How to Measure Gas Pressure at the Generator, Static and Under Load

Why this matters

Static pressure tells you gas is present. Running pressure at full load tells you the supply can actually feed the engine. Most fuel-starvation callbacks come from a tech who read the first number and wrote "gas OK." The generator starts, exercises and passes a no-load check on a line that cannot carry it, then drops the house on the first outage night when the furnace and water heater are firing too. Two readings, ten minutes apart, at the same port, settle it.

Safety first

If you smell gas on arrival, nobody touches the unit. Everyone moves away from the generator and the house, no switches, no starting the engine, no phone used at the unit, and the call goes to the utility from a safe distance. Testing resumes only after the leak is found and fixed.

The engine must not start while a hand is in the enclosure. Before opening the fuel compartment, take the controller out of automatic and lock out auto-start per the shop's procedure. See related: The Generator Auto-Start Lockout Before Service SOP. The unit gets started deliberately later, from the controller, with hands and hoses clear of the fan and the exhaust.

Hot parts and exhaust. During the running reading, the exhaust side and muffler are hot enough to burn; route the manometer hose away from them and stand on the intake side. The unit is outdoors at its sited clearances, and the CO survey belongs to its own procedure. See related: How to Survey Carbon Monoxide at Openings During a Generator Run.

What you are connecting to

 GAS SUPPLY from meter
      |
  [manual shutoff valve]   <- close this to fit the gauge
      |
      +--------------+   tee: gas leaves out the side
      |              |
  [nipple + cap]  [flexible connector]
  sediment trap,     |
  straight down   UNIT FUEL INLET
                     |
                     +-- [1/8 NPT test port, plug out]
                     |       |
                     |    barb -> hose -> MANOMETER
                     |                    (in. w.c.)
              [fuel solenoid valves]
                     |
              [demand regulator] -> mixer -> engine

The supply test port is the one upstream of the unit's own solenoids and regulator. Some units carry a second port downstream of the regulator for an outlet or manifold reading; that is a different measurement with a different spec, and the maker's manual says which port means what. Read the wrong port and both numbers on the sheet are meaningless.

Step 1: Get the spec before you get the gauge

Pull the maker's installation manual for this model and fuel and write down the supply pressure range at the inlet, in inches of water column, both the minimum while running at full load and the maximum static. Natural gas and propane carry different ranges on the same unit. If the manual gives only a single figure, note whether it is a minimum running pressure or a nominal; the difference decides whether a reading passes.

Why first: a tech who reads the gauge before knowing the range tends to accept whatever looks normal for a furnace. A generator's demand regulator often wants a narrower window than a house appliance tolerates.

Step 2: Shut off, fit the gauge, prove the fitting

1. Controller off and auto-start locked out.
2. Close the manual shutoff upstream of the sediment trap.
3. Remove the test port plug. A little gas escapes from the trapped section; that is expected and is why the valve is closed first.
4. Zero the manometer with its hose open to air, then thread in a barb fitting and push the hose on.
5. Open the shutoff slowly.
6. Brush leak-detection solution on the barb fitting threads and the hose connection. Bubbles mean fix it now, before anything runs.

Use a digital manometer or a water-filled U-tube that reads at least 0 to 20 inches of water column with tenths. A gauge graduated in psi cannot read this; 7 inches of water column is about a quarter of a psi.

Step 3: Read static pressure

With the generator off and nothing flowing through its line, read and record. This is the delivered pressure at the point of use with zero flow through the generator line.

Two things to compare it against. First, the maker's maximum: a static pressure above the maximum can overwhelm a demand regulator and needs a line regulator ahead of the unit, set by the fitter. Second, what the house should have: most low-pressure residential systems deliver around 7 inches at the meter outlet. A static reading well under that at the generator, with nothing running anywhere, points upstream before the engine ever starts.

Step 4: Start, load to full, read running pressure

With hands and hose clear, release the auto-start lockout per the procedure, leave the controller out of automatic, and start the unit in manual from the controller. Let it reach speed. Read the no-load running pressure; it should sit close to static, because an idling generator draws little gas.

