Gas Pressure Good Static Drops Under Load Decision Tree

Why this matters

A gas system that reads correct static pressure at the appliance but collapses the moment the burner fires is a volume-delivery problem, and it starves appliances, drops them out on flame failure, and forces incomplete combustion that risks carbon monoxide. Static pressure is the line at rest; the pressure under load is what the appliance actually gets, and a gap between them means the piping or the regulator cannot deliver the gas volume the load demands. Reading only static and walking away leaves the customer with an appliance that fails under real conditions. The tree below uses static manometer readings, dynamic readings under full firing load, and progressive isolation to find whether the regulator, the meter, or undersized piping is choking volume.

Symptom presentation

The customer reports an appliance that lights then drops out, a furnace or water heater that nuisance-faults on flame failure, low burner flames, or multiple gas appliances that misbehave when they run together. A manometer on the appliance gas valve reads a correct static inlet pressure with everything off, then sags below the appliance minimum when the burner fires, especially with other gas loads running. The signature is the spread between a good static reading and a failing dynamic reading under load.

Quick checks

1. Connect a manometer to the appliance gas valve inlet tap and record static inlet pressure with all gas off.
2. Fire the appliance to full input and record dynamic inlet pressure; compare to the appliance minimum on the rating plate.
3. Fire all gas appliances simultaneously and re-read dynamic pressure at the worst-case appliance, usually the farthest or largest.
4. Read pressure at the meter or service regulator outlet under the same full load to see if the drop is upstream or in the house piping.
5. Confirm the appliance manifold (outlet) pressure against the rating plate once inlet is verified.

Isolation tree

Branch A: Drop appears across the house piping, regulator outlet holds

The service regulator delivers, but the house piping cannot pass the volume.

• Undersized branch or trunk piping for the connected load: the run cannot carry the cubic feet per hour demanded. A dynamic drop that worsens as more appliances fire is the classic undersized-pipe signature.
• A long run with too many fittings adds equivalent length and pressure loss; size against the NFPA 54 longest-length method.
• A partially closed appliance shutoff or a restricted flex connector chokes the single appliance only.

Branch B: Drop appears at the meter or service regulator outlet

The supply side cannot deliver under load.

• A failing or undersized service regulator sags under full house demand; the outlet pressure itself drops when load is applied.
• A clogged regulator vent or a frozen vent screen restricts the diaphragm response, causing the outlet to droop under load.
• An undersized meter for the total connected load limits deliverable volume; coordinate with the gas utility.

Branch C: Drop only with multiple appliances firing

The system serves one appliance but not the combined load.

• Total connected load exceeds the pipe sizing or the regulator or meter capacity. Sum the input ratings in cubic feet per hour and check against the system capacity.
• A two-stage system where the second-stage regulator is undersized for simultaneous demand.

Branch D: Single appliance only, others fine

The restriction is local to that appliance.

• A clogged appliance regulator or a sediment-fouled gas valve inlet, often from pipe scale or a missing sediment trap upstream.
• A kinked or undersized appliance connector that passes static but cannot carry the firing-rate volume.
• A partially closed appliance shutoff valve never reopened fully after past service.

Branch E: Pressure recovers when an upstream appliance shuts off

A capacity ceiling shared across the system.

• The appliance only drops out when a specific larger appliance, often a furnace or a tankless heater, is also firing. This confirms the combined load exceeds the pipe, regulator, or meter capacity rather than a fault at the dropping appliance.
• Sum every appliance input rating in cubic feet per hour and compare the total to the system capacity at the installed pipe size and longest length; a total over the table value confirms the ceiling.

Confirming diagnosis

Confirm by reading dynamic pressure at progressive points under the same full load. Place the manometer at the service regulator outlet, then at the trunk, then at the appliance inlet, each time with the worst-case load firing. The point where pressure falls off under load brackets the restriction: a droop at the regulator outlet condemns the supply side, a droop that develops between the regulator and the appliance condemns the house piping sizing, and a droop confined to one appliance condemns its connector, shutoff, or regulator. Calculate the total connected load in cubic feet per hour and compare to the NFPA 54 capacity table for the installed pipe size, length, and gas type; an undersized result confirms a piping deficiency rather than a regulator fault.

Remediation

• Undersized house piping: resize and repipe the deficient run to carry the connected load per the NFPA 54 longest-length method, accounting for fitting equivalent length, not just straight pipe.
• Failing or undersized service regulator: have the gas utility replace or upsize the service regulator and clear the vent screen; regulator vent and meter work is utility scope, not the plumber's, in most jurisdictions.
• Undersized meter: coordinate a meter upsize with the utility for the total connected load when the summed cubic-feet-per-hour input exceeds the meter capacity.
• Local appliance restriction: replace the kinked or undersized connector, open a partially closed shutoff, or service the appliance gas valve and inlet regulator.
• Two-stage system second-stage shortfall: upsize or replace the undersized second-stage regulator that sags under simultaneous demand.
• Re-verify dynamic inlet pressure within the appliance range under full simultaneous load, then confirm manifold pressure against the rating plate and run a combustion analysis after any change.

References

• NFPA 54 National Fuel Gas Code (gas piping sizing, longest-length method, pressure drop allowances)
• International Fuel Gas Code (IFGC) Chapter 4 Gas Piping Installations
• Uniform Plumbing Code (UPC) Fuel Gas provisions and Chapter 12 where adopted
• CSA Z21 / appliance manufacturer rating-plate inlet and manifold pressure specifications