirement.
2) If the spec says “Inlet pressure” (at the equipment inlet)
- Verify: Dynamic pressure under full load (all appliance burners ON; ideally other appliances on the same branch ON, too).
- Pass: Reading is within the stated range or ≥ the stated minimum.
- Additionally: Confirm inlet static pressure (all OFF) does not exceed the listed maximum and the valve/regulator ratings.
3) If both inlet and manifold values are listed
- Inlet (static): ≤ maximum allowed.
- Inlet (dynamic): ≥ minimum required under load.
- Manifold (dynamic): Set to the rating-plate value at high fire.
Field procedure (5 quick steps)
- Warm up the manometer and connect to the correct test port (inlet or manifold).
- Record static pressure (all burners OFF).
- Fire the unit to high and turn on other appliances on the same line (worst-case load).
- After 60–120 seconds (stabilization), record dynamic pressure.
- Compare to the spec: adjust the appliance regulator only for manifold setpoint—never to “mask” a low inlet supply.
Troubleshooting cues
- Can’t hit manifold setpoint at high fire: inlet dynamic is low—fix upstream (service regulator, piping size, meter capacity), not the air shutter.
- Yellow/lazy flame under load with normal static: inlet dynamic sags—look for undersized piping, clogged regulators/filters, or too many appliances on one run.
- Static above max rating (e.g., >14″ w.c. on many valves): correct before firing to avoid valve/regulator damage.
- Multi-burner appliances: verify pressure uniformity—check a port near the far end of the manifold at high fire.
Why dynamic matters
Static is a no-flow snapshot. Real combustion happens under flow, where pressure drops across meters, regulators, valves, and piping. That’s why operational specs (inlet minimums and manifold setpoints) are dynamic by design.
Safety note
Always follow the appliance rating plate, OEM manuals, and local codes. If readings don’t conform under load, do not compensate with an overly open air shutter—solve the gas supply issue first.