Then load it to its full rating, which is the reading that matters. Two ways:

• A portable load bank sized to the generator's rated output. This is the reliable way, because you control the load. See related: How to Connect a Portable Load Bank to a Residential Standby.
• The house load, by simulating an outage and turning on the heavy loads. This works only if the house can actually pull near the generator's rating, and most houses on a mild day cannot.

Hold full load for at least two minutes so the reading settles, then record the lowest steady value. At the same time, if any gas appliance in the house can be fired (furnace, water heater, range burners), fire it. The real outage puts all of them on the line together, and the test should too.

Pressure drop rises faster than load. On a low-pressure system, the pipe's pressure drop grows roughly with flow to the power of about 1.85, the relationship behind the capacity tables in NFPA 54 and the IFGC, for the same pipe and gas. Half load gives about 0.5^1.85, roughly 0.28 of the full-load drop on the pipe's share. A tech who tests at half load and sees a 1-inch drop has not seen a 2-inch problem; he has seen something closer to a 3.6-inch one. That is why the reading has to be at full load, or else it understates the problem badly.

Step 5: Restore and prove it

1. Remove the load, let the unit run unloaded for its cooldown, then stop it from the controller and lock out auto-start again.
2. Close the manual shutoff.
3. Remove the barb fitting and reinstall the test port plug with a thread sealant rated for gas.
4. Open the shutoff and leak-test the plug with solution. No bubbles before you move on.
5. Remove the lockout, return the controller to automatic, and confirm the display shows the unit ready in auto. A unit left in off after a service visit is a no-start in the next outage.

Worked example: a 3.1-inch drop at full load

A 22 kW air-cooled standby on natural gas, two years old, owner reports it "struggled" in a short outage last winter. The installation manual (illustrative values for this example) gives a minimum of 5.0 inches water column at the inlet test port while running at full load, and a maximum static of 13.0.

Static drops to full load by 7.2 - 4.1 = 3.1 inches. The running reading of 4.1 is below the 5.0 minimum. Static at 7.2 is well under the 13.0 maximum, so static passes; a tech who stopped there would have signed off a unit that cannot run the house.

The 3.1-inch drop is the finding. The house was piped to the common 0.5-inch design drop, so something is costing about six times what the design allowed.

Where the drop lives. The fitter or utility reads the meter outlet at the same moment, under the same load:

• Meter outlet holds about 7.0 under load. The meter and service regulator are fine; 2.9 of the 3.1 inches (7.0 - 4.1) is lost in the piping between the meter and the unit. Undersized pipe, a too-long run of small-diameter flexible connector, or a partly closed valve. In this example the fitter found 1-inch pipe where a takeoff called for 1-1/4. See related: How to Size Natural Gas Pipe to a Standby Generator.
• Meter outlet itself falls to about 4.5 under load. The service regulator or meter cannot deliver the combined flow. That is the utility's equipment and the utility's call.

Route the repair. The generator tech does not re-pipe or adjust the service regulator. The readings, with the load and the time, go on the work order and to the fitter or utility. After the repair, the full-load reading is repeated at the same port on the same load, and it has to land at or above 5.0.

What would change the call

A pressure inside the range but a large drop. Static 9.0, full load 6.2: the 6.2 passes, but a 2.8-inch drop on a line designed for 0.5 means a cold night with every appliance at full input may push it lower. Note it and recommend the fitter look, but do not fail the unit on a passing reading.

Propane. The method is identical; the range is different (propane systems typically deliver around 11 inches water column at the second-stage regulator outlet, and the maker's range follows that), and a falling pressure in cold weather can be the tank, not the pipe. See related: How to Check a Propane Tank for Generator Runtime and Vaporization.

A unit that hunts or droops with good pressure. If the supply holds inside the range at full load and the engine still struggles, the supply is cleared and the problem moves into the fuel train: solenoid, demand regulator or mixer. See related: How to Replace a Standby Generator Fuel Regulator.

References

• The generator maker's installation manual for the inlet supply pressure range by fuel and the test port locations
• NFPA 54, National Fuel Gas Code, or the International Fuel Gas Code, in the edition your jurisdiction has adopted
• See related: The Generator Auto-Start Lockout Before Service SOP; Gas Supply Sizing and Connection Standard; How to Size Natural Gas Pipe to a Standby Generator